Effect Of Pore Pressure Research Articles

Modeling study of sandstone permeability under true triaxial stress based on backpropagation neural network, genetic programming, and multiple regression analysis

Permeability evolution of sandstone is of great significance in the development of tight sandstone gas reservoirs. Traditional laboratory tests have the disadvantages of high cost and long testing time. Therefore, the present study employed use artificial intelligence systems, i.e., backpropagation neural network (BPNN), genetic programming (GP), and multiple regression analysis to construct prediction models of sandstone permeability based on the coupling effect of true triaxial stress field and pore pressure. The results showed that the permeability prediction obtained from the systems fit well with the experimental data, and evidenced that permeability increased with pore pressure and decreased with increase in principal stress. Sensitivity analysis showed that the pore pressure has the greatest influence on sandstone permeability under different true triaxial stress. The effect of anisotropic principal stress on permeability exhibited σ1 > σ2 > σ3 under fixed pore pressure. Further assessment based on a combination of five evaluation indexes showed that the prediction accuracy of the BPNN model was better.

A permeability model for anisotropic coal masses under different stress conditions

The heterogeneous and anisotropic nature of coal can have an important impact on its permeability. Coal permeability is usually directly related to the stress and hence, it is of great significance to understand the evolution of anisotropic coal permeability under different stress conditions. In this work, based on the stress-strain relationship of coal and the interaction between coal fractures and matrix, an anisotropic permeability model was constructed considering different stress conditions. The proposed model is further improved by considering the combined influence of the effective stress, pore pressure and slippage effect. The proposed permeability model is verified using available experimental data from previously published studies, which has shown close agreement with the experimental results in predicting coal permeability under different stress conditions. Cleat compressibility exhibited anisotropy and was dependent on the orientation of gas flow. The variation rule between coal permeability and effective stress was determined to be in accordance with a negative exponential function. The model clearly illustrates strong anisotropy in both magnitude and variation of directional permeability.

Development of an Electrical Resistivity Tomography Monitoring Concept for the Svelvik CO<sub>2</sub> Field Lab, Norway

Abstract. Within the ERA-NET co-funded ACT project Pre-ACT (Pressure control and conformance management for safe and efficient CO2 storage – Accelerating CCS Technologies), a monitoring concept was established to distinguish between CO2 induced saturation and pore pressure effects. As part of this monitoring concept, geoelectrical cross-hole surveys have been designed and conducted at the Svelvik CO2 Field Lab, located on the Svelvik ridge at the outlet of the Drammensfjord in Norway. The Svelvik CO2 Field Lab has been established in summer 2019, and comprises four newly drilled, 100 m deep monitoring wells, surrounding an existing well used for water and CO2 injection. Each monitoring well was equipped with modern sensing systems including five types of fiber-optic cables, conventional- and capillary pressure monitoring systems, as well as electrode arrays for Electrical Resistivity Tomography (ERT) surveys. With a total of 64 electrodes (16 each per monitoring well), a large number of measurement configurations for the ERT imaging is possible, requiring the performance of the tomography to be investigated beforehand by numerical studies. We combine the free and open-source geophysical modeling library pyGIMLi with Eclipse reservoir modeling to simulate the expected behavior of all cross-well electrode configurations during the CO2 injection experiment. Simulated CO2 saturations are converted to changes in electrical resistivity using Archie's Law. Using a finely meshed resistivity model, we simulate the response of all possible measurement configurations, where always two electrodes are located in two corresponding wells. We select suitable sets of configurations based on different criteria, i.e. the ratio between the measured change in apparent resistivity in relation to the geometric factor and the maximum sensitivity in the target area. The individually selected measurement configurations are tested by inverting the synthetic ERT data on a second coarser mesh. The pre-experimental, numerical results show adequate resolution of the CO2 plume. Since less CO2 was injected during the field experiment than originally modeled, we perform post-experimental tests of the selected configurations for their potential to image the CO2 plume using revised reservoir models and injection volumes. These tests show that detecting the small amount of injected CO2 will likely not be feasible.

