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Rock pore structure coupled with fluid pressure plays an important role in controlling fault slip behavior. Observation of fluid-induced seismicity in geoenergy extraction has raised fundamental questions about the physics of fault rock structure and fault frictional stability in the presence of fluid. Here, we change the pore structure of faults by thermal treatment and report on the frictional stability of granite faults with pore evolution and pore fluid pressure in velocity stepping experiments under the rate-and-state framework, where the variation of pore fluid is monitored. The experiments under constant fluid pressure show that pore structure propagation leads to an increase in friction coefficient from 0.71 to 0.78. As the degree of pore propagation increases, the drained fault exhibits a transition from velocity strengthening to weakening behavior. The decrease in frictional stability could be caused by the coupling between the pore fluid and the well-connected pores, namely “fluid oscillation”. Pore pressure overpressurization could develop and cause non-uniform stress distribution along the fault surface due to pore fluid oscillation at velocity steps. The time required to equilibrate fluid pressure could be prolonged by fluid oscillation, leading to intrinsic velocity strengthening behavior appearing as velocity weakening. The decrease in rate-and-state parameter with elevating pore fluid pressure on high-porosity fault corroborates the fluid-induced fault destabilization. The fluid oscillation at the greater pore pressure could be responsible for fault reactivation. Therefore, the coupling effect of rock pore structure with pore fluid could be a potential mechanism governing fault frictional stability. [ABSTRACT FROM AUTHOR]

Predicting pore pressure is critical in every exploration venture because it prompts safe drilling, fluid design, casing emplacements, improved wellbore dependability, and water driven cracking enhancement. Hence, the objective of this paper was to predict and evaluate the pore pressure characteristics of a selected field in the Niger Delta Basin of Nigeria using appropriate standard methods. The results of the 1D pore pressure model from the Eaton and Bower's sonic techniques revealed that the deviation from Normal Compaction Trend started at about 8350 ft as a result of overpressure. The determined break angle from the Fracture gradient formular, utilizing the Effective Stress Ratio (ESR), Overburden Pressure (OVP) and the anticipated Pore Pressure (PP) was estimated to be between 0.65 psi/ft and 0.725 psi/ft. Due to the use of overbalanced drilling in the Niger-Delta, wellbore pressure should be higher than the Hydrostatic pressure, but not as high as the fracture pressure. Therefore, the safe drilling window should likely be from 0.613 psi/ft to 0.645 psi/ft for the study area. [ABSTRACT FROM AUTHOR]

Wave erosion of slopes can easily trigger landslides into marine environments and pose severe threats to both the ecological environment and human activities. Therefore, near-shore slope monitoring becomes crucial for preventing and alerting people to these potential disasters. To achieve a comprehensive understanding, it is imperative to conduct a detailed investigation into the dynamics of wave erosion processes acting on slopes. This research is conducted through flume tests, using a wave maker to create waves of various heights and frequencies to erode the slope models. During the tests, seismic signals, acoustic signals, and pore pressure generated by wave erosion and slope failure are recorded. Seismic and acoustic signals are analyzed, and time-frequency spectra are calculated using the Hilbert–Huang Transform to identify the erosion events and signal frequency ranges. Arias Intensity is used to assess seismic energy and explore the relationship between the amount of erosion and energy. The results show that wave height has a more decisive influence on erosion behavior and retreat than wave frequency. Rapid drawdown may potentially cause the slope to slide during cyclic swash and backwash wave action. As wave erosion changes from swash to impact, there is a significant increase in the spectral magnitude and Power Spectral Density (PSD) of both seismic and acoustic signals. An increase in pore pressure is observed due to the rise in the run-up height of waves. The amplitude of pore pressure will increase as the slope undergoes further erosion. Understanding the results of this study can aid in predicting erosion and in planning effective management strategies for slopes subject to wave action. [ABSTRACT FROM AUTHOR]

This paper presents two case studies dealing with undesirable impacts of overburden drilling of casings for end-bearing piles to bedrock. Monitored pore-water pressures and ground settlements are used to document and assess the influence from rotary percussive drilling with "down-the-hole" (DTH) hammers. The studies show that drilling with high-pressure air-driven DTH hammers may cause considerable erosion and soil volume loss adjacent to the drill bit and along the casing, resulting in settlements of the surrounding ground. The risk of soil volume loss increases when the drilling is carried out in erodible soils such as silt and fine sands. The volume loss is found to be caused by a combined air-lift pump effect and a Venturi suction effect. Monitoring pore pressures in the vicinity of the drilling may be used to reduce soil volume loss and prevent damaging settlements. Results from drilling with water-driven DTH hammer showed significantly less ground settlements and influence on pore pressures compared to using an air-driven hammer. The study suggests that the drilling parameters flow rate and penetration rate, and the cross-sectional area of the pile casing can be combined in a non-dimensional methodology to assess the mass balance of drill cuttings when drilling with water flushing. A design framework is suggested to guide overburden drilling in urban settings to reduce potential impact on the surroundings. [ABSTRACT FROM AUTHOR]

As a waterless fracturing fluids for gas shale stimulation with low viscosity and strong diffusibility, supercritical CO2 is promising than the water by avoiding the clay hydration expansion and reducing reservoir damage. The permeability evolution influenced by the changes of the temperature and stress is the key to gas extraction in deep buried shale reservoirs. Thus, the study focuses on the coupling influence of effective stress, temperature, and CO2 adsorption expansion effects on the seepage characteristics of Silurian Longmaxi shale fractured by supercritical CO2. The results show that when the gas pressure is 1-3 MPa, the permeability decreases significantly with the increase in gas pressure, and the Klinkenberg effects plays a predominant role at this stage. When the gas pressure is 3-5 MPa, the permeability increases with the increase in gas pressure, and the influence of effective stress on permeability is dominant. The permeability decreases exponentially with the increase in effective stress. The permeability of shale after the adsorption of CO2 gas is significantly lower than that of before adsorption; the permeability decreases with the increase in temperature at 305.15 K-321.15 K, and with the increase in temperature, the permeability sensitivity to the temperature decreases. The permeability is closely related to supercritical CO2 injection pressure and volume stress; when the injection pressure of supercritical CO2 is constant, the permeability decreases with the increase in volume stress. The results can be used for the dynamic prediction of reservoir permeability and gas extraction in CO2-enhanced shale gas development. [ABSTRACT FROM AUTHOR]

Rates of seismicity in the Delaware Basin of Texas and New Mexico increased from 10 earthquakes per year of local magnitude (ML) 3.0 and above in 2017 to more than 185 in 2022, coincident with increasing oil and gas production and wastewater re‐injection into strata shallow or deeper than producing intervals. Events of large magnitude—up to ML 5.4 to‐date—occur on faults extending into formations above the basement that have received more than four billion barrels of injection. Here, we demonstrate the link between injection geology, pore pressure evolution, fault stability, and induced seismicity in this region. We find that the injection targets are largely dolomitized platform carbonates with low (<5 vol.%) matrix porosity and fracture‐enhanced permeability with inherent heterogeneity in flow properties. A comprehensive, three‐dimensional geological model populated with reservoir properties is used for fluid flow modeling, with global calibration supplemented by dynamic injectivity data. Pore pressure changes with deep injection are up to 5 MPa from 1983 to 2023, increasing the native pore pressure state by 10% locally. Modeling results show that earthquakes occurring at distances of up to 30 km from deep injection have experienced small (<0.1 MPa) pore pressure increases, indicating that the faults hosting these earthquakes are highly sensitive to changes in effective stress and have lower frictional stability than the 0.6 generally assumed. These results serve as a critical step in understanding the stress changes that induce earthquakes in one of the most seismically active and geologically complex basins in the US. Plain Language Summary: Earthquakes have become more frequent in areas with historically little seismicity and are coincident in space and time with an increase in oil and gas production and the re‐injection of wastewater. Larger‐magnitude earthquakes occur on deep faults and generally within the crystalline basement. They are likely caused by wastewater injection into carbonate rocks above the basement, causing changes in pressure and stress on nearby faults. Results from this modeling work show that deep fluid injection increases pressure by up to 10% near deep injection wells. Key Points: Injection of oil field wastewater into deep strata in the Delaware Basin causes pore pressure changes of up to 5 MPaEarthquakes occurring in crystalline basement are likely induced by pore pressure diffusion associated with regional deep injectionMany basement‐rooted faults hosting earthquakes are optimally oriented, sensitive, and slip with small pore pressure perturbations [ABSTRACT FROM AUTHOR]

