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Shale gas is an important supplement to the supply of natural gas resources and plays an important role on the world’s energy stage. The efficient implementation of hydraulic fracturing is the key issue in the exploration and exploitation of shale gas. The existence of bedding structure results in a distinct anisotropy of shale rock formation. The anisotropic behaviors of shale rock have important impacts on wellbore stability, hydraulic fracture propagation, and the formation of complex fracture networks. This paper briefly reviews previous work on the anisotropic mechanical properties of shale rock and their relation to hydraulic fracturing in shale reservoirs. In this paper, the research status of work addressing the lithological characteristics of shale rock is summarized first, particularly work considering the mineral constituent, which determines its physical and mechanical behavior in essence. Then the anisotropic physical and mechanical properties of shale specimens, including ultrasonic anisotropy, mechanical behavior under uniaxial and triaxial compression tests, and tensile property under the Brazilian test, are summarized, and the state of the literature on fracture toughness anisotropy is discussed. The concerns of anisotropic mechanical behavior under laboratory tests are emphasized in this paper, particularly the evaluation of shale brittleness based on mechanical characteristics, which is discussed in detail. Finally, further concerns such as the effects of bedding plane on hydraulic fracturing failure strength, crack propagation, and failure pattern are also drawn out. This review study will provide a better understanding of current research findings on the anisotropic mechanical properties of shale rock, which can provide insight into the shale anisotropy related to the fracture propagation of hydraulic fracturing in shale reservoirs.

Abstract Brittleness of rocks plays an important role in the evaluation of unconventional oil and gas reservoir network fracturing and many rock engineering application fields. However, there is still no commonly accepted definition of the concept of brittleness. In this study, the brittleness differences between rocks are understood as the differences of internal microunit strength cracking characteristics under an external load. Because rock ruptures are regarded as a fracturing problem of a double body system, the region where no cracking occurs during the uniaxial compression loading process is regarded as a body I system with elastic deformation characteristics; the region that breaks first and induces the integral fracture of the rock is regarded as a body II system in which the structural strength of the microunit follows the deformation characteristic of the Weibull distribution. An energy quasi‐static equilibrium equation for rock fractures of a double body system under an external load was established. A new index for evaluating brittleness was established based on the energy mutation process of the quasi‐static equilibrium equation of double body system. Sandstone, red sandstone, mudstone, sandy mudstone, shale, limestone, and granite were selected as the research objects, while the correlation curve between the new brittleness index, the Weibull distribution characteristic parameters, and the mutation break point was calculated. The brittleness index studied in this paper reveals the evolution process of integral fractures induced by the internal fractures of rocks, which is important for further understanding the essence of brittleness. These results can provide a new method for further studying the formation process of local fracture networks.

Abstract Rock strength criterion is a basic subject in mining engineering and petroleum engineering. It has important guiding significances to the process of underground energy exploitation. Different rocks have different strength envelopes. Most strength criteria have only one type of envelope in π plan. Thus, these criteria cannot predict different rock strengths accurately. Most criteria consider only the hydrostatic stress and the Lode angle effects and neglect their interaction. However, based on the experimental results in this study, there is an evident interaction between the two effects. In this study, a new strength criterion is proposed considering the hydrostatic stress and Lode angle effects, as well as the effect of hydrostatic stress on the Lode angle effect. Applicable results of previous experimental data showed that the proposed strength criterion has good applicability for different rocks. Compared with other strength criteria, this criterion has the advantage of being able to adjust the strength of the Lode angle effect based on the hydrostatic stress. This advantage allows the strength criterion to show different types of strength envelopes in the π plan and to accommodate different rocks under different stress states. The effect of hydrostatic stress on the Lode angle effect of rock may be caused by the proportion of particles destroyed in the rock.

