Pore Compressibility Research Articles

Time-dependent coal permeability: Impact of gas transport from coal cleats to matrices

Coal seams are usually fractured reservoirs where the factures/cleats are fluids flow channels, and the matrix blocks serve as the storage space. The gas transport within the matrix has crucial impacts on the variation of cleat aperture and thus affects the reliable determination of coal permeability. Most literature on laboratory measurements and the development of relevant analytical models focus on response gas pressure dependent coal permeability change, under the assumption that gas transport in coal is in equilibrium. This means that these models are not suitable for the assessment of transient permeability when gas transport in coal is unsteady flow. In this paper, we real-time measured the entire process of the coal deformation induced by gas injection, and further obtained the diffusion-dependent curve of coal permeability against time by modifying the typical pressure-pulse-decay approach. An improved permeability model was then proposed by incorporating an Internal Deformation Coefficient into the pore compressibility of coal seams. The results show that the gas inflow can decline the pressure differential between the downstream pressure and the upstream pressure, during which the deformation of coal experiences a transition from early rapid expansion to latter slow expansion. As the gas was diffusing into coal matrices, the coal permeability was observed to first decrease and then became stable. It is also found that Internal Deformation Coefficient increases with the injection gas pressure. This indicates that the matrix blocks preferentially swell toward cleats. The modified permeability model achieves a much better fit of the coal permeability-pore pressure relationship when compared with two widely used permeability models. This approach can be extended to evaluate the deformation compatibility between coal cleats and matrices.

Dual Compressibility Characteristics of Lignite, Subbituminous, and High-Volatile Bituminous Coals: A New Insight into Permeability

Pore compressibility is an important physical property of coal, which controls geomechanical behavior and hydrocarbon reserve evaluation of coal seams. However, the representation of pore compressibility is usually oversimplified in coalbed methane (CBM) reservoir analysis. This study proposes the adoption of the dual compressibility concept, where both accessible porosity (connected pore) and the inaccessible portion of the coal compressibility contribute to the storage and transport capacities of coal reservoirs, provided the availability of mercury porosimetry data. We characterize dual compressibility as a function of confining pressure and identified the effect of factors, such as coal rank, moisture content, and porosity. It was found that pore compressibility reduces decreased one order of magnitude, from 10−3–10−2 to 10−4–10−3 MPa−1, with confining pressure increasing up to 413.3 MPa. The calculated compressibility of accessible pores is one-to-two orders of magnitude higher than the inaccessible portion of the coal. Using a compressibility correction on pore volume connectivity and permeability estimates showed a 23% reduction in those estimates. Results suggest that high pore compressibility affects storage and transport capacity more significantly than previously regarded. We propose a new model to estimate coal permeability as a function of confining pressure and accessible pore compressibility. The model uses integrated intrusion correction and pore compressibility values from the Katz–Thompson method. Pore volume and permeability response with pressure were evaluated according to coal rank, macro-lithotype, and seam. This work provides an improved methodology to estimate hydrocarbon reserves and CBM reservoir performance through more accurate pore volume and permeability evaluations from mercury porosimetry data.

Synergistic Effects of Micronano Structures on Porosity and the Permeability of Shale Under Varying Effective Stresses and Temperatures: A Case Study of Fresh Outcrops from Lower Silurian Longmaxi Formation Shale in the Southern Sichuan Basin, China.

To study the changes in porosity-permeability and the characteristics of the pore structure of shale under stress and high temperature, the Lower Silurian Longmaxi Formation shale in the southern Sichuan Basin, China, was investigated under conditions of continuous pressurization and heating. In addition, the pore compression coefficients and permeability stress sensitivity coefficients were analyzed and quantified. The mineral composition of these black shales was analyzed using scanning electron microscopy (SEM) and nuclear magnetic resonance (NMR). Our results reveal that the porosity and permeability of the shales exponentially decreases with increasing stress, reflecting that microfracture development during increasing stress causes the pores in these shale samples to enlarge. However, the pore compressibility and stress sensitivity coefficient vary for each sample, and the quantitative results indicate an overall decrease with increasing stress, suggesting that the shale deformation is nonlinear during stress release. Based on the mineral composition analyses and SEM measurements, we conclude that the nonuniform changes during stress release are related to the complexity of the shale mineral compositions and the different intercrystalline/ interlaminar pores of the different minerals, which are affected in different ways by pressure. The NMR measurements reveal that the mesopores are most developed in the shale samples, the pore volumes of the micro- and macropores are small, and the nanoscale pores are mainly from 1-60 nm in diameter. The different types of pore sizes decrease with increasing stress, indicating that the porosities measured experimentally reflect the synergistic effects of the different pore sizes on porosity. As the temperature increases, the permeability of the shale decreases significantly, which is primarily caused by the thermal expansion effect. The high clay mineral content of the shales also causes their permeabilities to be sensitive to temperature.