The semi-circular breakwater (SBW) has been implemented at numerous global locations due to its outstanding wave dissipation effectiveness and high structural performance. This study extends prior research by investigating the seabed dynamic response and hydrodynamic response characteristics around perforated SBWs. A coupled numerical model is developed to integrate waves, a semi-circular breakwater, and a sandy seabed. Wave behavior is simulated using Reynolds-averaged Navier–Stokes equations with a k-ε turbulence closure scheme, and the seabed response is numerically simulated using Biot's full-dynamic (u-w) equations. After verifying computational accuracy, a series of tests is conducted to explore the effects of marine environments and SBW characteristics. Test results reveal a positive correlation between seabed response and wave height, wave period, and perforation number, while showing a negative correlation between seabed response and water depth and perforation rate. The basic perforation type is more effective than front and rear perforation types in maintaining a stable flow field and seabed response. These findings provide insights for designing SBWs for effective wave dissipation and seabed stability in complex marine environments, offering valuable recommendations for future designs. [ABSTRACT FROM AUTHOR]

Predicting pore pressure is critical in every exploration venture because it prompts safe drilling, fluid design, casing emplacements, improved wellbore dependability, and water driven cracking enhancement. Hence, the objective of this paper was to predict and evaluate the pore pressure characteristics of a selected field in the Niger Delta Basin of Nigeria using appropriate standard methods. The results of the 1D pore pressure model from the Eaton and Bower’s sonic techniques revealed that the deviation from Normal Compaction Trend started at about 8350 ft as a result of overpressure. The determined break angle from the Fracture gradient formular, utilizing the Effective Stress Ratio (ESR), Overburden Pressure (OVP) and the anticipated Pore Pressure (PP) was estimated to be between 0.65 psi/ft and 0.725 psi/ft. Due to the use of overbalanced drilling in the Niger-Delta, wellbore pressure should be higher than the Hydrostatic pressure, but not as high as the fracture pressure. Therefore, the safe drilling window should likely be from 0.613 psi/ft to 0.645 psi/ft for the study area.

Abstract A two-dimensional unsteady seepage model for coal using a finite element program is developed, and the temporal variations of key factors such as water pressure and hydraulic gradient are analyzed in this paper. Additionally, the triaxial rock mechanical experiment and utilized pneumatic fracturing equipment on raw coal samples to investigate both hydraulic and pneumatic fracturing processes are conducted. Through these experiments, the relationship between pressure and crack formation and expansion are examined. The analysis reveals that the pore pressure gradient at the coal inlet reaches its peak during rapid surges in water pressure but diminishes over time. Conversely, the pore pressure gradient at the outlet side exhibits a gradual increase. Hydraulic fracturing is most likely to occur at the water inlet during sudden increases in water pressure. Besides, as the permeability of coal decreases, the duration for seepage stabilization prolongs due to the intensified pore pressure gradient resulting from sudden increases in water pressure. Moreover, an extended period of high hydraulic gradient further increases the risk of hydraulic fracturing. The experimental findings indicate that coal samples initially experience tensile failure influenced by water and air pressure. Subsequently, mode I cracks form under pressure, propagating along the fracture surface and becoming visible. The main types of failure observed in hydraulic and pneumatic fracturing are diametrical tensile failure, and the development of fractures can be categorized into three distinct stages, which contains the initial stage characterized by slight volume changes while water pressure increases, the expansion stage when pressure reaches the failure strength, and the crack closure stage marked by little or even decreasing volume changes during pressure unloading. The acoustic emission signal accurately corresponds to these three stages.

Objective The MS 6.8 earthquake that struck Luding County, Sichuan Province, on September 5, 2022, and its aftershocks have drawn widespread attention, especially concerning the potential risks of induced seismicity associated with the construction of high-dam reservoirs in regions with high seismic intensity. Previous studies have explored the possible link between reservoirs and seismic activity, without reaching a definitive conclusion. This study aims to assess the impact of water storage in the Dagangshan Reservoir on the surrounding strata and its correlation with recent seismic events. Methods Numerical simulation methods were employed using high-precision digital elevation model (DEM) data, fault data, and reservoir water level information to develop a three-dimensional poroelastic finite element numerical model extending from the surface to a depth of 25 km. By analyzing the hydrogeological conditions, lithology of rock masses, and groundwater dynamic changes, this study evaluated the seismic hazard risk of major faults, such as the Moxi Fault, and calculated variations in Coulomb stress and strata pore pressure at the hypocenter during the occurrence of the earthquake. Results The study indicates that, during the MS 6.8 Luding earthquake on September 5, 2022, the pore pressure at the epicenter reached 5 kPa, and the Coulomb stress increased by 3.6 kPa, suggesting that the impoundment of the Dagangshan Reservoir contributed to an increased risk along the Moxi Fault on the northwestern side of the reservoir. Using the source parameters of the MS 5.6 aftershock that occurred on January 26, 2023, it was observed that the impoundment caused a change in Coulomb stress at the epicenter of -0.69 kPa and a pore pressure of approximately 0.32 kPa. It is evident that the reservoir impoundment had a relatively minor impact on the fault activity where the MS 5.6 aftershock occurred and even exhibited a certain inhibitory effect. Moreover, seismic activity was mainly concentrated in two areas on the western side of the reservoir, with both the seismicity and expected magnitudes in these regions reflecting a higher risk of earthquakes. Conclusion This study demonstrates a correlation between the impoundment activities of the Dagangshan Reservoir and the occurrence of the Luding County earthquake and its aftershocks. The spatial distribution characteristics of the earthquakes align with the geological stress adjustment patterns following the reservoir impoundment, which played a promotive role in the occurrence of the MS 6.8 main shock, leading to an increased risk of earthquakes in the Moxi Fault region. This finding is significant for understanding the mechanisms of reservoir-induced earthquakes, subsequent aftershock analyses, and earthquake disaster prevention and mitigation efforts. Significance The results of this study provide new insights into the complex relationship between reservoir water storage and seismic events. This study offers a scientific basis for future assessments of seismic risks of reservoir design and operation, contributing to improved accuracy in earthquake early warnings and efficiency in disaster prevention and mitigation efforts.

Purposes Increasing the permeability of coal reservoirs is a key technology to improve the recovery rate of coalbed methane, and in-situ thermal injection mining is an effective method to enhance coalbed methane production. Methods In order to explore the change rule of coal reservoir permeability under the coupling of heat, fluid, and solid, pyrolysis seepage experiments of anthracite with different burial depths and temperatures were conducted, and theoretical analysis and research were conducted on the experimental results. Findings The results showed that: 1) Under three-dimensional effective stress, the permeability of pyrolysis anthracite varies with temperature and pore pressure in a "V" shape, with a significant rebound phenomenon, which decreases with the increase of burial depth. When the pore pressure is less than the critical pore pressure and the temperature is lower than 70 ℃, the permeability of the coal body mainly depends on the slippage effect. When the pore pressure is greater than the critical pore pressure and the temperature is higher than 70 ℃, the permeability mainly depends on thermal expansion and the opening degree of pore pressure on the fracture, and the impact of thermal stimulation on the permeability is much greater than that of pore pressure. 2) The critical pore pressure is between 1 and 1.5 MPa, the influence of slippage effect on permeability gradually decreases with the increase of temperature, and the gradient of influence reduction increases with the increase of burial depth. 3) The fracture opening coefficient increases with the increase of temperature and burial depth, but the higher the temperature, the smaller the change in the sensitivity of permeability to pore pressure under different burial depths. Conclusions These research results further enrich the theory of in-situ thermal injection mining of coalbed methane, and provide valuable data reference for enhanced permeability of coalbed methane.