A state-dependent hypoplastic model for methane hydrate-bearing sands

The mechanical behaviors of methane hydrate-bearing sands (MHBS) are largely affected by the presence of methane hydrate, temperature, and pore pressure. In this study, we present a simple hypoplastic model for MHBS. Methane hydrate saturation is included as a state parameter affecting the mechanical behaviors of MHBS. A new phase parameter is introduced to account for the coupled effects of temperature and pore pressure on the mechanical behaviors of MHBS. The phase parameter can be determined by a simple function of temperature and pore pressure. Comparison of the predictions with experiments shows that the model is able to capture the salient behaviors of MHBS.

Study on Effects of Low Viscosity Fluid Pre-injection on Hydraulic Fracture Complexity in Tight Heterogeneous Glutenite Reservoirs

Tight heterogeneous glutenite reservoir is typically not easy to form complex hydraulic fracture (HF) due to its poor physical properties, poor matrix seepage capacity, and small limit discharge radius and undeveloped natural fracture system. To improve the HF complexity and the stimulated reservoir volume (SRV), a novel stimulation technology called CO2 miscible fracturing has been introduced and its fracturing mechanism has been studied. The CO2 miscible fracturing modifies the in situ stress field by injecting low viscosity fluid to increase the HF complexity and SRV. Therefore, a series of numerical simulations based on a hydro-mechanical-damage model were carried out to study the effects of low viscosity fluid pre-injection on pore pressure, stress field, and fracturing effect in tight heterogeneous glutenite reservoirs. The results indicate that the low viscosity fluid injection can effectively increase the pore pressure around the wellbore and reduce the effective stress of the glutenite. The FCI and SRV increase with the increase of the pre-injection amount of the low viscosity fluid. The HF complexity and SRV can be improved by pre-injecting low viscosity fluid to transform the in situ stress field. The field application of this technology in a well of Shengli Oilfield showed that low-viscosity fluid pre-injection can effectively increase the width of the fractured zone, improve the SRV, and optimize the fracturing effect.

Numerical modelling of hydraulic fracture propagation in poro-viscoelastic formation

Modelling of hydraulic fracturing often deals with elastic deformation of rocks and fractures. Some rocks, such as the rock-salt and shale, however, displayed viscoelastic behaviour in both field investigation and lab creep tests. This time-dependent deformation poses a challenge in modelling and characterising of fracture propagation in such formations, thus adding complexity in a multi-physics scenario. In this paper, we intend to numerically model fracture propagation in viscoelastic formation and investigate the effect of creep on crack propagation and crack geometry. To incorporate pore pressure effect, we couple fluid diffusion with shale matrix viscoelasticity in the hydraulic fracturing propagation model. Therefore, a procedure for numerical modelling hydraulically-driven fracture propagation in poro-viscoelastic formation is developed. The method uses a poro-viscoelasticity theory to describe the fluid diffusion and matrix creep in solid skeleton, in conjunction with cohesive law based damage criteria to simulate crack growth and fluid flow in the crack. The modelling took shale as the representative sample. The viscoelastic properties were obtained via creep tests of shale samples from Roseneath formation, Cooper Basin, Australia. Results show that the hydraulically induced fracture tends to be wider and longer in creep formation. What's more, the propagation pressure is also lower than that in poroelastic formation. Fluid diffusion from crack to the matrix is more efficient due to modulus decay. For mixed-mode fracturing, the creep of the formation facilitates the process of reorientation with higher fracture propagation speed between kink points.

Experimental investigation of rate dependency of freeze bond strength

The effects of deformation rate on the shear strength of freeze bonds have been investigated through a series of Asymmetric Four-Point Bending (AFPB) experiments. Two submersion times of 30 min and 24 h have been considered, while the deformation rate has been varied from 0.01 to 100 mm/s. Ice specimens with an initial temperature of −18 °C have been subjected to a constant confinement of 25 kPa during formation and shearing of the freeze bonds. Failure of the bonds proved to be highly dependent on deformation rate, where more strain was observed prior to failure of the bond for slower deformation rates. The shear strength of the bond for both submersion times showed a general decreasing trend with increased deformation rate, which is of significant importance in characterizing the properties of ice ridge/rubble strength. Previous ice rubble strength experiments, as well as ice ridge keel gouging tests, have reported the same failure rate-dependency, highlighting the role of freeze bond failure on rate effects. Compressive strength measurements of solid ice presented in the literature have been observed to follow similar trends within the range of strain rates used in present study, where the failure mode appears to transition from ductile to brittle with increasing deformation rate. While more work is needed to understand the mechanisms associated with the rate dependency of freeze bond, this general observation suggests the rate dependency of ice ridges/rubble may indeed be related to the behavior of ice as a material, rather than being associated primarily with factors such as inertial and pore pressure effects, which have been previously proposed as the principal controlling mechanism.

