Influence Of Pore Pressure Research Articles

Improving Precision in Assessments for CO2 Geological Storage: Investigating the Influence of Pore Pressure on Rock Properties

Improving Precision in Assessments for CO₂ Geological Storage: Investigating the Influence of Pore Pressure on Rock Properties

An Analytical Model and Its Application in Reliability Analysis of Formation Failure during Hydrate Production in Deep-sea Areas

Methane hydrates (MHs) have potential economic and environmental significance. However, due to the sharp reduction in the mechanical properties of methane hydrate-bearing sediments (MHBS) caused by hydrate dissociation, the risk of formation failure in marine MHs exploitation is higher than that in conventional oil and gas exploitation. Formation failure in marine hydrate production probably leads to serious sand production or subsequent formation instability. A simplified and efficient analytical model is proposed at first for prediction of formation failure as well as the displacement around the wellbore during hydrate production, and then the reliability analysis for formation failure is performed by the Advanced First Order Second Moment Method based on the analytical solutions. This model considers the key factors affecting the formation failure, including 1) the partial coupling of multiple fields (the influence of pore pressure on mechanical field and the influence of hydrate dissociation on pore pressure, temperature and mechanical field), 2) the effect of gas/water or heat absorption caused by hydrate dissociation on pore pressure or temperature, 3) the change of mechanical properties of formation induced by hydrate dissociation and 4) the casing-formation interaction. The analytical solutions are verified and validated by numerical models. According to the analysis, the casing is sufficiently safe in the cross section due to its high stiffness, while the formation would be failure at the wellbore or dissociated front due to the stiffness/strength deterioration induced by hydrate dissociation. Hydrate dissociation leads to an increase in formation failure probability by approximately 20%. The probability of formation failure at the wellbore is about 30% higher than that at the dissociated front. The uncertainties in the internal friction angle and elastic modulus ratio have the greatest effect on formation failure probability.

The impact of supercritical CO2 exposure time on the effective stress law for permeability in shale

Carbon dioxide (CO2) sequestration in shale formations is regarded as an effective approach for reducing CO2 emissions. Permeability is critical for CO2 geological sequestration as it impacts migration of CO2 in shale formations. The effective stress coefficient for permeability (χ) scales the relative influence of pore pressure on permeability compared to confining pressure. This study examined the changes in χ of dry and wet shale samples with different times of supercritical CO2 (ScCO2) exposure. The results suggest that, as ScCO2 exposure time increases from 0 to 40 days, the χ of dry shale increases from 0.8304 to 0.9563, and that of wet shale increases from 0.8379 to 1.0134. Wet shale exhibits a greater increase in χ than dry shale for the same exposure time. These changes in χ may result from CO2-shale interaction that converts smaller-sized pores in shale into larger-sized pores, based on nuclear magnetic resonance (NMR) measurements. Furthermore, the presence of water intensifies the response of shale to ScCO2, thereby enhancing this transformation. Our findings provide insights into the accurate prediction of shale permeability evolution during CO2 storage, and can assist in optimizing CO2 injection strategies as well as evaluating the potential for CO2 sequestration.

Thermal Expansion and Adsorption Characteristics of Coal Based on Temperature Change and Its Influence on the Slippage Effect

The slippage effect is an important phenomenon of coalbed methane (CBM) migration. As the exploitation of CBM, the slippage effect becomes more and more obvious. For exploring the evolution of the slippage effect as CBM extraction by heat injection, the thermal expansion test and the temperature increasing desorption test under different stress states, as well as the seepage test under variable temperature conditions, were conducted by operating the high-temperature multifunctional triaxial test system. The effect rules for temperature and pore pressure on the slippage effect were analyzed, and then the evolution mechanism of the slippage factor at the united action of pore pressure and temperature was comprehensively analyzed based on thermal expansion deformation and adsorption characteristics. The results showed that: first, the influence of pore pressure on the slippage effect is mainly caused by the molecular mean free path and effective stress when pore pressure is greater than 0.8 MPa in the range of 30–150 °C. With the pore pressure increase, the slippage effect decreases. Second, the temperature influence on the slippage effect is mostly induced by the variation of cracks in coal due to thermal expansion deformation created by temperature. With the increase in temperature, the slippage factor average increases first from 30 to 60 °C is 19 times and then decreases from 60 to 90 °C is 36%. Finally, at the combined influence of temperature and pore pressure, the slippage effect is mainly influenced by thermal expansion deformation, desorption deformation, and effective stress. At 30–60 °C, the slippage factor is principally affected by thermal expansion deformation. At 60–90 °C, the slippage factor is mostly influenced by desorption deformation and effective stress. The research results provide a foundation for the technology of CBM exploitation by heat injection.