Stress sensitivity evaluation of ultra-low permeability sandstone reservoir and its influence on oilfield development

Luohe ultra-low permeability sandstone reservoir is a hot block in Yanchang oilfield, which is a potential point for increasing production and reservoir. In view of the current situation that there is no unified stress sensitivity evaluation standard for ultra-low permeability sandstone in the study area, taking the ultra-low permeability sandstone in Luohe district as the research object, the stress sensitivity evaluation of ultra-low permeability sandstone is carried out by using experimental analysis as the main means. The results show that it is more accurate to evaluate porosity by using pore stress sensitivity coefficient instead of pore compressibility coefficient. With the increase of net overburden pressure, the porosity stress sensitivity decreases gradually; the permeability stress sensitivity is evaluated by variable confining pressure. With the increase of confining pressure, the permeability damage decreases. With the decrease of confining pressure, the permeability damage increases, but it can not recover to the original value, so the permeability damage is irreversible; in the low bottom hole pressure stage, stress sensitivity has a greater impact on oil well productivity, while in the high bottom hole pressure stage, stress sensitivity has a smaller impact on oil well productivity; advanced water injection can reduce the adverse effect of stress sensitivity on the development of ultra-low permeability sandstone and maximize the economic benefits. The research results clarify the method of stress sensitivity evaluation, and provide guidance for efficient water injection in the next step.

Техногенные факторы воздействия на продуктивные отложения месторождений углеводородов и их связь с потенциальным оседанием земной поверхности

The article presents the results of the studies of the assessment of subsidence of the Earth’s surface above hydrocarbon fields, obtained by mathematical modeling and on the basis of field observations. The purpose of these studies is the substantiation of changes in the initial physical-mechanical and strength properties of productive deposits in the process of exposure to them by various methods of development, comparison of estimates of subsidence of the Earth’s surface obtained by calculation and geodetic methods, taking into account the heterogeneity of the geological structure of the fields and the technogenic factors of impact. It is shown that the value of the calculated surface subsidence over the field, obtained on single data of the coefficient of pore compressibility of rocks and without taking into account the impact of the applied methods of development, does not allow an objective assessment of this process over the field area. The conclusion is made about the necessity of repeated profile leveling and gravimetric observations for a real assessment of the subsidence process of the Earth’s surface, which allows to control the industrial and environmental safety of natural and natural-technogenic objects in the field.

Experimental Study of Mesostructure Deformation Characteristics of Unsaturated Tailings with Different Moisture Content

A portion of the accumulated tailings in a tailings pond exhibits an unsaturated state. The mechanical properties of unsaturated tailings affect the safety and stability of tailings dams. To investigate the effect of moisture content on the deformation characteristics of unsaturated tailings in the mesoscale, a special testing apparatus is applied to experimentally study the settlement deformation and mesostructure evolution of unsaturated tailings under continuous load. The results show that the mesostructure deformation of unsaturated tailings with different moisture contents under load is the same and can be divided into four stages: pore compression, elastic deformation, structure change, and further compaction. However, the critical pressures of the four stages are significantly different; there is an optimal moisture content corresponding to the maximum deformation resistance. Moreover, the influence of the liquid bridge regime on the mesostructure deformation of unsaturated tailings is discussed in this paper.

The effects of porosity and permeability changes on simulated supercritical CO2 migration front in tight glutenite under different effective confining pressures from 1.5 MPa to 21.5 MPa

