Decrease In Pore Pressure Research Articles

Shale gas simulation considering natural fractures, gas desorption, and slippage flow effects using conventional modified model

This paper presents the theory and application to modify the conventional simulator to describe the effects of gas adsorption and gas slippage flow in shale gas. Because of the local desorption of gas and the assumptions of gas desorption instantaneously with the decrease in pore pressure, we define one fictitious immobile “pseudo” oil with dissolved gas. The dissolved gas–oil ratio is calculated from the Langmuir adsorption isotherm constants and shale gas properties. Additional modifications required in the input data are the porosity and relative permeability curves to account for the existence of “pseudo” oil. The input rock table considers the changes of rock permeability versus pressure to describe the gas slippage flow effects. In addition, dual-porosity dual-permeability models coupled with local grid refinement method are used to distinguish the impacts of natural fractures and hydraulic fractures on shale gas production with the comparison of vertical well, fractured vertical well, horizontal well, and multistage fractured horizontal well production. This proposed simulation approach shows enough accuracy and outstanding time efficiency. Results show that ignoring gas desorption and slippage flow effects would bring significant error in shale gas simulation The existence of natural fractures also imposes great effects on the productivity of shale gas.

Quantitative 3D spatial characterization and flow simulation of coal macropores based on μCT technology

The macropores in coal are an important link connecting nanometre-scale pores and millimetre-scale fractures. The 3D spatial characterization and flow simulation of macropores laid the foundation for the study of Coal Bed Methane (CBM) migration in pore-fractures. A 3D digital coal model with macropores (>13.85μm) was constructed by using μCT scanning combined with Avizo image processing, and the equivalent pore network model (PNM) was extracted to calculate the absolute permeability of the coal samples. The CBM flow process in macropores is simulated by COMSOL, and the streamline distribution for the pressure field and velocity field are revealed. The absolute permeability in the X-, Y-, and Z-directions is calculated. The results showed that the macropores have good connectivity in 3D space, and the shapes of the macropores are either spherical or sheet-like. The mean coordination number of the pore throat is 2.55 and the mean throat radius is 43.76μm. The differences between the permeability simulation calculation method and representative elementary volume (REV) selection scale lead to different errors in the measured results, but the permeability values are still of the same order of magnitude. Among them: when REV is 200×200×200 voxels, the equivalent PNM calculation result and the test result of actual coal sample have small error, when REV is the 50×50×50 voxels, the finite element simulation result and the actual coal sample have a relatively large error. Along the gas flow direction, the pore pressure decreases, the flow rate becomes larger; because of the heterogeneity of pore structure, the micro-fracture velocity increased rapidly, especially where the fractured channel was narrow, and the CBM flow rate suddenly increased. In some complicated pore structures this inevitably leads to a gas stagnation and no flow occurred.

Laboratory Investigation of Coal Deformation Behavior and Its Influence on Permeability Evolution During Methane Displacement by CO2

Geological sequestration of CO2 in coal seams is of significant interest to both academia and industry. A thorough laboratory investigation of mechanical and flow behaviors is crucial for understanding the complex response of coalbeds to CO2 injection-enhanced coalbed methane recovery (CO2-ECBM) operation. In this work, systematic experiments were carried out on cylindrical coal core specimens under different uniform confining stresses. The coal deformation caused by variations in effective stress as well as the sorption-induced matrix swelling/shrinkage was monitored. The competitive gas sorption characteristics and permeability evolution during the process of methane displacement by CO2 were also investigated. The measured volumetric strain results indicate that sorption-induced strain is the dominant factor in the coal deformation. The relationship between the volumetric strain and the adsorbed gas volume has been revealed to be a linear function. Experimental results obtained under different stress conditions suggest that higher confining stress suppresses the increase in both volumetric strain and the adsorbed gas volume. Furthermore, both methane displacement and CO2 injection are reduced when applying higher confining stresses. In addition, the permeability enhancement is heavily suppressed at higher confining stress. At a certain confining stress, a characteristic “U-shaped” trend of permeability is presented as a function of decreasing pore pressure. This study contributes to the understanding of coal deformation and its impact on permeability evolution under uniformly stressed condition, which has practical significance for CO2 sequestration and CO2-ECBM operation in the Qinshui basin.