Deep rock is under a complex geological environment with high geo-stress, high osmotic pressure, and strong dynamic disturbance, under the action of the three, the rock body is more prone to damage and rupture, inducing sudden water surges, seepage, blowouts and other engineering geologic hazards. Therefore, investigating the rock dynamics of the rock under hydraulic-mechanical coupling is one of the prerequisites for conducting rock engineering construction. In recent years, many scholars have obtained some fruitful research results in the study of rock dynamics properties under the consideration of water and different stress states. In order to provide more comprehensive guidance for engineering construction and facilitate the subsequent research, the above work is reviewed and summarized in terms of experimental setups, test results, and the mechanism of the confining pressure and water content. Firstly, the basic principle of the split Hopkinson pressure bar (SHPB) system and the device improvements used to simulate the deep rock storage environment are introduced, including the confine-ment-coupled SHPB system and pore-pressure (osmotic pressure)-coupled SHPB system. The advantages and shortcomings of each type of device in the study of rock dynamics under hydraulic-mechanical coupling are briefly analyzed. Secondly, the dynamic mechanical response characteristics of rocks hydraulic-mechanical coupling considering different stress states (uniaxial confining, triaxial confining) are summarized. The dynamic mechanical response of deep rocks under fixed preset pore pressure and osmotic pressure coupling and its law of variation with pore water pressure and osmotic pressure are described in detail. Subsequently, the mechanism of confining pressure on the dynamic properties of the rock is outlined, and the influence law under different stress states is analyzed. The strengthening and weakening microscopic mechanism and quantitative expression of the dynamic mechanical properties of the rock by water are recapped. Finally, the dynamic response of deep rocks under hydraulic-mechanical coupling is summed up, and the further experimental research work and the research direction of deep rock dynamics are proposed.

Abstract To further investigate the weakening effect of pore water pressure on intact rock mechanics properties and characteristics of fracture surface after failure, direct shear tests of sandstone were conducted under different pore pressure. A 3D scanner was employed to digitize the morphology of the post-shear fracture surface. The variogram function was applied to quantify the anisotropic characteristics of post-shear fracture surface. The relationship between deformation during shear failure of intact rock and quantitative parameters of fracture surface after shear failure was initially established. It can be found that amplitudes of the sinusoidal surface determine the maximum value of variogram, and period affect lag distance that reach the maximum value of variogram. Test results revealed that the increase of pore pressure has obvious weakening effect on shear strength and deformation of rock. Moreover, the increase of pore pressure makes the shear fracture surface flatter. It can be obtained that both Sill max and Range max are positively related to shear strain, but negatively related to normal strain.

Episodic Tremors and Slip (ETS) are highly susceptible and sensitive to external stress perturbations. The tidal and remote triggering phenomena of tremors are well-documented globally, however, the significance of the delayed triggering mechanism remains elusive. In this paper, the possibilities of the tremor modulation by the Lamb waves induced from the Hunga Tonga-Hunga Ha’apai volcanic eruption (SW Pacific) on 15 January 2022 have been explored. The increasing activity of tremors after eruption has been explored at the study region of Cascadia subduction zone where such tremor do not correlate with tidal stress perturbations or remote triggering by far-off or near-by earthquakes. The increasing tremor activities are observed during the propagation of the Lamb wave cycles (L1, L2, L3, L4) and more interestingly during inter-ETS period. From the seismic waveform analysis, we observe two coherent packets of teleseismic energy on 15 January 2022, which corresponds to arrival of surface and Lamb waves, respectively. The delayed triggering of tremors may be linked either with the teleseismic surface waves or Lamb waves from the volcanic explosion, or both. Although we cannot rule out coincidence, the delayed triggering by Lamb waves appears to be consistent with magnitude-dependent time delay and 2-D coupled pore pressure induced diffusion model.

A two-dimensional unsteady seepage model for coal using a finite element program is developed, and the temporal variations of key factors such as water pressure and hydraulic gradient are analyzed in this paper. Additionally, the triaxial rock mechanical experiment and utilized pneumatic fracturing equipment on raw coal samples to investigate both hydraulic and pneumatic fracturing processes are conducted. Through these experiments, the relationship between pressure and crack formation and expansion are examined. The analysis reveals that the pore pressure gradient at the coal inlet reaches its peak during rapid surges in water pressure but diminishes over time. Conversely, the pore pressure gradient at the outlet side exhibits a gradual increase. Hydraulic fracturing is most likely to occur at the water inlet during sudden increases in water pressure. Besides, as the permeability of coal decreases, the duration for seepage stabilization prolongs due to the intensified pore pressure gradient resulting from sudden increases in water pressure. Moreover, an extended period of high hydraulic gradient further increases the risk of hydraulic fracturing. The experimental findings indicate that coal samples initially experience tensile failure influenced by water and air pressure. Subsequently, mode I cracks form under pressure, propagating along the fracture surface and becoming visible. The main types of failure observed in hydraulic and pneumatic fracturing are diametrical tensile failure, and the development of fractures can be categorized into three distinct stages, which contains the initial stage characterized by slight volume changes while water pressure increases, the expansion stage when pressure reaches the failure strength, and the crack closure stage marked by little or even decreasing volume changes during pressure unloading. The acoustic emission signal accurately corresponds to these three stages. [ABSTRACT FROM AUTHOR]

A growing number of large data sets have created challenges for the oil and gas industry in predicting reservoir parameters and assessing well productivity through efficient and cost-effective techniques. The design of drilling plans for a high-pressure tight-sand reservoir requires accurate estimations of pore pressure (Pp) and reservoir parameters. The objective of this study is to predict and compare the Pp of Huizhou Sag, Pearl River Mouth Basin, China, using conventional techniques and machine learning (ML) algorithms. We investigated the characteristics of low-permeability reservoirs by observing well-logging data sets and cores and examining thin sections under a microscope. In the reservoir zone, the average hydrocarbon saturation is 55%, and the average effective porosity is 11%. The tight sandstone reservoirs consist of fine- to extremely fine-grained argillaceous feldspathic sandstone. The mean absolute error for reservoir property prediction is 1.3%, 2.2%, and 4.8%, respectively, for effective porosity, shale volume, and water saturation. Moreover, the ML algorithm was employed to cross-check the validity of the prediction of Pp. Combining conventional and ML techniques with the core data demonstrates a correlation coefficient (R2) of 0.9587, indicating that ML techniques are the most effective in testing well data. This study shows that ML can effectively predict Pp at subsequent depths in adjacent geologically similar locations. Compared to conventional methods, a substantial data set and ML algorithms improve the precision of Pp predictions. [ABSTRACT FROM AUTHOR]

Pore pressure (PP) information plays an important role in analysing the geomechanical properties of the reservoir and hydrocarbon field development. PP prediction is an essential requirement to ensure safe drilling operations and it is a fundamental input for well design, and mud weight estimation for wellbore stability. However, the pore pressure trend prediction in complex geological provinces is challenging particularly at oceanic slope setting, where sedimentation rate is relatively high and PP can be driven by various complex geo-processes. To overcome these difficulties, an advanced machine learning (ML) tool is implemented in combination with empirical methods. The empirical method for PP prediction is comprised of data pre-processing and model establishment stage. Eaton's method and Porosity method have been used for PP calculation of the well U1517A located at Tuaheni Landslide Complex of Hikurangi Subduction zone of IODP expedition 372. Gamma-ray, sonic travel time, bulk density and sonic derived porosity are extracted from well log data for the theoretical framework construction. The normal compaction trend (NCT) curve analysis is used to check the optimum fitting of the low permeable zone data. The statistical analysis is done using the histogram analysis and Pearson correlation coefficient matrix with PP data series to identify potential input combinations for ML-based predictive model development. The dataset is prepared and divided into two parts: Training and Testing. The PP data and well log of borehole U1517A is pre-processed to scale in between [-1, þ1] to fit into the input range of the non-linear activation/transfer function of the decision tree regression model. The Decision Tree Regression (DTR) algorithm is built and compared to the model performance to predict the PP and identify the overpressure zone in Hikurangi Tuaheni Zone of IODP Expedition 372. [ABSTRACT FROM AUTHOR]

