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This study employed a comprehensive approach utilizing X-ray diffraction, scanning electron microscopy (computed), computed tomography (CT) three-dimensional scanning, uniaxial compressive testing, acoustic emission (AE), and digital image correlation to investigate the micromorphology, mechanical properties, macro–microscopic fractal characteristics, failure modes, and mineral composition changes in acid–base coal samples from underground reservoirs. The findings from this study indicate that, in acidic environments, calcite undergoes acidolysis to form calcium chloride (CaCl2), whereas kaolinite reacts with alkaline substances to produce albite (Na2Al2Si2O8). The average elastic modulus of the coal samples treated with strong acids or alkalis decreased by 51.08% and 38.17%, respectively, when compared to the naturally dried samples. After strong acid treatment (pH = 2), the post-peak modulus decreased to its lowest value of 1.78 GPa. The fracture threshold in the plastic phase of the acidic coal samples decreased by approximately 60% when compared to that of the naturally dried samples, with the cumulative AE energy reaching a maximum of 2.46 × 1010 aJ. CT image segmentation revealed that with higher H+ concentrations, the sliced fracture fractal dimension increased from 1.01 to 1.25, indicating enhanced fracture disorder. The evolution of the spatial and energy fractal dimensions of the AE, analyzed using the box-counting method and correlation dimension, showed that in strong acidic and alkaline environments, the spatial fractal dimension before the peak failure was higher, pre-disposing the formation of highly disordered and smaller-scale fractures. During the fracture development stage, the time-series energy correlation dimension of the acid–base coal samples experienced two significant fluctuations, with a rapid decrease before the peak, indicating a transition from disorderly expansion to orderly fracturing. In the final stage of fracturing, the strongly acidic samples predominantly developed microcracks. The stronger the acidity or alkalinity, the more likely it is that high-energy (greater than 104 aJ) AE shear events will occur, with the mixed mode of tensile and shear failure tending to concentrate in the areas of high shear strain, thereby enhancing the macroscopic fragmentation orderliness. This research is crucial in regard to the safety assessment of dam bodies or coal pillars in acidic and alkaline coal mines in Northwestern China, particularly for predicting their stability. [ABSTRACT FROM AUTHOR]

This work systematically reviews the complex mechanisms of CO2-water-rock interactions, microscopic simulations of reactive transport (dissolution, precipitation and precipitate migration) in porous media, and microscopic simulations of CO2-water-rock system. The work points out the key issues in current research and provides suggestions for future research. After injection of CO2 into underground reservoirs, not only conventional pressure-driven flow and mass transfer processes occur, but also special physicochemical phenomena like dissolution, precipitation, and precipitate migration. The coupling of these processes causes complex changes in permeability and porosity parameters of the porous media. Pore-scale microscopic flow simulations can provide detailed information within the three-dimensional pore and throat space and explicitly observe changes in the fluid-solid interfaces of porous media during reactions. At present, the research has limitations in the decoupling of complex mechanisms, characterization of differential multi-mineral reactions, precipitation generation mechanisms and characterization (crystal nucleation and mineral detachment), simulation methods for precipitation-fluid interaction, and coupling mechanisms of multiple physicochemical processes. In future studies, it is essential to innovate experimental methods to decouple "dissolution-precipitation-precipitate migration" processes, improve the accuracy of experimental testing of minerals geochemical reaction-related parameters, build reliable characterization of various precipitation types, establish precipitation-fluid interaction simulation methods, coordinate the boundary conditions of different physicochemical processes, and, finally, achieve coupled flow simulation of "dissolution-precipitation-precipitate migration" within CO2-water-rock systems. [ABSTRACT FROM AUTHOR]

Abstract The stability of coal pillar dams is crucial for the long-term service of underground reservoirs storing water or heat. Chemical damage of coal dams induced by ions-attacking in coal is one of the main reasons for the premature failure of coal dams. However, the diffusion process of harmful ions in coal is far from clear, limiting the reliability and durability of coal dam designs. This paper investigates sulfate diffusion in coal pillar through experimental and analytical methods. Coal specimens are prepared and exposed to sulfate solutions with different concentrations. The sulfate concentrations at different locations and time are measured. Based on experimental data and Fick’s law, the time-dependent surface concentration of sulfate and diffusion coefficient are determined and formulated. Further, an analytical model for predicting sulfate diffusion in coal pillar is developed by considering dual time-dependent characteristics and Laplace transformations. Through comparisons with experimental data, the accuracy of the analytical model for predicting sulfate diffusion is verified. Further, sulfate diffusions in coal dams for different concentrations of sulfate in mine water are investigated. It has been found that the sulfate concentration of exposure surface and diffusion coefficient in coal are both time-dependent and increase with time. Conventional Fick’s law is not able to predict the sulfate diffusion in coal pillar due to the dual time-dependent characteristics. The sulfate attacking makes the coal dam a typical heterogeneous gradient structure. For sulfate concentrations 0.01–0.20 mol/L in mine water, it takes almost 1.5 and 4 years for sulfate ions to diffuse 9.46 and 18.92 m, respectively. The experimental data and developed model provide a practical method for predicting sulfate diffusion in coal pillar, which helps the service life design of coal dams.

The city of Matera, as several other ancient historical cities and towns, is recovering a large part of its ancient parts developing a policy of urban regeneration. In the early 90s, during the restoration works of the main square, Piazza Vittorio Veneto, it was re-discovered a huge underground space used as water reservoir named Palombaro Lungo. It was built linking ancient pre-existing smaller caves and completed at the end of the nineteenth century making it impermeable with the cocciopesto (opus signinum) technique. The hypogeum is dug in a Plio-Pleistocene rock called Gravina Calcarenite. It is a calcareous sandstone weak rock with good mechanical properties, low permeability and easy to be dug. The internal geometry of the hypogeum and the rock thicknesses above the caves was reconstructed by the integration of topographic and GPR surveys. The paper presents a geo-mechanical model of Palombaro Lungo underground reservoir and evaluation about the stability of this structure. The stability analysis were developed using the numerical code UDEC for several transect of Palombaro Lungo assuming the calcareous sandstone blocks as deformable, trying to address the evolution of the stress–strain conditions. The case study can be considered as an example of general interest for the study of rupestrian underground reservoir of a longed form, where one planimetric dimension strictly prevails over the other. Numerical simulations showed a stress–strain state compatible with the fractures detected in situ and confirmed the absence of instability problems in the groundwater reservoir. [ABSTRACT FROM AUTHOR]