Laboratory investigation of the effect of the pore pressure on argillaceous siltstone permeability

The permeability of argillaceous siltstone is a critical parameter for the sustainable development of natural gas hydrates in the ocean beds. In the laboratory, the pressure pulse method is frequently used to measure permeability. However, the influence of pore pressure on permeability test results of argillaceous siltstone by pulse method is not established, resulting in inconsistencies between permeability test results in the laboratory and those in deep seawater (water depth ≥ 500 m). First, the effect of bound gas content in argillaceous siltstone on the rate of volume change in the test medium is considered, and the computation model of the pressure pulse method is validated. Next, the effect of pore pressure on the permeability of argillaceous siltstone is investigated while keeping the effective stress of the sample constant. The experimental results show that as pore pressure increases, the permeability of argillaceous siltstone increases gradually. The permeability measured in a 6 MPa pressure environment is two orders of magnitude greater than that measured in a 1 MPa pressure environment and the increased range is piecewise linear. The reasons for the increased permeability are investigated using the hydrophilicity of the argillaceous siltstone materials. Under high pressure, the hydrophobic properties of montmorillonite, quartz and kaolinite reduce the seepage resistance of argillaceous siltstone. The experimental results will serve as a foundation for future research, improving the accuracy of argillaceous siltstone permeability estimates.

Development and Influence of Pore Pressure around a Bucket Foundation in Silty Soil

Silty soil is common in the seabed of eastern coastal areas of China. The behaviors of the silty soil and a bucket foundation installed within it need more study. In this work, model tests of a bucket foundation in silty soil were performed. The development of the excess pore water pressures in the different positions around the bucket was measured. Different loading conditions, with a change in the horizontal cyclic load amplitude ratio, horizontal cyclic frequency, and vertical load ratio, were considered. The effects of the pore water pressure on the shear strength of the soil around the bucket and the horizontal bearing capacity of the bucket foundation were investigated. The results show that the normalized pore water pressures close to the bucket wall at depths between 0.1 L and 0.3 L exhibit distinct change under the cyclic load. Consistent with the distribution of the pore water pressure, the degradation of the undrained shear strength is more obvious with a greater load amplitude ratio, a greater load frequency, and a smaller vertical load. The degradation rate of the static horizontal ultimate bearing capacity is in a range of 1.57% to 14.90%, under different loading conditions.

Combined control of fluid adsorption capacity and initial permeability on coal permeability

The variations of strain and permeability of coal were systematically studied through the physical simulation of N2 and water injection. The effects of fluid adsorption capacity and initial permeability on strain, permeability and the dominant effect of pore pressure were discussed. The adsorption strain and strain rate of coal during water injection are significantly higher than those during N2 injection. An edge of free adsorption exists in the early phase of N2 and water injection, which is related to fluid saturation. Within this boundary, the strain rate and pore pressure are independent. Moreover, the injection time of initial stage accounts for about 20% of the total injection time, but the strain accounts for 70% of the total strain. For water injection, this boundary is about half of water saturation of coal. Besides, the influence of pore pressure on permeability is complex, which is controlled by adsorption capacity and initial permeability of coal. When the initial permeability is large enough, the effect of adsorption strain on permeability is relatively weak, and the promoting effect of pore pressure on fluid migration is dominant. Therefore, the permeability increases with increasing pore pressure. When the initial permeability is relatively low, the pore pressure may have a dominant role in promoting fluid migration for the fluid with weak adsorption capacity. However, for the fluid with strong adsorption capacity, the adsorption strain caused by pore pressure may play a leading role, and the permeability reduces first and then ascends with increasing pore pressure.

