Reservoir Response Research Articles

Pore-scale T2-based numerical investigation on dynamics and wettability in mixed-wet shale oil reservoirs

Oil recovery in shale reservoirs is low due to the dynamics and wettability characteristics in mixed-wet shale oil reservoirs. Nuclear magnetic resonance (NMR) logging, a nondestructive and noninvasive technique, effectively evaluates the continuous dynamics and wettability in these reservoirs. The NMR numerical investigation can characterize the effects of dynamics and wettability, including varying wet regions and wet angles, on NMR responses, providing new insights into the frequency-dependent of T2-based petrophysical parameters. The NMR relaxation theory for mixed-wet shale oil reservoirs was proposed, and the relevant parameters were determined. The dynamics and wettability were characterized using the Shan Chen Lattice Boltzmann method, with constraints based on digital core technology. For the first time, the random walk method was employed to simulate the effects of water-wet regions with varying proportions, echo spacings, and wet angles on NMR responses in mixed-wet shale oil reservoirs at different frequencies. The proportions of water-wet regions, magnetic field frequencies, and echo spacings significantly influence porosity and T2LM, indicating that pore structure governs the dynamics and wettability and that petrophysical parameters can be characterized by their frequency dependence in mixed-wet shale oil reservoirs.

Numerical simulation of the physical response of shale oil reservoirs under microwave and steam synergy and single heating methods

This article is the first to simulate the changes in temperature, porosity, permeability, and displacement of oil shale reservoirs under microwave followed by steam and steam followed by microwave and to analyze the oil shale reservoirs under single steam and microwave in comparison, aiming to find out the most suitable way for in-situ exploitation of oil shale among the four types of heating methods. From the results of COMSOL Multiphysics field software, the average temperatures of the reservoir under 600 W single microwave heating, 650 °C single steam heating, steam followed by microwave heating, and microwave followed by steam heating were 570.15, 528.53, 725.66, and 524.36 °C, respectively, at a heating time of 500 days; the average porosity was 16, 18.692, 19.52, and 17.744%, respectively; the average permeability was 4.343 × 10−16, 2.05 × 10−11, 1.738 × 10−11, and 1.09 × 10-11 m2, respectively; and the average displacement of the reservoir was 3.3 cm, 3.11 cm, 3.38 cm, and 2.65 cm, respectively. The temperature of the oil shale reservoir increases with time for four heating methods, and the heating distance becomes larger. The reservoir’s porosity, affected by steam followed by microwave and single steam, showed a decreasing then increasing trend over 100 days and leveled off after 300 days, while with single microwave and microwave followed by steam, it showed a linear increase. Permeability exhibits minimal variation at reservoir temperatures ranging from 200 to 300 °C but experiences a significant increase above 300 °C. The displacement of the reservoir gradually increases under the four applied methods. The oil shale reservoir treated with steam followed by microwave outperforms the other three methods in heating speed, range, pore connectivity, and oil and gas circulation, offering a new method for future research on oil shale in-situ extraction technologies.

Dynamic stress response and fatigue characteristics of tight sandstone reservoirs with pulsating hydraulic fracturing

Dynamic stress response and fatigue characteristics of tight sandstone reservoirs with pulsating hydraulic fracturing

Feasibility Study on Geothermal Dolomite Reservoir Reinjection with Surface Water in Tianjin, China

Reinjection is thought to be the most effective way to maintain reservoir pressure and production capacity for hydrothermal resources. The use of external water injection to replenish deep geothermal reservoirs is a new approach in China to addressing the problems of declining groundwater levels and energy depletion caused by the excessive and uneven exploitation of geothermal resources. However, the key challenge and focus of the feasibility assessment of this method lies in the chemical compatibility of the external water with the native geothermal reservoir water and surrounding rocks. In this paper, we discuss the geochemical response of a dolomite reservoir to lake water injection based on experiments on water–rock interaction in the Wumishan formation in the Dongli Lake area of Tianjin. The results show that after reactions with dolomite, the TDS of the reacted water decreases, indicating the occurrence of precipitation. According to the calculation results obtained using the PHQREEC program, the precipitation amount is found to be quite limited. Geochemical analysis indicates that at the initial stage of the reactions, plagioclase dissolves and releases alkaline metals like Ca-, Na-, SiO2- and Al-bearing compositions, leading to the oversaturation and precipitation of dolomite and calcite. As the reaction progresses, a portion of the dolomite dissolves, while the calcite continues to precipitate at a later stage. Illite precipitates and its effects on reservoir structure depend on its shape. Based on the experimental data, it can be concluded that the dolomite reservoir will be slightly affected by the reinjection of lake water; however, it is still a good method for the sustainable development of geothermal resources.