The stability of coal pillar dams is crucial for the long-term service of underground reservoirs storing water or heat. Chemical damage of coal dams induced by ions-attacking in coal is one of the main reasons for the premature failure of coal dams. However, the diffusion process of harmful ions in coal is far from clear, limiting the reliability and durability of coal dam designs. This paper investigates sulfate diffusion in coal pillar through experimental and analytical methods. Coal specimens are prepared and exposed to sulfate solutions with different concentrations. The sulfate concentrations at different locations and time are measured. Based on experimental data and Fick's law, the time-dependent surface concentration of sulfate and diffusion coefficient are determined and formulated. Further, an analytical model for predicting sulfate diffusion in coal pillar is developed by considering dual time-dependent characteristics and Laplace transformations. Through comparisons with experimental data, the accuracy of the analytical model for predicting sulfate diffusion is verified. Further, sulfate diffusions in coal dams for different concentrations of sulfate in mine water are investigated. It has been found that the sulfate concentration of exposure surface and diffusion coefficient in coal are both time-dependent and increase with time. Conventional Fick's law is not able to predict the sulfate diffusion in coal pillar due to the dual time-dependent characteristics. The sulfate attacking makes the coal dam a typical heterogeneous gradient structure. For sulfate concentrations 0.01–0.20 mol/L in mine water, it takes almost 1.5 and 4 years for sulfate ions to diffuse 9.46 and 18.92 m, respectively. The experimental data and developed model provide a practical method for predicting sulfate diffusion in coal pillar, which helps the service life design of coal dams. [ABSTRACT FROM AUTHOR]

Investigation of reservoir or aquifer pressure response is mainly based on the analytical solution of the one-dimensional diffusivity equation, and it has limiting assumptions that deviates the analytical model from the reality of the reservoir. Based on the fact that in underground reservoirs the properties of the reservoir rock vary from point to point, by considering permeability as a function of the location, the general equation of fluid flow is expressed in the porous medium. The reservoir quality and permeability decrease from the crest of the reservoir to its flanks. To the best of the authors' knowledge, there is no accurate analytical solution that could predict reservoir pressure behavior and well production rate in non-homogeneous reservoirs. Additionally, the position-dependent permeability could be obtained using the developed analytical solution, unlike the average permeability calculated by traditional well-testing methods. Previous models focused on numerical and semi-analytical solutions in a semi-infinite length. In this paper, by considering linear and quadratic functions in terms of location, the nonlinear diffusivity equation is defined, and then, an exact analytical solution of the diffusivity equation in a finite length and for different boundary conditions is developed. Also, in order to validate the proposed analytical model, the finite difference method is used, and the results of the analytical and numerical models are compared. Overlooking the position-dependent permeability changes could lead to error values up to 100% in predicting well production rate and additionally reservoir pressure at different points. [ABSTRACT FROM AUTHOR]

Million cubic meters from abandoned mines worldwide could be used as subsurface reservoirs for large scale energy storage systems, such as adiabatic compressed air energy storage (A-CAES). In this paper, analytical and three-dimensional CFD numerical models have been conducted to analyze the thermodynamic performance of the A-CAES reservoirs in abandoned mines during air charging and discharging processes. Unlike other research works, in which the heat transfer coefficient is considered constant during the operation time, in the present investigation a correlation based on both unsteady Reynolds and Rayleigh numbers is employed for the heat transfer coefficient in this type of application. A tunnel with a 35 cm thick concrete lining, 200 m3 of useful volume and typical operating pressures from 5 to 8 MPa were considered. Fiber-reinforced plastic (FRP) and steel were employed as sealing layers in the simulations around the fluid. Finally, the model also considers a 2.5 m thick sandstone rock mass around the concrete lining. The results obtained show significant heat flux between the pressurized air and the sealing layer and between the sealing layer and concrete lining. However, no temperature fluctuation was observed in the rock mass. The air temperature fluctuations are reduced when steel sealing layer is employed. The thermal energy balance through the sealing layer for 30 cycles, considering air mass flow rates of 0.22 kg s−1 (charge) and −0.45 kg s−1 (discharge), reached 1056 and 907 kWh for FRP and steel, respectively. In general, good agreements between analytical and numerical simulations were obtained. [ABSTRACT FROM AUTHOR]

Strong explosive eruptions of volcanoes throw out mixtures of gases and ash from high-pressure underground reservoirs. Investigating these subsurface reservoirs may help to forecast and characterize an eruption. In this study, we compare seismic tomography results with remote sensing and petrology data to identify deep and subaerial manifestations of pre-eruptive processes at Bezymianny volcano in Kamchatka shortly before its violent explosion on December 20, 2017. Based on camera networks we identify precursory rockfalls, and based on satellite radar data we find pre-eruptive summit inflation. Our seismic network recorded the P and S wave data from over 500 local earthquakes used to invert for a 3D seismic velocity distribution beneath Bezymianny illuminating its eruptive state days before the eruption. The derived tomography model, in conjunction with the presence of the high-temperature-stable SiO2 polymorph Tridymite in juvenile rock samples , allowed us to infer the coexistence of magma and gas reservoirs revealed as anomalies of low (1.5) and high (2.0) Vp/Vs ratios, respectively, located at depths of 2–3 km and only 2 km apart. The reservoirs both control the current eruptive activity: while the magma reservoir is responsible for episodic dome growth and lava flow emplacements, the spatially separated gas reservoir may control short but powerful explosive eruptions of Bezymianny. [ABSTRACT FROM AUTHOR]