AbstractDepending on rock pore compressibility, a change in effective confining pressure (ECP) can have a significant influence on supercritical CO2 (SC‐CO2) migration characteristics in natural reservoirs. In this study, a tight glutenite sample was used to conduct porosity/permeability measurements under different ECPs of 1.5, 5.5, 9.5, 13.5, 17.5 and 21.5 MPa. Then a SC‐CO2 drainage core flooding experiment, which was monitored using nuclear magnetic resonance (NMR) technique, was conducted at an ECP of 5.5 MPa. Measurement results show that the porosity and permeability of the sample were comparatively low (at an ECP of 1.5 MPa, 8.3% and 2.4 mD, respectively). With increasing ECP, the porosity/permeability decreased rapidly initially then more slowly at the larger ECP value. NMR results shows that SC‐CO2 preferentially displaced water creating flow channels inside the sample. At SC‐CO2 breakthrough, the average residual water saturation was 69.86%. Following breakthrough, SC‐CO2 continued to displace the water creating more substantial flow channels until they were sufficient to transport the SC‐CO2 at the fixed flow rate, resulting in a residual water saturation of 42.72%. A two‐dimensional computational model was then established based on these experimental results to simulate the fluid behaviors at an ECP of 5.5 MPa, and then the model was extended to different ECP values ranging from 1.5 to 21.5 MPa. Numerical simulation results show that SC‐CO2 displaced water in an inverted triangle‐like shape. Under different ECPs of 1.5, 5.5, 9.5, 13.5, 17.5 and 21.5 MPa, the simulated breakthrough times were 36.8, 44.6, 54.8, 56.4, 59.0 and 59.4 mins with an average SC‐CO2 saturation of 13.18, 16.71, 18.71, 19.72, 21.99 and 22.19%, respectively. SC‐CO2 breakthrough occurred quicker for lower ECP values resulting in a smaller SC‐CO2 saturation. Likewise, with larger ECP value, SC‐CO2 breakthrough occurred later, and a higher SC‐CO2 saturation was seen. © 2020 Society of Chemical Industry and John Wiley & Sons, Ltd.

Dynamic flow resistivity and sound absorption of compressed fibrous porous materials: Experimental and theoretical

Fibrous porous materials are the preferred absorbers for many applications since they can absorb impact energy and sound waves. The microscopic morphology and the relationship between the compressed state and sound absorption characteristics are studied for compressed fibrous porous materials. A series of specimens of fibrous porous materials are prepared and compressed to different thicknesses, and their pore size distributions are analyzed. The average pore size, dynamic flow resistivity, and acoustic impedance of the compressed fibrous porous materials are derived theoretically. The effects of porosity, thickness, average pore size, compression ratio, and combinations of these parameters on the dynamic flow resistivity and sound absorption coefficient of compressed fibrous porous materials are discussed theoretically. It is demonstrated that the compression ratio decreases the sound absorption coefficient of fibrous porous materials. The compression ratio and average pore size are important factors for the dynamic flow resistivity and sound absorption coefficient. Our results could help establish a theoretical foundation for understanding the microscopic morphology of compressed fibrous porous materials and aid the design of fibrous porous materials for various applications.

Porosity and Permeability Evolution with Deviatoric Stress of Reservoir Sandstone: Insights from Triaxial Compression Tests and In Situ Compression CT

Porosity and permeability are the two most important characteristics of underground gas storage in sandstone reservoirs. Injection of gas into reservoir rocks will cause rock deformation. The deformation will influence the porosity and permeability properties of the rocks. We investigate the evolution of these two properties of storage sandstone by triaxial compression tests and a uniaxial in situ compression CT test. As the deviatoric stress increases, the sandstone is compressed firstly (porosity reduction) and then dilates (porosity enhancement). With the increase in confining stress, the occurrence of volumetric dilation will be delayed. Trapped porosity of this sandstone at different deviatoric stresses is very small (0.122%-0.115%) which indicates that nearly all pores are connected. During the compression stage, the decrease in permeability is related to compression of pores and microcracks. During the volumetric dilation stage, it is related to increase in tortuosity. This interpretation can be confirmed by observations of in situ compression CT. The permeability evolution estimated by pore network modeling is consistent with macroscopic testing results.

Effect of permeability and loading conditions on dehydration of sludge in its pressurization process

The mechanical dehydration method is generally used for reducing the moisture content in sludge. However, there is little understanding of the physical and mechanical properties and dehydration mechanism of sludge during pressurization. In this paper, characteristics of sludge were studied from the perspective of soil mechanics. The water permeability, loading conditions, and dehydration performance of the sludge during the pressurization process were analyzed through a series of geotechnical laboratory experiments. The results showed that the pore structure, compressibility coefficient, consolidation coefficient, and permeability coefficient of the sludge are related to the dehydration rate. The conditioners that mixed with sludge could increase the dewatering rate by reducing the sludge viscosity, changing the water status, and increasing the permeability coefficient. The two conditioners used in this study increased the dehydration rate of the sludge from 27.5 to 52.6% and 63.9%, respectively. Unlike the seepage consolidation of ordinary clay, which has only one stage, the seepage consolidation of sludge could be divided into three stages. The removed water in the first stage is mainly free water; the discharge in the second stage is the combination of free water and bound water, and bound water was mainly discharged in the third stage. The water content at the end of each stage corresponds to the liquid limit, plastic limit, and shrinkage limit of the sludge. Our study provides a novel insight for understanding the deep dewatering of sludge and is benefit to solve the problem of sludge dewatering.