Thermo-poroelastic wellbore strengthening modeling: An analytical approach based on fracture mechanics

One of the most important problems encountered during drilling operation is lost circulation through the natural/induced fractures, which increases the operational time and costs. Wellbore strengthening (WBS) by plugging the fractures using lost control materials (LCM) is an effective strategy to overcome this problem. Various analytical studies have been previously conducted to model this process. Although the formation pore pressure and its temperature variations have been identified as influential factors in wellbore stability evaluations, in the majority of previous models, the effect of temperature and poroelastic properties have not been taken into account. In this paper, a thermo-poroelastic analytical model has been presented based on fracture mechanics to add the stresses resulting from the changes in pore pressure and fluid temperature to the previously analytical elastic equations. According to the obtained results, wellbore cooling due to the circulation of drilling fluid with a temperature less than the formation temperature produces tensile stresses around the wellbore and increases the probability of the growth of pre-existing fractures or initiation of new ones. On the other hand, considering the effect of formation pore pressure can significantly reduce the stress intensity factor (SIF) level at the fracture tip. Comparison of the outputs of the thermo-poroelastic model with those of the elastic ones shows that ignoring the impacts of temperature and pore pressure during modeling can result in parameter underestimation or overestimation. In addition, the results of a comprehensive sensitivity analysis indicate that geomechanical parameters (in situ stress anisotropy, Young's modulus, and Poisson's ratio), geometrical parameters (location of LCM plug and fracture length), and poroelastic parameters (Biot's coefficient) considerably affect the success of WBS approach and the final fracture pressure.

Modelling the drying shrinkage of porous materials by considering both capillary and adsorption effects

This paper presents a poromechanical model for drying of unsaturated porous media valid for a large range of relative humidity. Using the proper laws of thermodynamics, this model is derived and permits to account for different effects that contribute to the effective stress development: the average pore pressure effect, the energy of the interfaces effect, the surface adsorption effect and the Shuttleworth effect. The majority of the input parameters of this model are simply assessed by using two commonly known techniques for the characterization of pores structures applied on experimental desorption isotherms: the B.E.T theory (Brunauer et al., 1938) and the BJH technique (Barrett et al., 1951). Another input parameter (linked to the Shuttleworth effect) is fitted on experimental drying shrinkage strains. This model is tested and validated with experimental data for different porous materials - hardened ordinary cement paste, high- performance concrete and Vycor glass - found in the literature. The obtained results show a satisfactory evaluation of the drying shrinkage strains for all three tested materials, with the possibility of considering zero fitting parameter. Compared to other poromechanical models found in the literature such as the classical Biot-Bishop (Biot, 1941) model and the (Coussy et al., 2003) model, our model appears to be capable of displaying the transition at a certain relative humidity between the capillary pressure effects and the surface adsorption effect, which manifests itself by a plateau in the drying shrinkage strains curve at this value of relative humidity.

Effects of Cyclic Variations of Pore Pressure on the Behaviour of a Gneiss Residual Soil

Many slope failures in residual soils have been attributed to increased soil pore pressure. During the rainy season, water infiltration occurs, which can lead the slope to rupture by increasing the pore pressure. However, some landslides have been observed with pore pressures below those required for failure. Hypotheses for such ruptures include the occurrence of creep or fatigue due to cyclic variations in the piezometric level on the slope. This paper presents a study on the effect of pore pressure on the behaviour of the residual soil of the Soberbo hillside in Rio de Janeiro, Brazil, subjected to monotonic and cyclic pore pressure. Triaxial tests that simulate the increasing piezometric level (constant shear drained—CSD tests), drained creep (DC-CSD tests) and cyclic pore pressure variation (CPP tests) in slopes were carried out. The CSD test results showed that the failure due to increasing pore pressure is essentially dilatant and confirmed the peak strength obtained from conventional drained triaxial tests. Sudden failure was observed in all CSD tests at the end of the pore pressure increase phase. The results show that the failure of the slope may occur due to drained creep and the loss of soil shear strength under cycling pore pressure.

