A Numerical Simulation Study of Temperature and Pressure Effects on the Breakthrough Pressure of CO_(2)in Unsaturated Low-permeability Rock Core

Gas breakthrough pressure is a key parameter to evaluate the sealing capacity of caprock,and it also plays important roles in safety and capacity of CO_(2)geological storage.Based on the published experimental results,we present numerical simulations on CO_(2)breakthrough pressure in unsaturated low-permeability rock under 9 multiple P-T conditions(which can keep CO_(2)in gaseous,liquid and supercritical states)and thus,a numerical method which can be used to accurately predict CO_(2)breakthrough pressure on rock-core scale is proposed.The simulation results show that CO_(2)breakthrough pressure and breakthrough time are exponential correlated with P-T conditions.Meanwhile,pressure has stronger effects on experimental results than that of temperature.Moreover,we performed sensitivity studies on the pore distribution indexλ(0.6,0.7,0.8,and 0.9)in van Genuchten-Muale model.Results show that with the increase ofλ,CO_(2)breakthrough pressure and breakthrough time both show decreasing trends.In other words,the larger the value ofλis,the better the permeability of the caprock is,and the worse the CO_(2)sealing capacity is.The numerical method established in this study can provide an important reference for the prediction of gas breakthrough pressure on rock-core scale and for related numerical studies.

The stress-strain-permeability behaviour of clay rock during damage and recompaction

Characterisation and understanding of the stressestrainepermeability behaviour of a clay host rock during damage and recompaction are essential for prediction of excavation damaged zone and for assessment of its impact on the repository safety. This important issue has been experimentally studied in triaxial compression tests on the Callovo-Oxfordian clay rock in this study. The samples were sequentially loaded by(1) hydrostatic precompaction to close up sampling-induced microcracks,(2)applying deviatoric stresses to determine damage and permeability changes, and(3) recompression along different loading paths to examine reversibility of the damage. The critical stress conditions at the onset of dilatancy, permeability percolation, failure strength, and residual strength are determined. An empirical model is established for fracturing-induced permeability by considering the effects of connectivity and conductivity of microcracks. The cubic law is validated for the variation of permeability of connected fractures with closure. The experiments and results are also presented and discussed.

Digital core approach to the effects of clay on the electrical properties of saturated rocks using lattice gas automation

Clay has a significant influence on the relationship between resistivity index I and water saturation Sw （i.e, I-Sw relationship） of reservoir rocks because it complicates the current paths of these rocks. It is difficult to reveal the physical mechanisms of these clay effects on the conductivities of various rocks by physical laboratory measurements because the pore structure, micro distribution and content of clay inside a rock can not be observed and controlled during the experiments. We present a digital rock approach to study these clay effects on the electrical transport properties of reservoir rocks at pore scale using lattice gas automation （LGA） method. The digital rock samples are constructed with the information of grain size distribution from SEM images of reservoir rocks. The LGA is then applied on these digital rocks fully saturated with fluids to simulate the electrical transport properties for revealing the effects of volume and distribution patterns of clay on the non-Archie behaviors of the I-Sw relationship. The very good agreement between the simulated results and the laboratory measurements clearly demonstrates the validity of the LGA in numerical research of rock physics. Based on these studies, a new model has been developed for quantitatively describing the relationship between the saturation exponent and the volume of clay （Vsh）. This development may improve the evaluation for the fluid saturations in reservoir rocks.

A New Type of REE Deposit Found in Clay Rock at the Top of the Permian Emeishan Basalt in the Yunnan-Guizhou Area

Objective A set of REE-rich clay rocks is formed at the top of the Emeishan basalt in the eastern Yunnan-Western Guizhou area,accompanied by Nb,Zr,Ga.It is a new type of REE deposit and has great resource potential.This paper introduces its metallogenic conditions and element occurrence states,discusses the genetic mechanism.