Study of the Damage Mechanics and Dewatering Recovery Programs for the Shield Tunnel under the Yangtze River

A case is presented about damages due to leakage and inrushing water that developed in the segmented pre-casted concrete liner of a shield tunnel under the Yangtze River. Various possible causes were considered to explain the damage mechanism. The opening and distribution due to differential settlement caused by the leakage of biogas or water in the shield tunnel were analyzed. The damage mechanism of the aquifuge, which occurred under the tunnel through exertion of a high piezometric head in the lower confined aquifer, was analyzed. The tensile-shear damage mechanism of the aquifuge was studied, and the judgment formulas, which were used to determine whether surface water bursting occurred, were derived. Two dewatering recovery programs were simulated by the finite difference method in conjunction with MODFLOW software. From the simulation results, the hydraulic gradient and settlements caused by decreased pore pressure were calculated. The optimal dewatering scheme and protection measurements are suggested.

Construction of simplified design p–y curves for liquefied soils

In practice, laterally loaded piles are most often analysed using a ‘beam-on-non-linear-Winkler-foundation’ approach, whereby the soil–structure interaction is modelled by means of p–y curves. Although well-calibrated p–y curves exist for non-liquefied soils (e.g. soft clay and sand), the profession still lacks reliable p–y curves for liquefied soils. In fact, the latter should be consistent with the observed strain-stiffening behaviour exhibited by liquefied samples in both element and physical model tests. It is recognised that this behaviour is induced by the tendency of the liquefied soil to dilate upon undrained shearing, which ultimately results in a gradual decrease in excess pore pressure, and consequent increase in stiffness and strength. The aim of this paper is twofold. First, it proposes an easy-to-use empirical model for constructing stress–strain relationships for liquefied soils. This only requires three soil parameters which can conveniently be determined by means of laboratory tests. Second, it introduces a method for the construction of p–y curves for liquefiable soils from the proposed stress–strain model, based on the scaling of stress and strain into compatible soil reaction p and pile deflection y, respectively. The scaling factors for stress and strain are computed following an energy-based approach that is analogous to the upper-bound method used in classical plasticity theory. To validate the proposed p–y curves, results from a series of centrifuge tests are employed to back-calculate p–y curves for liquefied soils. The latter are compared with those obtained from the proposed method and the conventional p-multiplier approach.

Elastic and inelastic deformation of fluid-saturated rock.

In situ rock is often saturated with fluid, the presence of which affects both elastic parameters and inelastic deformation processes. Techniques were developed for testing fluid-saturated porous rock under the limiting conditions of drained (long-term), undrained (short-term) and unjacketed (solid matrix) response in hydrostatic, axisymmetric and plane-strain compression. Drained and undrained poroelastic parameters, including bulk modulus, Biot and Skempton coefficients, of Berea sandstone were found to be stress dependent up to 35 MPa mean stress, and approximately constant at higher levels of loading. The unjacketed bulk modulus was measured to be constant for pressure up to 60 MPa, and it appears to be larger than the unjacketed pore bulk modulus. An elasto-plastic constitutive model calibrated with parameters from drained tests provided a first-order approximation of undrained inelastic deformation: dilatant hardening was observed due to pore pressure decrease during inelastic deformation of rock specimens with constant fluid content.This article is part of the themed issue 'Energy and the subsurface'.

Comment on “A Historical Review of Induced Earthquakes in Texas” by Cliff Frohlich, Heather DeShon, Brian Stump, Chris Hayward, Matt Hornbach, and Jacob I. Walter

Researchers (Frohlich et al. , 2016) from the University of Texas at Austin and Southern Methodist University recently developed a new question‐based system for categorizing Texas earthquakes as either natural or man‐made. Unfortunately, this new test is a significant step backward. Frohlich et al. (2016) appears to reject the science‐based, integrated subsurface engineering and geomechanics approach as advocated by more than 80 national and international subject matter experts. The test also runs counter to the approach recommended by the National Academies and the U.S. Environmental Protection Agency (EPA), and it arbitrarily skews the conclusion away from natural causes. This article will explore these shortcomings and provide some recommendations for improvements. A question‐based system, where researchers answer conditional questions to determine if an earthquake may be man‐made, is a useful tool for exploring an inherently complicated subject. An earlier question‐based system developed by Davis and Frohlich (1993) focused on downhole pressure and fluid pathways: The premier U.S. scientific organization has also articulated the importance of subsurface factors. In its comprehensive 2013 report, the National Research Council of the National Academies concluded: Increase of pore pressure above ambient value due to injection of fluids and decrease in pore pressure below ambient value due to extraction of fluids have the potential to produce seismic events. For such activities to cause these events, a certain combination of conditions has to exist simultaneously …