Through modern technology we can use the void pore space of underground rock formations for a growing number of socially beneficial purposes. These run the gamut from unconventional oil and gas production to climate change mitigation. The common law of property and tort, however, has struggled to keep up with advancing technology in this area. Significant questions remain about the nature of property rights in pore space. Of particular interest are the limits, if any, on an owner's right to use pore space for beneficial purposes when it extends beneath the land of another. For example, may A hydraulically fracture an oil well on her property if the fractures extend beneath B's land? May C store anthropogenic carbon dioxide for climate change mitigation in a common reservoir that extends beneath the land of d, E, F, and G if they do not consent? If so, what, if any, compensation does C owe to the others? These and similar scenarios pose urgent questions for a wide range of landowners, industries, environmental interests, courts, and policymakers across the nation. This Article searches for answers to these pressing questions in the doctrinal histories of similar common pool natural resources. The Article reviews the development of common law rights in water and oil and gas to synthesize lessons for shaping the content and limits of rights in pore space. Then, applying these lessons to the current state of pore space rights, the Article explains that rights in pore space are established by a default rule of prior use and are absolute, subject to little, if any, limitation. As demand for the resource continues to grow, however, owners, and, ultimately, courts will likely search for ways to limit the absolute extent of pore space rights to avoid a tragedy of the pore space commons. In searching for doctrinal mechanisms to make pore space rights limited, or correlative, the Article predicts that courts will be tempted to choose between establishing limits by strict, formalist rides of proportionality (which favor certainty), on the one hand, and instrumentalist, utilitarian standards of reasonable use (which favor development of the resource), on the other. This Article identifies an underexplored doctrine from oil and gas law that would define the limits of pore space rights without resort to purely instrumentalist or formalist doctrines. The "fair-opportunity doctrine " articulated here would permit an owner to use any quantity of pore space anywhere in a common reservoir, so long as it does not interfere with the lawful existing operations of other owners or deprive other owners of a fair opportunity to either participate in the proposed operations or conduct like operations from their respective land. [ABSTRACT FROM AUTHOR]

Determining the pressure dependence of dynamic moduli in unconsolidated sediments is still an open problem in applied geophysics. This is because several petrophysical parameters affect the elastic response of the granular medium during compression. Effective medium theories based on the Hertz–Mindlin contact law estimate the effective moduli from petrophysical parameters. Among them, the Pride and Berryman model assumes that new contacts between grains are progressively created during compression. Furthermore, the gaps around rattlers are distributed following a power law with distance and the global strain can change either linearly or quadratically with the local strain. This identifies two types of strain accumulation. Quadratic strain accumulation is associated with grain rotation. We simplified this model by assuming a flat distribution of gaps around rattlers and we applied this simplified model to published ultrasonic measurements. By means of these measurements, we studied how the strain-accumulation mechanism affects the coordination number during isotropic compression. The coordination numbers were estimated by applying a DEM-based correction to the average-strain model. We observe that the majority of the experimental trends lay between the linear and the quadratic accumulation trends. Based on this result, we assume that the strain accumulation is a combination of the two mechanisms and we propose a formula to estimate the contribution of each mechanism. Furthermore, we note that, in the studied datasets, rotation affects larger grains (diameter approximately 500 μm) more than smaller grains (diameter approximately 100 μm). If further validated, this correlation could guide the determination of pressure trends for sands. [ABSTRACT FROM AUTHOR]

Million cubic meters from abandoned mines worldwide could be used as subsurface reservoirs for large scale energy storage systems, such as adiabatic compressed air energy storage (A-CAES). In this paper, analytical and three-dimensional CFD numerical models have been conducted to analyze the thermodynamic performance of the A-CAES reservoirs in abandoned mines during air charging and discharging processes. Unlike other research works, in which the heat transfer coefficient is considered constant during the operation time, in the present investigation a correlation based on both unsteady Reynolds and Rayleigh numbers is employed for the heat transfer coefficient in this type of application. A tunnel with a 35 cm thick concrete lining, 200 m3 of useful volume and typical operating pressures from 5 to 8 MPa were considered. Fiber-reinforced plastic (FRP) and steel were employed as sealing layers in the simulations around the fluid. Finally, the model also considers a 2.5 m thick sandstone rock mass around the concrete lining. The results obtained show significant heat flux between the pressurized air and the sealing layer and between the sealing layer and concrete lining. However, no temperature fluctuation was observed in the rock mass. The air temperature fluctuations are reduced when steel sealing layer is employed. The thermal energy balance through the sealing layer for 30 cycles, considering air mass flow rates of 0.22 kg s−1 (charge) and −0.45 kg s−1 (discharge), reached 1056 and 907 kWh for FRP and steel, respectively. In general, good agreements between analytical and numerical simulations were obtained.

In coal mines, underground reservoir systems can increase the availability of water and are an effective technical approach for the protection and utilization of water resources. The stability of coal pillar dams is the key factor in the safety and stability of these underground water storage systems. However, coal pillar dams must operate in complex environments that combine dynamic-static superimposed stress fields and water immersion; moreover, coal pillar dams subjected to both stress and seepage are more susceptible to damage and even collapse. In this study, a seepage-stress coupling model of a coal pillar dam was constructed using the Universal Distinct Element Code (UDEC) simulation software. This model provides a platform for analyzing the characteristics of fracture development in surrounding rock in active mines and the coupled development of crack fields and seepage fields in coal pillar dams. Methods were developed for (1) calculating the water content for the coal pillar dam numerical simulation model and (2) reducing water immersion weakening. The maximum seepage width of a coal pillar dam subjected to water immersion was obtained, and a damage and failure evolution mechanism for coal pillar dams experiencing flooding was developed. The results provide a scientific basis for enhancing the stability control of coal pillar dams and are of great significance for realizing water conservation in coal mines. [ABSTRACT FROM AUTHOR]

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Coupling watershed and lake models is a powerful approach for providing lake or reservoir inputs and for simulating how land use scenarios may impact lake ecosystems. SWAT (Soil and Water Assessment Tool) is one of the most widely used watershed models for this purpose. However, as outlets for watershed delineation in SWAT should be in the river network, modellers may – when modelling the inputs to a waterbody boundary – make choices leading to imprecise results. The impact of these choices on the accuracy of a coupled model can be highly relevant as they have a direct effect on the size of the delineated watershed. In this paper, we describe the design and development of SWAT2lake, a tool that remedies such imprecise delineation of watersheds to lakes and reservoirs. SWAT2lake has been designed as a plugin for the QGIS software that works in parallel with QSWAT and it is available through wet.au.dk. [ABSTRACT FROM AUTHOR]