The existing erosion models of abrasive gas jet tend to neglect the effects of the rebounding abrasive. To address this shortcoming, abrasive wear tests were conducted on limestone by using an abrasive gas jet containing different types of particles and with different standoff distances. The results indicate that erosion pits have the shape of an inverted cone and a hemispherical bottom. An annular platform above the hemispherical bottom connects the bottom with the side of the pit. The primary cause of the peculiar pit shape is the flow field geometry of the gas jet with its entrained particles. There is an annular region between the axis and boundary of the abrasive gas jet, and it contains no abrasive. Particles swirling around the axis form a hemispherical bottom. After rebounding, the abrasive with the highest velocity enlarges the diameters of both the hemispherical bottom and erosion pit and induces the formation of an annular platform. The surface features of different areas of the erosion pit are characterized using a scanning electron microscope (SEM). It can be concluded that the failure modes for different locations are different. The failure is caused by an impact stress wave of the incident abrasive at the bottom. Plastic deformation is the primary failure mode induced by rebounding particles at the sides of the hemispherical bottom. The plastic deformation induced by the incident abrasive and fatigue failure induced by the rebounding abrasive are the primary failure modes on the annular platform. Fatigue failure induced by rebounding particles is the primary mode at the sides of the erosion pits. The rock failure mechanism that occurs for particles with different hardness is the same, but the rock damaged by the hard abrasive has a rougher surface.

Coalbed methane recovery enhanced by hydraulic or nonaqueous fracturing methods has been studied for decades, and it is of significance to evaluate fracturing results and scope for field applications. Monitoring variation in velocity is one way to explain fracturing effects. However, the existence of residual water or gas within cracks or pores may affect velocity measurements, and the correlation between velocity and inherent coal attributes (such as density and porosity) has not been studied comprehensively. In this paper, coal of different ranks (lignite, bituminite, and anthracite) was prepared under water and gas saturation to approximately simulate the residual water and gas in cracks under field applications. Correlations between the velocity and coal attributes were studied. For both water- and gas-saturated cores, the diverse velocity distributions were highly correlated to rank and saturation media. The longitudinal ultrasonic pulse velocity (UPVp) and transverse ultrasonic pulse velocity (UPVs) of different cores were distributed differently. For coal saturated with water or gas, the UPVp values of lignite, bituminite, and anthracite had positive linear correlations with the corresponding UPVs values. The discrete velocity ratio data were fit as negative linear correlations with UPVs, and different coals had different declining degrees, the difference of which might be attributed to the characteristics of structural cracks and the inherent properties of the coal, such as grain size and pore shape, which result in decreasing coal integrity and strength. Moreover, the difference in acoustic resistance between coal and fluids might have an inverse impact on the acoustic energy, and a larger difference might cause a large amount of energy to dissipate and finally cause the velocity to decrease. Under water and gas saturation conditions, the UPVp showed a positive linear correlation with density and a negative linear correlation with porosity. Finally, a potential field application was designed on the relations between the velocity and the elastic parameters to estimate fracturing effects by monitoring the petrophysical parameters of coal lithologies.

In the process of waterflooding technology in the Jilin oilfield, local radial compressive stress caused by rock deformation results in local casing collapse. According to statistics regarding casing-deformation characteristics, a certain number of these characteristics are approximately parabola-shaped at the radial-deformation bottom, and the boundary of the whole deformation area is approximately symmetrical and double-parabola-shaped. The main work of this article focused on occurrences of such casing deformation. Assuming that, in the process of casing deformation, external work is totally converted into energy consumption due to the deformation, the variation regularity of bearing capacity under local radial load was obtained. In the Qing-1 stratum of the Jilin oilfield, by selecting casing with radial collapse deformation parameters of 41/2″J55, 51/2″J55, 41/2″N80, and 51/2″N80, radial bearing capacity was calculated. Study results showed that the casing bearing-capacity value was reduced by 39.69% compared with the current API 5C3 standard when under the action of a local radial load. The casing collapsed due to the impact of local radial loads produced by mudstone creep. A series of relationships between radial bearing strength and casing parameters were also obtained. The research results are of significant academic value for the compilation of casing design codes or standards under local radial loading.

