Porosity, permeability and rock mechanics of Lower Silurian Longmaxi Formation deep shale under temperature-pressure coupling in the Sichuan Basin, SW China

To investigate the porosity, permeability and rock mechanics of deep shale under temperature-pressure coupling, we selected the core samples of deep shale from the Lower Silurian Longmaxi Formation in the Weirong and Yongchuan areas of the Sichuan Basin for porosity and permeability experiments and a triaxial compression and sound wave integration experiment at the maximum temperature and pressure of 120 ℃ and 70 MPa. The results show that the microscopic porosity and permeability change and the macroscopic rock deformation are mutually constrained, both showing the trend of steep and then gentle variation. At the maximum temperature and pressure, the porosity reduces by 34%–71%, and the permeability decreases by 85%–97%. With the rising temperature and pressure, deep shale undergoes plastic deformation in which organic pores and clay mineral pores are compressed and microfractures are closed, and elastic deformation in which brittle mineral pores and rock skeleton particles are compacted. Compared with previous experiments under high confining pressure and normal temperature,the experiment under high temperature and high pressure coupling reveals the effect of high temperature on stress sensitivity of porosity and permeability. High temperature can increase the plasticity of the rock, intensify the compression of pores due to high confining pressure, and induce thermal stress between the rock skeleton particles, allowing the reopening of shale bedding or the creation of new fractures along weak planes such as bedding, which inhibits the decrease of permeability with the increase of temperature and confining pressure. Compared with the triaxial mechanical experiment at normal temperature, the triaxial compression experiment at high temperature and high pressure demonstrates that the compressive strength and peak strain of deep shale increase significantly due to the coupling of temperature and pressure. The compressive strength is up to 435 MPa and the peak strain exceeds 2%, indicating that high temperature is not conducive to fracture initiation and expansion by increasing rock plasticity. Lithofacies and mineral composition have great impacts on the porosity, permeability and rock mechanics of deep shale. Shales with different lithologies are different in the difficulty and extent of brittle failure. The stress-strain characteristics of rocks under actual geological conditions are key support to the optimization of reservoir stimulation program.

Computed X-ray Tomography Investigation of Porosity and Permeability of the Liujiagou Formation Sandstone Exposed to CO_(2)-Saturated Brine

In order to improve CO_(2) capture,utilization and storage(CCUS) to solve carbon emission,sandstone from the Triassic Liujiagou Formation(LF) from the Ordos Basin in China was investigated using permeability tests and computed X-ray tomography(CT) scanning.The presence of reactive minerals within the geological CO_(2) sequestration target storage formation can allow reaction with injected CO_(2),which changes the porosity and permeability of the LF beds,affecting storage effectiveness.To investigate the effect of chemical reactions on the pore structure and permeability of sandstone cores representing the LF CO_(2) storage,tests were conducted to analyze the changes in porosity and permeability of sandstone cores induced by CO_(2)-saturated brine at different reaction times(28-day maximum reaction period).Porosity and permeability of the sandstone increased after reaction with CO_(2)-saturated brine due to mineral dissolution.The sandstone exhibited an increase in porosity and permeability after 15 days of reaction with CO_(2)-saturated brine.Moreover,there was an increase in the volume of large pores in the sandstone after the 28-day period.The pore network of the sandstone was established through CT results,and the porosity calculated based on the obtained pore network was close to that measured in the test,demonstrating the feasibility to use CT to study the evolution of the microstructure of sandstone after long-time exposure to CO_(2)-saturated brine.

