Petrophysical properties of deep Longmaxi Formation shales in the southern Sichuan Basin, SW China

Deep shale layer in the Lower Silurian Longmaxi Formation,southern Sichuan Basin is the major replacement target of shale gas exploration in China.However,the prediction of"sweet-spots"in deep shale gas reservoirs lacks physical basis due to the short of systematic experimental research on the physical properties of the deep shale.Based on petrological,acoustic and hardness measurements,variation law and control factors of dynamic and static elastic properties of the deep shale samples are investigated.The study results show that the deep shale samples are similar to the middle-shallow shale in terms of mineral composition and pore type.Geochemical characteristics of organic-rich shale samples(TOC>2%)indicate that these shale samples have a framework of microcrystalline quartz grains;the intergranular pores in these shale samples are between rigid quartz grains and have mechanical property of hard pore.The lean-organic shale samples(TOC<2%),with quartz primarily coming from terrigenous debris,feature plastic clay mineral particles as the support frame in rock texture.Intergranular pores in these samples are between clay particles,and show features of soft pores in mechanical property.The difference in microtexture of the deep shale samples results in an asymmetrical inverted V-type change in velocity with quartz content,and the organic-rich shale samples have a smaller variation rate in velocity-porosity and velocity-organic matter content.Also due to the difference in microtexture,the organic-rich shale and organic-lean shale can be clearly discriminated in the cross plots of P-wave impedance versus Poisson’s ratio as well as elasticity modulus versus Poisson’s ratio.The shale samples with quartz mainly coming from biogenic silica show higher hardness and brittleness,while the shale samples with quartz from terrigenous debris have hardness and brittleness less affected by quartz content.The study results can provide a basis for well-logging interpretation and"sweet spot"prediction of Longmaxi Formation shale gas reservoirs.

Origin of carbonate minerals and impacts on reservoir quality of the Wufeng and Longmaxi Shale, Sichuan Basin

The Ordovician-Silurian Wufeng and Longmaxi Shale in the Sichuan Basin were studied to understand the genesis and diagenetic evolution of carbonate minerals and their effects on reservoir quality. The results of geochemical and petrological analyses show that calcite grains have a negative Ce anomaly indicating they formed in the oxidizing environment of seawater. The high carbonate mineral contents in the margin of basin indicate that calcite grains and cores of dolomite grains appear largely to be of detrital origin. The rhombic rims of dolomite grains and dolomite concretions with the δ^(13)C of –15.46‰ and the enrichment of middle rare earth elements were formed during the sulfate-driven anaerobic oxidation of methane. The calcite in radiolarian were related to the microbial sulfate reduction for the abundant anhedral pyrites and δ^(13)C value of –11.34‰. Calcite veins precipitated in the deep burial stage with homogenization temperature of the inclusions ranging from 146.70 ℃ to 182.90 ℃. The pores in shale are mainly organic matter pores with pore size mainly in the range of 1–20 nm in diameter. Carbonate minerals influence the development of pores through offering storage space for organic matter. When calcite contents ranging from 10% to 20%, calcite grains and cement as rigid framework can preserve primary pores. Subsequently, the thermal cracking of liquid petroleum in primary pores will form organic matter pores. The radiolarian were mostly partially filled with calcite, which combining with microcrystalline quartz preserved a high storage capacity.

Effect of height-diameter ratio on the mechanical characteristics of shale with different bedding orientations

Geological exploration cores obtained from shale gas wells several kilometers deep often show different height-diameter ratios(H/D)because of complex geological conditions(core disking or developed fractures),which makes further standard specimen preparation for mechanical evaluation of reservoirs difficult.In multi-cluster hydraulic fracturing,shale reservoirs between planes of hydraulic fractures with different lengths could be simplified to have different H/D ratios.Discovering the effect of H/D on the mechanical characteristics of shale specimens with different bedding orientations will support mechanical evaluation tests of reservoirs based on disked geological cores and help to optimize multicluster fracturing programs.In this study,we performed uniaxial compression tests and acoustic emission(AE)monitoring on cylindrical Longmaxi shale specimens under five bedding orientations and four H/D ratios.The experimental results showed that both the H/D-dependent mechanical properties and AE parameters demonstrated significant anisotropy.Increasing H/D did not change the uniaxial compressive strength(UCS)evolution versus bedding orientation,demonstrating a V-shaped relationship,but enhanced the curve shape.The stress level of crack damage for the specimens significantly increased with increasing H/D,excluding the specimens with a bedding orientation of 0°.With increasing H/D,the cumulative AE counts of the specimens with each bedding orientation tended to exhibit a stepped jump against the loading time.The proportion of low-average-frequency AE signals(below 100 kHz)in specimens with bedding orientations of 45°and 60°increased to over 70%by increasing H/D,but it only increased to 40%in specimens with bedding orientations of 0°,30°,and 90°.Finally,an empirical model that can reveal the effect of H/D on anisotropic UCS of shale reservoir was proposed,the anisotropic proportion of tensile and shear failure cracks in specimens under four H/D ratios was classified based on the AE data,and the effect of H/D on the anisotropic crack growth of specimens was discussed.

