Increase In Effective Stress Research Articles

The development of unit shaft resistance along driven piles in subsiding soil

The influence of bitumen coating on the development of unit shaft resistance along driven steel and precast concrete piles resulting from subsiding surrounding soft soil (gyttja) induced by fill placement at terrain was investigated. All piles were instrumented with conventional discrete-point vibrating wire strain gauges and distributed fibre optic sensors to achieve high-resolution strain measurements. The magnitude of the mobilised unit shaft resistance along uncoated piles was observed to be primarily related to an increase in effective stress resulting from the dissipation of excess pore water pressures. The unit shaft resistance along bitumen-coated piles was found to be primarily related to the rate of relative movement between pile and soil, which highlights the effectiveness of bitumen coating in reducing shaft resistance.

A new analytical method for estimating hydraulic parameters of a nonlinear consolidated aquitard with variable drawdown in adjacent aquifers

Estimation of hydraulic parameters and groundwater depletion of aquitards is necessary to evaluate the groundwater resources and land subsidence. In this study, an analytical solution for water release from a nonlinear consolidated aquitard was derived by approximating the drawdown history as a continuous piecewise linear drawdown function (PLD) with several stages, while the water level in adjacent aquifers varies arbitrarily over time. According to dimensionless and parameter sensitivity analyses, groundwater depletion of an aquitard increases with an increase in compression index, aquitard thickness, and water drawdown rate in adjacent aquifers, decreases with an increase in initial void ratio and effective stress, and is not affected by the consolidation coefficient. In addition, we proposed a type-curve fitting method to calculate the hydraulic parameters of the aquitard under the condition of PLD in adjacent aquifers. The proposed method was applied to estimate the hydraulic parameters of aquitards at two field sites in the Yangtze River Delta in China, i.e., Changzhou and Shanghai. Results show that the compression index of the aquitards in Changzhou and Shanghai are 0.074 and 0.24, respectively, while the consolidation coefficients are 7.62 × 10-7 m2/s and 2.16 × 10-6 m2/s, respectively. The hydraulic conductivity and specific storage estimated by the proposed method are in good agreement with previous studies and the results of the geological method of Konikow and Neuzil (2007). The proposed analytical method for estimating hydraulic parameters and groundwater depletion of aquitards will contribute to the sustainable management of groundwater resources in multi-layer aquifer systems.

Action mechanisms of abnormal fluid pressure on physical properties of deep reservoirs: A case study on Jurassic Toutunhe Formation in the southern margin of Junggar Basin, NW China

Considering the action mechanisms of overpressure on physical changes in skeleton particles of deep reservoir rocks and the differences in physical changes of skeleton particles under overpressure and hydrostatic pressure, the sandstone of the Jurassic Toutunhe Formation in the southern margin of Junggar Basin was taken as an example for physical modeling experiment to analyze the action mechanisms of overpressure on the physical properties of deep reservoirs. (1) In the simulated ultra-deep layer with a burial depth of 6000–8000 m, the mechanical compaction under overpressure reduces the remaining primary pores by about a half that under hydrostatic pressure. Overpressure can effectively suppress the mechanical compaction to allow the preservation of intergranular primary pores. (2) The linear contact length ratio under overpressure is always smaller than the linear contact length ratio under hydrostatic pressure at the same depth. In deep reservoirs, the difference between the mechanical compaction degree under overpressure and hydrostatic pressure shows a decreasing trend, the effect of abnormally high pressure to resist the increase of effective stress is weakened, and the degree of mechanical compaction is gradually close to that under hydrostatic pressure. (3) The microfractures in skeleton particles of deep reservoirs under overpressure are thin and long, while the microfractures in skeleton particles of deep reservoirs under hydrostatic pressure are short and wide. This difference is attributed to the probable presence of tension fractures in the rocks containing abnormally high pressure fluid. (4) The microfractures in skeleton particles under overpressure were mainly formed later than that under hydrostatic pressure, and the development degree and length of microfractures both extend deeper. (5) The development stages of microfractures under overpressure are mainly controlled by the development stages of abnormally high pressure and the magnitude of effective stress acting on the skeleton particles. Moreover, the development stages of microfractures in skeleton particles are more than those under hydrostatic pressure in deep reservoir. The multi-stage abnormally high pressure plays an important role in improving the physical properties of deep reservoirs.