The role of water lubrication in critical state fault slip

The conventional driving mechanisms including elevating the pore pressure or far-reaching poroelastic effects, may not necessarily explain the injection-triggered seismicity that can be up to 35–90 km far away from injection wells. In this experimental study, we examined the mechanism of water lubrication for the triggered seismicity where the fluid pressure was negligible compared to the in-situ stresses. The results show that fault slip can occur upon saturation when the ratio of the applied shear stress to the fault peak shear strength exceeds a critical threshold, which is defined to quantitatively assess the critical stress state for faults. The microscale slip test using atomic force microscopy reveals the mechanism responsible for the significant reduction of friction between granite surfaces from dry to wet state. A nanometer-thick water layer was attached on the granite surfaces, causing the shear plane shifting from the direct granite/granite interface to the water layer, and meanwhile the adhesive force between granite surfaces was significantly reduced by water intercalation, both of which played the dominate role in triggering fault slip. In practice, slip may be associated with in-situ fluid flow into far-field faults, which causes the increase of water and lubricates the fault zone.

Tight gas production model considering TPG as a function of pore pressure, permeability and water saturation

Threshold pressure gradient has great importance in efficient tight gas field development as well as for research and laboratory experiments. This experimental study is carried out to investigate the threshold pressure gradient in detail. Experiments are carried out with and without back pressure so that the effect of pore pressure on threshold pressure gradient may be observed. The trend of increasing or decreasing the threshold pressure gradient is totally opposite in the cases of considering and not considering the pore pressure. The results demonstrate that the pore pressure of tight gas reservoirs has great influence on threshold pressure gradient. The effects of other parameters like permeability and water saturation, in the presence of pore pressure, on threshold pressure gradient are also examined which show that the threshold pressure gradient increases with either a decrease in permeability or an increase in water saturation. Two new correlations of threshold pressure gradient on the basis of pore pressure and permeability, and pore pressure and water saturation, are also introduced. Based on these equations, new models for tight gas production are proposed. The gas slip correction factor is also considered during derivation of this proposed tight gas production models. Inflow performance relationship curves based on these proposed models show that production rates and absolute open flow potential are always be overestimated while ignoring the threshold pressure gradients.

Molecular Insight into Microbehaviors of n-Decane and CO2 in Mineral Nanopores

Nanopores are abundant in unconventional reservoirs. Understanding the microbehaviors of oil confined at such a small scale is favorable to the enhancement of oil recovery. In this study, molecular dynamics simulations were performed to investigate the adsorption behaviors and structural properties of n-decane + CO₂ mixtures confined within quartz, calcite, and clay nanopores. First, comparing the peak of n-decane density distribution in the first adsorbed layer and the interactions between rock surfaces and n-decane, we found that the adsorption strength of n-decane on various minerals follows the order calcite > kaolinite > montmorillonite > quartz > illite. Second, the distributions of n-decane on different mineral surfaces were analyzed, revealing that the strong adsorption of calcite is due to the strong interactions between Ca atoms in calcite and n-decane, and the flexible H atoms on the surfaces of quartz and kaolinite make n-decane distribute evenly. Then, the effects of pore size, filling fraction and pressure on the structural properties of n-decane were explored. Results show that the adsorbed layers remained unchanged when the pore size is big enough for n-decane (∼3 nm) to form a bulk region, while the peak in the adsorbed layers and the number of adsorbed layers vary with the pore size when the pore size becomes smaller. The reduction of alkane filling fraction decreases the density of n-decane in the region far from the wall in advance, and then the density in the adsorbed layers. The density of n-decane in the adsorbed layer decreases greatly with the increase of pressure (by adding CO₂), showing good oil displacement by CO₂. This work provides new molecular insights to CO₂ enhanced oil recovery.