Characterization of reservoir properties and pore structure based on micro-resistivity imaging logging: Porosity spectrum, permeability spectrum, and equivalent capillary pressure curve

According to the capillary theory,an equivalent capillary model of micro-resistivity imaging logging was built.On this basis,the theoretical models of porosity spectrum(Ф_(i)),permeability spectrum(K_(i))and equivalent capillary pressure curve(pe)were established to reflect the reservoir heterogeneity.To promote the application of the theoretical models,the Archie's equation was introduced to establish a general model for quantitatively characterizing bi,K,and pei.Compared with the existing models,it is shown that:(1)the existing porosity spectrum model is the same as the general equation of gi;(2)the Ki model can display the permeability spectrum as compared with Purcell's permeability model;(3)the per model is constructed on a theoretical basis and avoids the limitations of existing models that are built only based on the component of porosity spectrum,as compared with the empirical model of capillary pressure curve.The application in the Permian Maokou Formation of Well TsX in the Central Sichuan paleo-uplift shows that the Ф_(i),K_(i),and p_(ci) models can be effectively applied to the identification of reservoir types,calculation of reservoir properties and pore structure parameters,and evaluation of reservoir heterogeneity.

Mechanical and hydraulic properties of carbonate rock:The critical role of porosity

Carbonate rocks are extensively used in civil infrastructure and play a critical role in geoenergy geoengineering,either as hydrocarbon reservoirs or potential repositories for CO_(2)geological storage.Carbonate genesis and diagenetic overprint determine the properties of carbonate rocks.This study combines recent data gathered from Madison Limestone and an extensive dataset compiled from published sources to analyze the hydraulic and mechanical properties of limestone carbonate rocks.Physical models and data analyses recognize the inherently granular genesis of carbonate rocks and explain the strong dependency of physical properties on porosity.The asymptotically-correct power model in terms of(1-Ф/Ф*)a is a good approximation to global trends of unconfined stiffness E and unconfined compressive strength UCS,cohesive intercept in Mohr-Coulomb failure envelopes,and the brittle-toductile transition stress.This power model is the analytical solution for the mechanical properties of percolating granular structures.We adopted a limiting granular porosityФ*=0.5 for all models,which was consistent with the loosest packing of monosize spheres.The fitted power model has exponent(α=2)in agreement with percolation theory and highlights the sensitivity of mechanical properties to porosity.Data and models confirm a porosity-independent ratio between unconfined stiffness and strength,and the ratio follows a log-normal distribution with mean(E/UCS)≈300.The high angle of internal shear strength measured for carbonate rocks reflects delayed contact failure with increased confinement,and it is not sensitive to porosity.Permeability spans more than six orders of magnitude.Grain size controls pore size and determines the reference permeability k^(*)at the limiting porosityФ*=0.5.For a given grain size from fine to coarse-grained dominant carbonates,permeability is very sensitive to changes in porosity,suggesting preferential changes in the internal pore network during compaction.

Porosity and permeability variations of a dam curtain during dissolution

During reservoir operation,the erosion effects of groundwater change the porosity and permeability of the dam curtain,causing changes to the seepage field.To understand where the changes take place and to what degree the porosity and permeability change,a multi-field coupling model was built and solved.The model takes into account seepage,solution concentration,and solid structure.The model was validated using uplift pressure monitoring data.Then,the variations in curtain porosity,seepage flow,and loss quantity of Ca(OH)2 were calculated.The key time nodes were obtained through curve fitting of the variation of seepage flow with the BiDoseResp function.The results showed that the model could reflect the attenuation trend of curtain performance well.The process and position of the erosion were not homogeneous.Although erosion mainly occurred at the top and bottom of the curtain,it was most developed at the top.The erosion effects developed slowly during the early stage,much fast during the middle and late stages,and culminated in complete dissolution.The model results and the daily monitoring data can provide a scientific basis for the safe operation and management of reservoirs.

Stress dependent permeability and porosity of low-permeability rock

The seepage property of low-permeability rock is of significant importance for the design and safety analysis of underground cavities. By using a self-developed test system, both permeability and porosity of granite from an underground oil storage depot were measured. In order to study the influence of rock types on permeability, a tight sandstone was selected as a contrast. The experimental results suggested that the porosity of this granite is less than 5% and permeability is low to 10–20 m^2 within the range of effective stress. During the loading process, both exponential relationship and power law can be utilized to describe the relationship between effective stress and permeability. However, power law matches the experimental data better during the unloading condition. The stress dependent porosity of granite during loading process can be described via an exponential relationship while the match between the model and experimental data can be improved by a power law in unloading paths. The correlation of permeability and porosity can be described in a power law form. Besides, granite shows great different evolution rules in permeability and porosity from sandstone. It is inferred that this difference can be attributed to the preparing of samples and different movements of microstructures subjected to effective stress.