Implications for fault reactivation and seismicity induced by hydraulic fracturing

Evaluating the physical mechanisms that link hydraulic fracturing (HF) operations to induced earthquakes and the anticipated form of the resulting events is significant in informing subsurface fluid injection operations. Current understanding supports the overriding role of the effective stress magnitude in triggering earthquakes, while the impact of change rate of effective stress has not been systematically addressed. In this work, a modified critical stiffness was brought up to investigate the likelihood, impact, and mitigation of induced seismicity during and after hydraulic fracturing by developing a poroelastic model based on rate-and-state fraction law and linear stability analysis. In the new criterion, the change rate of effective stress was considered a key variable to explore the evolution of this criterion and hence the likelihood of instability slip of fault. A coupled fluid flow–deformation model was used to represent the entire hydraulic fracturing process in COMSOL Multiphysics. The possibility of triggering an earthquake throughout the entire hydraulic fracturing process, from fracturing to cessation, was investigated considering different fault locations, orientations, and positions along the fault. The competition between the effects of the magnitude and change rate of effective stress was notable at each fracturing stage. The effective stress magnitude is a significant controlling factor during fracturing events, with the change rate dominating when fracturing is suddenly started or stopped. Instability dominates when the magnitude of the effective stress increases (constant injection at each fracturing stage) and the change rate of effective stress decreases (the injection process is suddenly stopped). Fracturing with a high injection rate, a fault adjacent to the hydraulic fracturing location and the position of the junction between the reservoir and fault are important to reduce the Coulomb failure stress (CFS) and enhance the critical stiffness as the significant disturbance of stresses at these positions in the coupled process. Therefore, notable attention should be given to the injection rate during fracturing, fault position, and position along faults as important considerations to help reduce the potential for induced seismicity. Our model was verified and confirmed using the case of the Longmaxi Formation in the Sichuan Basin, China, in which the reported microseismic data were correlated with high critical stiffness values. This work supplies new thoughts of the seismic risk associated with HF engineering.

Influence of confining pressure on permeability and structural properties of selected sedimentary, igneous, and metamorphic rocks

As part of the work, studies of the rock’s permeability to gases were carried out using the original measuring apparatus, which makes it possible to study gas seepage through a porous medium under confining pressure conditions corresponding to in situ. Samples of selected sandstone, sapropelic coal, marble, granite, limestone, and spongiolite rocks were used for permeability studies. The permeability of these rocks was determined in relation to helium (He) and carbon dioxide (CO2) in various values of the confining pressure: 1, 5, 10, 15, and 30 MPa. The obtained variability ranges of permeability coefficients allowed to assign the tested samples to particular classes, from poor and tight permeable rocks, where k∞ < 1 mD and k∞ < 0.1 mD (granite, marble), through good permeable rocks with a value of 10 < k∞ < 100 mD (limestone, spongiolite, sandstone), to very good permeable rocks with coefficient k∞ > 100 mD (coal). The Klinkenberg slippage effect was twice as large for He compared to CO2, and as permeability increased, the slippage effect disappeared. The Walsh model was used to analyze the obtained results, based on which it was found that the highest impact of effective stress was observed for a granite sample, the smallest for sapropelic coal, where an increase in effective stress by about 30 MPa reduced the permeability of coal to He by 50% and to CO2 by 30%. Changes in the structural properties of rocks as a result of subjecting them to gas seepage processes under confining pressure conditions were also examined. Open porosity, specific surface area, pore size distribution, and mean pore diameter in the samples were determined. In most of the studied rocks, a decrease in porosity and a reduction in the pore space of the rocks were observed after permeability tests under confining pressure conditions.