Pore pressure prediction is a critical parameter in petroleum engineering and is essential for safe drilling operations and wellbore stability. However, traditional methods for pore pressure prediction, such as empirical correlations, require selecting appropriate input parameters and may not capture the complex relationships between these parameters and the pore pressure. In contrast, artificial neural networks (ANNs) can learn complex relationships between inputs and outputs from data. This paper presents a new empirical correlation for predicting pore pressure using ANNs. The proposed method uses 42 datasets of well log data, including temperature, porosity, and water saturation, to train ANNs for pore pressure prediction. The trained model, with the Bayesian regularization backpropagation function, predicts the pore pressure with an average absolute percentage error (AAPE) and correlation coefficient (R) of 4.22% and 0.875, respectively. The trained ANN is then used to develop a new empirical correlation that relates pore pressure to the input parameters considering the weights and biases of the optimized ANN model. To validate the proposed correlation, it is applied to a blind dataset, where the model successfully predicts the pore pressure with an AAPE of 5.44% and R of 0.957. The results show that the proposed correlation provides accurate and reliable predictions of pore pressure. The proposed method provides a robust and accurate approach for predicting pore pressure in petroleum engineering applications, which can be used to improve drilling safety and wellbore stability. [ABSTRACT FROM AUTHOR]

Earthquakes impact the stability of municipal solid waste (MSW) landfills, especially those with high water levels, and may further lead to disastrous landslides. Numerical analysis offers an efficient and cost-effective way to study the seismic stability of a landfill. In this study, the finite difference nonlinear analysis method was employed to meticulously evaluate the dynamic response of landfills under varying water levels and seismic intensities. The analysis was guided by the seismic instability and centrifuge test outcomes. The rationality of the computational model was verified by examining the responses of acceleration and pore pressure. Subsequently, the time history curve of the dynamic safety factor was derived from the dynamic response of landfills. The results indicated that a landfill was more susceptible to large earthquake effects, and its stability decreased as the water level rose, with the safety factor decreasing to a critical point under the coupling effect of strong earthquakes and high water levels. In contrast, the stability of the landfill with low water levels was good under weak earthquake conditions, with only a slight decrease in the safety factor observed. The seismic stability of a landfill was significantly influenced by both accumulative deformation and negative excess pore pressure. A certain degree of hysteresis in the landfill's instability was also observed compared to the earthquake loading process. The time history curve of the safety factor can offer a comprehensive insight into seismic stability under diverse conditions. Additionally, future research efforts are needed to better determine the values of strength parameters of MSW in seismic analysis. [ABSTRACT FROM AUTHOR]

The prediction of in situ stress based on azimuthal seismic data has extensive use in horizon crushability evaluation. Nonetheless, existing in situ stress seismic prediction models do not consider overburden pressure and tectonic strain, which limit the prediction accuracy. To this end, we propose a decoupled overburden pressure in situ stress prediction method under tectonic constraints. The key to this method is to consider that the overburden pressure could act on the rock skeleton and pore fluid, i.e. generating effective pressure and pore pressure; the pore pressure can be estimated using Eaton's method, and then the effective stress can be obtained. The relationship between tectonic strain and effective pressure is constructed based on Hooke's law, where tectonic strain can be calculated from curvature attributes extracted from seismic data. Introducing pore pressure and deriving a model for calculating the maximum and minimum horizontal stresses and the difference in horizontal stress ratio for orthotropic media (OA). When the Thomson anisotropy parameters and the pore pressure are neglected, the proposed model can be degraded to a conventional horizontal transverse isotropy medium in situ stress prediction model, which proves the validity of the model. The results of sensitivity analysis experiments affirm the need to decouple overburden pressure and account for tectonic strain when predicting in situ stress reasonably. Finally, single-well and azimuthal seismic prediction were carried out by using the a priori information from well logging and seismic data. [ABSTRACT FROM AUTHOR]

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Abstract Rates of seismicity in the Delaware Basin of Texas and New Mexico increased from 10 earthquakes per year of local magnitude (ML) 3.0 and above in 2017 to more than 185 in 2022, coincident with increasing oil and gas production and wastewater re‐injection into strata shallow or deeper than producing intervals. Events of large magnitude—up to ML 5.4 to‐date—occur on faults extending into formations above the basement that have received more than four billion barrels of injection. Here, we demonstrate the link between injection geology, pore pressure evolution, fault stability, and induced seismicity in this region. We find that the injection targets are largely dolomitized platform carbonates with low (

There is a large amount of CBM in the long flame coal. With the continuous increase of mining depth, it is necessary to explore the complex permeability features between CBM and groundwater in the coal reservoir to reduce the difficulty of CBM mining and improve the efficiency of CBM mining. Taking the long flame coal in the Weijiamao mining area of Zhungeer Banner, Ordos, Inner Mongolia as the experimental object, the TCXS-II coal rock gas water relative permeability tester is used to conduct the long flame gas water phase permeability experiment. The non steady state method is used to obtain the gas water phase permeability features of long flame coal under different effective stresses, pore pressures, and temperatures during the gas water drive process. The results show the following points. ① When the effective stress increases from 3.7 MPa to 7.7 MPa, the increase in gas phase relative permeability decreases, while the decrease in water phase relative permeability slightly increases. The increase of effective stress will have an inhibitory effect on the permeability of the fluid, and the inhibitory effect on water phase seepage is greater than that on gas phase seepage. The residual water saturation increases with the increase of effective stress. ② When the pore pressure increases from 2 MPa to 6 MPa, the decrease in the relative permeability curve of the water phase slows down, and the increase in the relative permeability curve of the gas phase becomes more obvious. The range of gas water co-permeation becomes wider, the saturation of the isotonic point increases, and the residual water saturation decreases. ③ When the temperature rises from 20 ℃ to 80 ℃, the increase in gas phase relative permeability and the decrease in water phase relative permeability gradually increase. The range of gas water co-permeation becomes wider, the residual water saturation shows a decreasing trend, and the gas phase permeability flow rate shows an increasing trend. The research results can provide theoretical basis and experimental reference for the research of CBM extraction technologies such as hydraulic fracturing and thermal injection in long flame coal reservoirs.

Vertical drains are used to accelerate consolidation of clays in ground improvement projects. Smear zones exist around these drains, where permeability is reduced due to soil disturbance caused by the installation process. Hansbo solution is widely used in practice to consider the effects of drain discharge capacity and smear on the consolidation process. In this study, a computationally efficient diameter reduction method (DRM) obtained from the Hansbo solution is proposed to consider the smear effect without the need to model the smear zone physically. Validated by analytical and numerical results, a diameter reduction factor is analytically derived to reduce the diameter of the drain, while achieving similar solutions of pore pressure dissipation profile as the classical full model of the smear zone and drain. With the DRM, the excess pore pressure u obtained from the reduced drain in the original undisturbed soil zone is accurate enough for practical applications in numerical models. Such performance of DRM is independent of soil material property. Results also show equally accurate performance of DRM under conditions of multi-layered soils and coupled radial-vertical groundwater flow.