The goal of this study is to investigate the integrity of gypsum-containing rocks during underground mining using non-destructive testing (NDT) anchors by the shock-spectral method. Studies were conducted in-situ at a gypsum deposit in the roof of the transport drift and cleaning chamber of a mine. Steel-polymer anchors were studied. The anchors were fastened in the hole with polymer resin and were fastened outside the hole with washers and nuts. The anchors were free in the middle. The vibrational response after striking the protruding end of an anchor was recorded, a spectrum was calculated, and the frequency (F) of the spectral maximum and the acoustic quality factor (Q) were determined. Q was calculated as the ratio of F to the frequency band at the 1 / 2 level of the spectrum maximum. With a tensioned anchor, it was determined that at the free length of the anchor, half of the wavelength was placed (F is high) and the outer layer of roof rocks determined the measured data. With a weakened anchor, a quarter wavelength was placed on its length (F is low) and the inner layer of roof rocks determined the measured data. All anchors were divided into two groups. When F ≤ 1050 Hz the inner layers determined the Q-factor, and at F > 1050 Hz the external layers determined the Q-factor. The average value of Q-factors of tensioned anchors (outer layers) were below the average value of Q-factors of weakened anchors (inner layers). Large Q values were assumed to correspond to higher rock integrity and strength. The ratio of the average value of the Q-factors of the external layer to the average value of the Q-factors of the inner layer was used to assess roof rock integrity. Smaller values of this ratio were characterized by lower rock integrity and a tendency to collapse and fall. [ABSTRACT FROM AUTHOR]

Researches were conducted on the following three typical areas: the central area of Chaobai River underground reservoir, where groundwater was overdrawn seriously but was affected by the South-to-North water recharge; Tianzhu in Shunyi District and Wangsiying in Chaoyang District, where land subsidence had developed for many years but the withdrawing of groundwater had been limited or prohibited since the South-to-North Water Transfer Project was finished. The dynamic changes of groundwater levels and land subsidence before and after the Yangtze River water entering Beijing in these typical areas were analyzed quantitatively and the inherent mechanism of different geological environment effects in different areas was analyzed preliminarily. The analysis shows that the center area of Chaobai River underground reservoir has a quick response to the South-to-North water recharge, and the groundwater level in this area rises rapidly during recharging period, but drops once the recharge is stopped, so this area is still the center of the groundwater funnel. In Tianzhu area, the phreatic water level did not show a clear descending trend in two years, and the confined water level dropped by about 1m and the land subsidence rate slowed down to 34 mm/ a. As to Wangsiying area, the phreatic water level has remained at the same level for nearly 10 years, and the confined water level in all layers has been declining in recent two years, among them, the dynamic amplitude of shallow artesian water level is less than 0. 5m and that of deep artesian water levels is about 1m. The rate of land subsidence in the area is reduced from 64mm/ a (2014) to 55 mm/ a (2016). [ABSTRACT FROM AUTHOR]

Once a geothermal site has been selected for exploration, a full understanding of the formation temperature distribution on the surface and in the underground reservoir is always needed to assess the geothermal capability and productivity. Commercially available electronic measuring instruments cannot overcome the highest downhole temperatures obtained in the world’s deepest and hottest geothermal reservoirs. In this study, fiber optic Bragg grating (FBG) measurement technology is utilized applied in an attempt to replace more expensive electronic sensors and to obtain more accurate downhole pressure and temperature data. This has been our target since 2013. For this purpose, a portable pressure and temperature (P/T) optic fiber wire-line instrument prototype was developed, and it can measure up to 1500 m in depth. The prototype was named SINOTECH-OPTIC-P/T in 2015. It has been tested in various well types, including a hot spring well and a pioneering geothermal exploration well (JY-01). The primary results have proved to be satisfactory. This paper summarizes the temperature and pressure measurement results obtained in JY-01 well at Sanshing geothermal site in I-lan County, Taiwan. The maximum measured well depth was 1200 m, and data measured includes a downhole temperature profile with the maximum temperature of around 75 °C. [ABSTRACT FROM AUTHOR]

Geothermal energy is an unconventional renewable energy source with enormous potential for exploitation, of which only a very small part is currently used today. Worldwide, countries with a high geothermal potential are Iceland, New Zealand and in Europe there are Hungary, Romania and Serbia. In these countries, as well as in many others, there are geological conditions that allow to geothermal phenomena to manifest on the surface of the Earth. The limited exploitation of this energy resource is still limited by existing technologies and by the high cost of deep-water equipment. For a good image of the Romania's geothermal potential, we present, in our paper, the temperature map at 3000 m deep and the geothermal flow map on the surface. These maps made with the Surfer program, based on the data presented on geophysical portal on the WEB page of the Geological Institute of Romania. [ABSTRACT FROM AUTHOR]

Emulsions are suspensions of droplets ubiquitous in oil recovery from underground reservoirs. Oil is typically trapped in geological porous media where emulsions are either formed in situ or injected to elicit oil mobilization and thus enhance the amount of oil recovered. Here, we briefly review basic concepts on geometrical and wetting features of porous media, including thin film stability and fluids penetration modes, which are more relevant for oil recovery and oil-contaminated aquifers. Then, we focus on the description of emulsion flow in porous media spanning from the behaviour of single droplets to the collective flow of a suspension of droplets, including the effect of bulk and interfacial rheology, hydrodynamic and physico-chemical interactions. Finally, we describe the particular case of emulsions used in underground porous media for enhanced oil recovery, thereby discussing some perspectives of future work. Although focused on oil recovery related topics, most of the insights we provide are useful towards remediation of oil-contaminated aquifers and for a basic understanding of emulsion flow in any kind of porous media, such as biological tissues. [ABSTRACT FROM AUTHOR]

Centrifuge experiments on seismic performance of relatively stiff underground reservoir structures in dry sand are modeled numerically. The capabilities of numerical simulations with calibrated equivalent linear soil properties in capturing the main features of experimentally measured responses are explored for both low and high amplitude earthquake motions. The scattering effects of the centrifuge container boundaries are also investigated by modeling the same soil deposit resting on an elastic bedrock and extending infinitely laterally, using the domain reduction method. It is observed that the calibrated equivalent linear soil models perform well in predicting accelerations, racking, and bending strains on the buried structure, even for high amplitude motions for which significant soil nonlinearity is expected. While not as accurate, the seismic lateral earth pressures predicted with these models are in fair agreement with direct measurements made with tactile sensors. The mismatches in earth pressures are likely due to local nonlinearities of soil and frictional contact, which were absent from the numerical models. It is also observed that the scattering effects of the container boundaries become more significant closer to the soil surface, and their characteristics are seen to depend on both the side boundaries and the embedded structure's stiffness. [ABSTRACT FROM AUTHOR]