Multi-scale reservoir response characteristics of coalbed methane development through stress relief via horizontal well cavity completion in tectonically deformed coals

China’s coalbed methane (CBM) resources hold significant potential. However, the widespread presence of soft and low-permeability tectonically deformed coal (TDC) presents a challenge to the efficient development of CBM. The development of CBM through stress relief via horizontal well cavity completion (HWCC) offers a promising innovative solution to this problem. To elucidate the multi-scale reservoir response characteristics of CBM development through stress relief via HWCC in TDCs, this article comprehensively employs particle discrete element and finite element numerical simulation methods, and carries out the numerical simulation studies of cyclic seepage at the specimen scale, HWCC at the coal seam scale, and CBM development at the coal seam group scale. The main findings are summarized as follows. Unlike the stress-compression specimens, which exhibited overall failure, the differential distribution of pressure gradients within the specimens during the gas seepage experiments resulted in the development of microcracks concentrated at the bottom of the specimens. With the escalation of coal body structural damage, the permeability stress compression coefficient during the elastic deformation stage shows an increasing trend. Upon entering the plastic deformation stage, the specimen’s permeability shifts from a decreasing to an increasing trend. Increasing the lower limit pressure ratio, upper limit pressure ratio, and cycle rate of gas injection pressure could accelerate the failure efficiency of the specimens. Increasing the lower limit pressure ratio has a more pronounced effect on the TDC specimens, while increasing the upper limit pressure ratio has a more significant impact on the primary undeformed and cataclastic coal specimens. In TDCs, the phenomenon of wellbore collapse and cavity expansion becomes more pronounced. The cavity volume tends to increase with the increase in the initial well diameter, in-situ stress, and alternating bottom-hole pressure differences. The HWCC could enhances reservoir permeability and production in both the cavity completed and adjacent coal seams. For two horizontal wells with cavity completions in the same layer, excessively close well spacing is unfavorable for fostering inter-well interference and reservoir pressure drop. When the HWCC is applied to thin coal seams, the enhancement of permeability and production is limited. Reasonable optimization of drainage rates could effectively mitigate the reservoir permeability velocity-sensitive damage. The research findings extend related theories and provide a robust reference for subsequent practical engineering applications.

Induced seismicity in the Kiskatinaw area of Northeastern British Columbia, Canada: Empirical investigation of hydraulic diffusivity and induced seismic response of the fractured reservoir

This empirical study investigates the mechanistic reasoning behind the occurrence of large magnitude fluid-injection-induced seismic events during hydraulic fracturing in the Kiskatinaw area. The data unveiled atypical non-parabolic spatio-temporal distributions of induced seismic event hypocenters—previously unreported in existing literature. These distributions were predominantly associated with larger magnitude events. Distinctly, our research delves into the concept of hydraulic diffusivity within fractured reservoirs, interpreting the observed patterns in these unique spatio-temporal hypocentral growth distributions. The study reveals that in unconventional fractured reservoirs of this region smaller seismic events are linked to active stages via low hydraulic diffusive and highly hydraulically connected, dispersed, fracture network, while larger magnitudes can associate with highly diffusive and concentrated fractured pathways with limited hydraulic connectivity. This was attributed to the contrasting storativity of connected fracture networks impacting fluid pressure propagation pace to hydraulically connected seismogenic faults. Furthermore, the data pointed to an inverse relationship between hydraulic diffusivity and the number of hydraulically connected structures to the active stage, leading to higher pressure build-ups and larger seismic event magnitudes at greater diffusivity levels. This understanding offers insights into the variances in seismic responses across stages and wells. Intriguingly, unlike standard hydraulic fracturing of unconventional reservoir models emphasizing on tensile fracture generation, our findings underscore the significant role of pre-existing natural fractures in inducing shear slip during fluid injection. The seismic energy release to hydraulic energy input ratio observed was considerably higher than in settings with more massive rocks, aligning with results reported for enhanced geothermal operations. Conclusively, fluid injection in certain fractured reservoirs of the KSSMA can lead to significant pressure buildup perturbations, causing larger seismic events, while in others, a multitude of smaller events prevails, highlighting the complex interplay of hydraulic diffusivity, fracture intensity, and connectivity in determining seismic responses.