During CO2 injection into deep saline aquifers, the overlying caprock may be subjected to geochemical reactions which can alter the leakage pathways for injected CO2. Thus, it is crucial to identify the supercritical CO2 (scCO2) flow behaviour via fractures in caprock and its permeability to estimate the permanence of injected CO2. The objective of this study is to find the effect of scCO2 flow on fractured caprock permeability. A fractured siltstone sample was saturated in deionized water and conducted scCO2 permeability tests using a high-precision advanced core flooding apparatus under different injection pressures and confinements. Next, the siltstone sample was saturated in 10% w/w NaCl brine and conduced scCO2 permeability tests as described earlier. The results show that the brine-saturated sample has low permeability compared to water-saturated siltstone sample. The reason would be the deposition of evaporites during scCO2 flow through the fractured sample. This is known as CO2 dry-out phenomenon or absorbing moisture into the scCO2, making the remaining brine saturated with salts. Thus, the CO2 back-migration through the caprock discontinuities becomes minimized due to CO2 dry-out phenomenon, which is an advantage for the caprock integrity in deep saline aquifers. In addition, aquifers with high salinity contents show significant dry-out phenomenon because pore fluid easily becomes supersaturated with salts due to evaporation of moisture into the scCO2.

Coal deformation is one of the leading problems for carbon dioxide (CO2) sequestration in coal seams especially with respect to different-phase CO2 injection. In this paper, a series of core flooding tests were conducted under different confining stresses (8⁻20 MPa), injection pressures (1⁻15 MPa), and downstream pressures (0.1⁻10 MPa) at 50 °C temperature to investigate the effects of coal deformation induced by adsorption and effective stress on sub-critical, super-critical, and mixed-phase CO2 permeability. Due to the linear relationship between the mean flow rate and the pressure gradient, Darcy Law was applied on different-phase CO2 flow. Experimental results indicate that: (1) Under the same effective stress, sub-critical CO2 permeability > mixed-phase CO2 permeability > super-critical CO2 permeability. (2) For sub-critical CO2 flow, the initial volumetric strain is mainly attributed to adsorption-induced swelling. A temporary drop in permeability was observed. (3) For super-critical CO2 flow, when the injection pressure is over 10 MPa, effective-stress-generated deformation is dominant over the adsorption-induced strain and mainly contributes to the volumetric strain change. Thus, there is a linear increase of the volumetric strain with mean pore pressure and super-critical CO2 permeability increased with volumetric strain. (4) For mixed-phase CO2 flow, coupling effects of adsorption-induced swelling and effective stress on the volumetric strain were observed but effective stress made more of a contribution. CO2 permeability consistently increased with the volumetric strain. This paper reveals the swelling mechanism of different-phase CO2 injections and its effect on coal permeability.

Although the mechanical behaviors and flow aspects of sandstone have been previously investigated, studies of the effect of the intermediate principal stress (σ2) on the strength, deformation, and permeability of sandstone are lacking. In this work, the mechanical behaviors and permeability of sandstone under true triaxial stress conditions were investigated using a newly developed true triaxial geophysical apparatus. The experimental results showed that with increasing σ2, the peak strength, octahedral effective normal stress, and octahedral effective shear stress of the sandstone increased, and the rate of increase decreased. This is because a larger intermediate principal stress coefficient b has an inhibitory effect on rock strength. In our study, as the ratio of σ2/σ3 increased, the specimen entered compressive strain in the σ2 direction during the first stress drop. The stress and strain path deviations occur during rock failure. The amount of deviation increased as the σ2 increased before the peak stress. This phenomenon indicates that elastic mechanics are not suitable for understanding this sandstone rock during its failure. The permeability evolution of the sandstone under true triaxial stress conditions was measured and analyzed to investigate the effect of σ2. During the complete true triaxial stress-strain experiments, the variation we found in gas seepage velocity could be divided into two stages. Before the first pressure drop, the gas seepage velocity was mainly affected by volume strain. After the first pressure drop, the seepage velocity was affected by the deviator strain, which can change the seepage channels.

