Decrease In Pore Pressure Research Articles

Colloidal-induced permeability degradation assessment of porous media

The paper presents the methodology and results of coreflooding, taking into account the decrease in the permeability caused by the migration of natural colloids during fluid injection. As a porous medium, core samples of consolidated slightly clayey sandstone from one of the oil fields of the Komi Republic (Russia) were used. The coreflooding methodology implied a cyclic change in the injection rate and excluded the influence of other factors such as creep and chemical mobilisation of colloids on permeability. It has been established that a decrease in pore pressure leads to a more intense decrease in permeability during injection, which most likely indicates additional mobilisation of colloids. An analysis of the coreflooding results showed that porous media with lower permeability are more sensitive to colloid migration and pore-pressure changes.

Numerical analysis of consolidation behavior of soil-bentonite backfill in a full-scale slurry trench cutoff wall test

The stress state of the SB backfill in a full-scale cutoff wall test during the construction and consolidation was simulated by a finite-element model. The applicability of the model was demonstrated by good agreements between modeled and field monitored earth pressures and pore pressure in the backfill. Based on the analyzed results, the backfill consolidation process can be described as a four-staged cycle of load transfer: (1) the effective stress of the backfill increases due to the dissipation of excess pore pressure caused by self-weight consolidation of the backfill; (2) the increased effective stress results in settlement of the backfill and an increase of sidewall friction between the backfill and the sidewall interface; (3) the increased sidewall friction results in the transfer of backfill weight to the sidewalls and a decrease of the consolidation stress on the backfill; and (4) the decreased consolidation stress attenuates the decrease of excess pore pressure, influencing the subsequent consolidation. The cycle from (1) to (4) continues until the consolidation is completed.

Coupled THMC modeling of dissociation induced deformation of gas hydrate bearing media

Methane gas recovery from hydrate bearing sediment (HBS) by depressurization causes significant pore pressure decrease, loss of solid load bearing hydrate from pores and small temperature drop. The decrease in pore pressure increases the effective compressive stress arising from overburden load on the HBS and thus causes the settlement of the HBS. The reduction of medium strength by the loss of load bearing hydrate and thermal contraction also influences the deformation of HBS. We have developed a THMC solver to simulate the complex geomechanics problem along with hydrate phase change kinetics, non-isothermal multiphase flow in porous media, and the alteration of porosity, permeability and bearing capacity. The deformation behavior of the hydrate-bearing media depends on the host sediment and hydrate morphology, effective porosity and critical porosity. We have included the influence of loss of load bearing hydrate and compaction due to increase of effective compressive stress as the result of pore pressure decrease, fraction of hydrate morphology, effective porosity and critical porosity on the effective medium bulk and shear moduli using a modified effective medium model. The effects of hydrate saturation decrease and effective compressive pressure increase on the deformation of HBS are thoroughly investigated. We have performed numerical simulations to understand the deformation behavior for different porosities and matrix supporting hydrate fractions. Our results show that the deformation is about 20% less when the fraction of matrix supporting hydrate increases from 0.1 to 0.4 because of increased soil bonding.

Colloid Migration as a Reason for Porous Sandstone Permeability Degradation during Coreflooding

During coreflooding under stationary conditions, permeability is determined by the Darcy formula; thus, the apparent permeability is determined, which characterizes the average hydraulic conductivity of the core, but does not take into account the processes occurring at the microlevel. Transient processes during fluid injection regime change are important for understanding the evolution of permeability and underlie such phenomena as permeability degradation and its hysteresis. Our paper presents the coreflooding methodology and the results showing that during changes in injection conditions, the permeability of samples can significantly decrease due to the migration of colloids. In the developed methodology, coreflooding conditions exclude the influence of other factors on permeability, such as creep, the chemical reactions of rocks and fluids, and chemical mobilization of colloids. It has been established that the decrease in permeability occurs only when fluid is injected through the core, which is indirect evidence of pore blockage by colloids. The article also attempts to evaluate the effect of pore pressure on the sensitivity of the porous medium permeability to the amount of injected fluid. A decrease in pore pressure leads to a more intense decrease in permeability during injection, which most likely indicates additional mobilization of colloids, while the narrowing of pore channels does not affect permeability. Analysis of coreflooding results showed that porous media with lower permeability are more sensitive to colloid migration and pore pressure changes. It has also been found that the sensitivity of porous media to colloid migration is greatly affected by the pore pressure gradient, while media with higher permeability are less sensitive to colloid migration.