When designing the parameters for the development of oil and gas field at significant depths, crucial to comprehend how certain factors affect the behavior of reservoir rocks and host rocks. These factors include the high level of rock stress, the ambient temperature field, and the hydro- and gas-dynamic processes within the mass. The impact of one or a combination of these factors can result in alterations to the construction, structure, composition, and properties of the rock mass and, ultimately leading to a mismatch between the design solutions and the actual conditions. The purpose of the research is to establish a methodology for conducting laboratory studies that investigate the impact of the mode of occurrence of oil and gas field reservoirs at great depths on the properties of rock samples. The research objectives encompass a theoretical analysis and the identification of the principal factors influencing rock behavior and changes in internal structure. Additionally, the objectives include developing laboratory research methods that comprehensively simulate these factors and conducting trial experiments to assess their effects. As part of the project, tests were conducted on sandstone samples collected from depth ranging from 3.5 to 4 km within the hydrocarbon field. These studies were performed while simulating thermobaric reservoir conditions, which include temperature, rock pressure, and reservoir pressure. The results of these experiments, aimed at examining the behavior of rock samples as closely as possible to their natural reservoir occurrence at depth of 3.5–4 km, are presented. It has been observed that rock samples of the same lithology, collected from nearly identical depths, can exhibit significant differences in deformation characteristics, both in the pre- and off-limit regions of loading. The findings from these studies provide the initial data for the development and refinement of geomechanical model behavior for materials that take into account not only fracture strength criteria but also dilatancy processes at various stages of rock deformation. Increasing lateral pressure within the range of 0 to to 55 MPa causes relatively minor change in ultrasonic vibration velocities, typically ranging from 1 to 10%. This makes it challenging to determine the necessity of utilizing these results for indirectly assessing changes in rock properties within the mass. Nevertheless, within the context of geophysical studies, considering variations in velocity values enhances the quality of result interpretation, especially given the substantial geometric dimensions of the rock masses under investigation. Research into the acoustic emissions of rocks in a complex stressed state enables the monitoring of spatial micro- and macrofracturing processes throughout the entire loading phase of samples. This provides a more comprehensive understanding of changes in their internal structure. The article delves into the factors that impact structural changes in oil and gas field rocks, particularly as their development extends to greater depths. The study outlines methodological approaches that facilitate the investigation of physical and mechanical properties of rock samples, while accurately modeling complex thermobaric conditions. Additionally, the it describes the technical specifications of the testing equipment, ensuring the closest possible replication of the actual conditions of reservoir rock occurrences. Lastly, the study reveals key features related to the deformation and fracture of rock samples during testing under lateral pressures of 55 MPa and pore pressures of 30 MPa, along with the creation of temperature fields up to 100 °C.

Wave erosion of slopes can easily trigger landslides into marine environments and pose severe threats to both the ecological environment and human activities. Therefore, near-shore slope monitoring becomes crucial for preventing and alerting people to these potential disasters. To achieve a comprehensive understanding, it is imperative to conduct a detailed investigation into the dynamics of wave erosion processes acting on slopes. This research is conducted through flume tests, using a wave maker to create waves of various heights and frequencies to erode the slope models. During the tests, seismic signals, acoustic signals, and pore pressure generated by wave erosion and slope failure are recorded. Seismic and acoustic signals are analyzed, and time-frequency spectra are calculated using the Hilbert–Huang Transform to identify the erosion events and signal frequency ranges. Arias Intensity is used to assess seismic energy and explore the relationship between the amount of erosion and energy. The results show that wave height has a more decisive influence on erosion behavior and retreat than wave frequency. Rapid drawdown may potentially cause the slope to slide during cyclic swash and backwash wave action. As wave erosion changes from swash to impact, there is a significant increase in the spectral magnitude and Power Spectral Density (PSD) of both seismic and acoustic signals. An increase in pore pressure is observed due to the rise in the run-up height of waves. The amplitude of pore pressure will increase as the slope undergoes further erosion. Understanding the results of this study can aid in predicting erosion and in planning effective management strategies for slopes subject to wave action.

Pore pressure is essential data in drilling design, and its accurate prediction is necessary to ensure drilling safety and improve drilling efficiency. Traditional methods for predicting pore pressure are limited when forming particular structures and lithology. In this paper, a machine learning algorithm and effective stress theorem are used to establish the transformation model between rock physical parameters and pore pressure. This study collects data from three wells. Well 1 had 881 data sets for model training, and Wells 2 and 3 had 538 and 464 data sets for model testing. In this paper, support vector machine (SVM), random forest (RF), extreme gradient boosting (XGB), and multilayer perceptron (MLP) are selected as the machine learning algorithms for pore pressure modeling. In addition, this paper uses the grey wolf optimization (GWO) algorithm, particle swarm optimization (PSO) algorithm, sparrow search algorithm (SSA), and bat algorithm (BA) to establish a hybrid machine learning optimization algorithm, and proposes an improved grey wolf optimization (IGWO) algorithm. The IGWO-MLP model obtained the minimum root mean square error (RMSE) by using the 5-fold cross-validation method for the training data. For the pore pressure data inWell 2 andWell 3, the coefficients of determination (R²) of SVM, RF, XGB, and MLP are 0.9930 and 0.9446, 0.9943 and 0.9472, 0.9945 and 0.9488, 0.9949 and 0.9574. MLP achieves optimal performance on both training and test data, and the MLP model shows a high degree of generalization. It indicates that the IGWO-MLP is an excellent predictor of pore pressure and can be used to predict pore pressure. [ABSTRACT FROM AUTHOR]

Pore fluid plays a crucial role in many granular flows, especially those in geophysical settings. However, the transition in behaviour between dry flows and fully saturated flows and the underlying physics that relate to this are poorly understood. In this paper, we report the results of small-scale flume experiments using monodisperse granular particles with varying water content and volume in which the basal pore pressure, total pressure, flow height and velocity profile were measured at a section. We compare the results with theoretical profiles for granular flow and with flow regimes based on dimensional analysis. The runout and the centre of mass were also calculated from the deposit surface profiles. As the initial water content by mass was increased from zero to around 10%, we first observed a drop in mobility by approximately 50%, as surface tension caused cohesive behaviour due to matric suction. As the water content was further increased up to 45%, the mobility also increased dramatically, with increased flow velocity up to 50%, increased runout distance up to 240% and reduced travel angle by up to 10° compared to the dry case. These effects can be directly related to the basal pore pressure, with both negative pressures and positive pore pressures being measured relative to atmospheric during the unsteady flow. We find that the initial flow volume plays a role in the development of relative pore pressure, such that, at a fixed relative water content, larger flows exhibit greater positive pore pressures, greater velocities and greater relative runout distances. This aligns with many other granular experiments and field observations. Our findings suggest that the fundamental role of the pore fluid is to reduce frictional contact forces between grains thus increasing flow velocity and bulk mobility. While this can occur by the development of excess pore pressure, it can also occur where the positive pore pressure is not in excess of hydrostatic, as shown here, since buoyancy and lubrication alone will reduce frictional forces. [ABSTRACT FROM AUTHOR]

Hydraulic fracturing is one of the most important enhanced oil recovery technologies currently used to develop unconventional oil and gas reservoirs. During hydraulic fracture initiation, fluid seeps into the reservoir rocks surrounding the wellbore, inducing rock deformation and changes in the stress field. Analyzing the fluid–solid coupling mechanism around the wellbore is crucial to the construction design of fracturing technologies such as pulse fracturing and supercritical carbon dioxide fracturing. In this study, a new transient fluid–solid coupling model, capable of simulating the pore pressure field and effective stress field around the wellbore, was established based on the Biot consolidation theory combined with the finite difference method. The numerical results are in excellent agreement with the analytical solutions, indicating the reliability of the model and the stability of the computational approach. Using this model, the influence of seepage parameters and reservoir properties on the fluid–solid coupling around the open-hole wellbore was investigated. The simulation results demonstrate that, during wellbore pressurization, significant changes occur in the pore pressure field and effective stress field near the wellbore. The fluid–solid coupling effect around the wellbore returns to its initial state when the distance exceeds four times the radius away from the wellbore. As the fluid viscosity and wellbore pressurization rate decrease, the pore pressure field and effective circumferential stress (ECS) field around the wellbore become stronger. Adjusting the fluid viscosity and wellbore pressurization rate can control the effect of seepage forces on the rock skeleton during wellbore fluid injection. For the same injection conditions, rocks with q higher Young's modulus and Poisson's ratio exhibit stronger pore pressure fields and ECS fields near the wellbore. This model furnishes a dependable numerical framework for examining the fluid–solid coupling mechanism surrounding the open-hole wellbore in the initiation phase of hydraulic fractures. [ABSTRACT FROM AUTHOR]