Influence of fly ash fineness and shape on the porosity and permeability of blended cement pastes

The effects of the fineness and shape of fly ash on the porosity and air permeability of cement pastes were investigated. Pulverized coal combustion （PCC） fly ash and fluidized bed coal combustion （FBC） fly ash classified into three different finenesses were used. River sand with particle size distribution similar to that of fly ash was also used for comparison. Portland cement was replaced with fly ash and ground sand at the dosages of 0, 20wt%, and 40wt%. A water-to-binder ratio （w/b） of 0.35 was used throughout the experiment. The results show that the porosity and air permeability of the pastes are influenced by the shape, fineness, and replacement level of fly ash. The porosity and air permeability of FBC fly ash pastes are higher than those of PCC fly ash pastes. This is due to the higher irregular shape and surface of FBC fly ash compared to the spherical shape and relatively smooth surface of PCC fly ash. The porosity increases with the increase in fly ash replacement level and decreases with the increase in its fineness. The permeability of PCC fly ash pastes decreases with the increase in replacement level and fineness, while for FBC fly ash, the permeability increases with the increase in replacement level. Decreases in porosity and permeability are due to a combined effect of the packing of fine particles and the reaction of fly ash.

How moisture loss affects coal porosity and permeability during gas recovery in wet reservoirs?

Moisture in coal seams changes gas adsorption capacity, induces coal deformation, and affects coal porosity. However, fewer studies have investigated the dynamic process of moisture loss. In this study, a fully coupled multi-physical model for coal deformation, gas flow and moisture loss was implemented. It validated the coal-gas-moisture interactions of the decay of gas adsorption capacity and the coal shrinkage.Subsequently, the proposed model was applied to a simulation of coal seam gas recovery from wet reservoir and solved using the finite method in COMSOL Multiphysics 3.5. Analyses of the component factors and the sensitive parameters of moisture loss on coal porosity and permeability were comprehensively studied at last. The results reveal that moisture loss enhances coal porosity and permeability. The decay of gas adsorption capacity decreases coal permeability while the coal shrinkage promotes it. The decrease of the adsorption decay coefficient and the increase of the initial density of saturated water vapor and water evaporation constant can enhance the permeability of wet coal seams.

Theoretical method for calculating porosity and permeability under self-regulating effect

In order to study the effect of stress-sensitivity and matrix shrinkage on coalbed methane production,equivalent matrix particle model is proposed considering the process of adsorption and desorption.Calculating mathematical models for calculating porosity and permeability which considered matrix shrinkage by combining diameter model and desorption were established.The calculations of porosity and permeability under self-regulating effect were obtained by combining traditional stress-sensitivity equations.The changes of porosity and permeability in different reservoirs were calculated and analyzed through a variety of basic parameters.The results show that high coal rank reservoir has the biggest range ability of porosity and permeability under the same pressure difference conditions,followed by the middle rank and the low rank.The research observed the positive relationship between stress-sensitivity and declining period of porosity and permeability in low rank coal reservoir,and the inverse relationship between matrix shrinkage and declining period of porosity and permeability.The stronger the matrix shrinkage is,the earlier declining period and rise period will occur.

The primary controlling parameters of porosity, permeability,and seepage capability of tight gas reservoirs:a case study on Upper Paleozoic Formation in the eastern Ordos Basin,Northern China