Dislocation-density-related constitutive model and its applicability to cyclic deformation and damage behavior of 316LN SS at 823 K

In the present investigation, an in-depth understanding of the relationship between dislocation kinetics and cyclic plasticity, along with the description of yield asymmetry behavior during loading and unloading in nuclear grade 316LN austenitic stainless steel at 823 K, was examined by employing a constitutive framework using the evolution of physically-based internal-state variables. The formulation encompassed a set of first-order simultaneous differential equations that governed the evolution of dislocation density, mean free path of dislocations, and back stress. These variables were interdependent with the power law which described the kinetics of plastic strain rate. The evolution equations were numerically integrated, and the simulated data were fitted to the experimental data up to stabilized cycle at a temperature of 823 K and total strain amplitudes of ±0.004, ±0.006, ±0.008 and ± 0.010. The predicted cyclic stress response as a function of the cyclic number displayed the continuous cyclic hardening from the first cycle to the initiation of stabilized cycle. Further, good agreement between the predicted and experimental hysteresis loops was found at different cycle numbers. The predicted dislocation density and tensile peak values of back/effective stress increased with cyclic hardening and approached the value of saturation at the stabilized cycle. On the other hand, a rapid decrease in mean free path with cycle number was noticed. An increase in total strain amplitude resulted in a systematic increase in dislocation density, peak stress, back stress and effective stress. At the total strain amplitudes above ±0.006, the predicted higher rates of dislocation accumulation and annihilation, and the faster evolution rate of back stress towards saturation led to the development of stable dislocation substructure configurations with a clear distinction between dislocation high/low dense regions. Furthermore, the applicability of the model was extended by including an empirical relationship between the damage variable and cycle number within the power law, thereby enabling predictions of the cyclic stress-strain response beyond the stabilized cycle.

Seismically progressive motion mechanism of earth slopes considering shear band porewater pressure feedback under earthquake excitations

The complicated seismic response of earth slopes and the transition mechanism from progressive to catastrophic motion of landslides have long been recognized as one of the most challenging research topics in geotechnical earthquake engineering. In this paper, this fundamental conundrum will be tentatively unveiled from a new perspective combining the seismic slope displacement analysis with (mechanical) dynamic porewater pressure feedback of the shear band soils against the earthquake excitation. We present both the conceptual scheme and the calculation framework to numerically study the dynamic changes of porewater pressure, shear strength parameters, and hydraulic properties of the shear band soils. Their significant influences to the change of earthquake-induced landslide movement modes are systematically investigated. Different dilation angles are chosen to characterize the shear dilation and shear contraction processes in the shear band soil, while the dynamic porewater pressure feedback in the shear band soil during the continuous seismic displacement is investigated. The results show that the seismic slope displacement and failure strongly depend on the mechanical feedback of the shear band soil. When the dilation angle is positive, the porewater pressure feedback in the shear band during the slope movement is negative, and the effective resistance stress increases, which is not susceptible to catastrophic motion. On the other hand, when the dilation angle is negative, the positive porewater pressure feedback may lead to catastrophic failure. At last, the effects of shear softening/hardening combined with dilation angle evolution on the ultimate slope movement mode are also put forward.

Development of reduced volume endosseous cuspid tooth implant using topology optimization by SIMP technique for improved osseointegration

The article aims to design and develop a topology-optimized endosseous cuspid tooth implant of the maxilla region. The manuscript presents a numerical analysis of the resulting von Mises stresses and effective strain resulting in the topology-optimized implant with occlusal loading of 110 N. Solid Isotropic Material with Penalization (SIMP) method is employed for topology optimization and four different models, namely model-1, model-2, model-3, and model-4, are developed based on volume reduction rates of 8%, 16%, 24%, and 32%, respectively. FEA results highlight that the maximum stress and strain in the screw increases with volume reduction rates. The comparative analyses of the resulting stresses in the compact and cancellous bone along with the strain in the screw led to the conclusion that model-1, model-2, and model-3 resulted in moderate stresses on compact and cancellous bone compared to the original model of the implant. However, the screw and bones are subjected to maximum stress and strain in the model-4. The study concludes that model-2, with 16% reduced volume and 14.2% reduced mass as compared to the original implant, may be considered as the optimized design of the model. The resulting model offers a significant reduction in the weight and volume with a minor increase in effective stress and strain without negatively impacting the functionality and bio-mechanical performance of the implant. The optimized dental implant prototype is also fabricated as a proof of concept by the Fused Deposition Modelling process.