Preparation and characterization of glass ceramic foams based on municipal solid waste incineration ashes using secondary aluminum ash as foaming agent

Glass ceramic foam with high strength was developed using municipal waste incineration bottom ash, fly ash, pickling sludge and secondary aluminum ash as raw materials without foaming agent. The effect of the secondary aluminum ash content and alkalinity on the pore morphology, porosity, water absorption, compression strength and bulk density of glass ceramic foams has been evaluated. The results showed that the increase of secondary aluminum ash content is beneficial to gas generation, but it will reduce the formation of liquid phase, resulting in the variation of pore morphology. In the process of alkalinity adjustment, the degree of polymerization of glass decreased first and then increased, which is similar to the change of max pore size, providing guidance for quantitative analysis of pore formation. The optimized sample reached porosities up to 63.02%, bulk density of ~1.42 g/cm3, and excellent compressive strength ~41.93 MPa. The analysis of self-foaming mechanism proved that secondary aluminum ash can be used as a self-foaming waste results from the oxidation reaction of AlN. In this work, a clean and sustainable utilization method has been proposed for recycling multiple solid wastes by preparing glass ceramic foams with a potential application for construction insulation materials with load-bearing. Moreover, an innovative exploration of secondary aluminum ash as the self-foaming waste in high temperature environmental was proposed.

A new straight-line reserve evaluation method for water bearing gas reservoirs with high water production rate

A material balance equation (MBE) is usually used to estimate hydrocarbon reserves, to judge driving mechanism, and to predict the dynamic performance of oil and gas reservoirs. Few straight-line methods have been proposed for estimating original gas in place (OGIP) of water bearing gas reservoirs with high water production until now. For this type of gas reservoir, gas and mobile water mostly coexist in the same formation because of low permeability. Lots of water is produced even though there is no bottom or edge aquifer. Thus, the effect of water production on average pressure should not be ignored.In this work, an MBE for water bearing gas reservoirs considering the effects of water compressibility, pore compressibility, and water production, is derived step by step from the basic material balance principle, and the corresponding straight-line reserve evaluation method is developed. Then the proposed reserve evaluation method is validated against a computer modelling group (CMG) simulation case and two field cases using pressure transient analysis (PTA) method in the dynamic data analysis software KAPPA Ecrin. Finally, the necessity and importance of the proposed reserve evaluation method in water bearing gas reservoirs are discussed.Validation cases prove that the proposed straight-line reserve evaluation method for water bearing reservoirs is reliable and rational. The OGIP of water bearing reservoirs can be evaluated directly using the y-intercept divided by the minus slope of the straight line. If ignoring the effect of water production, the OGIP of water bearing gas reservoirs will be underestimated, especially for gas wells with high water production rate and short producing time.

Compressibility of Different Pore and Fracture Structures and Its Relationship with Heterogeneity and Minerals in Low-Rank Coal Reservoirs: An Experimental Study Based on Nuclear Magnetic Resonance and Micro-CT

Accurate characterization of the compressibility of pores and fractures in coal reservoirs is of great significance for predicting permeability and for optimizing the production of coalbed methane (CBM). However, for low-rank coal (Ro,max < 0.65%), compressibility and the influences of different pore structures and minerals on compressibility have not been studied thoroughly. To solve this problem, four low-rank coal samples were collected from the south of Junggar Basin. Low-field nuclear magnetic resonance is used to study the compressibility of pore and fracture in low-rank coal and its relationship with the heterogeneity. In addition, the effects of fracture structure and minerals on compressibility were studied by combining with micro-CT. The results show that the effects on the compression space of the low-rank coal with the increase in effective stress mainly come from the fracture and seepage pore volume, and the compressibility becomes weaker. The low-rank coal reservoirs with well-developed fractures and seepage pores show better compressibility. The compressibility of the pores and fractures in low-rank coal is noticeably affected by the mineral content, mineral shape, and mineral arrangement. When the fracture development is similar, the coal reservoirs with a high mineral content and with many fractures filled by minerals show poor compressibility. There is a negative linear correlation of the compressibility of pores and fractures with the fractal dimension of the seepage space in low-rank coal. Moreover, the closing rate of the pores and microfractures in low-rank coal with significant heterogeneity in the seepage space is faster under confining pressure. The research results are beneficial to the optimization of CBM production measures in low-rank coal reservoirs.