Thermo-hydro-mechanical behavior of clay rock for deep geological disposal of high-level radioactive waste

In the context of deep geological disposal of radioactive waste in clay formations, the thermo-hydro- mechanical （THM） behavior of the indurated Callovo-Oxfordian and Opalinus clay rocks has been extensively investigated in our laboratory under repository relevant conditions： （1） rock stress covering the range from the lithostatic state to redistributed levels after excavation; （2） variation of the humidity in the openings due to ventilation as well as hydraulic drained and undrained boundary conditions; （3） gas generation from corrosion of metallic components within repositories; and （4） thermal loading from high-level radioactive waste up to the designed maximum temperature of 90 ~C and even beyond to 150 ~C, Various important aspects concerning the long-term barrier functions of the clay host rocks have been studied： （1） fundamental concept for effective stress in the porous clay-water system; （2） stress- driven deformation and damage as well as resulting permeability changes; （3） moisture influences on mechanical properties; （4） self-sealing of fractures under mechanical load and swelling]slaking of clay minerals upon water uptake; （5） gas migration in fractured and resealed claystones; and （6） thermal impact on the hydro-mechanical behavior and properties, Major findings from the investigations are summarized in this paper,

Examination of effective stress in clay rock

This paper examines the effective stress in indurated clay rock theoretically and experimentally.A stress concept is derived from the analysis of the microstructure and of the pore water in the highly-indurated Callovo-Oxfordian and Opalinus clay rocks,and subsequently validated by various experiments performed on these claystones.The concept suggests that the interparticle or effective stress in a dense clay ewater system is transferred through both the adsorbed interparticle pore water in narrow pores and the solidesolid contact between non-clay mineral grains.The experiments show that the adsorbed pore water in the claystones is capable of bearing deviatoric effective stresses up to the failure strength.The applied stresses are for the most part or even totally transferred by the bound pore water,i.e.the swelling pressure in the interparticle bound pore water is almost equivalent to the effective stress.This stress concept provides a reasonable view to the nature of the effective stress in argillaceous rock and forms the fundamental basis for studies of the hydro-mechanical properties and processes in clay formations.

Thermal effects on clay rocks for deep disposal of high-level radioactive waste

Thermal effects on the Callovo-Oxfordian and Opalinus clay rocks for hosting high-level radioactive waste were comprehensively investigated with laboratory and in situ experiments under repository relevant conditions:(1) stresses covering the range from the initial lithostatic state to redistributed levels after excavation,(2) hydraulic drained and undrained boundaries, and(3) heating from ambient temperature up to 90℃-120℃ and a subsequent cooling phase. The laboratory experiments were performed on normal-sized and large hollow cylindrical samples in various respects of thermal expansion and contraction, thermally-induced pore water pressure, temperature influences on deformation and strength, thermal impacts on swelling, fracture sealing and permeability. The laboratory results obtained from the samples are consistent with the in situ observations during heating experiments in the underground research laboratories at Bure and Mont-Terri. Even though the claystones showed significant responses to thermal loading, no negative effects on their favorable barrier properties were observed.

Patterns of Clay Minerals Transformation in Clay Gouge, with Examples from Revers Fault Rocks in Devonina Niqiuhe Formation in The Dayangshu Basin

The role of authigenic clay growth in clay gouge is increasingly recognized as a key to understanding the mechanics of berittle faulting and fault zone processes,including creep and seismogenesis,and providing new insights into the ongoing debate about the frictional strength of brittle fault(Haines and van der Pluijm,2012).However,neither the conditions nor the processes which

Clay Mineralogical and Geochemical Studies of Detrital Rocks of the Upper Cretaceous Salt-bearing Strata,Simao Basin,SW China:Implication for Provenance and Source Weathering

1 Introduction Geological studies established on several sections in Lanping-Simao basin have shown that the salt-bearing strata of Mengyejing formation(Yunlong Fm.in Lanping basin)are constituted by an alternation of salt layers and interbedded facies.The latter consists mainly of mudstones,and mudstone-rich conglomerate.The mineralogy and geochemistry of salt-bearing beds and