Tight sandstone gas(hereafter"tight gas")has become a subject of unconventional gas exploration globally.The large-scale development and use of tight gas resources in the USA,in particular,facilitated the rapid rebound of natural gas production in the USA,in addition to driving the rapid development of tight gas worldwide.In the eastern Ordos Basin,the Upper Paleozoic feature includes multiple layers of gas,a shallow depth,and notable potential for exploration and development.However,the reservoirs in the area are relatively tight,exhibit strong heterogeneity,and possess a complex micropore structure,thus restricting the eff ective economic development of oil and gas.Thus,research on the primary parameters controlling pore throat structure and the seepage capability of low-permeability reservoirs will be beneficial for the effcient exploration and development of natural gas in the eastern Ordos Basin.The parameters of reservoir porosity and percolation ability,as well as permeability,were analyzed using systematic sampling of the of the Upper Paleozoic Benxi,Taiyuan,and Shanxi Formations in the eastern Ordos Basin,constant-rate mercury injection experiments,nuclear magnetic resonance analysis,and gas–water-phase experimental studies.The results indicate that reservoir porosity is controlled by the effective pore volume and number,whereas permeability is controlled by the largest throat radius,rather than the average.The effective pore volume controls the movable fluid saturation,while reservoir percolation capability is controlled by the effective pore volume,irreducible water saturation,and size of the gas–water two-phase seepage zone.

Resistivity response to the porosity and permeability of low rank coal

Laojunmiao coal samples from the eastern Junggar basin were studied to understand the relationship between coal resistivity and the physical parameters of coal reservoirs under high temperatures and pressures.Specifically,we analysed the relationship of coal resistivity to porosity and permeability via heating and pressurization experiments.The results indicated that coal resistivity decreases exponentially with increasing pressure.Increasing the temperature decreases the resistivity.The sensitivity of coal resistivity to the confining pressure is worse when the temperature is higher.The resistivity of dry coal samples was linearly related to φ~m.Increasing the temperature decreased the cementation exponent(m).Increasing the confining pressure exponentially decreases the porosity.Decreasing the pressure increases the resistivity and porosity for a constant temperature.Increasing the temperature yields a quadratic relationship between the resistivity and permeability for a constant confining pressure.Based on the Archie formula,we obtained the coupling relationship between coal resistivity and permeability for Laojunmiao coal samples at different temperatures and confining pressures.

Feasibility Study on Chemical Flooding in Super High Porosity and Permeability Heavy Oil Reservoir

In this paper, the feasibility study of chemical flooding is carried out for ultra-high porosity and high permeability heavy oil field with permeability higher than 10 μm2 and porosity greater than 35%. The viscosity-concentration relationship of four kinds of oil flooding systems such as hydrolyzed polyacrylamide, structural polymer A, structural polymer B and gel was studied. The results showed that the viscosity of ordinary polymer and structural polymer B was lower compared with other two types of oil displacement agents, and the viscosity of structural polymer A was higher. The higher the concentration, the higher the viscosity retention rate. The gel system has the highest viscosity and best anti-shear ability. The resistance coefficient and residual resistance coefficient of structural polymer A and gel system were further studied. The results show that permeability, velocity and polymer concentration all affect the resistance coefficient and residual resistance coefficient. From the point of view of resistance establishment ability, it is considered that structural polymer A is not suitable for permeability formation above 10 μm2. Gel system has stronger ability to establish resistance coefficient than structural polymer A flooding system, and it is more feasible for formation system with permeability above 10 μm2.

Reflection of plane seismic waves at the surface of double-porosity dual-permeability materials

The present work deals with the reflection of plane seismic waves at the stress-free plane surface of double-porosity dualpermeability material. The incidence of two main waves（i.e., P1 and SV） is considered. As a result of the incident waves,four reflected（three longitudinal and one shear） waves are found in the medium. The expressions of reflection coefficients for a given incident wave are obtained as a non-singular system of linear equations. The energy shares of reflected waves are obtained in the form of an energy matrix. A numerical example is considered to calculate the partition of incident energy for fully closed as well as perfectly open pores. Effect of incident direction on the partition of the incident energy is analyzed with the change in wave frequency, wave-induced fluid-flow, pore-fluid viscosity and double-porosity structure. It has been confirmed from the numerical interpretation that during the reflection process, conservation of incident energy is obtained at each angle of incidence.