Multiphysics coupling study of near-wellbore and reservoir models in ultra-deep natural gas reservoirs

Understanding the changes of the near-wellbore pore pressure associated with the reservoir depletion is greatly significant for the development of ultra-deep natural gas reservoirs. However, there is still a great challenge for the fluid flow and geomechanics in the reservoir depletion. In this study, a fully coupled model was developed to simulate the near-wellbore and reservoir physics caused by pore pressure in ultra-deep natural gas reservoirs. The stress-dependent porosity and permeability models as well as geomechanics deformation induced by pore pressure were considered in this model, and the COMSOL Multiphysics was used to implement and solve the problem. The numerical model was validated by the reservoir depletion from Dabei gas field in China, and the effects of reservoir properties and production parameters on gas production, near-wellbore pore pressure and permeability evolution were discussed. The results show that the gas production rate increases nonlinearly with the increase in porosity, permeability and Young’s modulus. The lower reservoir porosity will result in the greater near-wellbore pore pressure and the larger rock deformation. The permeability changes have little effect on geomechanics deformation while it affects greatly the gas production rate in the reservoir depletion. With the increase in the gas production rate, the near-wellbore pore pressure and permeability decrease rapidly and tend to balance with time. The reservoir rocks with higher deformation capacity will cause the greater near-wellbore pore pressure.

A Model for the Apparent Gas Permeability of Shale Matrix Organic Nanopore Considering Multiple Physical Phenomena

The flow of shale gas in nano scale pores is affected by multiple physical phenomena. At present, the influence of multiple physical phenomena on the transport mechanism of gas in nano-pores is not clear, and a unified mathematical model to describe these multiple physical phenomena is still not available. In this paper, an apparent permeability model was established, after comprehensively considering three gas flow mechanisms in shale matrix organic pores, including viscous slippage Flow, Knudsen diffusion and surface diffusion of adsorbed gas, and real gas effect and confinement effect, and at the same time considering the effects of matrix shrinkage, stress sensitivity, adsorption layer thinning, confinement effect and real gas effect on pore radius. The contribution of three flow mechanisms to apparent permeability under different pore pressure and pore size is analyzed. The effects of adsorption layer thinning, stress sensitivity, matrix shrinkage effect, real gas effect and confinement effect on apparent permeability were also systematically analyzed. The results show that the apparent permeability first decreases and then increases with the decrease of pore pressure. With the decrease of pore pressure, matrix shrinkage, Knudsen diffusion, slippage effect and surface diffusion effect increase gradually. These four effects will not only make up for the permeability loss caused by stress sensitivity and adsorption layer, but also significantly increase the permeability. With the decrease of pore radius, the contribution of slippage flow decreases, and the contributions of Knudsen diffusion and surface diffusion increase gradually. With the decrease of pore radius and the increase of pore pressure, the influence of real gas effect and confinement effect on permeability increases significantly. Considering real gas and confinement effect, the apparent permeability of pores with radius of 5 nm is increased by 13.2%, and the apparent permeability of pores with radius of 1 nm is increased by 61.3%. The apparent permeability model obtained in this paper can provide a theoretical basis for more accurate measurement of permeability of shale matrix and accurate evaluation of productivity of shale gas horizontal wells.