Fracability of unconventional gas reservoirs is an important parameter that governs the effectiveness of subsequent gas extraction. Since reservoirs are saturated with various pore fluids, it is essential to evaluate the alteration of fracability of varyingly saturated rocks. In this study, varyingly saturated (dry, water, and brine with 10%, 20% and 30% NaCl by weight) siltstone samples were subjected to uniaxial compressive loading to evaluate their fracability variation. Acoustic emission (AE) and ARAMIS photogrammetry analyses were incorporated to interpret the crack propagation. SEM analysis was carried out to visualize the micro-structural alterations. Results show that siltstone strength and brittleness index (BI) are reduced by 31.7% and 46.7% after water saturation, due to water-induced softening effect. High NaCl concentrations do not reduce the siltstone strength or brittleness significantly but may contribute to a slight re-gain of both values (about 3–4%). This may be due to NaCl crystallization in rock pore spaces, as confirmed by SEM analysis. AE analysis infers that dry siltstone exhibits a gradual fracture propagation, whereas water and brine saturated specimens exhibit a hindered fracturing ability. ARAMIS analysis illustrates that high NaCl concentrations causes rock mass failure to be converted to shear failure from splitting failure, which is in favour of fracability.

The failure mechanism of heterogeneous rocks (geological materials), especially under hydraulic conditions, is important in geological engineering. The coupled mechanism of flow-stress-damage should be determined for the stability of rock mass engineering under triaxial stress states. Based on poroelasticity and damage theory, a three-dimensional coupled model of the flow-stress-damage failure process is studied, focusing mainly on the coupled characteristics of permeability evolution and damage in nonhomogeneous rocks. The influences of numerous mesoscale mechanical and hydraulic properties, including homogeneity, residual strength coefficient, loading rates, and strength criteria, on the macro mechanical response are analyzed. Results reveal that the stress sensitive factor and damage coefficient are key variables for controlling the progress of permeability evolution, and these can reflect the hydraulic properties under pre-peak and post-peak separately. Moreover, several experiments are conducted to evaluate the method in terms of permeability evolution and failure process and to verify the proposed two-stage permeability evolution model. This model can be used to illustrate the failure mechanics under hydraulic conditions and match different rock types. The relation of permeability with strain can also help confirm appropriate rock mass hydraulic parameters, thereby enhancing our understanding of the coupled failure mechanism in rock mass engineering.

Because the limitations of water-based fracturing fluids restrict their fracturing efficiency and scope of application, liquid CO2 is regarded as a promising substitute, owing to its unique characteristics, including its greater environmental friendliness, shorter clean-up time, greater adsorption capacity than CH4 and less formation damage. Conversely, the disadvantage of high leak-off rate of CO2 fracturing due to its very low viscosity determines its applicability in gas shales with ultra-low permeability, accurate measurement of shale permeability to CO2 is therefore crucial to evaluate the appropriate injection rate and total consumption of CO2. The main purpose of this study is to accurately measure shale permeability to CO2 flow during hydraulic fracturing, and to compare the leak-off of CO2 and water fracturing. A series of permeability tests was conducted on cylindrical shale samples 38 mm in diameter and 19 mm long using water, CO2 in different phases and N2 considering multiple influencing factors. According to the experimental results, the apparent permeability of shale matrix to gaseous CO2 or N2 is greatly over-estimated compared with intrinsic permeability or that of liquid CO2 due to the Klinkenberg effect. This phenomenon explains that the permeability values measured under steady-state conditions are much higher than those under transient conditions. Supercritical CO2 with higher molecular kinetic energy has slightly higher permeability than liquid CO2. The leak-off rate of CO2 is an order of magnitude higher than that of water under the same injection conditions due to its lower viscosity. The significant decrease of shale permeability to gas after water flooding is due to the water block effect, and much longer clean-up time and deep water imbibition depth greatly impede the gas transport from the shale matrix to the created fractures. Therefore, it is necessary to substitute water-based fracturing fluids with liquid or super-critical CO2 in clay-abundant shale formations.