Experimental research on deformation response of hot dry rock (HDR) geothermal reservoir during hydraulic modification at 300 °C and depth 2500 m

In the stages of reservoir construction and geothermal exploitation, the rock deformation is an important factor in the study of the reservoir stability. However, the existing laboratory research in this area pay far less attention to the macroscopic deformation of the reservoir rock mass. Hydraulic fracturing experiments have here been conducted at a temperature of 300 °C, an axial pressure of 65 MPa, and a confining pressure of 60 MPa. The quantitative relationship between the granite strain (axial, radial, and volumetric) and the water injection pressure during hydraulic fracturing has then for the first time been determined experimentally. Meanwhile, the deformation response of granite geothermal reservoir during hydraulic modification has also been discussed. As a result, the maximum opening of the hydraulic fractures was in the range of 10−1 μm - 10−2 μm and the maximum volumetric strain rate of the rock mass was 1.71 × 10−4 s−1. Microseismic events preferably occurred during periods of high volumetric strain rates when the injection pressure fluctuated repeatedly. During geothermal exploitation, the structural changes caused by hydraulic fractures and thermal cracks resulted in greater potential for contraction of the rock mass. These research results provide a reference for geothermal reservoir reconstruction and stability assessment.

A new expression for fluid factor using AVO intercept and gradient: Theory and application on gas sand reservoirs from offshore Australia

Numerous amplitude variation with offset (AVO) fluid indicators have proven to be sensitive to the presence of hydrocarbons. Mathematically, these fluid indicators measure the deviation of seismic responses of hydrocarbon reservoirs from their background in a specific domain. We develop a new expression for the fluid factor commonly used in AVO analysis and interpretation. The expression is a function of the common AVO intercept and gradient, along with a weighting coefficient that suppresses the contribution of lithology and other factors unrelated to fluid content. The coefficient also assists in calibrating the fluid factor to the local geology. We compare the performance of our fluid factor with existing fluid factors. All the fluid factors are tested on a worldwide collection of P- and S-wave velocity and density measurements. The testing is followed by application on synthetic data before applying the attribute on real data from the Northwest Shelf of Australia. In the latter application on the real data, we use two wells for calibration purposes, and the third as a blind well. Our fluid factor demonstrated high capability in detecting the targeted gas zones at the three well locations and identifying new prospective zones.

Application of seismic attribute analysis in Ordovician reservoir prediction in Hechuan Tongnan exploration area, Sichuan Basin

Abstract Exploration has revealed that the Ordovician in Sichuan Basin has good conditions for oil and gas accumulation in the peripheral areas of the paleo-uplift, but it is difficult to predict the carbonate reservoirs due to their strong heterogeneity and complex planar distribution. Based on seismic, drilling, logging and analysis data, this paper makes full use of 3D seismic data from the Hechuan Tongnan exploration area to study the relationship between petrophysical parameters and geophysics parameters, the seismic response characteristics of carbonate reservoir are determined by forward modeling. On the basis of detailed interpretation, several seismic attributes with high correlation coefficients with reservoir response characteristics in this area were selected and seismic attribute reservoir prediction research was carried out in the Hechuan Tongnan exploration area, Sichuan Basin Province, through correlation intersection analysis of seismic attributes and intersection analysis of seismic attributes and well-point porosity, the optimal seismic attributes for reservoir prediction are optimized, and the reservoir distribution is characterized, it provides an important basis for risk exploration in this area.