Subsidence: Causes, Effects, and Mitigations in Geothermal Field

Subsidence is the motion of the ground surface as it moves down relatively. It can occur in a wide range of area. It is an impact of production of large mass and volume of saturation from the reservoir. It doesn't happen especially in geothermal fields only, but also in oil and gas industry. Large fluid volume production leads to the decrease of pore pressure inside reservoir. This decline disturbs the pressure stability and overburden pressure compress the pores. It results the drop in ground surface. The decrease in ground surface level induces a devastating effect in the construction of some facilities, such as building, pipeline, canal, and river. It may interrupt the balance in the ecosystems nearby. Good management and several survey methods (such as levelling and gravity) will reduce the risk of subsidence and the other effects related to it. This discussion output can be used as a guide for minimizing subsidence impact in the geothermal field in general.

An investigation of pore water pressure and consolidation phenomenon in the unfrozen zone during soil freezing

Accurate modeling of soil deformation depends on the consideration of both frost heave deformation in the frozen zone and consolidation deformation in the unfrozen zone. Pore water pressure investigations are important for revealing these two deformation behaviors. Herein, we aim to reveal the consolidation process by measuring and analyzing changes in pore water pressure in the unfrozen zone. Using a custom-made pore water pressure gauge, we performed a series of real-time pore water pressure measurements in the unfrozen zone of silty clay and sandy soil samples that were exposed to closed and open freezing systems. The results show that the pore water pressure in the unfrozen zone generally increases at first and then decreases. The temperature changes in the unfrozen zone have no significant influence on the changes in pore water pressure, and the variations of the pore water pressure are mainly controlled by the stress and hydraulic boundary (changes in pore water pressure) conditions at the freezing front. Furthermore, changes in the pore water pressure are affected by several parameters, including soil type, water supply condition, initial moisture content, measured soil layer depth, and hydraulic conductivity. Soil consolidation is mainly caused by change in effective stress, which results from increase in total stress or decrease in pore pressure. Based on the observed pore water pressure variations and its numerical simulations, we propose that consolidation in the unfrozen zone during soil freezing includes compression-induced consolidation, which results from an increase in frost heaving stress, and vacuum-induced consolidation, which results from a decrease in pore water pressure. Each consolidation pattern plays an important role in the different stages of soil freezing. Compression-induced consolidation primarily occurs during the early stage of soil freezing, while vacuum-induced consolidation mainly occurs during the later stage of soil freezing.

A two-phase two-layer model for fluidized granular flows with dilatancy effects

We propose a two-phase two-thin-layer model for fluidized debris flows that takes into account dilatancy effects, based on the closure relation proposed by Roux & Radjai (Physics of Dry Granular Media, 1998, Springer, pp. 229–236). This relation implies that the occurrence of dilation or contraction of the granular material depends on whether the solid volume fraction is respectively higher or lower than a critical value. When dilation occurs, the fluid is sucked into the granular material, the pore pressure decreases and the friction force on the granular phase increases. On the contrary, in the case of contraction, the fluid is expelled from the mixture, the pore pressure increases and the friction force diminishes. To account for this transfer of fluid into and out of the mixture, a two-layer model is proposed with a fluid layer on top of the two-phase mixture layer. Mass and momentum conservation are satisfied for the two phases, and mass and momentum are transferred between the two layers. A thin-layer approximation is used to derive average equations, with accurate asymptotic expansions. Special attention is paid to the drag friction terms that are responsible for the transfer of momentum between the two phases and for the appearance of an excess pore pressure with respect to the hydrostatic pressure. For an appropriate form of dilatancy law we obtain a depth-averaged model with a dissipative energy balance in accordance with the corresponding three-dimensional initial system.