Thermally induced damage has an important influence on rock mechanics and engineering, especially for high-level radioactive waste repositories, geological carbon storage, underground coal gasification, and hydrothermal systems. Additionally, the wide application of geothermal heat requires knowledge of the geothermal conditions of reservoir rocks at elevated temperature. However, few methods to date have been reported for investigating the micro-mechanics of specimens at elevated temperatures. Therefore, this paper uses a cluster model in particle flow code in two dimensions (PFC2D) to simulate the uniaxial compressive testing of Australian Strathbogie granite at various elevated temperatures. The peak strength and ultimate failure mode of the granite specimens at different elevated temperatures obtained by the numerical methods are consistent with those obtained by experimentation. Since the tensile force is always concentrated around the boundary of the crystal, cracks easily occur at the intergranular contacts, especially between the b-b and b-k boundaries where less intragranular contact is observed. The intergranular and intragranular cracking of the specimens is almost constant with increasing temperature at low temperature, and then it rapidly and linearly increases. However, the inflection point of intergranular micro-cracking is less than that of intragranular cracking. Intergranular cracking is more easily induced by a high temperature than intragranular cracking. At an elevated temperature, the cumulative micro-crack counts curve propagates in a stable way during the active period, and it has no unstable crack propagation stage. The micro-cracks and parallel bond forces in the specimens with elevated temperature evolution and axial strain have different characteristics than those at lower temperature. More branch fractures and isolated wider micro-cracks are generated with increasing temperature when the temperature is over 400 °C. Therefore, the total number of cracks is almost constant when the temperature is below 400 °C; next, it linearly increases when the temperature is over 400 °C. This trend is the same as that observed by experimentation.

An experimental study was performed to investigate the effect of subcritical carbon dioxide (CO2) adsorption on mechanical properties of shales with different coring directions. Uniaxial compressive strength (UCS) tests were conducted on shale samples with different CO2 adsorption time at a pressure of 7 MPa and a temperature of 40 °C. The crack propagation and the failure mechanism of shale samples were recorded by using acoustic emission (AE) sensors together with ARAMIS technology. According to the results, samples with parallel and normal bedding angles present reductions of 26.7% and 3.0% in UCS, 30.7% and 36.7% in Young’s modulus after 10 days’ adsorption of CO2, and 30.3% and 18.4% in UCS, 13.8% and 22.6% in Young’s modulus after 20 days’ adsorption of CO2. Samples with a normal bedding angle presented higher brittleness index than that with a parallel bedding angle. The strain distributions show that longer CO2 adsorption will cause higher axial strains and lateral strains. The AE results show that samples with a parallel angle have higher AE energy release than the samples with a normal angle. Finally, samples with longer CO2 adsorption times present higher cumulative AE energy release.

During CO2 sequestration in deep coal seams, the coal mass may be subjected to various fluid (CO2, N2, etc.) saturations. Therefore, in order to maintain the long-term integrity of the process, it is necessary to identify the mechanical responses of preferable coal seams for various fluid saturations. To date, many studies have focused on the CO2 saturation effect on coal mass strength and less consideration has been given to the influence of other saturation mediums. Hence, this study aims to investigate coal’s mechanical responses to water and N2 saturations compared to CO2 saturation and to determine the effect of coal-rank. A series of unconfined compressive strength (UCS) tests was conducted on Australian brown and black coal samples saturated with water and N2 at various saturation pressures. An advanced acoustic emission (AE) system was utilized to identify the changes in crack propagation behaviors under each condition. According to the results, both CO2 and water act similarly with coal by enhancing the ductile properties of the coal mass and this mechanical weakening is greater for high-rank coal. Conversely, N2 saturation slightly enhances coal strength and delays crack propagation in coal and this strength enhancement can be improved by increasing the N2 saturation pressure.

This study combines convolutional neural networks, spatial pyramid pooling, and long short-term memory networks (LSTM) with self-attention (SA) mechanisms (abbreviated as CSAL) to address the problem of production dynamics prediction in tight reservoirs during the CO2 water-alternating-gas (CO2-WAG) injection process. By integrating DenseNet and SPP modules, this method effectively captures and processes complex spatial features in tight reservoirs. Concurrently, the LSTM enhanced with SA mechanisms improves the prediction capability of temporal data during the CO2-WAG process. Experimental results demonstrate that the CSAL model performs excellently in both the training and testing phases, achieving a coefficient of determination (R2) exceeding 0.98, significantly enhancing the model's prediction accuracy. Compared to models without attention mechanisms, the CSAL model increases the R2 value in time series prediction by 10%. Furthermore, employing the Ensemble Smoother with Multiple Data Assimilation algorithm, the CSAL model achieves high-precision history matching, significantly reducing the error between predicted values and actual observations. This study validates the application potential and superiority of the CSAL model in the CO2-WAG process in tight reservoirs. [ABSTRACT FROM AUTHOR]