Reflection, transmission, and AVO response of inhomogeneous plane waves in thermoporoelastic media with two-temperature equations of heat conduction

We develop a modified fluid-saturated thermoporoelastic model by introducing two temperature equations to account for the temperature differences between the solid skeleton and the pore filling. The modified two-temperature generalized thermoporoelastic (TTG) equation is an extension of the classical single-temperature Lord-Shulman (LS), Green-Lindsay, and generalized LS theories. It predicts four compressional waves and one shear wave based on the analysis of inhomogeneous plane waves. We study the exact reflection and transmission (R/T) coefficients at the interface separating two thermoporoelastic half-spaces and develop an amplitude-variation-with-offset (AVO) approximation. Comparison with the Biot poroelastic case of the water/oil contact shows that the TTG model reproduces the exact R/T results. The AVO response of oil, gas, and real CO2 geosequestration reservoirs illustrates the practical applicability of our model and provides the theoretical basis for the exploration of high-temperature resources.

Prediction of deep low permeability sandstone seismic reservoir based on CBAM-CNN

The prediction of sand body is a key focus in evaluating reservoir distribution, playing a crucial role in oil and gas exploration and development. To clarify the development characteristics of the sand body in the Huangyan structural belt of the Xihu Sag and effectively identify the hidden channel, a new approach is proposed. This approach incorporates a Convolutional Block Attention Module (CBAM) into a Convolutional Neural Network (CNN). Well logging and seismic data were used to predict the distribution of sand body in the lower section of the Huagang Formation (H12). Firstly, based on known sand-stratum ratio data and well-side seismic data, the seismic attributes that are sensitive to the reservoir response are preferred through correlation analysis, which are used to construct a Polynomial Linear Regression (PLR) model to obtain the preliminary sand distribution prediction results. Secondly, the grid is divided according to this result. Then, three sample selection methods, namely proportional, fixed-range, and random, are used to construct three sample sets in conjunction with the geologic model guide. Finally, CBAM-CNN, CNN, Random Forest (RF), Support Vector Machines (SVM), and Backpropagation Neural Network (BPNN) are utilized for sand body prediction. The results indicate that when using the same model, the sample set selected by the fixed-range selection method yielded the best prediction outcome. When using the same sample set, the CBAM-CNN model outperformed the others. Among various strategies, training the CBAM-CNN model with the sample set derived from the fixed-range selection method led to the highest test set R2 of 0.913. The results of the sand body distribution indicate that fine river channels are more continuous, and reservoir boundaries are clearer, significantly outperforming other schemes. This outcome can serve as a guide for further reservoir delineation.

Multi-scenario prediction and attribution analysis of carbon storage of ecological system in the Huaihe River Basin, China.

Evaluating the impact of large-scale human activities on carbon storage through land use changes is of growing interest in terrestrial ecosystem assessments. The Huaihe River Basin, a vital Chinese grain production area, has undergone marked land use changes amid socio-economic acceleration. Evaluating the impacts of land use change on carbon storage and future carbon sequestration is imperative for regional ecosystem sustainability and Chinese food security, simultaneously, furnishing data support to regional land use planning and decision-making processes. Nonetheless, the mechanisms linking land use changes to carbon storage and the future carbon reservoir responses remain unclear. We utilized a multi-source dataset and representative scenarios, integrating PLUS, InVEST models, and Geodetector to assess land use change impacts on carbon storage in the Huaihe River Basin (2000-2030). The data indicates the following: (1) from 2000 to 2020, cultivated land decreased by 28,344.69 km2, construction land increased by 26,914.56 km2, and other land types changed little. (2) Land use change resulted a carbon loss of 1.17 × 108 t, primarily due to the expansion of construction land. (3) All four simulation scenarios exhibited diminished carbon storage relative to 2020, with the economic development scenario recording the lowest at 4.98 × 109 t and the ecological protection scenario the highest at 5.06 × 109 t. (4) Elevation predominantly drives carbon storage changes, with its interaction with NPP having the greatest impact. The factors synergistically enhance their explanatory power. The research provides a scientific basis for strategies aimed at augmenting regional carbon sequestration and refining low-carbon land management, safeguarding ecosystem stability.