Experimental study on the effective stress law and permeability of damaged sandstone under true triaxial stress

The effective stress law and the permeability play a notable role in dealing with the gas-solid coupling problem of porous media . The key parameter to quantify the influence of pore pressure to effective stress is the effective stress coefficient α ij . However, current relevant researches are mainly focused on the pore elastic flow characteristics in the elastic stage or hydrostatic pressure under conventional triaxial stress state, which are difficult to reflect the actual formation stress condition. To this end, the present study was conducted to investigate the influence of induced damage on the effective stress coefficient and the permeability of sandstone under true triaxial stress. The results showed that the effective stress coefficient and the permeability were closely related to the damage evolution and fracture propagation . α ij exhibited obvious anisotropy, α 1 was larger than α 2 and α 3 , and α 3 > α 2 appeared in the horizontal direction. α ij increased with increasing the fracture density and opening, even greater than unity. Increasing the initial horizontal stress could reduce α ij under the damage state. The elastic modulus E ij also exerted anisotropy during loading, E 1 basically exhibited an increasing trend followed by a decrease as the axial strain increased, E 2 and E 3 were continue to degrade with increasing the axial strain. Loading the deviatoric stress could cause damage, which induced the initiation and propagation of microcracks , and in turn led to the nonlinear stress-strain relationship and volume expansion of the rock, so the permeability increased, and it had increased sharply before reaching the peak strength. Under the influence of σ 2 , α ij first increased and then decreased with increasing the intermediate principal stress coefficient b under the damage stage. The porosity effect also affected the relationship between α ij and the permeability related to the induced damage, and α ij revealed an increasing trend as the permeability increased. • Effective stress coefficient a ij increases with fracture growth, even beyond unity. • a ij and E ij exhibit anisotropy with loading. a 1 is the largest, and a 3 > a 2 . • Increasing the horizontal stress can reduce a ij related to the induced damage. • a ij first decreases and then increases with increasing b under the damage state. • a ij increases with the increase in permeability attributed to the induced damage.

Coupling Mechanism of Coal Body Stress–Seepage around a Water Injection Borehole

In China, coal seam water injection technology is widely used in the prevention and control of mine gas and dust disasters. The evolution law of stress–seepage field in the range of hydraulic influence in the process of coal seam water injection is the most basic theoretical problem in coal seam water injection technology. Therefore, this paper first uses the radial seepage experimental system to test the permeability of raw coal samples, and then numerically inverts the stress–seepage evolution law of the coal body around the water injection hole. The results show that the coal seam permeability decreases exponentially with the increase in the experimental overburden pressure. The dynamic water intrusion causes the pressure relief area and the stress concentration area to transfer to the deep part of the coal body, and abnormal stress occurs near the water injection hole wall. The composite stress area formed by the superposition of stress and pore water pressure reduces the permeability of the hole wall. During the radial development of water injection, the pore water pressure dropped sharply and eventually tended to zero, while Darcy’s velocity decreased significantly (within the pressure relief zone, it rapidly decreased from 7.26 × 10−3 m s−1 to 2.5 × 10−3 m s−1 (by 65.56%)). Near the initiation point, the maximum step-down height of Darcy’s velocity decreased from 6.73 × 10−3 m s−1 to 5.27 × 10−3 m s−1 (by 22.73%). This can make it clear that the seepage velocity presents a non-Darcy seepage phenomenon under the influence of pore pressure.

Analysis of wellbore stability of gas underbalanced drilling technology

Gas underbalanced drilling technology has the advantages of high ROP and little damage to oil and gas reservoirs. In recent years, it has been widely used in the exploration and development of oil and gas fields in China. Under normal air drilling conditions, the pore pressure is basically zero, so it is necessary to establish the wellbore stability model under the condition of zero pore pressure to provide theoretical support for the wellbore stability analysis of air drilling. However, gas underbalanced drilling also has wellbore instability, more sand deposition at the bottom of the well, and even the malignant accident of sticking. Firstly, through the analysis of the traditional wellbore stability model, the influence of pore pressure is removed, and the wellbore stability model suitable for air drilling is obtained. At the same time, it is proposed that the sand deposition is mainly caused by the collapse material when the collapse ellipse is formed on the shaft wall, and the formation of the collapse ellipse is conducive to maintaining the stability of the shaft wall. For large and medium-sized gravel layers, the influence of cementation strength on wellbore stability should be considered.