The Influence of the Porous Structure of Activated Coke for the Treatment of Gases from Coal Combustion on Its Mechanical Strength

This study investigated influences of the open/close states of pores and porosity distribution of activated coke on the mechanical strength of common activated coke for the purification of coal-fired flue gas by analyzing pore structure, abrasive resistance, and compression strengths of 9 types of desulfurization and denitration activated cokes. Research conclusions are conducive to disclosing the influences of porosity characteristics of activated coke for the purification of coal-fired flue gas on mechanical strength, decreasing the physical consumption of activated coke in the recycling of flue gas purification systems, and lowering the purification cost of coal-fired flue gas. According to research results, pores in the ranges of 0–2 nm and 2–500 nm of activated coke are further developed after recycling using the coal-fired flue gas purification system, and the average compression strength of activated coke is about 70% of the added fresh activated coke. However, the abrasive resistance of the recycled activated coke which has a smooth surface is higher than that of the fresh activated coke. Open pores are the main cause of reduced compression strength of activated coke. Open pores in the range of 2–500 nm can destroy the compression strength of activated coke the most. The open/close states of pores cause no significant impacts on the abrasive resistance of activated coke, but pores with diameters ranging from 0–2 nm can destroy the abrasive resistance of activated coke most significantly.

In Vitro Mechanical and Biological Properties of 3D Printed Polymer Composite and β-Tricalcium Phosphate Scaffold on Human Dental Pulp Stem Cells.

3D printed biomaterials have been extensively investigated and developed in the field of bone regeneration related to clinical issues. However, specific applications of 3D printed biomaterials in different dental areas have seldom been reported. In this study, we aimed to and successfully fabricated 3D poly (lactic-co-glycolic acid)/β-tricalcium phosphate (3D-PLGA/TCP) and 3D β-tricalcium phosphate (3D-TCP) scaffolds using two relatively distinct 3D printing (3DP) technologies. Conjunctively, we compared and investigated mechanical and biological responses on human dental pulp stem cells (hDPSCs). Physicochemical properties of the scaffolds, including pore structure, chemical elements, and compression modulus, were characterized. hDPSCs were cultured on scaffolds for subsequent investigations of biocompatibility and osteoconductivity. Our findings indicate that 3D printed PLGA/TCP and β-tricalcium phosphate (β-TCP) scaffolds possessed a highly interconnected and porous structure. 3D-TCP scaffolds exhibited better compressive strength than 3D-PLGA/TCP scaffolds, while the 3D-PLGA/TCP scaffolds revealed a flexible mechanical performance. The introduction of 3D structure and β-TCP components increased the adhesion and proliferation of hDPSCs and promoted osteogenic differentiation. In conclusion, 3D-PLGA/TCP and 3D-TCP scaffolds, with the incorporation of hDPSCs as a personalized restoration approach, has a prospective potential to repair minor and critical bone defects in oral and maxillofacial surgery, respectively.

Log-based rock compressibility estimation for Asmari carbonate formation

Rock compressibility has many applications in the upstream petroleum industry, for example, reservoir material balance calculations and geomechanics, as related to possible formation compaction and subsidence. When measurements on core are not available, empirical correlations may need to be considered, derived for specific fields. In the study presented, the suitability of the log-based method for evaluating rock compressibility is investigated, with the aim of deriving a suitable correlation for Asmari carbonate, Southwest Iran. A stepwise log-based method of rock compressibility determination is presented, including the construction of a geomechanical earth model, MEM. In comparing a log approach with core-derived results, it was found that the log-based method showed a rather reliable estimation of pore compressibility for carbonates studied, except for intervals with extremely large wellbore washout. Although the selected procedure is similar to that used for sandstone by other researchers, involving in some cases complex procedures, a relatively good matching result could be obtained with a relatively simple procedure. To validate the results from this study, a comparison of compressibility values is made with several industry correlations. An application of formation compressibility for estimating possible formation compaction and associated subsidence is also presented. However, a minimal effect is indicated in this case, as the carbonates studied are rather consolidated. In summary, this study presents a new correlation for rock compressibility for Southwestern Iranian carbonate oilfields (Asmari carbonate), validating the applicability of a log-based technique.