Prediction of Subsidence Over Oil and Gas Fields with Use of Influence Functions (Case Study: South Pars Gas Field, Iran)

Withdrawal of oil and gas from reservoirs causes a decrease in pore pressure and an increase in the effective stress which leads to reservoir compaction. Reservoir compaction can be result in surface subsidence through the elastic response of the subsurface layers. To determine the subsidence above a hydrocarbon field, the reservoir compaction must first be calculated, and then, the effect of the reservoir compaction on the surface is modeled. The use of the uniaxial compaction theory is a more customary and accepted method for determining the amount of reservoir compaction induced by pressure depletion. Surface deformation above compacting reservoirs can be efficiently modeled using influence functions. In this paper, Knothe and Geertsma influence functions were used in a simple approach for subsidence modeling of the South Pars gas field. In this approach, we used circular network, called influence network, for determining the whole reservoir compaction effect on the field surface. In addition, the subsidence of the South Pars gas field is modeled using the AEsubs software, and the results are compared to the predicted subsidence by the influence functions.

Effect of gas bubbles on pore pressure response in peat beneath a railway embankment

This paper presents the pore pressure behaviour observed within peat beneath a newly constructed railway embankment. Piezometers installed at different depths beneath the structure showed episodic increases in pore pressure above hydrostatic pressure followed by a rapid pore pressure decrease. It was postulated that this fluctuation in pore pressure was due to the movement and expulsion of gases within the peat. Investigations were conducted in the laboratory to replicate this pore pressure response. The peat specimens were subjected to isotropic consolidation, within a triaxial cell, at constant confining pressure while measuring the change in the specimen’s pore pressure and volume. The results of this investigation indicate that the gas bubbles move through the channeled network of voids towards the drainage boundary where they restrict the flow of water, which results in the increase of pore pressure towards an upper limit or escape pressure, ultimately causing an expulsion of gas bubbles. This expulsion of bubbles results in a rapid drop in pore pressure and volume change within the specimen. The implications of the volume change associated with this pore pressure behaviour are discussed.

Study on the damage mechanics and dewatering recovery programs for shield tunnel

A case is presented about damages due to leakage and inrushing water that developed in the segmented precasted concrete liner of a shield tunnel under the Yangtze River. Various possible causes were considered to explain the damage mechanism. The opening and distribution due to differential settlement caused by the leakage of biogas or water in the shield tunnel were analyzed. The damage mechanism of the aquifuge, which occurred under the tunnel through exertion of a high piezometric head in the lower confined aquifer, was analyzed. The tensile-shear damage mechanism of the aquifuge was studied, and the judgment formulas, which were used to determine whether surface water bursting occurred, were derived. Two dewatering recovery programs were simulated by the finite difference method (FDM) in conjunction with MODFLOW software. From the simulation results, the hydraulic gradient and settlements caused by decreased pore pressure were calculated. The optimal dewatering scheme and protection measurements are suggested.

Closing an open system: Pore pressure changes in permeable edifice rock at high strain rates

A permeable or open system will react as a closed system if the rocks implicated are deformed on a timescale that precludes fluid movement. Closed system (“undrained”) deformation therefore leads to a failure mode dependent change in pore pressure: microcracking (dilatant behaviour) and cataclastic pore collapse (compactant behaviour) will decrease and increase pore pressure, respectively. In the dilatant regime (i.e., in the shallow edifice, <1km depth), a decrease in pore pressure will serve to strengthen rock—a process termed dilatancy hardening. However, it is shown here, using undrained triaxial deformation experiments, that the high initial porosity and microcrack density of typical edifice-forming andesites prevent dilatancy hardening. This allows the rock proximal to the magma-filled conduit in the shallow edifice to remain weak during periods of unrest when high magma strain rates could be transferred to the adjacent country rock. Although the propensity for fracturing will likely reduce the structural integrity of the edifice, fracturing of the shallow edifice may improve the outgassing efficiency of the nearby magma-filled conduit. The increase in pore pressure during undrained deformation in the compactant regime (i.e., in the deep edifice, >1km depth) could lead to pore pressure embrittlement and fracturing. Indeed, the experiments of this study show that the pore pressure increases during progressive compaction in a closed system. However, the pore pressure is prevented from reaching the critical value required to promote a dilatant response (i.e., fracturing) for two reasons. First, the rate of compaction (i.e., porosity decrease) slows as the sample is deformed at a constant strain rate, a consequence of the decay in effective pressure. Second, the emergence of microcracking as the rock approaches the compactant–dilatant transition acts as a negative feedback and prevents the rock from transiting into the dilatant field. At this point, local porosity increases due to dilatant microcracking and local porosity decreases due to cataclastic pore collapse are balanced and the rock deforms without further changes to porosity or pore pressure. This will prevent potentially destabilising brittle failure deeper in the edifice during the high strain rates that may accompany unrest and, although it precludes the formation of efficient outgassing pathways in the form of fractures, undrained deformation in the compactant regime will prevent a reduction in porosity and permeability and may therefore facilitate lateral outgassing of the conduit into the country rock. We assess the conditions (strain rate and permeability) required for drained or undrained deformation by defining a dimensionless Darcy number. Closed system or undrained deformation is likely commonplace within a volcano (strain rates in the rock adjacent to an active volcanic system can be high and textural heterogeneities can serve as barriers to fluid flow) and therefore forms an important component for a complete understanding of the mechanical response of an edifice to the stress perturbations accompanying unrest.