A fully coupled thermo-poroelastic model for energy extraction in naturally fractured geothermal reservoirs: sensitivity analysis and flow simulation

The development of a novel method for modelling fluid flow and heat transfer in naturally fractured geothermal reservoirs represents a significant advancement in geothermal energy research. This Study presents a hybrid approach, which combines discrete fracture and single continuum techniques, to effectively capture the complex interactions between fluid flow and heat transfer in geothermal fractured reservoirs. In addition, the incorporation of the local thermal nonequilibrium method for simulating heat transmission accounts for the disparities in temperature between the rock matrix and the fluid, providing a more realistic representation of heat transfer processes. The study also presents a fully coupled thermo-poro-elastic framework that integrates fluid flow and heat transfer to comprehensively evaluate reservoir responses to injection/production scenarios. This coupled approach allows for the prediction of changes in reservoir properties, such as permeability and porosity, under varying fluid pressure and temperature conditions. The application of the proposed model to evaluate a geothermal reservoir’s long-term response to injection/production scenarios provides valuable insights into the reservoir’s behaviour and potential energy production capacity. The sensitivity analysis further enhances the model’s utility by identifying the key reservoir parameters that significantly influence the thermal depletion of the reservoir. Overall, this novel modelling approach holds promise for improving the understanding and management of naturally fractured geothermal reservoirs, contributing to the optimization of geothermal energy extraction strategies.

Jamaican fruit bat (Artibeus jamaicensis) insusceptibility to mucosal inoculation with SARS-CoV-2 Delta variant is not caused by receptor compatibility

The ancestral sarbecovirus giving rise to SARS-CoV-2 is posited to have originated in bats. While SARS-CoV-2 causes asymptomatic to severe respiratory disease in humans, little is known about the biology, virus tropism, and immunity of SARS-CoV-2-like sarbecoviruses in bats. SARS-CoV-2 has been shown to infect multiple mammalian species, including various rodent species, non-human primates, and Egyptian fruit bats. We show that SARS-CoV-2 can utilize Jamaican fruit bat (Artibeus jamaicensis) ACE2 spike for entry in vitro. Therefore, we investigate the Jamaican fruit bat as a possible in vivo model to study reservoir responses. We find that SARS-CoV-2 Delta does not efficiently replicate in Jamaican fruit bats in vivo. We observe infectious viruses in the lungs of only one animal on day 1 post-inoculation and find no evidence of shedding or seroconversion. This is possibly due to host factors restricting virus egress after aborted replication. Furthermore, we observe no significant immune gene expression changes in the respiratory tract but do observe changes in the intestinal metabolome after inoculation. This suggests that, despite its broad host range, SARS-CoV-2 is unable to infect all bat species, and Jamaican fruit bats are not an appropriate model to study SARS-CoV-2 reservoir infection.

Isotopic study of honey documents widespread plant uptake of old carbon in North America

A comprehensive understanding of carbon cycling pathways in the soil-plant system is needed to develop models that accurately predict global carbon reservoir responses to anthropogenic perturbations. Honey is a carbon-rich natural food produced by wild and managed pollinating insects all over the world; the composition of a single sample is a function of millions of pollinator-plant interactions. We studied the 13C/12C and Δ14C of 121 honey samples sourced from the United States, and found a significant older carbon contribution. The effect is observed from 25 to 45° latitude, not correlated with 13C/12C, and consistent with a previously published study on European honeys. In specific cases, the measured values were up to 20 ‰ (Δ14C) higher than the expected atmospheric 14CO2 value for the given year, which shows a significant older carbon contribution. We hypothesize that the older carbon is from plant liquids derived in part from soil carbon or stored nonstructural carbohydrates from plants, which shifts the calibrated age of the sample by 5 years or more. Our work is the first to describe the widespread occurrence of older carbon in honey and shows that radiocarbon measurements can be a powerful tool to trace carbon allocations in terrestrial food webs and detect the atmosphere-soil-plant carbon cycle contributions.