Stress and pressure dependent permeability of shale rock: Discrete element method (DEM) simulation on digital core

Shale permeability will change with the variation of reservoir pressure and formation stress, especially when natural fractures occur failure due to pressure and stress perturbation during hydraulic fracturing, the permeability will change dramatically. So far, however, there is no comprehensive theoretical method could estimate the rock permeability change with natural fractures failure under true-triaxial stress condition. This paper aims to propose a numerical method to investigate the influence of pore pressure, triaxial stress, and natural fractures failure on the shale rock permeability. Firstly, this paper built a digital shale core cylinder by combining the discrete element method (DEM) and fluid-solid coupling model. Then, all the micro parameters of the digital rock were delicately calibrated according to the quasi-triaxial compression tests data of real rock core collected from Bakken shale formation, making the macro-mechanical property and seepage characteristic of digital rock are consistent with real shale rock. Then, based on the calibrated micro-parameters, we constructed a digital core cube to simulate the permeability test under true triaxial conditions. Finally, some natural fractures, as the weak planes, were preset in digital core, and the shale rock permeability change was analyzed with natural fractures failure. It shows that the intact shale permeability will decrease with triaxial stress increasing or pore pressure decreasing. Besides, the permeability of shale will be affected by the natural fractures’ property and failure states. In general, as the triaxial stress increases, the natural fractures in shale rock cube will occur failure gradually, meanwhile, its permeability will rise remarkably in the beginning. However, after most of the natural fractures have occurred failure under high-stress or high-pressure, shale permeability will turn to decrease as the stress continues to increase. This paper explores the inner mechanism and exterior behavior of shale permeability dependence on triaxial stress, pore pressure, and natural fractures failure. • A digital core modeling method is proposed based on the discrete element method. • The digital shale core is constructed and it is identical to a real shale core from Bakken formation. • A series of permeability tests simulation are conducted on the digital shale core cube under true-triaxial stress conditions. • The mechanism of shale permeability dependence on stress, pressure, and natural fractures failure is explored. • This research is beneficial to the fracturing design in shale reservoirs.

Investigation on Creep Behavior and Permeability Evolution Characteristics of Sandstone under Different Pore Pressures

To investigate the influence of pore pressure ( σ w ) on the creep behavior and permeability of red sandstone, triaxial creep test with permeability test under different pore pressures was conducted using MTS 815 testing system. The experimental results demonstrate that water has significant weakening effect on the long-term mechanical properties of sandstone, and the long-term strength of sandstone gradually decreases with increase in pore pressures. All permeability-time curves demonstrate a “decreasing-increasing” trend, but two different permeability evolution trends during the steady creep stage are observed, which are related to deviatoric stress. The permeability of both intact ( k 0 ) and fractured sandstone ( k f ) samples increases with the increase in pore pressure, which are in consistent with the failure mode analysis of fractured sandstone samples. However, while the relationship between k 0 and σ w is positive linear, it is a positive exponential function relationship between k f and σ w .

Research on Change Laws of Pore Water Pressure of Soft Clay in the Large-scale Wave Flume

The study on the interaction between wave structure and soft clay foundation is carried out in the large scale wave flume trough of the Academy of communications and transportation under the construction of the research on the failure mechanism and design parameters of the port hydraulic structure under the harsh hydrology condition. The remolding technology of the soft clay foundation of the Yangtze River mouth is studied. The model test scale of is 1:5. In the test, the strength of the original soil can be reduced to the similarity of the test phenomenon. The soil strength is reduced by increasing the water content of the original soil, and a series of tests are made on the characteristics of the original soil. The strength of soil increases with the increase of static time. The early growth is fast and the later growth is slow. In the experiment, the soil pressure and pore water pressure sensor were arranged to observe the change rule of soil pressure and pore water pressure under the action of wave. By arranging water pressure sensor and laser displacement sensor, the force and settlement of the structure of the semicircle caisson were observed. In this paper, the variation rule of earth pressure and pore water pressure under different wave heights and longtime waves are studied. The pore pressure and soil pressure change with the wave action, the influence of pore pressure increases with the increase of wave height. Maximum pore pressure occurs at a certain depth of soil. The change in pore pressure is irregular. After a certain value is reached, the soil is softened and destroyed.