NMR Analysis Method of Gas Flow Pattern in the Process of Shale Gas Depletion Development

Shale gas reservoirs have pores of various sizes, in which gas flows in different patterns. The coexistence of multiple gas flow patterns is common. In order to quantitatively characterize the flow pattern in the process of shale gas depletion development, a physical simulation experiment of shale gas depletion development was designed, and a high-pressure on-line NMR analysis method of gas flow pattern in this process was proposed. The signal amplitudes of methane in pores of various sizes at different pressure levels were calculated according to the conversion relationship between the NMR T 2 relaxation time and pore radius, and then, the flow patterns of methane in pores of various sizes under different pore pressure conditions were analyzed as per the flow pattern determination criteria. It is found that there are three flow patterns in the process of shale gas depletion development, i.e., continuous medium flow, slip flow, and transitional flow, which account for 73.5%, 25.8%, and 0.7% of total gas flow, respectively. When the pore pressure is high, the continuous medium flow is dominant. With the gas production in shale reservoir, the pore pressure decreases, the Knudsen number increases, and the pore size range of slip flow zone and transitional flow zone expands. When the reservoir pressure is higher than the critical desorption pressure, the adsorbed gas is not desorbed intensively, and the produced gas is mainly free gas. When the reservoir pressure is lower than the critical desorption pressure, the adsorbed gas is gradually desorbed, and the proportion of desorbed gas in the produced gas gradually increases.

Long-term effect of desorption-induced matrix shrinkage on the evolution of coal permeability during coalbed methane production

In a coal reservoir, matrix shrinkage caused by gas desorption is regarded as a significant factor that can influence natural fracture permeability. It is considered to be a constant and lasting effect throughout coalbed methane (CBM) production based on previous investigations. However, experimental measurements of long-term permeability change have demonstrated that adsorption-induced strain has a time-dependent effect on permeability evolution. This study presents long-term investigations on permeability evolution of coal reservoir during CBM production. A fully coupled two-phase flow model for gas-water is specially proposed. Furthermore, the independent impacts of the matrix swelling during gas adsorption are characterized by a strain-rate-based permeability model. The competing effect between pressure depletion and desorption shrinkage on the permeability change is evaluated during the process of long-term production. The calculations of the proposed model are respectively consistent with experimental and field data and the findings show that matrix shrinkage is the most important factor in permeability evolution towards the wellbore. The effect of matrix shrinkage on permeability enhancement is significant in the primary stage and vanishes with uniform depressurization in the matrix. As a result, matrix shrinkage dominates permeability first, followed by effective stress. A sensitivity analysis of mechanical properties and flow properties on permeability evolution was performed and has demonstrated that the larger the matrix shrinkage-induced strain is, the higher the permeability enhancement is, which can be also enhanced with greater initial permeability and a lower diffusion coefficient of the matrix and the impact of the negative strain induced by decreasing pore pressure is solely related to the bulk modulus. The range of permeability variation is narrowed when porosity is high, which implies that the influence of matrix shrinkage and effective stress on permeability is suppressed. This research will advance the understanding of the permeability change during long-term CBM production.

Wellbore Integrity: An Integrated Experimental and Numerical Study to Investigate Pore Pressure Variation during Cement Hardening under Downhole Conditions

Summary Understanding the cement hardening process and determining the development of the state of stress in the cement under specific downhole conditions are challenging but fundamental requirements to perform an accurate prediction of wellbore integrity. As an essential component of the state of stress, the temporal variation of cement pore pressure is a critical factor that affects the occurrence of cement failure. In this study, we present a novel laboratory setup to measure the cement pore pressure variation during hardening under representative downhole conditions, including the pressure, temperature, and water exchange between the cement and formation. The pore pressure measurements are further incorporated with a staged finite element analysis (FEA) approach to investigate the state of stress development during cement hardening and to evaluate cement failure under different operations and after different wait-on-cement (WOC) periods. The laboratory measurements show that the external water supply from the formation significantly impedes the pore pressure drop in the cement. The numerical results indicate that the accelerated pore pressure decrease obtained without considering downhole conditions elevates the contact pressure at the cement-formation interfaces significantly and moderately increases the von Mises stress in the cement. The numerical results further predict that the accelerated pore pressure decrease leads to an overestimation of shear failure during pressure testing and steamflooding operations but an underestimation of debonding failure during severe fluid loss and injection-related cooling processes. Based on the results of the integrated laboratory and numerical approach, qualitative and quantitative suggestions are provided for field operations to inhibit wellbore integrity risk during the wellbore life cycle.