When studying discharges in closed mines with relevant environmental impact, the first step is to undertake a complete hydrogeological study of the mining system. In this study, some inactive and flooded coal mines in NW Spain were characterized. The water resources (recharge) of each mine were evaluated considering the infiltration of effective rainfall over mined areas, the groundwater contribution, and the losses from surface watercourses. The studied mine drainage corresponds to a mining reservoir of 5 connected mines. The resources of this reservoir were estimated in 54 L s − 1 for an average year, coming mainly from rainfall as well as losses from irrigation systems during the summer. The volume of voids of the reservoir was estimated by using different approaches: the calculation of the void left by the mining activity and the volume of infiltrated water during the mine flooding. In this particular case, the volume of voids below the discharge level (water reserves) are estimated to be around 1 Mm 3 , which makes them useful for storage and regulation of water, as well as for the production of energy. The acid mine water discharge is of poor quality, and affects the receiving watercourses. Strategies, such as the reduction of the mine's water flow, avoiding water level fluctuations, and passive treatment systems could be applied to diminish these negative effects. The methodology explained here is believed to be suitable for application in closed mines where there is not much information available. [ABSTRACT FROM AUTHOR]

Present-day catchments adjacent to sedimentary basins may preserve geomorphic elements that have been active through long intervals of time. Relicts of ancient catchments in present-day landscapes may be investigated using mass-balance models and can give important information about upland landscape evolution and reservoir distribution in adjacent basins. However, such methods are in their infancy and are often difficult to apply in deep-time settings due to later landscape modification. The southern Barents Sea margin of N Norway and NW Russia is ideal for investigating source-to-sink models, because it has been subject to minor tectonic activity since the Carboniferous, and large parts have eluded significant Quaternary glacial erosion. A zone close to the present-day coast has likely acted as the boundary between basin and catchments since the Carboniferous. Around the Permian-Triassic transition, a large delta system started to prograde from the same area as the present-day largest river in the area, the Tana River, which has long been interpreted to show features indicating that it was developed prior to present-day topography. We performed a source-to-sink study of this ancient system in order to investigate potential linkages between present-day geomorphology and ancient deposits. We investigated the sediment load of the ancient delta using well, core, two-dimensional and three-dimensional seismic data, and digital elevation models to investigate the geomorphology of the onshore catchment and surrounding areas. Our results imply that the present-day Tana catchment was formed close to the Permian-Triassic transition, and that the Triassic delta system has much better reservoir properties compared to the rest of Triassic basin infill. This implies that landscapes may indeed preserve catchment geometries for extended periods of time, and it demonstrates that source-to-sink techniques can be instrumental in predicting the extent and quality of subsurface reservoirs. [ABSTRACT FROM AUTHOR]

Drill-in fluid loss is the most important cause of formation damage during the drill-in process in fractured tight reservoirs. The addition of lost-circulation material (LCM) into drill-in fluid is the most popular technique for loss control. However, traditional LCM selection is mainly performed by use of the trial-and-error method because of the lack of mathematical models. The present work aims at filling this gap by developing a new mathematical model to characterize the performance of drill-in fluid-loss control by use of LCM during the drill-in process of fractured tight reservoirs. Plugging-zone strength and fracture-propagation pressure are the two main factors affecting drill-in fluid-loss control. The developed mathematical model consists of two submodels: the plugging-zone-strength model and the fracture-propagation-pressure model. Explicit formulae are obtained for LCM selection dependent on the proposed model to control drill-in fluid loss and prevent formation damage. Effects of LCM mechanical and geometrical properties on loss-control performance are analyzed for optimal fracture plugging and propagation control. Laboratory tests on loss-control effect by use of different types and concentrations of LCMs are performed. Different combinations of acid-soluble rigid particles, fibers, and elastic particles are tested to generate a synergy effect for drill-in fluid-loss control. The derived model is validated by laboratory data and successfully applied to the field case study in Sichuan Basin, China. [ABSTRACT FROM AUTHOR]

Changes in co-seismic water levels associated with the Gorkha Nepal earthquake (25 April 2015, Mw 7.8) were recorded in the Jingle well in Shanxi Province China (longitude E112.03 ° , latitude N38.35 ° , about 2769 km from epicenter). Based on the observed water levels, we clearly identified signals relating to P, S and surface waves. However, the water temperature recorded at a depth of 350 m shows no co-seismic changes. A spectrum analysis of co-seismic variations of water level shows that the oscillation frequency and amplitude of water level in the borehole are determined by the natural frequency of the borehole, which is not associated with the propagation of seismic waves. The borehole-aquifer system shows a large amplification associated with ground vibrations generated by earthquakes. Considering the local hydro-geological map and the temperature gradient of the Jingle well, a large volume “groundwater reservoir” model can be used to explain these processes. Due to seismic wave propagation, the volume of a well-confined aquifer expands and contracts forming fractures that change the water flow. In the well-confined aquifer, water levels oscillate simultaneously with high amplitude ground shaking during earthquakes. However, the water in the center of the “underground reservoir” remains relatively stationary, without any changes in the water temperature. In addition, a possible precursor wave is recorded in the water level at the Jingle well prior to the Gorkha earthquake. [ABSTRACT FROM AUTHOR]

For successful CO2 storage in underground reservoirs, the potential problem of CO2 leakage needs to be addressed. A profoundly improved understanding of the behavior of fractured cement under realistic subsurface conditions including elevated temperature, high pressure and the presence of CO2 saturated brine is required. Here, we report in situ X-ray micro computed tomography (μ-CT) studies visualizing the microstructural changes upon exposure of cured Portland cement with an artificially engineered leakage path (cavity) to CO2 saturated brine at high pressure. Carbonation of the bulk cement, self-healing of the leakage path in the cement specimen, and leaching of CaCO3 were thus directly observed. The precipitation of CaCO3, which is of key importance as a possible healing mechanism of fractured cement, was found to be enhanced in confined regions having limited access to CO2. For the first time, the growth kinetics of CaCO3 under more realistic well conditions have thus been estimated quantitatively. Combining the μ-CT observations with scanning electron microscopy resulted in a detailed understanding of the processes involved in the carbonation of cement. [ABSTRACT FROM AUTHOR]