Study on the Rock Fracture during Fluid Injection Using a Coupled Flow‐Stress‐Damage (FSD) Model: Insight into the Stress Shadow Effect

In order to investigate the influence of pore pressure on hydraulic fracturing behavior in the local and whole model, the coupled flow‐stress‐damage (FSD) analysis system RFPA‐Flow was used to study the influence of rock heterogeneity, natural stress ratio, double‐hole spacing, and water pressure gradient on the stress shadow effect. The numerical results show that the tensile crack induced by pore water pressure is significantly affected by the pore water pressure and water pressure gradient. The larger the pore pressure gradient is, the more asymmetrical the crack development pattern and the smaller the instability pressure of the model. In addition, the shape of hydraulic fracture becomes much more irregular with the increase in rock heterogeneity. The number and shape of tip microcracks under the influence of local water pressure are closely related to the homogeneity of rock. Moreover, when the natural stress difference is large, the hydraulic fracture propagates parallel to the maximum principal stress; when the stress field is close and the spacing between two holes is less than 5 times the diameter, the propagation direction of hydraulic fractures between holes is perpendicular to the maximum principal stress. It is found that no hydraulic fractures occur between the two holes when the distance between holes is greater than 5 times the diameter.

Influence of pore pressure on natural frequency wandering of structures under earthquake conditions

Temporary seismic frequency wandering of buildings during and after seismic events has often been observed and has not yet been investigated, in particular with regards to the effects of the pore pressures generated within the soil mass. This paper investigates the influence of delayed soil response on the observed temporary natural frequency wandering induced by seismic actions. The focus is on the pore pressure generation within the soil mass and its impact on the observed wanderings. Our findings show that the natural frequency of the system is lower shortly after the seismic event, but it returns to its initial value with time, as expected from measurements. The results are shown in terms of generated pore pressure, its dissipation over time, reduced mean effective pressure and nonlinear stress strain behaviour, all of them affecting the temporary changes in natural frequencies. Parametric studies on the influence of soil permeability, soil density and type of input motion are presented. Finally, a time history inclusive of an aftershock is investigated to explore the different soil-structure interaction mechanisms occurring during the main event and the subsequent aftershocks.

Statistical Simplification and Discrete Element Analysis of Seepage Model for Complex Fracture Network of Dam Foundation Rock Mass

In this paper, a model of seepage in a fracture network is analyzed by taking the rock mass of the dam foundation of Datengxia Hydropower Station, the No. 1 Project of the Ministry of Water Resources, as an example. Complex discontinuity systems are present in the rock mass of the Datengxia Hydropower Station dam foundation, and faults, soft interlayers, and structural fracture systems intersect with one another. A multi scalar structural analytical scheme that combines the deterministic analysis of large scale geological structures (i.e., faults and weak interlayers) with the statistical simulation of random discontinuities (i.e., structural fractures) is proposed in this paper to reduce the difficulty and uncertainty of the analysis of rock masses with complex structures. In particular, a chain analytical method from dominant group definition to statistical parameters (i.e., spacing and occurrence) is formed for the structural fracture system. In this way, the analysis of rock masses with complex structures is simplified to a regular and feasible scientific problem that could be solved by rock mass simulation. We then establish a seepage model of the rock mass in dam section 29# in UDEC by adopting the above structural simplification scheme. The quantitative analysis of seepage in complex structured rock masses under normal water storage and dynamic conditions is then realized. Results show that the statistical simplification of the rock mass structure and discrete element simulation can efficiently solve the seepage analysis of complex rock masses. The negative influence of pore pressure caused by seepage on the rock mass, especially under dynamic conditions, should be emphasized for safety assurance in the stability analysis of dam foundations.