Determination of gas-water seepage characteristics with consideration of dynamic pore-throat structure in a tight sandstone gas formation

In this paper, a pragmatic and integrated technique has been developed to experimentally determine the gas-water seepage characteristics with consideration of dynamic pore-throat structure in a tight sandstone gas formation due to a decrease in pore pressure. Conventional experiments were conducted to obtain porosity and permeability of core samples collected from a tight sandstone gas reservoir in the Ordos basin. Then, gas and water flow experiments integrated with the nuclear magnetic resonance (NMR) measurement were employed to obtain the dynamic characteristics of relative permeability curves due to changes in pore-throat structure, while gas permeabilities under different conditions are corrected with consideration of the slippage effect and dynamic pore-throat characteristics. Subsequently, irreducible water saturation and water blocking coefficient were obtained by combining the measured NMR T2 spectrum and gas-water volume, which were used to examine the effects of dynamic evolution of a pore-throat structure on the gas-water micro-distribution and percolation capacity. In the case that the decreased pore pressure inevitably leads to the contraction of pores and throats, although the absolute value of permeability decrease is larger for samples with higher permeability, the relative proportion of permeability decrease is larger for samples with lower permeability, indicating a stronger stress sensitivity. With such a decrease of pore pressure, gas and water relative permeability curves are shifted downwards to the right-hand side, during which the flowing ability of the gas-water system is weakened. The increase in irreducible water saturation is found to be greater in the larger throats, while the relative permeability characteristics and irreducible water saturation of a core sample with a higher permeability have larger changes due to the influence of the dynamic pore-throat structure under the same stress variation. The change in the pore-throat structure due to a decrease in pore pressure imposes the greatest impact on the seepage characteristics at the initial stage.

Observing Posteruptive Deflation of Hydrothermal System Using InSAR Time Series Analysis: An Application of ALOS‐2/PALSAR‐2 Data on the 2015 Phreatic Eruption of Hakone Volcano, Japan

AbstractFrom June 29 to July 1, 2015, a phreatic eruption occurred in Owakudani, the largest fumarole area in Hakone volcano, Japan. In this study, an interferometric synthetic aperture radar (InSAR) time series analysis of the Advanced Land Observing Satellite‐2 (ALOS‐2)/Phased Array type L‐band Synthetic Aperture Radar‐2 (PALSAR‐2) data was performed to measure deformation after the eruption. The results show that the central cones of the volcano have subsided since the eruption and its deflation source is located beneath the previously estimated bell‐shaped conductor, which is considered as a sealing layer confining a pressurized hydrothermal reservoir. Therefore, the InSAR results demonstrate the deflation of the hydrothermal system beneath the volcano. One possible cause of this deflation is compaction due to a decrease in pore pressure caused by rupture and fluid migration during and after the eruption.

Integrated source rock evaluation along the maturation gradient. Application to the Vaca Muerta Formation, Neuquén Basin of Argentina

AbstractThe Vaca Muerta Formation (Tithonian–early Valanginian) is the main source rock in the Neuquén Basin and the most important unconventional shale resource in South America. In the present study, organic geochemistry, electron microscopy and basin and petroleum system modelling (BPSM) were combined to evaluate source rock properties and related processes along a transect from the early oil (east) to the dry gas (west) window. The unit is characterized by high present‐day (1%–8% average) and original (2%–16% average) total organic carbon contents, which increase towards the base of the unit and basinal (west) settings. Scanning electron microscopy shows that organic pores derived from the transformation of type II kerogen. Isolated bubble pores are typical of the oil window, whereas bubble and densely distributed spongy pores occur in the gas stage, indicating that the maturity gradient exerts strong control on organic porosity. Organic geochemistry, pressure and porosity data were incorporated into a 2D basin petroleum system model that includes the sequential restoration of tectonic events and calculation of compaction trends, kerogen transformation, hydrocarbon generation and estimation of pore pressure through geologic time. The W–E regional model extends from the Agrio Fold and Thrust Belts to the basin border and allows us to evaluate the relationship between thermal maturity and timing of hydrocarbon generation from highly deformed (west) to undeformed (east) regions. Modelling results show a clear decrease in maturity and organic matter (OM) transformation towards the eastern basin margin. Maximum hydrocarbon generation occurred in the inner sectors of the belt, at ca. 120 Ma; long before the first Andean compression phase, which started during the Late Cretaceous (ca. 70 Ma). Miocene compression (15–7 Ma) promoted tectonic uplift of the inner and outer sectors of the belt associated with a reduction in thermal stress and kerogen cracking, as well as massive loss of retained fluids and a decrease in pore pressure. The OM transformation impacted (a) the magnitude of effective porosity associated with organic porosity development, and (b) the magnitude and distribution of pore pressure within the unit controlled by hydrocarbon generation and compaction disequilibrium. BPSM shows a progressive increase in effective porosity from the top to the base and towards the west region related to the original organic carbon content and maturity increasing along the same trend. Overpressure intervals with high organic carbon contents are the most prone to develop organic pores. The latter represent favourable sites for the storage of hydrocarbons in the Vaca Muerta Formation.