Geomechanical and water vapor absorption characteristics of clay-bearing soft rocks at great depth

The geological and physico-mechanical properties characterization of deep soft rocks is one of the critical scientific issues for deep soft rock engineering. In the present study, X-ray diffraction(XRD) analysis,scanning electron microscope(SEM), and mercury intrusion porosimetry experiments were carried out to investigate the mineral compositions, microstructure and porosity characteristics of the 13 claybearing soft rock samples collected from a deep coal mine in China. Water vapor absorption and uniaxial compressive experiments were also performed to examine water absorption characteristics and waterinduced strength degradation effect of the investigated deep soft rock samples. The results show that the dominant mineral components in mudstone, coarse sandstone and fine sandstone samples were calcite, quartz and clay respectively. The contents of clay minerals in all samples were relatively high and ranged from 12.3%(N-4) to 56.5%(XS-1). Water vapor absorption processes of all the soft rock samples follow an exponential law which is very similar to the water vapor absorption behavior of conglomerate samples reported in our earlier study. Correlation analyses also suggested that there were good positive correlation relationships between water absorptivity and clay minerals for both mudstone and sandstone samples. Furthermore, it was found that vapor absorption was not correlated with the porosity for mudstone, however, positive correlation relationship was found between them for sandstone. Correlation analysis between UCS, modulus of elasticity and water content demonstrated that both of them tend to decrease with the increase of their water content due to water absorption.

RESEARCH ON THE SEROSITY DURING INJECTING CLAY GROUTS INTO THE OVERLYING ROCK STRATA

On the basis of the mechanism study of injecting clay grouts into overlying strata, the clay grouts are researched in greater detail from three aspects. The flowing state of clay grouts in the strata——the pattern of different direction flowing around a point source is advanced and the flowing equation is put forward which is correspond with experiment result, and the corresponding mechanical model is set up which has its formulistic study, and the function of clay grouts is also discussed after the water in it has been lost, at the same time the concept of similar rock in effective supporting zone is given. It would draw great positive inspiration from what studied in this paper for studying on drawing down the surface subsidence by injecting.

Research on anisotropy of shale oil reservoir based on rock physics model

Rock physics modeling is implemented for shales in the Luojia area of the Zhanhua topographic depression. In the rock physics model, the clay lamination parameter is introduced into the Backus averaging theory for the description of anisotropy related to the preferred alignment of clay particles, and the Chapman multi-scale fracture theory is used to calculate anisotropy relating to the fracture system. In accordance with geological features of shales in the study area, horizontal fractures are regarded as the dominant factor in the prediction of fracture density and anisotropy parameters for the inversion scheme. Results indicate that the horizontal fracture density obtained has good agreement with horizontal permeability measured from cores, and thus confirms the applicability of the proposed rock physics model and inversion method. Fracture density can thus be regarded as an indicator of reservoir permeability. In addition, the anisotropy parameter of the P-wave is higher than that of the S-wave due to the presence of horizontal fractures. Fracture density has an obvious positive correlation with P-wave anisotropy, and the clay content shows a positive correlation with S-wave anisotropy, which fully shows that fracture density has a negative correlation with clay and quartz contents and a positive relation with carbonate contents.

Anisotropy rock physics model for the Longmaxi shale gas reservoir,Sichuan Basin,China

The preferred orientation of clay minerals dominates the intrinsic anisotropy of shale. We introduce the clay lamination （CL） parameter to the Backus averaging method to describe the intrinsic shale anisotropy induced by the alignment of clay minerals. Then, we perform the inversion of CL and the Thomsen anisotropy parameters. The direct measurement of anisotropy is difficult because of the inability to measure the acoustic velocity in the vertical direction in boreholes and instrument limitations. By introducing the parameter CL, the inversion method provides reasonable estimates of the elastic anisotropy in the Longmaxi shale. The clay content is weakly correlated with the CL parameter. Moreover, the parameter CL is abnormally high at the bottom of the Longmaxi and Wufeng Formations, which are the target reservoirs. Finally, we construct rock physics templates to interpret well logging and reservoir properties.