Bearing capacity of shallow foundation under cyclic load on cohesive soil

Abstract The bearing capacity of shallow foundations under static loads has been widely studied by various authors. In contrast, the bearing capacity under dynamic loads has been solved indirectly by adopting an equivalent pseudostatic approach or by means of reduction coefficients. The study of the problems related to cyclic loads has focused mainly on the behavior of granular soils. However, in the case of cohesive soils, these began to be studied in detail as a result of the Northridge, Kocaeli and Chi-Chi earthquakes that took place in the 90s. The object of the present work is to evaluate the influence of pore pressure in the calculation of bearing capacity of shallow foundation on cohesive soil and dynamically requested. This research is based on the results of cyclic simple shear tests carried out with samples from the Prat port (Barcelona). A pore-water pressure generation equation is proposed that depends on the effective vertical stress “in situ” (σ’ov), static shear stress (τo), cyclic shear stress (τc), and void ratio (e0). The formulation was implemented to the finite difference software FLAC2D, by which the bearing capacity of foundations can be determined. The proposed formulation can calculate the maximum cyclic load (CL) that a cohesive soil can resist before failure by cyclic softening, as a consequence of cyclic loads transferred by a shallow foundation. The practical application of this research is to provide a chart to calculate the bearing capacity of cohesive soils requested by cyclic loading (CL). This application takes into account the properties of the ground, the static load capacity (Phe) and the effective load outside the foundation ( q ). In addition, the formulation is validated in the application of a real case.

Experimental and numerical investigation on influence of pore-pressure distribution on multi-fractures propagation in tight sandstone

During the process of reservoir stimulation, how to create more fractures has become a key issue for the tight sandstone reservoir development. From the experimental investigation and numerical simulation, the effect of pore pressure distribution on fracture propagation was analyzed. The experiment consisted of 300 × 300 × 600 mm3 sandstone specimens was divided into two laboratory tests to investigate the fracture propagation of simultaneous fracturing and sequence fracturing subjected to true tri-axial confinement stresses. The experimental results showed that the fracture distribution was asymmetric due to the influence of pore pressure and stress field. Meanwhile, the numerical simulation results discovered that the increase of pore pressure can not only cause the deflection of fracture, but also increase the breakdown pressure and extension pressure of rock. When the pore pressure increment was over 4 MPa, the fracture deflected obviously in the process of extension, and the breakdown pressure and extension pressure can increase by 22.8% and 23.05%.

Numerical simulation of cement grouting of saturated soil during a mine shaft sinking using the artificial ground freezing

Cement grouting is widely applied in geotechnical engineering to reinforce water saturated soil and to control groundwater. The present paper is devoted to a numerical study of cement grout injection in a soil stratum during a construction of a vertical mine shaft using the artificial ground freezing (AGF) technology. For the study a thermo-hydro-mechanical model was developed. The governing equations of the model are derived based on the theory of porous media. The model takes into account the features of AGF and grouting processes such as a non-Newtonian behavior of cement grout flow, water-ice phase change and an influence of pore pressure and temperature on an evolution of a stress-strain of the soil. Three-dimensional numerical simulation of cement grouting of unfrozen soil between the shaft lining and the ice-soil wall was conducted using the model for various injection regimes. Results of the simulation have shown that the grout injection leads to a rise in temperature and volumetric strain in the soil. A mechanical impact of the injection procedure on the shaft lining is an increase in the mechanical pressure near the injection boreholes and a redistribution of the von Mises stress.

Nonlinear creep damage model considering effect of pore pressure and analysis of long-term stability of rock structure

Water changes natural seepage in the surrounding geological environment and weaken rock mass of valley during reservoir impounding, which result in large time-dependent deformation of foundation of dam. A nonlinear creep damage model considering water effect is proposed by taking into account the influence of pore pressure on the spherical stress tensor and yield surface. The elastic-viscoelastic model is consisted of a spring and several cascaded Kelvin models. The model is implemented in the ABAQUS user material subroutine. The accuracy of the model is verified by the triaxial creep test of saturated sandstone and dry sandstone. Then it is applied in the valley contraction analysis during impounding of Xiluodu high arch dam.