Numerical Simulation Research on Improvement Effect of Ultrasonic Waves on Seepage Characteristics of Coalbed Methane Reservoir

The matrix pores of a coalbed methane (CBM) reservoir are mostly nanoscale pores, with tiny pore throats and poor connectivity, which belong to the category of low–permeability gas reservoirs. The matrix particles and organic pore surfaces adsorb a large amount of CBM. These problems are the main reasons that limit the increase in CBM production. At present, the primary measure to increase CBM production is hydraulic fracturing. However, due to the technical characteristics and geological conditions of CBM reservoirs, applying this technology to CBM exploitation still has some key issues that need to be resolved. Therefore, it is essential to develop a new technology that can effectively increase the production of CBM. This paper proposed a method that uses ultrasonic waves to improve the seepage characteristics of CBM reservoir and theoretically verifies the feasibility of this idea using numerical simulation. In this paper, we firstly coupled the temperature, pressure, and seepage parameters of the CBM reservoir and built a CBM seepage model under the action of ultrasonic waves. Secondly, by comparing the numerical simulation results with the experiment, we verified the accuracy of the model. Finally, on the basis of the mathematical model, we simulated the change characteristics of pore pressure, reservoir temperature, permeability, and porosity under the action of ultrasonic waves. Research results show that under the action of ultrasonic waves, the pressure-drop funnel of CBM reservoir becomes more apparent. The boundary affected by the pressure drop also increases. With the increase of the action time of ultrasonic waves, the temperature of CBM reservoir also increases, and the action distance is about 4 m. With decreased pore pressure, the permeability and porosity of CBM reservoir significantly increase under the action of ultrasonic waves. With increased ultrasonic power, its effect on reservoir permeability and porosity becomes more significant.

Fault nucleation of tight sandstone by investigation of mechanical, acoustic, and hydraulic responses

To investigate the effect of water on the fracture mechanism and associated acoustic emission (AE) characteristics, triaxial compression experiments were performed on dry and water-saturated tight sandstone specimens sampled from the Sichuan Basin, China. The saturated specimen was precisely controlled by different drainage conditions at the two ends. The detailed spatiotemporal distribution of AE activities and the P-wave velocities were used to examine the initiation and propagation of microcracks, as well as fault nucleation process. There are significant differences in mechanical behavior, P-wave velocity, AE activities, and fault nucleation between the dry and saturated specimens. The mechanical behavior of the saturated specimen is characterized by water weakening, with 10% and 26% reductions in peak strength and elastic modulus, respectively, in comparison with those for the dry specimen. Water enhances inelastic axial deformation and strain softening. The evolution of P-wave velocity is associated with microcracking-induced dilatancy. The decrease in P-wave velocity is suppressed in the saturated specimen, especially at the drained end. Significant attenuation of AE energy is observed in the saturated specimen, resulting into a lower event rate compared with that for the dry specimen. It is shown that the saturated specimen has an extended fault nucleation duration and lower rupture velocity. The precursory phase of dynamic failure can be clearly mapped based on strain softening, accelerating AE event rate, and decreasing b-value, as well as decreasing pore pressure (in undrained condition) or increasing injection rate (in drained condition).