Benchmark for Nonlinear Consolidation of a Soil Deposit by Considering Depth-Dependent Initial Effective Pressure

Analytical solutions for 1D nonlinear consolidation of soils are widely applied to calculate consolidation settlement for their simplicity and convenience. Furthermore, analytical solutions can provide valid benchmark for numerical solutions of nonlinear consolidation. However, the existing analytical solutions for 1D nonlinear consolidation assume the initial effective stress to remain constant in the whole deposit. In practice, the initial effective stress increases with depth in a soil deposit. The existing analytical solutions for 1D nonlinear consolidation are lack of consideration about the variation of initial effective stress along with depth, so there is no benchmark for evaluating the corresponding numerical calculation. In this study, an analytical solution for nonlinear consolidation is developed under a special case, considering the linear increase of initial effective stress with the depth, which provides a benchmark for the corresponding numerical calculation of nonlinear consolidation. To facilitate the verification of the reliability of the numerical calculations of nonlinear consolidation, the pore pressure dissipation curves and settlement curves with time for conditions of PTIB (the case for a permeable top surface and an impermeable bottom surface is referred to as PTIB) and PTPB (the case for a permeable top surface and a permeable bottom surface is named as PTPB) are provided in this paper, thus providing some validation data for future calculations of nonlinear consolidation considering a linear increase in the initial effective stress with depth.

Effect of seepage on soil arching effect in deep shield tunnel

Shield tunneling and post-tunneling steady seepage are accompanied by stress and displacement variations, which could induce and influence the soil arching effect. Although there are many studies on the tunneling-induced soil arching effect, the research about the effect of seepage on soil arching effect is extremely lacking. In this study, a numerical model is firstly established and verified by field data. Then, a series of numerical models, whose simulation method of steady seepage is verified by adopting the conformal mapping technique, are established to study the soil arching evolution of deep-buried tunneling and post-construction steady seepage. The results indicate that seepage leads to an increase in effective vertical stress, which is consistent with the existing theory. The seepage weakens the soil arching effect resulting in the height of the arch zone reducing from 2.38D (D is the tunnel diameter) to 1.25D. The seepage leads to the further development of ground consolidation settlement, but the differential displacement in the soil mass decreases. The ground reaction curve in the steady seepage condition shows a bigger value than that after excavation. It is reasonable to control the ground loss ratio in the range of 0.5–1.0%, which can minimize overburden pressure with moderate ground deformation.

Dynamic Response Analysis of Airport Pavement under Impact Loading

The safety of the airport runway, as an infrastructure, is of considerable concern. The existing research has problems of hysteresis and unreasonable load application. In this paper, ANSYS is used to construct a coupled tire–pavement model to study the dynamic characteristics of airport asphalt pavements under impact loading. Taking the Boeing 737–800 as an example, the impact of an aircraft landing on an airport pavement is simulated by applying a dynamic load to the landing gear. The effects of tire pressure, landing pitch angle, and sinking speed on the dynamic response of the airport runway are investigated. The effects of each factor of the airport pavement are analyzed by comparing the maximum effective stress, effective strain, and displacement in the vertical direction at the same position of different structures. The results show that when the tire pressure is 2.0 MPa, the maximum values of effective stress, effective strain, and vertical displacement increase by 29.8%, 19.1%, and 22.2%, respectively, compared with 1.0 MPa. The maximum values of effective stress, effective strain, and vertical displacement at the 3.0 m/s sinking speed increase by 25.2%, 93.1%, and 77.1%, respectively, compared with that at the sinking speed of 1 m/s, which indicates that the effect of sinking speed on the dynamic response of the pavement is more significant. However, the change in the landing pitch angle has little impact on the response parameters of the pavement. Meanwhile, the flexural tensile stress at the bottom of the surface and the equivalent effect at the top of the soil foundation must be considered in the design of the airport pavement structure.