A new approach for measuring the permeability of shale featuring adsorption and ultra-low permeability

Based on the critical issues associated with shale permeability measurements, a new experimental approach by modifying the traditional pressure-pulse decay method was developed. In order to reduce the measurement error caused by the pore volumes of the traditional method, we made a new design that the upstream and downstream reservoir volumes can be changed, In addition, we added a by-pass pipe to measure the bidirectional permeability instantaneously, which can reduce the total test time significantly. Except for nitrogen or helium, methane was utilized to measure the shale permeability in this work, which can be more practical and better understand the real gas transport mechanisms in shale. Furthermore, we modified the conventional interpretation model of permeability measurements by incorporating the physical mechanism of gas adsorption. We also performed a series of experimental measurements and data analyses using different cores from pure shale, sand shale, to sandstone, which are from the Ordos basin (Chang 7 section) in China. The results show that: (1) the error caused by the pore volume errors of the traditional method is decreased by nearly half if variable reservoir volumes are used. The total test time is reduced by around 7 h by adding the by-pass pipe on the apparatus. The value of permeability measured with methane is higher than that measured with nitrogen while lower than that measured with helium. (2) The effective gas adsorption porosity increases with the increasing Langmuir pressure and decreasing pore pressure. If without considering gas adsorption, the measured permeability value will be underestimated, especially under lower pore pressure, higher adsorption capacity, and higher Langmuir pressure. (3) The total error is less than 10% using this new apparatus and the modified permeability interpretation method. The measured permeability values are reliable by comparing the measurements using the new apparatus and the standard instrument of ProPDP-200 under the same condition. (4) The influence of gas adsorption on permeability measurement in shale cannot be ignored, and the permeability is underestimated by up to 97% in pure shale while by only 7.5% in sandstone if the gas adsorption is not taken into account.

Analysis of the variation of the compressibility index (Cc) of volcanic clays and its application to estimate subsidence in lacustrine areas

Abstract. An analysis of the deformation conditions of lacustrine materials deposited at three sites in the volcanic valley of the Mexico City is presented. Currently geotechnical studies assume that compressibility of granular materials decreases in depth due to the lithostatic load. That means that the deeper the sample the more rigid is supposed to be, this assumption should be demonstrated by a decreased Compression Index (Cc) in depth. Studies indicate that Mexico City clays exhibit brittle behaviour, and have high water content, low shear strength and variable Cc values. Furthermore, groundwater withdrawal below the city causes a differential decrease in pore pressure, which is related to the physical properties of granular materials (hydraulic conductivity, grain size distribution) and conditions of formation. Our results show that Cc for fine grain materials (lacustrine) can be vertically variable, particularly when soils and sediments are the product of different volcanic materials. Lateral and vertical variations in the distribution of the fluvio-lacustrine materials, especially in basins with recent volcanic activity, may be assessed by Cc index variations. These variations can also be related to differential deformation, nucleation and propagation of fractures and need to be considered when modelling land subsidence.