Injecting fluid into subsurface reservoirs is a hydromechanical coupling process that can induce heaving of the ground surface. The presence of clayey interbeds in the reservoir can have considerable influence on the surface uplift. We used a numerical method to investigate this process. We found that different locations have different impacts on the surface uplift. We applied the orthogonal experimental design using the Taguchi method for extensive parametric analysis and determined the most influential factor. Finally, we analyzed the low-permeable effect of the interbed due to its low permeability, compared to the reservoir. [ABSTRACT FROM AUTHOR]

Underground Pumped Storage Hydropower (UPSH) is a potential alternative to manage electricity production in flat regions. UPSH plants will interact with the surrounding porous medium through exchanges of groundwater. These exchanges may impact the surrounding aquifers, but they may also influence the efficiency of the pumps and turbines because affecting the head difference between the reservoirs. Despite the relevance for an accurate efficiency assessment, the influence of the groundwater exchanges has not been previously addressed. A numerical study of a synthetic case is presented to highlight the importance of considering the groundwater exchanges with the surrounding porous medium. The general methodology is designed in order to be further applied in the decision making of future UPSH plants introducing each case specific complexity. The underground reservoir of a hypothetical UPSH plant, which consists in an open pit mine, is considered and modelled together with the surrounding porous medium. Several scenarios with different characteristics are simulated and their results are compared in terms of (1) head difference between the upper and lower reservoirs and (2) efficiency by considering the theoretical performance curves of a pump and a turbine. The results show that the efficiency is improved when the groundwater exchanges increase. Thus, the highest efficiencies will be reached when (1) the underground reservoir is located in a transmissive porous medium and (2) the walls of the open pit mine do not constrain the groundwater exchanges (they are not waterproofed). However, a compromise must be found because the characteristics that increase the efficiency also increase the environmental impacts. Meaningful and reliable results are computed in relation to the characteristics of the intermittent and expected stops of UPSH plants. The frequency of pumping and injection must be considered to properly configure the pumps and turbines of future UPSH plants. If not, pumps and turbines could operate far from their best efficiency conditions. [ABSTRACT FROM AUTHOR]

Azimuthal P-wave amplitude variation with offset (AVO) offers a method for the characterisation of a naturally fractured system in a reservoir. This information is important for the analysis of fluid flow during production of, for example, oil, petroleum and natural gas. This paper provides a modelling scheme by incorporating the squirt-flow model for the prediction of velocity dispersion and attenuation with azimuthal reflectivity method for the calculation of frequencydependent seismic responses. AzimuthalAVOresponses from a fractured poroelastic sandstone layer encased within shale are investigated based on the proposed method. Azimuthal reflections are a combination of the dynamic information including the contrast in anisotropic properties, anisotropic propagation and attenuation within the layer, as well as tuning and interferences. Modelling results indicate that seismic responses from the top of the sandstone layer are dominated by reflection coefficients, and show azimuthal variations at far offset which is consistent with conventional azimuthalAVOtheory. Reflections from the base, however, demonstrate complex azimuthal variations due to anisotropic propagation and attenuation of transmission waves within the layer. Tuning and interferences further complicate the azimuthalAVOresponses for thinner layer thickness. The AVO responses of top reflections show no azimuthal variations for lower fluid mobility, while those of base reflections show visible and stable azimuthal variations even at near and moderate offsets for different fluid mobility. Results also reveal that it would be practical to investigate wavetrains reflected from the fractured layers that are regarded as integrated units, especially for thinner layers where reflections from the top and base are indistinguishable. In addition, near-offset stacked amplitudes of the reflected wavetrains show detectable azimuthal variations, which may offer an initial look at fracture orientations before AVO analysis. [ABSTRACT FROM AUTHOR]

Integration of 3D and time-lapse 4D seismic data into reservoir modelling and history matching processes poses a significant challenge due to the frequent mismatch between the initial reservoir model, the true reservoir geology, and the pre-production (baseline) seismic data. A fundamental step of a reservoir characterisation and performance study is the preconditioning of the initial reservoir model to equally honour both the geological knowledge and seismic data. In this paper we analyse the issues that have a significant impact on the (mis)match of the initial reservoir model with well logs and inverted 3Dseismic data. These issues include the constraining methods for reservoir lithofacies modelling, the sensitivity of the results to the presence of realistic resolution and noise in the seismic data, the geostatistical modelling parameters, and the uncertainties associated with quantitative incorporation of inverted seismic data in reservoir lithofacies modelling. We demonstrate that in a geostatistical lithofacies simulation process, seismic constraining methods based on seismic litho-probability curves and seismic litho-probability cubes yield the best match to the reference model, even when realistic resolution and noise is included in the dataset. In addition, our analyses show that quantitative incorporation of inverted 3D seismic data in static reservoir modelling carries a range of uncertainties and should be cautiously applied in order to minimise the risk of misinterpretation. These uncertainties are due to the limited vertical resolution of the seismic data compared to the scale of the geological heterogeneities, the fundamental instability of the inverse problem, and the non-unique elastic properties of different lithofacies types. [ABSTRACT FROM AUTHOR]

The natural gas storage (underground reservoir) is an efficient process that combines constant supply of natural gas through pipelines with variable market demand which depends by the weather or economic considerations. During summer, when the transport capacity of the pipeline exceeds consumer demand, the natural gas is stored in order to be taken more often in winter when the gas consumption greatly increases. The natural gas extracted from the oil and gas deposits are used increasingly more to meet energy needs. The inflexibility gas production from these fields does not meet market demand variable. The underground storage of natural gas for adjust the gas supply to meet fluctuating demand, is used as a common technology since 1915. In this article will be analyzed the natural gas storage facility from Bălăceanca, both geologically and historically production conditions of economic efficiency point of view. [ABSTRACT FROM AUTHOR]