Investigating the Relationship between Porosity and Permeability Coefficient for Pervious Concrete Pavement by Statistical Modelling

A study evaluating the relationship between porosity and permeability coefficient for pervious concrete (PC) is presented. In addition, the effect of mixture design parameters particularly, water-to-cement ratio (W/C) and size of aggregate on the permeability coefficient of PC was investigated. The PC mixtures were made with 4 range of W/C and 2 range size of aggregate. PC mixes were made from each aggregate and were tested. The results showed that the W/C and aggregate size are key parameters which significantly affect the characteristic performance of PC. Permeability coefficient of coarse pervious concrete (CPC) is bigger than fine pervious concrete (FPC) and the porosity of CPC are bigger than porosity of FPC. A regression model (RM) along with analysis of variance (ANOVA) was conducted to study the significance of porosity distribution on permeability coefficient of PC. The statistical model developed in this study can facilitate prediction permeability coefficient of CPC and FPC as the sustainable pavements.

Reflection and refraction of plane waves at the boundary of an elastic solid and double-porosity dual-permeability materials

Phenomena of reflection and refraction of plane harmonic waves at a plane interface between an elastic solid and doubleporosity dual-permeability material are investigated. The elastic solid behaves non-dissipatively, while double-porosity dual-permeability materials behave dissipatively to wave propagation due to the presence of viscosity in pore fluids. All the waves(i.e., incident and reflected) in an elastic medium are considered as homogeneous(i.e., having the same directions of propagation and attenuation), while all the refracted waves in double-porosity dual-permeability materials are inhomogeneous(i.e., having different directions of propagation and attenuation). The coefficients of reflection and refraction for a given incident wave are obtained as a non-singular system of linear equations. The energy shares of reflected and refracted waves are obtained in the form of an energy matrix. A numerical example is considered to calculate the partition of incident energy among various reflected and refracted waves. The effect of incident direction on the partition of the incident energy is analyzed with a change in wave frequency, wave-induced fluid-flow, pore-fluid viscosity and double-porosity structure.It has been confirmed from numerical interpretation that during the reflection/refraction process, conservation of incident energy is obtained at each angle of incidence.

Modeling of multiphase flow in low permeability porous media:Effect of wettability and pore structure properties

Multiphase flow in low permeability porous media is involved in numerous energy and environmental applications.However,a complete description of this process is challenging due to the limited modeling scale and the effects of complex pore structures and wettability.To address this issue,based on the digital rock of low permeability sandstone,a direct numerical simulation is performed considering the interphase drag and boundary slip to clarify the microscopic water-oil displacement process.In addition,a dual-porosity pore network model(PNM)is constructed to obtain the water-oil relative permeability of the sample.The displacement efficiency as a recovery process is assessed under different wetting and pore structure properties.Results show that microscopic displacement mechanisms explain the corresponding macroscopic relative permeability.The injected water breaks through the outlet earlier with a large mass flow,while thick oil films exist in rough hydrophobic surfaces and poorly connected pores.The variation of water-oil relative permeability is significant,and residual oil saturation is high in the oil-wet system.The flooding is extensive,and the residual oil is trapped in complex pore networks for hydrophilic pore surfaces;thus,water relative permeability is lower in the water-wet system.While the displacement efficiency is the worst in mixed-wetting systems for poor water connectivity.Microporosity negatively correlates with invading oil volume fraction due to strong capillary resistance,and a large microporosity corresponds to low residual oil saturation.This work provides insights into the water-oil flow from different modeling perspectives and helps to optimize the development plan for enhanced recovery.

Experimental Study on Seismic Attenuation and Permeability of Large Porosity Rock

The large porosity areas widely present in the underground resulting from natural hazards or artificial damages. The porosity and permeability are suggested to be capable of estimating the mechanical and air flow conditions inside the porous layer in the underground. To accurately measure the porosity and permeability in the porous area is imperative. To address this issue, we experimentally modeled some porous samples in large porosities by using sandstone particles sieved to different sizes. Ultrasonic was employed to apply on the porous sandstone samples to characterize the seismic velocity and attenuation. Permeability was also measured simultaneously to find a correlation with the porosity. The results showed the seismic attenuation decrease as the reduction of frequency and increasing particle size at the same porosity. Seismic attenuation was strongly correlated to porosity and particle size. Velocity showed a good relationship with the porosity change. Permeability was highly dependent on the particle size especially in the higher porosity range. The results indicated that it is possible to find a relationship between the permeability and seismic attenuation via the porosity and particle size.