Impact of downhole pressure and fluid-access on the effectiveness of wellbore cement expansion additives

Autogenous shrinkage of wellbore cement widely impairs zonal isolation. MgO-based cement expansion additives (CEAs) can mitigate this shrinkage, or even impart net expansion, by creating porosity through displacive crystal growth-processes. However, both MgO hydration and autogenous shrinkage behaviour depend strongly on stress state. Evaluation of CEA performance in wellbore cements therefore requires testing under elevated pressures representative for subsurface environments. We report experiments addressing the chemical and bulk volume changes that occur in cement hydrating at 10 MPa confining pressure and 90 °C. Volumetric response was investigated as function of MgO concentration, external water supply, and pore pressure decrease through water consumption during reaction. Results show the bulk expansion achieved using MgO-based CEAs diminishes markedly with increasing effective confining pressure or, equivalently, upon restricting fluid supply. This reduced expansion-potential under pressure has profound implications for slurry design, notably regarding CEA-concentrations required to counteract micro-annulus formation while maintaining low cement permeability.

Quantitative Multiparameter Prediction of Fractured Tight Sandstone Reservoirs: A Case Study of the Yanchang Formation of the Ordos Basin, Central China

Summary Fractured reservoirs account for more than one-half of the global oil and gas output and thus play a pivotal role in the world’s energy structure. Under diagenesis, rocks become dense, and tectonic fractures easily form under subsequent tectonic movement. These tectonic fractures are the main seepage conduits of tight sandstone reservoirs and are important determinants of whether a tight sandstone reservoir can have high, stable oil and gas production. The influence of multistage tectonic movement has led to well-developed fractures in the Ordos Basin in central China. In the process of reservoir development, the effective stress on the fracture surface increases because of the decrease in pore pressure, and the fracture aperture, porosity, and permeability also change accordingly. Therefore, modeling of the dual porosity and dual permeability of fractured reservoirs requires a dynamic 4D modeling process related to time. In this paper, we propose a 4D modeling method of dual porosity and dual permeability in fractured tight sandstone reservoirs. First, the porosity and permeability distribution of the reservoir matrix are established based on reservoir modeling. Based on geomechanical modeling, the density and occurrence of natural fractures are predicted by the paleostress field. The in-situ stress field is used to analyze the fracture aperture, and the variation in the fracture aperture during the development process is analyzed along with the variation in the in-situ stress in the development process to realize 4D modeling of the porosity and permeability of fractured reservoirs. The total porosity of the fracture is 0 to 8 × 10−3%, and the principal value of the planar permeability of the fracture is 0 to 3 × 10−3 µm2; the principal value of the fracture permeability is concentrated in the direction of 65 to 70° east-northeast. The simulated fracture porosity stress sensitivity index is distributed between 0 and 0.2, and the fracture permeability stress sensitivity index is distributed between 0 and 0.4. The Young’s modulus of the rock, in-situ stress parameters, and sound velocity in the rock are important factors affecting the fracture stress sensitivity.

Research on the Formation Mechanism of Seepage Channels in Broken Rock Mass under the effects of Particle Migration

Seepage within broken rock mass widely exists in geotechnical engineerings, such as railways, slopes, dam foundations and mining. Seepage is often accompanied by particle migration, which usually cause structural instability and water inrush disasters. In order to study the formation mechanism of seepage channels in the brocken rock mass under the effects of particle migration, a series of seepage tests were designed on broken red sandstone. The effects of pore pressure and particle size distribution (PSD) on particle migration and porosity evolution were studied. The main conclusions are as follows: 1) By observing the particles collected during the seepage process, particles with size range of 0-2 mm, 2-5 mm and 5-8 mm appear in sequence, which means that with the formation of seepage channels, large size of rock particles can pass through the broken rock mass. 2) The cumulative mass of migrating particle increases with the increase of pore pressure and decrease of PSD; with the increasing duration of migration process, the pore pressure and PSD decrease. 3) The peak value of porosity increases with the increase of pore pressure and decrease of PSD. According to above results, the seepage channels of the samples with smaller PSD and larger pore pressure are wider. Besides, the formation speed of seepage channels is larger in sample under higher pore perssure. These can explain the higher risk of geological disasters (i.e., water inrush and structural instability) in broken rock mass within higher pore pressure and smaller PSD.