Abandoned and flooded mines constitute underground reservoirs which must be managed. When pumping is stopped in a closed mine, the process of flooding should be anticipated in order to avoid environmentally undesirable or unexpected mine water discharges at the surface, particularly in populated areas. The Candín-Fondón mining reservoir in Asturias (NW Spain) has an estimated void volume of 8 million m 3 and some urban areas are susceptible to be flooded if the water is freely released from the lowest mine adit/pithead. A conceptual model of this reservoir was undertaken and the flooding process was numerically modelled in order to estimate the time that the flooding would take. Additionally, the maximum safe height for the filling of the reservoir is discussed. [ABSTRACT FROM AUTHOR]

The seismic response of underground reservoir structures is a complex soil-structure interaction problem that depends on the properties of the earthquake motion, surrounding soil, and structure. More experimental and field data of the response of these structures under different boundary conditions are needed to validate analytical and numerical tools. This paper presents the results of four centrifuge experiments that investigate the seismic performance of reservoir structures, restrained from rotational movement at their roof and floor, buried in dry, medium-dense sand and compacted, partially saturated, silty sand. This study focuses on the influence of backfill soil properties, cover, and slope on accelerations, strains, lateral distortions, and lateral earth pressures experienced by the buried structure. The structure to far-field acceleration spectral ratios were observed to approach unity with added soil confinement, density, and stiffness. Both dynamic thrust and accelerations on the structure showed a peak near the effective fundamental frequency of the backfill soil. The addition of a shallow soil cover and stiffness slightly increased seismic earth pressures and moved their centroid upward, hence slightly amplifying seismic moments near the base. The added stiffness, density, and apparent cohesion of the compacted site-specific soil did not influence the magnitude of dynamic earth pressures significantly but often moved their centroid upward. A sloping backfill reduced the earth pressures and bending moments near the top of the wall because of the reduced soil mass. The trends in the experimental results indicate that new analytical procedures and design guidelines are needed to account for the backfill soil conditions and ground motions for which these underground structures must be designed. [ABSTRACT FROM AUTHOR]

The seismic performance of underground reservoir structures depends on the properties of the structure, soil, and ground motion as well as the kinematic constraints imposed on the structure. A series of four centrifuge experiments were performed to evaluate the influence of site response, structural stiffness, base fixity, and excitation frequency on the performance of relatively stiff reservoir structures buried in dry, medium-dense sand. The magnitude of seismic thrust increased and the distribution of seismic earth pressures changed from approximately triangular to parabolic with increasing structural stiffness. Heavier and stiffer structures also experienced increased rocking and reduced flexural deflection. Fixing the base of the structure amplified the magnitude of acceleration, seismic earth pressure, and bending strain compared to tests where the structure was free to translate laterally, settle, or rotate atop a soil layer. The frequency content of transient tilt, acceleration, dynamic thrust, and bending strain measured on the structure was strongly influenced by that of the base motion and site response, but was unaffected by the fundamental frequency of the buried structure (f structure ). None of the available simplified procedures could capture the distribution and magnitude of seismic earth pressures experienced by this class of underground structures. The insight from this experimental study is aimed to help validate analytical and numerical methods used in the seismic design of reservoir structures. [ABSTRACT FROM AUTHOR]

Earthquake absolute location errors which can be encountered in an underground reservoir are investigated. In such an exploitation context, earthquake hypocentre errors can have an impact on the field development and economic consequences. The approach using the stateof- the-art techniques covers both the location uncertainty and the location inaccuracy--or bias--problematics. It consists, first, in creating a 3-D synthetic seismic cloud of events in the reservoir and calculating the seismic traveltimes to a monitoring network assuming certain propagation conditions. In a second phase, the earthquakes are relocated with assumptions different from the initial conditions. Finally, the initial and relocated hypocentres are compared. As a result, location errors driven by the seismic onset time picking uncertainties and inaccuracies are quantified in 3-D. Effects induced by erroneous assumptions associated with the velocity model are also modelled. In particular, 1-D velocity model uncertainties, a local 3-D perturbation of the velocity and a 3-D geostructural model are considered. The present approach is applied to the site of Rittershoffen (Alsace, France), which is one of the deep geothermal fields existing in the Upper Rhine Graben. This example allows setting realistic scenarios based on the knowledge of the site. In that case, the zone of interest, monitored by an existing seismic network, ranges between 1 and 5 km depth in a radius of 2 km around a geothermal well. Well log data provided a reference 1-D velocity model used for the synthetic earthquake relocation. The 3-D analysis highlights the role played by the seismic network coverage and the velocity model in the amplitude and orientation of the location uncertainties and inaccuracies at subsurface levels. The location errors are neither isotropic nor aleatoric in the zone of interest. This suggests that although location inaccuracies may be smaller than location uncertainties, both quantities can have a cumulative effect. Besides, small velocity uncertainties applied to the whole 1-D profile can lead to large increase of the location uncertainties. However, local variations of the velocity field around the well may have negligible effects that would make such a feature undetectable with an absolute location method. Although the reference 1-D velocity model was built from well log data, the results show that it is not a good representative of a more realistic 3-D model including a fault and its associated block shift. The amplitude and distribution of the induced location inaccuracies are such that the positioning and the orientation of features delineated by seismicity are distorted and may be difficult to correctly interpret. [ABSTRACT FROM AUTHOR]

The article reports on developments related to groundwater management. Colorado Governor John Hickenlooper spoke at the 2016 National Ground Water Association Summit. A study shows that years of drought and over-pumping in Arizona and California has created underground water reservoirs. Plymouth University in England has reduced water consumption cost using an equipment from resource conservation specialist HWM.