Multi-Scale Insights on the Threshold Pressure Gradient in Low-Permeability Porous Media

Low-permeability porous medium usually has asymmetric distributions of pore sizes and pore-throat tortuosity, thus has a non-linear flow behavior with an initial pressure gradient observed in experiments. A threshold pressure gradient (TPG) has been proposed as a crucial parameter to describe this non-linear flow behavior. However, the determination of this TPG is still unclear. This study provides multi-scale insights on the TPG in low-permeability porous media. First, a semi-empirical formula of TPG was proposed based on a macroscopic relationship with permeability, water saturation, and pore pressure, and verified by three sets of experimental data. Second, a fractal model of capillary tubes was developed to link this TPG formula with structural parameters of porous media (pore-size distribution fractal dimension and tortuosity fractal dimension), residual water saturation, and capillary pressure. The effect of pore structure complexity on the TPG is explicitly derived. It is found that the effects of water saturation and pore pressure on the TPG follow an exponential function and the TPG is a linear function of yield stress. These effects are also spatially asymmetric. Complex pore structures significantly affect the TPG only in the range of low porosity, but water saturation and yield stress have effects on a wider range of porosity. These results are meaningful to the understanding of non-linear flow mechanism in low-permeability reservoirs.

Pore pressure when compacting two-phase soil

In modern conditions, in relation to the total amount of accumulated professional knowledge, the volume of active information resources increases, and construction practice is constantly enriched with new experimentally-theoretically based accurate knowledge. Therefore, various programs are widely used when solving problems in ground bases. Despite the fact that modern numerical methods allow solving problems of any complexity, it should be noted that experimental and analytical methods are still relevant, and it is an effective combination of methods that leads to the development of mechanics, an organic combination of experimental research methods with the enormous computational capabilities of modern supercomputers. Methods for modeling the action of bulk forces are widely used in problems of deformable solid mechanics. Many known solutions have limitations and are given for special cases. The authors present the theoretical foundations of the elastic analogy method for modeling the effect of pore pressure on the soil. When posing the question, the following assumption is made that the liquid filling the pores of the soil does not perceive shear deformation. Tangential stresses that occur in the ground are only perceived by the ground skeleton. The water that fills the pores does not resist tangential stresses. In this case, the deformation of the soil skeleton from the action of hydrostatic water pressure, respectively, is a volumetric deformation. In this case, the components of the ball stress tensor are equal to the pore pressure. The article considers the use of the theory of volume forces in modeling the effect of pore pressure in the process of compaction of two-phase soil. In this case, we consider a one-dimensional problem for the case of deformation (compaction) of a two-phase soil layer thickness under the action of a distributed load of constant intensity. We believe that the consolidated layer lies on a rocky undeformable base. The authors consider various drainage conditions for the surfaces of the consolidated layer. Expressions are given for determining stresses and precipitation with creep, for the instantaneous stress-strain state and the final stress-strain state.

Analysis of the Abutment Movements of High Arch Dams due to Reservoir Impoundment

The deformation of mountains induced by reservoirs around high arch dams may influence the safety of those arch dams, but the underlying mechanisms of this phenomenon have not yet been elucidated. The abutment movements of the Jinping-I and Xiluodu arch dams during reservoir impoundment are analyzed systematically in this paper, and the analysis reveals that impoundment led to basin irreversible uplifts. The underlying mechanisms are analyzed from the viewpoint of the constitutive relation of rock mechanics. Based on the theories of poroelasticity and poroplasticity, the effective stress principle of rock is deduced considering the effect of high pore pressure. The modified nonlinear finite element program TFINE is used to simulate the abutment movements of the Jinping-I arch dam, and the calculated values are in accordance with the measured values, verifying the validity of the revised effective stress principle.