Machine learning approaches identify immunologic signatures of total and intact HIV DNA during long-term antiretroviral therapy.

Understanding the interplay between the HIV reservoir and the host immune system may yield insights into HIV persistence during antiretroviral therapy (ART) and inform strategies for a cure. Here, we applied machine learning approaches to cross-sectional high-parameter HIV reservoir and immunology data in order to characterize host-reservoir associations and generate new hypotheses about HIV reservoir biology. High-dimensional immunophenotyping, quantification of HIV-specific T cell responses, and measurement of genetically intact and total HIV proviral DNA frequencies were performed on peripheral blood samples from 115 people with HIV (PWH) on long-term ART. Analysis demonstrated that both intact and total proviral DNA frequencies were positively correlated with T cell activation and exhaustion. Years of ART and select bifunctional HIV-specific CD4 T cell responses were negatively correlated with the percentage of intact proviruses. A Leave-One-Covariate-Out (LOCO) inference approach identified specific HIV reservoir and clinical-demographic parameters, such as age and biological sex, that were particularly important in predicting immunophenotypes. Overall, immune parameters were more strongly associated with total HIV proviral frequencies than intact proviral frequencies. Uniquely, however, expression of the IL-7 receptor alpha chain (CD127) on CD4 T cells was more strongly correlated with the intact reservoir. Unsupervised dimension reduction analysis identified two main clusters of PWH with distinct immune and reservoir characteristics. Using reservoir correlates identified in these initial analyses, decision tree methods were employed to visualize relationships among multiple immune and clinical-demographic parameters and the HIV reservoir. Finally, using random splits of our data as training-test sets, machine learning algorithms predicted with approximately 70% accuracy whether a given participant had qualitatively high or low levels of total or intact HIV DNA. The techniques described here may be useful for assessing global patterns within the increasingly high-dimensional data used in HIV reservoir and other studies of complex biology.

Characterizing spatial distribution of ice and methane hydrates in sediments using cross-hole electrical resistivity tomography

Electrical resistivity is an essential parameter for assessing the abundance of gas hydrates and predicting their location. However, imaging techniques are poorly developed for detecting the electrical resistivity distribution of hydrate-bearing sediments. In this study, a cross-hole electrical resistivity tomography (CHERT) array was established in the large-scale simulator for characterizing the spatial distribution of the sedimentary system. Ice- and methane hydrate-bearing sediments were investigated respectively using CHERT. The results show that CHERT effectively observed the significant influence of methane hydrate/ice formation and salt drainage on the variations in electrical parameters. Their formation leads to a sustained increase in average resistivity, while salt ion exclusion leads to an increase in pore water salinity, resulting in a decrease in local resistivity. Moreover, CHERT observed a range of resistivities from a few Ω·m to several thousand Ω·m for ice- and methane hydrate-bearing sediments, which is consistent with the resistivity response of hydrate reservoirs in the field. It is suggested that CHERT can be used for hydrate reservoir detection and combined with acoustic imaging to more accurately characterize the spatial distribution of hydrates. These results contribute to advancing the potential application of CHERT technology in multi-method and multi-scale gas hydrate monitoring initiatives.

Effects of microstructure on hydraulic fracturing and gas–water production in coal reservoirs: A case study of the Dahebian coalbed methane block in Western Guizhou, China

AbstractThe development of coalbed methane (CBM) in China is susceptible to the influence of microstructure. Therefore, it is crucial to understand the extension laws of hydraulic fractures and engineering responses in coal reservoirs affected by microstructure development. Utilizing the Dahebian block in western Guizhou as the study area, this investigation examines the coal reservoir characteristics, fracturing, and drainage engineering analysis of the well DC1 group in the region. The aim is to discuss the spatial distribution characteristics of hydraulic fractures, geological controlling factors influenced by microstructure, and their corresponding engineering responses. The results indicate that, for coal reservoirs unaffected by microstructure, the extension laws of the fracture network in both longitudinal and planar directions are influenced by burial depth and the regional stress field. In the microstructural belt, tectonic stress dominates, causing changes in the ground stress field. Consequently, the hydraulic fracture network deviates from the direction of the maximum principal stress during the extension process. When a secondary fracture is nearby, the hydraulic fracture network extends towards the shortest path radial secondary fracture direction, leading to a rapid increase in fracture width per unit length until it intersects with the secondary fracture. Additionally, the presence of secondary joints near the microfault structure decreases fracturing pressure and results in a dense distribution of the fracture network. This promotes the formation of a complex fracture network, favorable for fracturing. The extension of the fracture network in complex structural development areas is influenced by the microfault structure between wells, which is reflected in the fracturing construction pressure and fluid output. This accounts for the significant variations in the early drainage performance of CBM wells.