Geomechanical and multiphase flow characteristics are essential in recovering oil from naturally fractured rocks during hydrocarbon production because of changes in pore pressure and tension within the rock. It is a wellestablished fact that the geomechanical and multiphase flow characteristics of fractured rocks are interdependent on each other. Evaluation of these characteristics, for hydrocarbons displaced by water in fractured rocks under external stress loading, is severely lacking in published literature. This study aims to develop a novel numerical framework for a fully coupled model of fractured rocks, taking into consideration the pore pressure and porous media discontinuity at the fracture-matrix interface, along with an expanded Darcy's equation. The fully coupled Finite Element Method (FEM) and Computational Fluid Dynamics (CFD) model developed in this study is shown to accurately predict geomechanical and multiphase flow behaviour at the fracture-matrix interface. The results show that as external stress loading on the fractured rock increases, the porosity and permeability of the rock matrix decrease, capillary pressure at the fracture-matrix interface decreases, and the relative permeability curves shift to the right, indicating a water-soaked fracture-matrix interface. The findings of this study can be used to develop innovative strategies for enhanced oil recovery from fractured rocks. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Mine Automation is the property of Industry & Mine Automation Editorial Department and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

High overpressure is a critical drilling issue in the Lower Kutai Basin. Typical pore pressure prediction approaches involve an empirical relationship, such as Eaton's method using sonic log data. In areas with high geothermal gradients, such as the Lower Kutai Basin, there is evidence for additional overpressure from gas generation such that sediment unloading must be considered to interpret pore pressure correctly. In this paper a repeatable deterministic model is presented for pore pressure from sonic data and, using selected wells from the Lower Kutai Basin, also the use of the resistivity log in a similar model. In the Lower Kutai Basin, sonic logs are often absent from the logging suite or otherwise running over limited intervals, making an alternative log-based prediction method particularly valuable. As a caveat, shallow freshwater encroachment is reported in the Lower Kutai Basin, means the shallow resistivity data can be problematic to use to define both top of overpressure and a normal compaction trend. Care must therefore be taken if resistivity is to be used for the interpretation of unloaded pore pressure, and chiefly applied and this likely to be more successful where encroachment is less pronounced, such as pro-delta shales. Assuming the additional care needed in using resistivity data, this paper suggests that resistivity can be a useful tool for pore pressure prediction in unloaded shale at elevated temperatures within the Lower Kutai Basin. At present the technique has been applied to only a limited dataset due to data availability limitations, but it is hoped with further refinement it will form a helpful additional approach in the pore pressure prediction toolkit. [ABSTRACT FROM AUTHOR]

The authors have proposed a modified intensity–duration (I-D) method that incorporates field measurements of tensiometer to improve the accuracy of predicting the risk of slope failure. This method uses an indicator that considers the relationship of the duration from the time point at which the saturated zone is assumed to have formed to the average rainfall intensity during that period. The usefulness of this method has been verified, but its applicability to different slopes has not yet been investigated. Here, the authors collected long-term observations on a natural slope in Minamiashigara City, Kanagawa Prefecture, and examined the Modified I-D method using data on slope failures in the surrounding area. The authors also compared the results with plots of previous rainfall index (soil water index–accumulated rainfall in 60 min, effective rainfall amount with a half-life of 72 h–effective rainfall amount with a half-life of 1.5 h, and accumulated rainfall–accumulated rainfall in 60 min). The snake curves for rainfall events during slope failure and non-failure were clearly separated. The accuracy was high, confirming the applicability of the modified I-D method. [ABSTRACT FROM AUTHOR]

Due to the increase in earthquake activity in Iraq and Middle East during the last two decades, the study and understanding of probable destructive action and the best method to mitigate this effect became more important. So, many improvements and mitigation methods can be used. In this study, the use of permeation grout technique was adopted to prevent the existing soil condition in urban area by using cement kiln dust and bentonite clay. The tests were executed by using 1 g shaking table apparatus to simulate a sinusoidal motion (vibration) at specified different frequencies. The liquefaction phenomena were observed for loose saturated sand at 60 s, 25 s, and 10 s for 0.5 Hz, 0.75 Hz, and 1 Hz, respectively. After mitigation process, the soil liquefaction did not occur until 100 s, 60 s, and 30 s, for the same mentioned frequencies. Besides, the use of cement kiln dust decreases the liquefaction potential and increase the factor of safety. [ABSTRACT FROM AUTHOR]

Copyright of Iraqi National Journal of Earth Sciences is the property of Republic of Iraq Ministry of Higher Education & Scientific Research (MOHESR) and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Acta Sedimentologica Sinica is the property of Acta Sedimentologica Sinica Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

As a waterless fracturing fluids for gas shale stimulation with low viscosity and strong diffusibility, supercritical CO2 is promising than the water by avoiding the clay hydration expansion and reducing reservoir damage. The permeability evolution influenced by the changes of the temperature and stress is the key to gas extraction in deep buried shale reservoirs. Thus, the study focuses on the coupling influence of effective stress, temperature, and CO2 adsorption expansion effects on the seepage characteristics of Silurian Longmaxi shale fractured by supercritical CO2. The results show that when the gas pressure is 1–3 MPa, the permeability decreases significantly with the increase in gas pressure, and the Klinkenberg effects plays a predominant role at this stage. When the gas pressure is 3–5 MPa, the permeability increases with the increase in gas pressure, and the influence of effective stress on permeability is dominant. The permeability decreases exponentially with the increase in effective stress. The permeability of shale after the adsorption of CO2 gas is significantly lower than that of before adsorption; the permeability decreases with the increase in temperature at 305.15 K–321.15 K, and with the increase in temperature, the permeability sensitivity to the temperature decreases. The permeability is closely related to supercritical CO2 injection pressure and volume stress; when the injection pressure of supercritical CO2 is constant, the permeability decreases with the increase in volume stress. The results can be used for the dynamic prediction of reservoir permeability and gas extraction in CO2-enhanced shale gas development.

Pore pressure (PP) information plays an important role in analysing the geomechanical properties of the reservoir and hydrocarbon field development. PP prediction is an essential requirement to ensure safe drilling operations and it is a fundamental input for well design, and mud weight estimation for wellbore stability. However, the pore pressure trend prediction in complex geological provinces is challenging particularly at oceanic slope setting, where sedimentation rate is relatively high and PP can be driven by various complex geo-processes. To overcome these difficulties, an advanced machine learning (ML) tool is implemented in combination with empirical methods. The empirical method for PP prediction is comprised of data pre-processing and model establishment stage. Eaton's method and Porosity method have been used for PP calculation of the well U1517A located at Tuaheni Landslide Complex of Hikurangi Subduction zone of IODP expedition 372. Gamma-ray, sonic travel time, bulk density and sonic derived porosity are extracted from well log data for the theoretical framework construction. The normal compaction trend (NCT) curve analysis is used to check the optimum fitting of the low permeable zone data. The statistical analysis is done using the histogram analysis and Pearson correlation coefficient matrix with PP data series to identify potential input combinations for ML-based predictive model development. The dataset is prepared and divided into two parts: Training and Testing. The PP data and well log of borehole U1517A is pre-processed to scale in between [-1, +1] to fit into the input range of the non-linear activation/transfer function of the decision tree regression model. The Decision Tree Regression (DTR) algorithm is built and compared to the model performance to predict the PP and identify the overpressure zone in Hikurangi Tuaheni Zone of IODP Expedition 372.