A better understanding of the stress–strain behaviors of shale samples after shale-CO2 or shale-water–CO2 interactions is of great importance to CO2 enhanced shale gas exploitation and CO2 sequestrating in shale reservoirs. In this study, a constitutive model that combines with the modified Duncan–Chang model and Weibull distribution-based model is applied to investigate the stress–strain characteristics of low-clay shale samples treated by sub-/super-critical CO2 and sub-/super-critical CO2 + water for different times (10 days, 20 days, and 30 days). The results show that the model could describe well the crack closure stage, the elastic stage, and the inelastic stage of shale samples. The axial strain at the connection point between the two models varies from 28.51 to 43.36% of the axial strain at the failure point. Shale-CO2 or shale-water–CO2 interactions make shale samples more ductile at the crack closure stage, which can be depicted as the increase of initial elastic modulus during the imbibition process. The brittleness index values (BI) which are calculated based on the combined constitutive model increase with increasing soaking time for shale samples treated by sub-/super-critical CO2, and decrease with increasing soaking time for shale samples treated by sub-/super-critical CO2 + water. [ABSTRACT FROM AUTHOR]

Abstract This review summarizes current progress in molecular dynamic (MD) simulation in the context of geological high-level radioactive waste disposal (HLRW). Geological HLRW disposal is a long-term thermal-hydraulic-mechanical-chemical (THMC) coupling process; hence, it is difficult to model actual geological HLRW disposal conditions experimentally. MD simulation has been adopted as an alternative method to address geological HLRW disposal issues. This paper reviews the application of MD simulation in geological HLRW disposal in the last two decades. The swelling of clay minerals, and the transport and adsorption properties of ions are reviewed and discussed. The paper concludes with highlights of future directions of MD simulation in HLRW disposal. The application of MD simulation in geological HLRW disposal is still in its infancy and several problems remain to be solved. [ABSTRACT FROM AUTHOR]

With its low viscosity properties, CO2 has much greater penetration capacity into micro-fractures, and therefore has more potential to create expanded and effective fractures in shales during the hydraulic fracturing process. However, the feasibility of this technique is dependent on the accurate prediction of formation flow characteristics, given the high leak-off of CO2 at deep depths. The aim of this study is therefore to understand the flow behavior of CO2 in deep shale plays. A series of tri-axial permeability tests was conducted under both steady-state and transient conditions. The test results show much lower permeability values for liquid CO2 than gaseous CO2, and the permeability under transient conditions is much lower than that under steady-state conditions, due to the combined effects of the reduced slip-flow effect under low pressures and the temperature variation influence under steady-state conditions. Under steady-state conditions, unstable flow behavior occurred at higher injection pressure (≥9 MPa) possibly due to the fine mineral particle migration and the deposition of small drikold particles, which indicates the serious error in permeability calculation under steady-state conditions. Importantly, a greater than 1 effective stress coefficient (χ) for permeability in tested siltstone was observed, confirming the greater sensitivity of CO2 to pore pressure than confining pressure. [ABSTRACT FROM AUTHOR]

This paper proposes a coupled thermal-hydrological-mechanical damage (THMD) model for the failure process of rock, in which coupling effects such as thermally induced rock deformation, water flow-induced thermal convection, and rock deformation-induced water flow are considered. The damage is considered to be the key factor that controls the THM coupling process and the heterogeneity of rock is characterized by the Weibull distribution. Next, numerical simulations on excavation-induced damage zones in Äspö pillar stability experiments (APSE) are carried out and the impact of in situ stress conditions on damage zone distribution is analysed. Then, further numerical simulations of damage evolution at the heating stage in APSE are carried out. The impacts of in situ stress state, swelling pressure and water pressure on damage evolution at the heating stage are simulated and analysed, respectively. The simulation results indicate that (1) the v-shaped notch at the sidewall of the pillar is predominantly controlled by the in situ stress trends and magnitude; (2) at the heating stage, the existence of confining pressure can suppress the occurrence of damage, including shear damage and tensile damage; and (3) the presence of water flow and water pressure can promote the occurrence of damage, especially shear damage. [ABSTRACT FROM AUTHOR]