Effect of Contoured Surface, Excavation and Fill Construction to the 3D Settlement Analysis

Settlement analysis is commonly done by assuming a horizontal soil surface with homogeneous soil layer. This assumption may no longer applicable when construction is conducted on mountainous area where there are a lot of hills or sloping area. Not only due to initial stress is no longer uniform throughout the entire construction area, excavation and fill activities which are required for the land preparation change the stress state of the soil. These problems are aggravated by the fact that soil layer in most of the mountainous area is not horizontal. In this paper, the effect of contoured surface and construction activities such as excavation and fill will be described by using a parking lot building project constructed at the mountainous area. The analysis is carried by using finite element analysis through MIDAS GTS NX. The result will then be compared with the finite element analysis without considering contoured surface and construction activities. The result shows that excavation and fill activities highly affect the settlement profile. For the area where excavation activities are carried, there will be less settlement which is expected due to an increase in over consolidated ratio. On the other hand, fill area experience more settlement due to lower fill modulus an increase in effective stress around the fill area.

Evolutionary Model and Experimental Validation of Gas-Bearing Coal Permeability under Negative Pressure Conditions.

Coal bed methane drainage is the main approach to lower risks of coal seam while raising the efficiency in natural resource utilization. The negative pressure used for extraction in coal mines is largely determined empirically due to a lack of experimental research on how coal permeability changes under the combined influence of effective stress and negative pressure. This results in low gas extraction efficiency and concentration. In this paper, to study the effect law of complex stress and extraction on coal permeability during coal and gas co-mining, a test system was specially designed to determine the gas flow and coal permeability of coal samples under different stress paths and negative pressure conditions in the lab. The study analyzed the correlation between coal permeability, effective stress, and negative pressure and subsequently developed a permeability evolution model for gas-bearing coal under negative pressure conditions. The results showed that the permeability of coal increases with the increase in negative pressure and decreases with the increase in effective stress; the permeability of coal can be abruptly changed by changes in stress loading patterns; the established model of permeability evolution of gas-bearing coal can better reflect the correlation between permeability, effective stress, and negative pressure. The research outcomes offer a valuable theoretical foundation for the efficient extraction and utilization of methane in coal mines.

Research on the seepage properties of coal with different particle size proppant under cyclic loading

The selection of proppant particle size significantly impacts the gas output and gas production period of the extracting coalbed methane (CBM). This study combines theoretical analysis and permeability testing, based on the in situ stress distribution characteristics of the coal seam in Wangjiazhai Coal Mine, Guizhou Province, conducted on artificial fractures with different particle size proppant combinations during the cyclic loading and unloading. The findings indicate that the coal sample with two particle sizes of proppant has more permeability and smaller stress sensitivity coefficient than the coal sample with a single particle size proppant; as effective stress increases, the coal sample with the maximum permeability and the smallest stress sensitivity coefficient is placed with a proppant ratio of 20/40 mesh to 40/70 mesh of 1–3. The stress sensitivity coefficient and the permeability decrease with an increase in the number of confining pressure cycles. The increase in the proppant embedding depth has a hysteresis phenomenon with the increase in the effective stress, and the coal sample with a proppant ratio of 20/40 mesh to 40/70 mesh of 1–3 has the smallest embedded depth. The proppant will cause damage to the fracture surface of the coal seam. This study provides technical support for efficiently extracting the CBM resources that are difficult to exploit in Guizhou Province.

Centrifuge tests and numerical simulations on vertical bearing capacity of mat foundation on marine clay seabed

Mat foundation is a new type of jack-up wind power installation platform suitable for clay seabed, which avoids the problems of excessive insertion and extraction difficulties of spudcan foundation on soft clay seabed. Evaluating the vertical bearing capacity of the mat foundation on the clay seabed is of great significance for the safe operation of jack-up platform in marine environment. In this research, the vertical bearing characteristics of mat foundation on horizontal and sloping clay seabed were studied through centrifuge tests. Then, numerical simulations were conducted to study the vertical bearing capacity of the full-scale mat foundation on clay seabed, in which the effects of the large mat foundation size, pile leg insertion depth, preloading, consolidation degree of soil, and soil plasticity were analyzed. The results show that the vertical bearing capacity of the mat foundations on the sloping seabed is reduced, and the soil under the mat foundation shows obvious sliding failure. The contribution of the insertion depth of pile legs on the horizontal bearing capacity is more obvious than that of the vertical bearing capacity. The enhancement of the vertical bearing capacity is mainly due to the dissipation of the excess pore pressure and the increase of the mean effective stress of the soil. The effects of preloading, consolidation degree, and soil property on the vertical bearing capacity of the mat foundation are significant.