Purpose: to study the distribution of pore pressure under an underground dam located in a mountain valley for the atmospheric precipitation accumulation, while analyzing the effect of water levels in headwater and tailwater. At the same time, the factors of influence varied in the range: the water level in headwater from minus 8.2 to minus 1.2; water level in tailwater from minus 7.9 to minus 6.7. Materials and methods. The study of a complex system was carried out in two stages. At the first stage, the pore pressure under the underground dam, responsible for the seepage under it, was calculated using the finite element method based on the Midas GTX NX software package, which has the ability to take into account the complex joint operation of the underground dam and the soil mass. At the second stage, the resulting array of numbers, which reflects the physical processes occuring when the response function, which is taken as the pore pressure under the underground dam, is affected by disturbances, for which the water levels in headwater and tailwater of the dam are taken, was subjected to digital analysis. Results. The range of pore pressure values at the maximum headwater levels is 31.72 % over the entire range of tailwater levels. For the average levels of the headwater, the range of pore pressure values is 32.70 %, and for the minimum levels of the headwater, the range of pore pressure values is 39.41 %, also in the entire range of changes at the tailwater levels. Thus, the lower the headwater level, the greater the impact on the pore pressure of the tailwater level. Conclusions: with a change in the studied range of water levels in headwater or tailwater of the dam, i.e. factors of influence, with a decrease in the values of the impact factors, the degree of their influence on the pore pressure under the underground dam decreases, and with growth – increases, and it occurs almost linear.

Abstract To investigate the coupling effect of in situ stress and fluid pore pressure and the propagation law of damaged area during hydraulic fracturing and gas fracturing in heterogeneous fractured reservoirs, according to the effective stress principle and the smooth Rankine stress criterion, this paper establishes a coupled fluid–solid‐damage mathematical model of heterogeneous rock layers and carries out numerical simulations on the characteristic field changes of rock layers under water and nitrogen fracturing. The results show that the continuous injection of nitrogen or water can cause an increase in pore pressure in natural fractures and rock matrices, leading to rock damage and an increase in porosity. Under the same parameters, the damage area and length of the rock mass during nitrogen fracturing are larger than those of hydraulic fracturing, and the changes in pore pressure, permeability, and porosity are consistent with the damage area. The pore pressure near the connected natural fractures is relatively high, resulting in tensile strain; the pore pressure at isolated fractures is relatively low, and some even produce compressive strain. During nitrogen fracturing, the damage evolution is less affected by the in situ stress and the matrix permeability, while hydraulic fracturing is more affected by them. The damage area is the smallest when the horizontal in situ stress is equal. When the initial permeability of the matrix is low, the damaged area mainly follows the natural fractures and is distributed in a strip shape. As the permeability of the matrix increases, the fracturing fluid can enter the matrix, and the damaged area is in the shape of a block.

The 57-km long, new Gotthard Base Tunnel (GBT) crosses over a 1.1-km long stretch containing kakirites, i.e. weak rocks which, depending on the degree of tectonisation, may exhibit severe squeezing behaviour upon tunnel excavation. This tunnel stretch was already identified as a key problem in the early planning phase, necessitating intensive geological exploration and laboratory investigations over a 14-year period, including research and development in experimental techniques. The laboratory investigations for the GBT provided not only a considerable amount of data about the geotechnical parameters of the kakirites, but also valuable information about their basic behaviour and the adequacy of various testing techniques for this type of rock. Specifically, by using specially designed testing equipment and procedures, it was shown that, (1) conventional rock mechanics triaxial tests cannot provide reliable results for kakirites; (2) the kakirites obey the principle of effective stress in Terzaghi's classical form and consequently it is indispensable to measure the pore pressure during triaxial testing, analogously to the consolidated drained (CD) and consolidated undrained (CU) tests in soil mechanics; (3) the effects of a possible loss of water and desaturation during sampling, transport, storage and preparation of specimens can be successfully mitigated by subjecting the specimens to flow-through before the actual triaxial testing; and, (4) CD tests on partially saturated specimens that are obtained by means of flow-through can provide reliable and reproducible results. This saves a significant amount of time, allows a greater number of tests to be performed, and allows results to be obtained more quickly, which is particularly important for decision-making during tunnel advance.

In order to explore the anisotropic permeability of raw coal under low stress, taking the coal samples of Zhangcun Coal Mine, Tingnan Coal Mine and Xinjing Coal Mine as the research objects, using the improved triaxial seepage test device, the permeability tests under constant confining pressure and variable pore pressure along the direction of coal sample face cleat, butt cleat and vertical bedding were carried out. The results show that under the same confining pressure and axial pressure, the flow through the coal sample increases with the increase of osmotic pressure difference. Under the same osmotic pressure difference, the flow is closely related to the direction of seepage. The permeability in the face cleat direction is slightly larger than that in the butt cleat direction and much larger than that in the vertical bedding direction under the same average pore pressure. There is little difference in permeability between face cleat and butt cleat under low stress. It is not accident that the permeability in parallel bedding plane is similar, and the permeability of coal has the characteristics of transverse isotropy. The increase of coal stress is the main reason for the difference of permeability between butt cleat and face cleat.

With the continuous increase of coal mining depth, the response of coal mechanics and the mechanism of gas migration have become extremely complicated. In order to explore the coal damage evolution and gas seepage mechanism under the integrated operation of first extraction and subsequent mining in engineering, the K2 coal seam briquette sample of Chongqing Songzao Coal Mine was used as the research object. Using the triaxial servo seepage device of thermal-fluid-solid coupling of gas-bearing coal, the reduced pore pressure seepage test and the triaxial compression-seepage test were successively carried out on the same specimen. According to the elasto-plasticity theory, a statistical damage constitutive model that characterized the whole stress-strain relationship of coal was derived, and the permeability model of coal under consideration of damage was further constructed. The results of the research shown that, in the reduced pore pressure seepage test, the permeability of coal under constant external stress shown a trend of first rising gently and then rising sharply with the decrease of pore pressure. In this process, the change of coal permeability was affected by the competition between effective stress and gas desorption. In the process of the triaxial compression-seepage test, the characteristics of coal deformation stages under different confining stresses were basically similar. As the confining stress increased, the coal mechanics properties were strengthened. The coal permeability curve changed as a negative exponential function with the increasing axial strain . The damage variable curves and plastic strain curves shown a trend of first rising slowly and then rising sharply with the increase of axial strain, the damage evolution process was corresponded to the whole stress-strain curve of each stage of coal deformation and failure. The rationality of the constructed damage constitutive model and permeability model were verified by comparison with test data, which shown that the model can more accurately reflect the characteristics of coal deformation stages and the law of gas seepage.

[Introduction] With the increasing demand for clean energy, the offshore wind power sector has seen a spurt of progress in recent years, and the bucket foundation has become the preferred choice for offshore wind turbines considering its good economy, convenient construction, and recyclability. Due to the widespread distribution of seismic zones in China, the seismic performance of bucket foundation is a crucial consideration for structural design. The bucket foundation is featured by high structure stiffness, so that the probability of structure damage caused by earthquake is low, and the failure under earthquake is mainly caused by the liquefaction of the foundation soil. For this purpose, the paper focuses on the seismic performance of bucket foundation in sandy soil. [Method] The liquefaction resistance of sandy soil for bucket foundation was analyzed by shaking table tests in this paper. The study objects included four types of bucket foundation in sandy soil, namely mono-bucket foundation (MBF), composite bucket foundation (CBF), three-bucket jacket foundation (TBJF) and four-bucket jacket foundation (FBJF). [Result] By carrying out shaking table tests, the excess pore pressure ratios of sandy soil for different types of bucket foundation under earthquake are obtained, and the impact mechanism of bucket foundation on the anti-liquefaction performance of sand soil is clarified. [Conclusion] The bucket foundation can improve the liquefaction resistance of sand, since the additional load effect of the superstructure and the hoop effect of bucket skirt weakens its shear shrinkage. The test results of MBF are compared with those of CBF, and the test results of TBJF are compared with those of FBJF. It is found that the seismic performance of CBF and FBJF is respectively superior to that of MBF and TBJF.