Characterization of Natural Vuggy Fractured Porous Medium: Its Physicochemical Properties, Porosity and Permeability

The aim of this work was to obtain the physicochemical properties by SEM, XRD, FTIR analyses and the surface characteristics from carbonate outcrops cores such as pore diameter, surface area, porosity and permeability. The methods used to characterize them were Scanning Electron Microscopy, SEM;X Ray Diffraction, DRX;Fourier Transform Infrared Spectroscopy, FTIR. The porosity and permeability of natural vuggy fractured porous medium from core samples was determined obtained in the laboratory with conventional procedures. The cores have smooth and rough surfaces with porous with several sizes. Some crystals appear in preferential zones mainly composed by calcium, carbon and oxygen. Apparently into free spaces were found the organic materials, organic residues of crude oil. The cores have smooth and rough surfaces with porous with several sizes. Some crystals appear in preferential zones composed by calcium, carbon and oxygen. Apparently into free spaces were found the organic materials, organic residues of crude oil. The main inorganic compound in cores is calcite, (CaCO3). The porosity was for porous core 26% and for solid core 8.5%. The values obtained show that the cores have permeability where the fluid migrates through the particles at 2.23 × 10-4 cm/s.

Diagenesis characteristics and their influence on porosity and permeability of sandstone from Yingcheng Formation in Jinshan field

In order to understand the diagenesis and its influence on porosity and permeability of sandstones from Yingcheng Formation in Jinshan field,applying thin sections,casting and scanning electron microscope,the authors studied petrography,diagenesis,porosity and permeability of Yingcheng Formation sandstone reservoir. The results show that the reservoir of Yingcheng Formation is mainly composed of lithic arkoses. Sandstones experienced compaction,pressolution,cementation,metasomatism and dissolution. The primary pores of the sandstones are undeveloped; most of pores are the secondary pores,which are mainly intragranular dissolution pores in feldspar and debris. The compaction and cementation reduced the primary pores in sandstones,the porosity and permeability decreased; while the dissolution formed the secondary pores,resulting in the porosity and permeability increase. Due to compaction and cementation,the porosity and permeability are reduced with increasing of depth. There exists an abnormal enlargement of porosity and permeability within a depth range 2310--2450 m. In combination with analysis of petrology,the dissolution of feldspar and debris is most developed in the depth range. Therefore,compaction,cementation and dissolution are the important factors which affect the porosity and permeability of sandstones.

Physical simulation of fluid flow and production performance in extra-low permeability porous media

For extra-low permeability reservoirs, with a permeability of about 0.3×10?3 μm2, fluid flow and production performance in cores were studied. A long core holder with a multi-location piezometric measurement was specially designed. An artificial long core, about 700 mm long and with a cross section of 45mm×45mm, was used. In the experiment, pressure distribution along the core can be measured in real time. Single phase flow in the core was investigated. Different modes of production in long cores were also simulated including natural depletion, water flooding, and advanced water flooding. Through physical simulation, flow parameters were collected and production characteristics in extra-low permeability cores were studied. From experimental results, it can be seen that fluid flow in extra-low permeability cores is different from that in high permeability cores. Transmission of pressure in extra-low permeability cores is very slow, and it needs a long time for the pressure to become stable. The distribution curve of pressure along the core is nonlinear and the production rate in extra-low permeability reservoirs decreases sharply. The development effects of different production modes in extra-low permeability cores were compared with one another. Among the production modes, advanced water flooding has much potential for effective development of extra-low permeability reservoirs. Natural depletion and conventional water flooding can also be used in early production periods. In addition, the countermeasures and some ideas especially for the potential development of extra-low permeability reservoirs are suggested.