A State-Dependent Hypoplastic Bounding Surface Model for Gassy Sand

As a kind of partially saturated soil often containing dissolved gas with a high degree of saturation (S &gt; 85%), gassy sand sediment widely exists in the marine environment all over the world. Due to the effect of gas dissolution and exsolution on the pore fluid compressibility, its stress–strain and pore pressure responses are quite different from those of common saturated and unsaturated soils when subjected to undrained loading. Since almost all the gas bubbles are occluded in the pore water of offshore gassy sand, the matric suction may be neglected, and an undrained constitutive model for gassy sand is developed based on the existing hypoplastic bounding surface model for saturated sand. Both Boyle’s and Henry’s laws are employed in the model to characterize the equilibrium behavior of the gas compressibility and solubility, and then the equivalent compressibility coefficient of the pore fluid is obtained. To avoid unrealistic volumetric expansion, the concepts of the critical state and state-dependent dilatancy stress ratio are incorporated to describe its ultimate shear strength and dilatancy characteristics, respectively. Finally, the triaxial undrained test results on gas-charged sand from Hangzhou Bay are analyzed at three initial saturation degrees of 85%, 90%, and 100%, and two effective confining pressures of 50 kPa and 200 kPa. Moreover, carbon dioxide (CO2) was selected in the test, and the samples were loose with a relative density of 30%. It is noted that a good agreement is achieved between the simulation results and the experimental data, including the influence of gas content and confining pressure on the shear dilatancy and the mean effective stress increase at the beginning of the effective stress path, among others.

Characteristics of In Situ Stress and Reservoir Pressure in Deep Coal Seams and Their Influences on Reservoir Depletion: A Field Case Study

The production of coalbed methane resources is greatly dependent on the reservoir depletion process. To improve the depressurization efficiency of deep coalbed methane (DCBM) reservoirs, the fluid production characteristics and implications of in situ stress and reservoir pressure for reservoir depletion in the eastern margin of the Ordos Basin are analyzed. The results indicate that the duration of the single-phase water production stage is overall long, with the gas declining stage occurring too early. The daily gas production volume of DCBM wells is only a few hundred m3. For DCBM reservoirs, it is considered that in situ stress, initial reservoir pressure, and critical desorption pressure significantly affect the fluid production and depressurization process. A high in situ stress in deep coal seams is likely to damage the flow conductivity of fracture systems due to the increase in effective stress during reservoir depletion. Based on the in situ stress characteristics of deep coal seams and their implications for reservoir depletion, we propose that the depressurization of DCBM reservoirs during gas production can be promoted via a new method of stress release. Slots or cavities created in coal seams by high-pressure hydraulic jet can provide space for stress release. This novel depressurization method could help improve the gas production efficiency of DCBM reservoirs.

Experimental study of sandstone stress sensitivity under different fluids: Characteristics and mechanisms

Stress sensitivity is the phenomenon of permeability variation that caused by pore throat changes, opening or closure of fractures under forces, which is an important parameter in reservoir evaluation. Fractured sandstones under various fluids will produce different stress sensitivity, which can greatly influence oil and gas development. However, little is known on this topic currently. Hence, in this study, characteristics and mechanisms were investigated based on a series of experiments and theoretical analysis. The results indicate that: I, compared with unfractured rocks, fractured rocks show a stronger stress sensitivity. The permeability shows an obvious “decrease initially steep and finally gentle” pattern with the increase of effective stress. II, from purified water to low NaCl/CaCl2 mineralization fluids, the permeability damage of fractured rocks decreases gradually with the increase of mineralization degree; however, it increases with mineralization degree when rocks are under high mineralization fluids, and the damage degree is also high. The permeability damage degree is generally low when fractured rocks are under the action of kerosene. Mechanism analysis indicates that: I, fractured rocks have both fracture and pore systems, whose opening and closure can produce stronger stress sensitivity and more obvious nonlinear variation of permeability than unfractured rocks. II, under purified water conditions, the expansion of clay minerals is an important indicator and reason for the permeability damage. III, under low fluid mineralization degree conditions, the reaction between fluids and minerals will destroy rock structures, increase fracture aperture, and produce secondary pores, and finally decrease the permeability damage to a certain extent. IV, under high fluid mineralization degree conditions, crystals may be deposited, blocking up pores/fractures and producing high permeability damage. V, there are no reactions between minerals and kerosene, hence, the permeability damage degree is generally low. The results are expected to provide references for the exploration and development of fractured tight reservoirs in Kuqa Depression.