Alteration of ultrasonic signatures by stress-induced changes in hydro-mechanical properties of fractured rocks

In this study, we evaluate the changes in ultrasonic signatures (i.e., frequency content, velocity, amplitude, and attenuation) due to stress-induced alteration of fracture aperture/permeability. Flow-through experiments were performed on artificially-fractured phyllite specimens along with the concurrent measurements of ultrasonic signatures under different stress conditions. Increasing pore pressure led to fracture opening, as indicated by increases in both mechanical and hydraulic apertures. In addition, we observed that increase in confining pressure (and decrease in pore pressure) led to increases in ultrasonic velocities, ultrasonic amplitudes, and fracture specific stiffness, and decrease in ultrasonic attenuations. It was found that time-frequency partitioning depends on hydraulic aperture. The higher frequency band, for both P- and S-waves, was insensitive to the changes in stress conditions; the lower band was sensitive to the changes in stress conditions, as long as the hydraulic aperture was changing. Three-Element rheological and Power-Law models successfully predicted the time-dependent fracture displacement, with the former being more accurate at higher levels of pore pressures.

Permeability evolution in tectonic coal: The roles of moisture and pressurized water‐injection

AbstractWater often influences the exploitation of Coalbed methane (CBM) as well as the hydraulically enhanced permeability; it is therefore useful to objectively quantify the permeation properties of a CBM reservoir with different moisture contents. Based on the occurrence of CBM and the laws governing gas flow in CBM reservoirs, gas seepage experiments were carried out on the coal collected from the Baijiao mine in Sichuan, China. Various moisture contents were evaluated using natural coal samples, dry coal samples heated in an oven for various periods, and coal samples wet by soaking them in different pressure water under a constant effective confining stress. Results showed that: (1) The coal moisture decreased gradually with the drying time and increased with the soaking time. When coal is soaked in high pressure water (5–20 MPa), the moisture content in coal will increase from 2.60 to 3.85% with the increasing of water pressure. (2) The permeability of soaked coal decreased with the increasing of gas pressure, while the permeability of natural coal firstly decreased and then increased with the pore pressure decrease. (3) For the coal with higher moisture contents, gas permeability is more sensitive to the reduction of pore pressure, which may be due to the alteration of fracture structure induced by high‐pressure water injection. (4) Moisture content of coal affects the slippage effect in coal pores, and a higher moisture in coal will result in the decrease effective pore throat, incurring a stronger slippage flow and a higher apparent permeability of coal. © 2021 Society of Chemical Industry and John Wiley & Sons, Ltd.

A FRACTAL MULTIPHASE TRANSPORT MODEL IN SHALE POROUS MEDIA WITH MULTIPLE TRANSPORT MECHANISMS AND ROCK–FLUID INTERACTION

Fluid transport in shales is complex due to the various storage spaces and multiple transport mechanisms, especially for multiphase transport during flowback and early stage of production. This study proposes a gas-water relative permeability fractal model during a gas displacing water process in shale gas reservoirs, with incorporations of (1) real gas transport controlled by Knudsen Number (Kn) and second-order slip boundary, (2) slip length for water phase transport, (3) a mobile water film with varying thickness due to rock–fluid interaction and (4) stress-dependence. Specially, the varying thickness of water film is determined according to the extended Derjaguin–Landau–Verwey–Overbeek (DLVO) theory through van der Waals, electrostatic and structural force during a drainage process. Moreover, the organic matter (OM) and inorganic matter (IOM) pore structures are considered with individual pore/tortuosity fractal dimensions. The proposed model is verified by comparing with an analytical model and experimental data. Results show that the decreasing pore pressure during depressurization brings a decline in gas relative permeability, while the decreasing pore pressure has little impact on water relative permeability. The impact of pore and tortuosity fractal dimensions of OM can be ignored compared with that of IOM. Furthermore, neglecting the mobile water film with varying thickness during a gas drainage process leads to an overestimation of gas relative permeability, especially at smaller pore sizes. This work presents a comprehensive model to determine gas-water relative permeability in shales by considering fluids/reservoir properties and rock–fluid interaction in full, which reveals multiphase transport mechanisms in the unconventional reservoirs.