Estimating pore volume compressibility by spheroidal pore modeling of digital rocks

The real pores in digital cores were simplified into three abstractive types, including prolate ellipsoids, oblate ellipsoids and spheroids. The three-dimensional spheroidal-pore model of digital core was established based on mesoscopic mechanical theory. The constitutive relationship of different types of pore microstructure deformation was studied with Eshelby equivalent medium theory, and the effects of pore microstructure on pore volume compressibility under elastic deformation conditions of single and multiple pores of a single type and mixed types of pores were investigated. The results showed that the pore volume compressibility coefficient of digital core is closely related with porosity, pore aspect ratio and volumetric proportions of different types of pores. (1) The compressibility coefficient of prolate ellipsoidal pore is positively correlated with the pore aspect ratio, while that of oblate ellipsoidal pore is negatively correlated with the pore aspect ratio. (2) At the same mean value of pore aspect ratio satisfying Gaussian distribution, the more concentrated the range of pore aspect ratio, the higher the compressibility coefficient of both prolate and oblate ellipsoidal pores will be, and the larger the deformation under the same stress condition. (3) The pore compressibility coefficient increases with porosity. (4) At a constant porosity value, the higher the proportion of oblate ellipsoidal and spherical pores in the rock, the more easier for the rock to deform, and the higher the compressibility coefficient of the rock is, while the higher the proportion of prolate ellipsoidal pores in the rock, the more difficult it is for rock to deform, and the lower the compressibility coefficient of the rock is. By calculating pore compressibility coefficient of ten classical digital rock samples, the presented analytical elliptical-pore model based on real pore structure of digital rocks can be applied to calculation of pore volume compressibility coefficient of digital rock sample.

Transport property evolution during hydrostatic and triaxial compression of a high porosity sandstone

Permeability and porosity are two crucial properties related to underground gas storage. This research investigates the permeability and porosity evolution of a Vosges sandstone under different stress paths. Under hydrostatic stress, the influence of irreversible deformation on permeability is more significant than on porosity. The permeability is more sensible to confining pressure than porosity. Under non-hydrostatic loading, permeability decreases continuously with deviatoric stress. At volumetric compression stage, the decrease is due to compression and closure of pores and micro-cracks. At volumetric dilatancy stage, sand production in the shear band causes the permeability decrease. Sand production mainly occurs at volumetric dilatancy stage when closing to failure.

Thermal Pressurization Weakening in Laboratory Experiments

AbstractThermal pressurization (TP) is expected to be a dominant frictional weakening mechanism during earthquakes. However, most of our understanding of TP relies on theoretical studies. Our experimental setup allows us to test the mechanical response of experimental faults under elevated confining and pore pressures in the laboratory using a rotary‐shear apparatus. We observe dynamic weakening by TP of water‐saturated faults in Frederick diabase sliding at subseismic slip rates of 2.5–5 mm/s under an effective normal stress of 25–50 MPa and effective confining pressure of 20–49 MPa. No other weakening mechanism is activated at these conditions in dry samples. The permeability of the samples is varied by heat treatment before each experiment, and the frictional behavior is tested by velocity‐stepping experiments. Dynamic weakening was investigated after both low and high total fault displacements. For low‐displacement samples: (1) the magnitude and rate of weakening increase as the sample permeability decreases and (2) frictional heating is limited during sliding. These observations are consistent with the expected behavior of TP from theory. Weakening of high‐displacement samples is similar in magnitude to low‐displacement faults, but with slower rates of weakening and less dilation. The difference in weakening of the high‐displacement samples may reflect changes in pore compressibility in the fault zone, which would have a significant effect on the frictional response during TP. The activation of TP at modest slip rates in these experiments supports the hypothesis that TP is an active, and likely dominant, dynamic weakening mechanism during earthquakes in the early stages of slip.

Experimental study on the influence of temperature cycle on low-rank coal permeability

The formation temperature is a key factor affecting coalbed methane (CBM) migration in reservoirs. Both the prediction of CBM production and prevention of mine gas disasters require the understanding on the controlling mechanism of temperature on coal permeability. We experimentally examined the evolution of permeability for natural low-rank coal samples under various stresses and cyclic temperature conditions. Apparent permeability and intrinsic permeability decrease significantly when the temperature increases and they only partially recover after the temperature returns. The permeability loss decreases greatly with the increasing number of temperature cycles. The permeability loss due to the rising temperature and the irreversible permeability loss for a whole temperature cycle decrease prominently with increasing confining stress. The impacts of swelling/shrinking of coal matrix, roughness of surface, pore compressibility and weakly bound water in coal on temperature sensitivity of coal permeability are investigated.