Research on fabric characteristics and borehole instability mechanisms of fractured igneous rocks

There are favorable exploration prospects in igneous rock reservoirs. However, problems of borehole instability occur frequently during drilling igneous formations, which is a serious impediment to oil and gas exploration and production. The lack of systematic understanding of the inherent instability mechanisms is an important problem. A series of experiments were conducted on several igneous rock samples taken from the sloughing formations in the Tuha area in an attempt to reveal the inherent mechanisms of wellbore instability when drilling in fractured igneous rocks. Research methods involved slurry chemistry, analysis of micro-geological features （Micro-CT imaging, SEM）, and rock mechanics testing. The experimental results indicated that clay minerals were widely distributed in the intergranular space of the diagenetic minerals, crystal defects, and microcracks. Drilling fluid filtrate would invade the rock along the microcracks. Tile invasion amount gradually increased over time, which constantly intensified the hydration and swelling of clay minerals, leading to changes in the microscopic structure of igneous rocks. Primary and secondary microcracks can propagate and merge into single cracks and thus reducing rock cohesion and the binding force along cleavage planes. Based on this result the authors propose that a key towards solving wellbore instability in igneous formations is that specific micro-geological characteristics of the igneous rocks should be taken into consideration in the design of antisloughing drilling muds.

Mechanical behavior of sandy facies of Opalinus Clay under different loadconditions

The mechanical behavior of sandy facies of Opalinus Clay at the Mont Terri underground rock laboratory(URL)in Switzerland was investigated with drained and undrained triaxial compression and extension,cyclic compression,and creep tests.Samples were taken from boreholes drilled parallel to bedding.Most of the samples were reconditioned to minimize sampling effects of desaturation and micro-cracking.The compression was accomplished by increasing axial stress at constant radial stress.The extension was carried out by increasing radial stress at constant axial stress.Moreover,extension was also achieved by simultaneously increasing radial stress and decreasing axial stress under constant mean stress.The test results showed elastoplastic stress-strain behavior with volumetric compaction until onset of dilatancy at high deviatoric stresses above 80%-90%of the peak failure strength.The strength is dependent upon load path and mean stress.The strength under triaxial compression is higher than that under extension.The respective strength increases with increasing mean stress.Desaturation enhances the stiffness and strength of the claystone.The deformation and strength of the elaystone are time-dependent.Under constant deviatoric stress,the claystone crept continuously with time,which can be characterized by a transient phase and a following stationary phase,and even a tertiary phase at high deviatoric stresses to rupture.

Deformation mechanism of roadways in deep soft rock at Hegang Xing’an Coal Mine

Engineering geomechanics characteristics of roadways in deep soft rock at Hegang Xing'an Coal Mine were studied and the nature of clay minerals of roadway surrounding rock was analyzed. This paper is to solve the technical problems of high stress and the difficulty in supporting the coal mine, and provide a rule for the support design. Results show that mechanical deformation mechanisms of deep soft rock roadway at Xing'an Coal Mine is of ⅠABⅡABCⅢABCD type, consisting of molecular water absorption (the ⅠAB -type), the tectonic stress type + gravity deformation type + hydraulic type (the ⅡABC -type), and the ⅢABCD -type with fault, weak intercalation and bedding formation. According to the compound mechanical deformation mechanisms, the corresponding mechanical control measures and conversion technologies were proposed, and these technologies have been successfully applied in roadway supporting practice in deep soft rock at Xing'an Coal Mine with good effect. Xing'an Coal Mine has the deepest burial depth in China, with its overburden ranging from Mesozoic Jurassic coal-forming to now. The results of the research can be used as guidance in the design of roadway support in soft rock.