This paper reports on a comprehensive study of the CO2-EOR (Enhanced oil recovery) process, a detailed literature review and a numerical modelling study. According to past studies, CO2 injection can recover additional oil from reservoirs by reservoir pressure increment, oil swelling, the reduction of oil viscosity and density and the vaporization of oil hydrocarbons. Therefore, CO2-EOR can be used to enhance the two major oil recovery mechanisms in the field: miscible and immiscible oil recovery, which can be further increased by increasing the amount of CO2 injected, applying innovative flood design and well placement, improving the mobility ratio, extending miscibility, and controlling reservoir depth and temperature. A 3-D numerical model was developed using the CO2-Prophet simulator to examine the effective factors in the CO2-EOR process. According to that, in pure CO2 injection, oil production generally exhibits increasing trends with increasing CO2 injection rate and volume (in HCPV (Hydrocarbon pore volume)) and reservoir temperature. In the WAG (Water alternating gas) process, oil production generally increases with increasing CO2 and water injection rates, the total amount of flood injected in HCPV and the distance between the injection wells, and reduces withWAG flood ratio and initial reservoir pressure. Compared to other factors, the water injection rate creates the minimum influence on oil production, and the CO2 injection rate, flood volume and distance between the flood wells have almost equally important influence on oil production. [ABSTRACT FROM AUTHOR]

Rock fracture acoustic emission (AE) signals are commonly used non-destructive testing data in geological exploration, resource exploitation, and engineering fields. However, these signals are often accompanied by noise interference caused by environmental factors. In this study, we propose an enhanced model for denoising rock fracture AE signals, called simplified fully convolutional denoising autoencoder (SFCDAE). This model is based on the denoising autoencoder principle in the field of deep learning neural networks. The SFCDAE model consists of only seven layers, with minimal preprocessing of data input. By comparing denoising performance evaluation indicators, higher peak signal-to-noise ratio (PSNR) and lower root mean square error (RMSE) were achieved. On average, PSNR increased by 5.575% and RMSE decreased by 22.225%. Using simulated environmental noise to validate the model, it was found that the model has good robustness and can remove artefacts from sudden noise. The practical application value of the LSTM classification model was validated using data containing real experimental noise, resulting in a higher classification accuracy of 80.083%. These results indicate that the proposed model has better denoising performance compared to existing intelligent models and has certain practical value. [ABSTRACT FROM AUTHOR]

In view of the water swelling of mudstone and the creep induction function of formations in the process of oilfield water injection, the casing incurs collapse deformation under local lateral load. In this study, according to the actual collapse deformation characteristics of the casing in the second section of the Qing formation of the Songliao Basin in China, the yield surfaces of the casing collapse deformation are considered as plane plastic areas (half rhombus) with symmetric parabola shaped boundaries, and a mechanical model for the local lateral collapse deformation of casing is presented based on the principle of virtual work. Four types of casing, 4½″J55, 5½″J55, 4½″N80 and 5½″N80, are selected as examples. The relation of the casing intensity, the absolute reduction of intensity and the relative reduction ratio of intensity change with casing wall thickness, yield stress, radial maximum deformation, and deformation length are calculated and analyzed. The results show that the casing intensity of the casing is reduced under local lateral load, which is lower than the design standard value of the American Petroleum Association specification (API SPEC) 5CT. The relative reduction ratio declines linearly with the wall thickness of the casing wall as the yield stress increases, and increases linearly with increasing maximum deformation. In addition, the local lateral bearing capacity of the casing reaches the minimum value when the plastic deformation length reaches the critical value or the deformation quantity is less than the critical value. The conclusions provide scientific guidance for preventing casing failure accidents caused by deformation. [ABSTRACT FROM AUTHOR]