The classical groundwater seepage theory was established on the basis of the principle of water equilibrium, which cannot explain the phenomenon of well water level change caused by natural seismic activities and other external loads, and is not conducive to the in-depth understanding of the role of groundwater seepage movement in various environmental geological disasters. To solve this problem, a mathematical model of pore pressure change of a confined aquifer driven by seismic wave stress is constructed based on the fluid-structure coupling dynamic theory. The numerical verification of the model is realized by using the software Comsol. The change characteristics of well water level are inversely performed by using the Cooper theory, and the results are compared with the change characteristics of well water level caused by strong earthquakes in the Sichian-Yunnan region. The influencing factors of seepage movement of a confined aquifer under earthquake are studied by changing the simulation parameters. The results show that when seismic wave loads act on the confined aquifers, pore pressure oscillates in the same period as the seismic waves, and the amplitude, frequency and hydraulic slope of the seismic waves have significant effects on the pore pressure, while the coefficient of permeability and porosity have little effect on the change characteristics. At the initial stage of seismic wave loading, pore pressure increases rapidly, and then part of the water in the aquifer is slowly discharged. The load pressure gradually transfers to the granular framework, and the rate of change of pore pressure slows down and tends to reach a new equilibrium. The variation characteristics of well water level are closely related to pore pressure, and the oscillation period and variation pattern are consistent with pore pressure, but the amplitude is different. Generally, the oscillation rises and tends to be stable, which is basically the same with the variation pattern of well water level observed in the Sichuan-Yunnan region. The results are of valuable exploration significance for the establishment and improvement of groundwater seepage theory under stress, and can enrich and expand the research ideas and application fields of traditional groundwater dynamics and classical fluid-structure coupling theory.

Geomechanical and multiphase flow characteristics are essential in recovering oil from naturally fractured rocks during hydrocarbon production because of changes in pore pressure and tension within the rock. It is a well-established fact that the geomechanical and multiphase flow characteristics of fractured rocks are interdependent on each other. Evaluation of these characteristics, for hydrocarbons displaced by water in fractured rocks under external stress loading, is severely lacking in published literature. This study aims to develop a novel numerical framework for a fully coupled model of fractured rocks, taking into consideration the pore pressure and porous media discontinuity at the fracture-matrix interface, along with an expanded Darcy's equation. The fully coupled Finite Element Method (FEM) and Computational Fluid Dynamics (CFD) model developed in this study is shown to accurately predict geomechanical and multiphase flow behaviour at the fracture-matrix interface. The results show that as external stress loading on the fractured rock increases, the porosity and permeability of the rock matrix decrease, capillary pressure at the fracture-matrix interface decreases, and the relative permeability curves shift to the right, indicating a water-soaked fracture-matrix interface. The findings of this study can be used to develop innovative strategies for enhanced oil recovery from fractured rocks.

The capacity of bone to optimize its structure in response to mechanical loads has been widely observed. The mechanical load acting on a bone at the macroscopic level influences the bone cells, particularly osteocytes within the lacunae canalicular network (LCN). Osteocytes are responsive to a range of physical signals, including strain, interstitial fluid flow, and pore pressure. However, physiological tissue strain is known to be typically smaller than that required to directly induce bone formation. On the other hand, as per evidence provided by this study from the literature, models based on fluid flow alone cannot simultaneously predict bone formation at both the periosteal and endocortical surfaces. This suggests that another component of the osteocyte's mechanical environment, such as pore pressure, may play an essential role in bone adaptation, either alone or in combination with other stimuli, such as tissue strain and/or interstitial fluid flow. In vitro experiments have also confirmed that osteocytes respond to cyclic pore pressure and, thus, have a mechanism to sense the pressure, possibly because of its viscoelasticity.In this work, dissipation energy density, being irreversible work done per unit volume, has been successfully used as a greater stimulus to incorporate all of the parameters of mechanical environments of the LCN, such as waveforms of both fluid velocity and pore pressure, number of loading cycles. Mineral Apposition Rate (MAR) has also been mathematically derived to be proportional to the square root of the dissipation energy density minus its reference value. A hypothesis is accordingly proposed and successfully tested/validated for both endocortical and periosteal surfaces with respect to an in-vivo study on mouse tibia available in the literature. The constant of proportionality and the reference/threshold value of the dissipation energy density are determined through a nonlinear curve fitting. The mathematical/computational method thus developed is then successfully used to predict MAR at both endocortical and periosteal surfaces induced by a different loading condition.Computational implementation of the mathematical model has been done through a poroelastic finite element analysis of bone, where bone is assumed to be porous and filled with fluid, with a boundary condition that the periosteum is impermeable to the fluid and the endosteal surface maintains a reference zero pressure. This work also provides evidence for these assumptions to be true based on the state-of-the-art literature on related experimental studies. The currently developed model shows that the bone uses these conditions (assumptions) to its advantage, as the greater stimulus, i.e., the dissipation energy due to both fluid flow and pore pressure, are of a similar order at both the surfaces, and hence osteogenesis of the same order at both the surfaces.As a bottom line, the resulting model is the first of its kind as it has been able to correctly predict MAR at both endocortical and periosteal surfaces. This study thus significantly advances the modeling of cortical bone adaptation to exogenous mechanical loading.

AbstractA numerical hydro-mechanical damage model is proposed in this paper to investigate the stability and failure characteristics of artificial dam in coal mine underground water reservoir. The localized damage model for the coupled pore pressure and rock failure of a stressed rock is developed, and a local damage constitutive law is employed to describe its stress-strain relationship and fracture propagation. The Mohr-Coulomb criterion and maximum tension criterion are used to Judge the damage and failure of rock. Experimental data for rock samples are used to validate the proposed numerical model, and it accurately replicates the stress-strain curves and failure pattern compared with the experimental results. We then explore the influence of water pressures, cutting depth and dam strengths of artificial dam in coal mine underground water reservoir, and the simulation results show that the dam strength is the main factor affecting the strength of artificial dam. The modeling described herein is expected to assist in the management and optimization of underground water reservoirs.

Water ingress to tunnels may result in pore pressure drawdown and consolidation settlements in areas above tunnels founded on soft soil deposits, potentially causing damage to buildings and infrastructure. To limit pore pressure drawdown, requirements are set on water ingress to bedrock tunnels. To meet these requirements, pre-excavation grouting is often performed to reduce the hydraulic conductivity of the rock mass surrounding the tunnel. Real-time pore pressure monitoring may be used to document pore pressure drawdown during construction. However, the effect of tunnel water ingress can be difficult to distinguish from natural pore pressure fluctuations. This paper presents a tunnel case in Oslo, Norway, where time series modelling was applied to local pore pressure data using the transfer function model framework. The input to the models was daily meteorological data considering precipitation and evapotranspiration, and the output was simulated pore pressure levels with impulse response functions. The models were optimised with data from before tunnel excavation, and simulations were run during the tunnel excavation period. Simulated pore pressure levels were compared with observed pore pressure levels to assess tunnelling-induced drawdown. Model uncertainty ranges were used to produce upper, lower, and best estimates of the drawdown. The findings show that time series modelling with transfer function models may be used in tunnel projects to continuously assess the impact on the local groundwater environment, for better evaluation of the pre-grouting performance, and for quantifying both the temporary and long-term drawdown with increased accuracy. [ABSTRACT FROM AUTHOR]