Investigation on influence of drilling unloading on wellbore stability in clay shale formation

Wellbore collapse frequently happens in the clay shale formation.To maintain wellbore stability,appropriate mud pressure is a vital factor.When clay formation is opened,drilling unloading occurs,modifying rock structure and strength at the wall of borehole,which affects the selection of mud pressure.Currently,mechanism of drilling unloading is still poorly understood which in return will bring a concern to wellbore stability.Therefore,in this study,a combination of triaxial compressive test and ultrasonic wave test has been used to simulate drilling unloading and analyze its mechanism.Results indicate that more void space is created inside the clay shale sample due to unloading.This structure change leads to a decline of strength and acoustic amplitude.Additionally,unloading influence is depended on varying drilling unloading parameters.Small unloading range and fast unloading rate are able to enhance stability.With various degrees of unloading impact,collapse pressure equivalent density has a clear modification,proving that unloading is a non-negligible influencing factor of wellbore stability.Besides,the unloading effect is much stronger in large confining pressure,implying that more attention should be given to unloading when drilling is in extreme deep or high geostress formation.Findings in this paper can offer theoretical guidance for drilling in the clay shale formation.

Sealing of fractures in claystone

The sealing behavior of fractures in clay rocks for deep disposal of radioactive waste has been comprehensively investigated at the GRS laboratory. Various sealing experiments were performed on strongly cracked samples of different sizes from the Callovo-Oxfordian argillite and the Opalinus clay under rel- evant repository conditions. The fractured samples were compacted and flowed through with gas or synthetic pore-water under confining stresses up to 18 MPa and elevated temperatures from 20 ℃ to 90℃. Sealing of fractures was quantified by measurements of their closure and permeability. Under the applied thermo-hydro-mechanical （THM） conditions, significant fracture closure and permeability decrease to very low levels of 10^-19 to 10^-21 m^2 were observed within time periods of months to years. The properties of the resealed claystones are comparable with those of the intact rock mass. All test results suggest high sealing potentials of the studied claystones.

Advances in interaction mechanism of water(gas) on clay minerals in China

Dealing with large-scale deformations in soft-rock tunnels is a very important issue in soft-rock tunnel engineering. The mechanism of this large-scale deformation is closely related to the physical and chemical properties of soft rock, interaction between soft rock and water, and interaction between soft rock and gas contained in soft rock. In order to gain a better predictive understanding of the governing principles associated with this phenomenon, we used experimental and theoretical methods to study the effects of point defect on physical and chemical properties of soft rock and mechanism of interaction between water(gas) and soft rock. Firstly, we calculated the impurity formation energies and transition energy levels of defects by using the first-principle calculation, the results showed the microscopic mechanism of defects substitution in kaolinite and effects of defects on the structure of kaolinite. Moreover,comparing the experimental and theoretical results, we found the mechanism of interaction between water and soft rock. The results show that water is one of the most important factors which can induce various kinds of geological disasters. At last, the interaction between soft rock and surrounding gas as CO2, CH4 and CO is disused, the influence of surrounding gas on soft rock should not be ignored.

Flow simulation considering adsorption boundary layer based on digital rock and finite element method

Due to the low permeability of tight reservoirs,throats play a significant role in controlling fluid flow.Although many studies have been conducted to investigate fluid flow in throats in the microscale domain,comparatively fewer works have been devoted to study the effect of adsorption boundary layer(ABL)in throats based on the digital rock method.By considering an ABL,we investigate its effects on fluid flow.We build digital rock model based on computed tomography technology.Then,microscopic pore structures are extracted with watershed segmentation and pore geometries are meshed through Delaunay triangulation approach.Finally,using the meshed digital simulation model and finite element method,we investigate the effects of viscosity and thickness of ABL on microscale flow.Our results demonstrate that viscosity and thickness of ABL are major factors that significantly hinder fluid flow in throats.