Analytical orientation criteria for drilling and completion-induced fracture initiation considering fluid infiltration from the wellbore

Analytically-developed criteria are presented for the orientation of fracture initiation in porous-permeable rocks considering fluid infiltration from non-perforated and perforated wellbores. For non-perforated wellbores, this is a fully-exact analytical solution representing the initiation of drilling-induced tensile fractures (DITFs). For perforated wells, a set of analytical approximations are derived for fracture initiation representing completion-induced hydraulic fractures (CIHFs). DITF orientation is used to constrain the in-situ stress tensor, especially in the case of transverse DITFs, which are rarer than longitudinal DITFs. Results indicate that transverse CIHF initiation only occurs over a wellbore pressure-at-breakdown window, while longitudinal CIHF initiation occurs at comparatively higher wellbore pressures. Transverse CIHF initiation however, occurs more frequently than transverse DITFs, because the presence of perforations aids transverse fracture initiation. Low Biot's poroelastic coefficient and high Poisson's ratio values also aid transverse fracture initiation. The region of the in-situ stress states where transverse initiation is promoted can be shown in dimensionless plots. Case studies are run on seven prolific shale plays (Barnett, Bakken, Fayetteville, Haynesville, Niobrara, Marcellus, and Vaca Muerta) using breakdown pressures from the literature. Transverse CIHF initiation is found to be a highly likely outcome, as opposed to results from past studies that neglected pore pressure effects. Based on the findings of this study, recommendations are made to practitioners to enhance their chances on achieving transverse CIHF initiation in the field. These involve slow injection rates, oriented perforation strategy and strong bonding between the casing, cement and the surrounding rock to prevent fracturing fluid leakages.

An elastoplastic damage constitutive model of concrete considering the effects of dehydration and pore pressure at high temperatures

Pore pressure and thermal dehydration of concrete are two major factors which affect the spalling of concrete at high temperatures. Thermal dehydration can cause deterioration of concrete properties and promote the development of micro cracks in the skeleton of concrete. Pore pressure will also be influenced by dehydration when the hydrated water enters the micro pores. In the current study, a framework of elastoplastic damage constitutive model is developed to described the mechanical behaviour of concrete and pore pressure at high temperatures. Both load induced damage and thermal dehydrated damage are considered. The thermal dehydrated damage is temperature related, which reflects the impact of temperature on concrete properties. A model of pore pressure is developed by considering the influence of thermal dehydration and damage related permeability. The pore pressure field associated with vapor, liquid water and dry air is predicted and compared with experimental results.

Micromechanical modelling of short- and long-term behavior of saturated quasi-brittle rocks

This paper presents a physically-motivated micromechanical damage model for describing short- and long-term behaviors of saturated quasi-brittle rocks under drained condition. Rocks are considered composed of a pores-weakened elastic solid matrix and distributed microcracks. The system free energy is determined with the Mori-Tanaka homogenization method. Two main dissipative mechanisms, inelastic deformation due to frictional sliding and damage by crack propagation, are involved and strongly coupled in both stress-loading phases and creep phases under constant stresses. The effect of pore pressure is taken into account in the damage-friction coupling context. The microcracks induced rock damage is divided into two parts: instantaneous damage related directly to stress-induced microcrack growth, and time-dependent damage caused by subcritical cracking. The short- and long-term strengths of saturated brittle rocks are analytically deduced by damage-friction coupling analysis. By using the long-term strength, the creep parameter is calibrated in terms of a critical stress state identified in experiment. Comparisons between model’s predictions and laboratory tests are performed for two typical quasi-brittle rocks under triaxial compression tests and triaxial compression creep tests. It is shown that the proposed model can capture the main features of the mechanical behavior of quasi-brittle materials.

Analysis of the critical pool level of partially submerged slopes

SummaryPartially submerged c′ − φ′ slopes with a horizontal water table exhibit a critical pool level at which the factor of safety becomes a minimum. The phenomenon was first identified using finite element methods, but in this paper, a more thorough analytical investigation is presented. The approach described herein assumes a rigid block sliding with a circular failure mechanism, combined with optimization software to identify the critical circle. The method is initially validated against known slope solutions that assume circular and log‐spiral mechanisms and shown to give excellent agreement, especially for flatter slopes. The method is then applied to partially submerged slopes with a focus on the critical pool level. Through detailed investigation of the overturning and restoring moments in the stability analyses, the critical pool level phenomenon is shown to lie in the trade‐off between the destabilizing effects of internal pore pressures on soil strength and the stabilizing effect of external hydrostatic water pressures on the slope surface.