Experimental investigation on the permeability characteristics of methane hydrate-bearing clayey-silty sediments considering various factors

Permeability is a key physical parameter of hydrate-bearing sediments (HBS), which is an important basis for evaluating reservoirs and developing exploitation plans. However, few permeability studies of HBS conducted so far have involved effective stress and seepage pressure difference, which is inconsistent with the stress-seepage coupling process in actual hydrate exploitation. In this study, marine clay taken from the South China Sea and quartz sand were used to synthesize hydrate-bearing clayey-silty sediments (HBCSS). A series of water permeability tests were conducted to investigate the effects of effective stress, hydrate saturation and seepage pressure difference on the permeability of HBCSS. The results show that an increase in effective stress can cause a significant decrease in permeability, and this effect is particularly pronounced at the beginning of stress increase. The decreasing effective stress can only cause a slight rebound in permeability, as only part of the elastic strain can be recovered. The negative exponential relationship between hydrate saturation and permeability is controlled by the blockage mechanism of hydrate particles. The change in effective stress is the essence of the effect of seepage pressure difference on permeability, and this effect is attenuated with increasing hydrate cementation. Based on the experimental results, permeability models considering the coupling effect of effective stress and hydrate saturation are proposed.

Study on influencing factors and mechanism of pore compressibility of tight sandstone reservoir—A case study of upper carboniferous in ordos basin

A series of studies were carried out on 11 tight sandstone samples of Upper Carboniferous in Ordos Basin. Firstly, the deposit composition and pore structure characteristics are investigated based on analysis and experiments including cast thin section scanning electron microscope high-pressure mercury intrusion and nuclear magnetic resonance Then, combined with DP-P test, the stress-dependent permeability change and pore compressibility characteristics of sandstone reservoirs were studied to reveal the influencing factors and mechanism of reservoir pore compressibility. The detrital particles of the sandstone reservoir in the study area are mainly quartz (75.8%–89%), followed by fragments (3%–16.1%), and almost no feldspar. The content of interstitial materials is 6.5%–11.2%. The type I reservoirs mainly consist of mesopores and macropores, accounting for 60.57% and 32.84% respectively. Mesopores are dominated in Type II reservoirs, accounting for 78.98% of the total pore volume. There are almost no macropores, while a similar proportion of mesopores, micro mesopores and micropores in the type Ⅲ reservoirs. The study of pore compressibility shows that the pore compressibility coefficient decreases with the increase of effective stress, and the reduction rate shows the two-stage characteristics of rapid in the early stage and slow in the later stage. The pressure turning point is between 3 and 10 MPa. The average pore compressibility coefficient increases from type I to type Ⅲ reservoirs. The compressibility coefficient is directly proportional to the changing rate of the pore volume. The higher the content of rigid detrital particles, quartz and carbonate cement in sandstone, the smaller the pore compressibility coefficient, while the higher the content of ductile components such as soft rock fragments and clay minerals, the greater the pore compression coefficient. The pore-throat structure is closely related to the pore compressibility, reservoirs with low displacement pressure, T2glm value, and large average pore-throat radius show lower compressibility coefficient. In addition, the compressibility coefficient of the reservoir is positively correlated with DL (dimension of large pores such as mesopores and macropores), and negatively correlated with DS (the fractal dimension of micropores and micro mesopores). It is considered the pore compression of sandstone including two stages, viscoplastic destructive deformation of ductile components for the first and then the small-scale non-ideal elastic deformation on rigid particles.