FEM analysis of dynamic behavior of asphalt pavement structure weakened by grassroots with account of hydraulic and vehicle load coupling effects

This paper establishes the finite element model of saturated asphalt pavement with ANSYS. The thickness and elastic modulus of pavement surface layers and grassroots are taken as influencing factors to analyze the influence of these factors’ change on the pavement mechanical behavior. The results indicate that the increase of surface thickness leads to the shear stress's decrease that may relieve the internal shear stress concentrated phenomenon effectively. The increase of elastic modulus of surface makes the pore pressure decrease, and result in the shear stress's increase. The grassroots thickness has no obvious effect on the pore- water pressure and shear stress; the grassroots elastic modulus’ increase may increase the internal shear stress and decrease the pore pressure.

Poromechanics of stick‐slip frictional sliding and strength recovery on tectonic faults

AbstractPore fluids influence many aspects of tectonic faulting including frictional strength aseismic creep and effective stress during the seismic cycle. However, the role of pore fluid pressure during earthquake nucleation and dynamic rupture remains poorly understood. Here we report on the evolution of pore fluid pressure and porosity during laboratory stick‐slip events as an analog for the seismic cycle. We sheared layers of simulated fault gouge consisting of glass beads in a double‐direct shear configuration under true triaxial stresses using drained and undrained fluid conditions and effective normal stress of 5–10 MPa. Shear stress was applied via a constant displacement rate, which we varied in velocity step tests from 0.1 to 30 µm/s. We observe net pore pressure increases, or compaction, during dynamic failure and pore pressure decreases, or dilation, during the interseismic period, depending on fluid boundary conditions. In some cases, a brief period of dilation is attendant with the onset of dynamic stick slip. Our data show that time‐dependent strengthening and dynamic stress drop increase with effective normal stress and vary with fluid conditions. For undrained conditions, dilation and preseismic slip are directly related to pore fluid depressurization; they increase with effective normal stress and recurrence time. Microstructural observations confirm the role of water‐activated contact growth and shear‐driven elastoplastic processes at grain junctions. Our results indicate that physicochemical processes acting at grain junctions together with fluid pressure changes dictate stick‐slip stress drop and interseismic creep rates and thus play a key role in earthquake nucleation and rupture propagation.

Pore-scale modeling and analysis of surface diffusion effects on shale-gas flow in Kerogen pores

This paper investigates the influences of surface diffusion on shale-gas transport through our proposed model. The model simulates multiple flow mechanisms in Kerogen pores considering surface diffusion, its impact on slippage velocity, and nonlinear assembly of viscous flow and Knudsen diffusion contribution. Further, an improved gas slippage model was developed considering the additional viscous flux produced by surface diffusion. A sensitivity analysis was carried out using real Kerogen properties to identify the contributions of surface diffusion to gas permeability. The results show that surface diffusion gradually becomes a primary contributor to gas production with decreasing pore pressure and pore diameter and increasing surface diffusivity and adsorbed capacity of Kerogen. In particular, surface diffusivity and pore diameter are the two most important factors for surface diffusion. However, the importance of surface diffusion is only valid in the cases satisfying certain conditions. In this study, the influence of surface diffusion becomes strong and must be accounted for when the surface diffusivity exceeds 1E-7 m2/s and the pore diameter is less than 5 nm.

Influence of gas transport mechanisms on the productivity of multi-stage fractured horizontal wells in shale gas reservoirs

In order to investigate the influence on shale gas well productivity caused by gas transport in nanometer-size pores, a mathematical model of multi-stage fractured horizontal wells in shale gas reservoirs is built, which considers the influence of viscous flow, Knudsen diffusion, surface diffusion, and adsorption layer thickness. A discrete-fracture model is used to simplify the fracture modeling, and a finite element method is applied to solve the model. The numerical simulation results indicate that with a decrease in the intrinsic matrix permeability, Knudsen diffusion and surface diffusion contributions to production become large and cannot be ignored. The existence of an adsorption layer on the nanopore surfaces reduces the effective pore radius and the effective porosity, resulting in low production from fractured horizontal wells. With a decrease in the pore radius, considering the adsorption layer, the production reduction rate increases. When the pore radius is less than 10 nm, because of the combined impacts of Knudsen diffusion, surface diffusion, and adsorption layers, the production of multi-stage fractured horizontal wells increases with a decrease in the pore pressure. When the pore pressure is lower than 30 MPa, the rate of production increase becomes larger with a decrease in pore pressure.