Extraction of fluids from subsurface reservoirs induces changes in pore pressure, leading not only to geomechanical changes, but also perturbations in seismic velocities and hence observable seismic attributes. Time-lapse seismic analysis can be used to estimate changes in subsurface hydromechanical properties and thus act as a monitoring tool for geological reservoirs. The ability to observe and quantify changes in fluid, stress and strain using seismic techniques has important implications for monitoring risk not only for petroleum applications but also for geological storage of CO2 and nuclear waste scenarios. In this paper, we integrate hydromechanical simulation results with rock physics models and full-waveformseismic modelling to assess time-lapse seismic attribute resolution for dynamic reservoir characterization and hydromechanical model calibration. The time-lapse seismic simulations use a dynamic elastic reservoir model based on a North Sea deep reservoir undergoing large pressure changes. The time-lapse seismic traveltime shifts and time strains calculated from the modelled and processed synthetic data sets (i.e. pre-stack and post-stack data) are in a reasonable agreement with the true earth models, indicating the feasibility of using 1-D strain rock physics transform and time-lapse seismic processing methodology. Estimated vertical traveltime shifts for the overburden and the majority of the reservoir are within ±1 ms of the true earth model values, indicating that the time-lapse technique is sufficiently accurate for predicting overburden velocity changes and hence geomechanical effects. Characterization of deeper structure below the overburden becomes less accurate, where more advanced time-lapse seismic processing and migration is needed to handle the complex geometry and strong lateral induced velocity changes. Nevertheless, both migrated full-offset pre-stack and near-offset post-stack data image the general features of both the overburden and reservoir units. More importantly, the results from this study indicate that integrated seismic and hydromechanical modelling can help constrain time-lapse uncertainty and hence reduce risk due to fluid extraction and injection. [ABSTRACT FROM AUTHOR]

We investigate the flow-channeling phenomenon caused by thermal drawdown in fractured geothermal reservoirs. A discrete fracture network-based, fully coupled thermal-hydrological-mechanical simulator is used to study the interactions between fluid flow, temperature change, and the associated rock deformation. The responses of a number of randomly generated 2D fracture networks that represent a variety of reservoir characteristics are simulated with various injection-production well distances. We find that flow channeling, namely flow concentration in cooled zones, is the inevitable fate of all the scenarios evaluated. We also identify a secondary geomechanical mechanism caused by the anisotropy in thermal stress that counteracts the primary mechanism of flow channeling. This new mechanism tends, to some extent, to result in a more diffuse flow distribution, although it is generally not strong enough to completely reverse flow channeling. We find that fracture intensity substantially affects the overall hydraulic impedance of the reservoir but increasing fracture intensity generally does not improve heat production performance. Increasing the injection-production well separation appears to be an effective means to prolong the production life of a reservoir. [ABSTRACT FROM AUTHOR]

A unified permeability evolution model is proposed to capture the evolution of permeability induced by stress. This model is formulated within a conventional permeability-porosity power function, with special attentions being paid to the effects of shear deformation-induced tortuosity and compaction-induced closure of fissures through a permeability resistance parameter. This model contains a small amount number of parameters, which can be calibrated based on conventional experiments. The proposed model is validated through comparison between model simulation and experimental results for sandstones under a wide range of confining pressures. Good performance demonstrates the capability of the proposed permeability evolution model. [ABSTRACT FROM AUTHOR]

The current study investigated the feasibility of using compost as the heat source of an underground river reactor. This study compared the effect of heat on the temperature of bioreactor, biogas yield and organic matter removal efficiency at different organic loading rates between TP group (heat by compost) and CK group (without heat source).The following results could be obtained: in TP group, a seasonal average temperature of bioreactor was 25.2 °C, 34.6 °C, 25.6 °C and 19.3 °C in contrary to average ambient temperature which was 20.2 °C, 32.4 °C, 22.5 °C and 9.8 °C respectively in spring, summer, autumn and winter. Compared with CK group, biogas production in TP group was increased by 49.49%, 55.29%, 67.69% and 106.5% and organic matter removal efficiency in TP group was increased by 7%, 22.3%, 14.74% and 11.37% respectively. Meanwhile, using compost as heat source not only saved energy consumption but also could get revenue from biogas by 34,560 US $ and fertilizer by 55,040 US $ per year on a scale of 1000 m 3 . [ABSTRACT FROM AUTHOR]

Using empirical matched field processing (MFP), we compare 4 yr of continuous seismic data to a set of 195 master templates from within an active geothermal field and identify over 140 per cent more events than were identified using traditional detection and location techniques alone. In managed underground reservoirs, a substantial fraction of seismic events can be excluded from the official catalogue due to an inability to clearly identify seismic-phase onsets. Empirical MFP can improve the effectiveness of current seismic detection and location methodologies by using conventionally located events with higher signal-to-noise ratios as master events to define wavefield templates that could then be used to map normally discarded indistinct seismicity. Since MFP does not require picking, it can be carried out automatically and rapidly once suitable templates are defined. In this application, we extend MFP by constructing local-distance empirical master templates using Southern California Earthquake Data Center archived waveform data of events originating within the Salton Sea Geothermal Field. We compare the empirical templates to continuous seismic data collected between 1 January 2008 and 31 December 2011. The empirical MFP method successfully identifies 6249 additional events, while the original catalogue reported 4352 events. The majority of these new events are lower-magnitude events with magnitudes between M0.2-M0.8. The increased spatial-temporal resolution of the microseismicity map within the geothermal field illustrates how empirical MFP, when combined with conventional methods, can significantly improve seismic network detection capabilities, which can aid in long-term sustainability and monitoring of managed underground reservoirs. [ABSTRACT FROM AUTHOR]

Gas filtration from an underground reservoir through a layer of a porous medium due to an instantaneous increase in the gas pressure in the reservoir is studied. The problem is considered in a one-dimensional formulation in the general case where the temperatures of the gas and the porous medium are different and unstable, and in the case of a high specific heat of the solid phase and a high interfacial heat-transfer rate. The dynamics of the gas flow at the inlet and outlet of the underground reservoir is analyzed, the time of unloading of the system is estimated as a function of the permeability of the porous medium. It is shown that, depending on the properties of the porous layer, two characteristic gas flow regimes are possible: a fast discharge regime and a slow regime which is determined mainly by barodiffusion. [ABSTRACT FROM AUTHOR]

An unprecedented increase in earthquakes in the U.S. mid-continent began in 2009. Many of these earthquakes have been documented as induced by wastewater injection. We examine the relationship between wastewater injection and U.S. mid-continent seismicity using a newly assembled injection well database for the central and eastern United States. We find that the entire increase in earthquake rate is associated with fluid injection wells. High-rate injection wells (>300,000 barrels per month) are much more likely to be associated with earthquakes than lower-rate wells. At the scale of our study, a well's cumulative injected volume, monthly wellhead pressure, depth, and proximity to crystalline basement do not strongly correlate with earthquake association. Managing injection rates may be a useful tool to minimize the likelihood of induced earthquakes. [ABSTRACT FROM AUTHOR]