The influence of stress on the fracture and elastic properties of carbonate rocks controlled by strike-slip faults: a novel rock-physics modelling perspective

SUMMARY The fine-scale fractures within strike-slip faults substantially impact the flowing capacity. However, effective methods for their characterization are still lacking, making it challenging to predict hydrocarbon accumulation patterns. In this study, we conducted microscopic statistics, ultrasonic experiments and theoretical modelling to analyse the fracture density and elastic characteristics within the strike-slip fault and investigated the impact of stress. Our findings reveal that the fracture density in the fault core is 3–4 times higher than that in the damage zone, and the acoustic velocity is 13–18 per cent lower under atmospheric pressure. With the rising confining pressure, the fracture density initially decreases rapidly and then slowly, while the acoustic velocity follows the same increasing trend. The gradually slowing trend indicates that the majority of fractures close within the range of 0–20 MPa. Moreover, the stress sensitivity of the bulk modulus is higher than that of the shear modulus. The stress sensitivity is higher in the fault core than in the damage zone, which correlates strongly with the variation in fracture density. These indicate that the stress sensitivity in the fault-controlled rock is attributed to stress-induced fracture deformation, predominantly manifested as volumetric compression deformation. During the geological evolution, differences in tectonic faulting, fluid filling and compaction within the fault zone contribute to present heterogeneity in fracture density. Finally, our research demonstrates a strong correlation between theoretical prediction results and underground logging, drilling and core data. These findings can help predict the underground fracture distribution and elastic response of carbonate reservoirs controlled by strike-slip faults.

Geomechanical responses of sand–clay interbedded gas hydrate-bearing deposits under depressurization-driven gas production at site UBGH2-6 in Ulleung Basin, offshore Korea

This study explores geomechanical responses of sand-clay interbedded hydrate reservoirs subjected to depressurization. A thermo-hydro-mechanically (T-H-M) coupled simulation code was created to simulate T-H-M interactions in layered hydrate reservoirs, utilizing an explicit two-way coupling scheme. The code was then used to model a hydrate reservoir analogous to a hydrate deposit system in Ulleung Basin, Offshore Korea (Site UBGH2-6). Hydrate reservoirs comprising thin and interbedded sand-clay layers exhibit unique geomechanical responses due to the localized gas hydrate dissociation along sand-clay layer interfaces. The localized dissociation pattern leads to accelerated sediment consolidations along the layer interfaces. The simulation results also reveal development of shear deformation along the sand-clay interfaces, which implies the layer interfaces may act as the main sources of sand productions and fines migration within a depressurized hydrate reservoir. Additional simulations are conducted to further investigate the effects of depressurization rate and formation wettability on gas productivity. Although faster depressurization speeds up initial gas production, ultimately, the production rate is governed by the final bottom hole pressure, regardless of the depressurization rate. Nonetheless, slower depressurization appears more favorable, given its capacity to mitigate initial geomechanical responses, such as reservoir compaction and sand production, thus preserving the long-term production potential of a hydrate reservoir. On the other hand, a cyclic depressurization scheme also proves to be able to mitigate the level of initial geomechanical responses, but only at the cost of reduced gas productivity. A comparative analysis with varying wettability indicates that water-wet reservoirs may be more favorable target for gas hydrate exploitation than gas-wet reservoirs. The presented results enhance understanding of complex hydrate reservoir dynamics and contribute to advancement of sustainable and efficient hydrate production techniques.