A modified 3D mean strain energy density criterion for predicting shale mixed-mode I/III fracture toughness

The fracture toughness of rocks is a critical fracturing parameter in geo-energy exploitation playing a significant role in fracture mechanics and hydraulic fracturing.The edge-notched disk bending(ENDB)specimens are employed to measure the entire range of mixed-modeⅠ/Ⅲfracture toughness of Longmaxi shale.To theoretically interpret the fracture mechanisms,this research first introduces the detailed derivations of three established fracture criteria.By distinguishing the volumetric and distortional strain energy densities,an improved three-dimensional mean strain energy density(MSED)criterion is proposed.As the critical volumetric to distortional MSED ratio decreases,the transition from tensiondominated fracture to shear-dominated fracture is observed.Our results indicate that both peak load and applied energy increase significantly with the transition from pure mode I(i.e.,tension)to pure modeⅢ(i.e.,torsion or tearing)since mode-Ⅲcracking happens in a twisted manner and mode-Ⅰcracking occurs in a coplanar manner.The macroscopic fracture signatures are consistent with those of triaxial hydraulic fracturing.The average ratio of pure mode-Ⅲfracture toughness to pure mode-Ⅰfracture toughness is 0.68,indicating that the obtained mode-Ⅲfracture resistance for a tensionbased loading system is apparent rather than true.Compared to the three mainstream fracture criteria,the present fracture criterion exhibits greater competitiveness and can successfully evaluate and predict mixed-modeⅠ/Ⅲfracture toughness of distinct materials and loading methods.

Reservoir Characteristics and Preservation Conditions of Longmaxi Shale in the Upper Yangtze Block, South China

The Upper Ordovician-Lower Silurian Longmaxi Shale in the Upper Yangtze block represents one of the most important shale gas plays in China. The shale composition, porosity, organic thermal maturity, and methane sorption were investigated at the Qilongcun section in the Dingshan area, southeastern Sichuan Basin. The results show that the Upper Ordovician-Lower Silurian Longmaxi Shale contains： （1） sapropelic I organic matter; （2） a 40-m thick bedded sequence where total organic carbon （TOC） content is 〉 2%; （3） a 30-m thick layer at the base of the Longmaxi Shale with a brittle mineral content higher than 50%; and （4） a mean methane adsorption capacity of 1.80 cm3/g （7 MPa pressure）. A positive correlation between TOC and sorbed gas indicates that organic matter content exerts an important control on methane storage capacity. Based on the analysis of the shale reservoir characteristics, the lower member of the Longmaxi Shale can thus be considered a favorable stratum for shale gas exploration and exploitation. It has similar reservoir characteristics with the Longmaxi Shale in the Jiaoshiba area tested with a high-yield industrial gas flow. However, based on tectonic analysis, differences in the level of industrial gas flow between the low-yield study area and the high-yield Jiaoshiba area may result from different tectonic preservation conditions. Evidence from these studies indicates the shale gas potential of the Longmaxi Shale is constrained by the reservoir and preservation conditions.

Pore Types and Quantitative Evaluation of Pore Volumes in the Longmaxi Formation Shale of Southeast Chongqing, China

The common microscale to nanoscale pore types were introduced and divided into organic and inorganic pores to estimate their contributions to porosity in the Lower Silurian Longmaxi Formation shale of southeast Chongqing. Following the material balance principle, the organic porosity values, which changed with formation subsidence and thermal evolution, were calculated using chemical kinetics methods and corrected via the organic porosity correction coefficient, which was obtained from field emission scanning electron microscopy. Grain density values were determined using the contents and true densities of compositions in the shale samples. The total porosity was calculated based on the grain and bulk densities. The inorganic porosity was determined from the difference between the total porosity and organic porosity at the same depth. The results show that inorganic pores mainly contain microfractures, microchannels, clay intergranular pores, intercrystalline pores and intracrystalline pores in the Lower Silurian Longmaxi Formation shale of southeast Chongqing. Organic pores mainly include organopore and fossil pore. The total porosity, organic porosity and inorganic porosity of organic-rich shale samples can be quantitatively evaluated using this method. The total porosity, organic porosity and inorganic porosity values of the Longmaxi Formation shale samples from the well Pyl in southeast Chongqing lie in 2.75%-6.14%, 0.08%-2.52% and 1.41%-4.92% with average values of 4.34%, 0.95% and 3.39%, respectively. The contributions of the inorganic pores to the total porosity are significantly greater than those of the organic pores.

Physical Mechanism of Organic Matter-Mineral Interaction in Longmaxi Shale,Sichuan Basin,China

Objective Shale gas is as an important kind of unconventional natural gas,with a great resource potential,and its exploration and development has attracted much attention around the world.Organic matter（OM）pores are a common constituent in shales and form the dominant pore network of many shale gas systems.

Source Rock and Cap Rock Controls on the Upper Ordovician Wufeng Formation–Lower Silurian Longmaxi Formation Shale Gas Accumulation in the Sichuan Basin and its Peripheral Areas

Objective The Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation is one of the priority interval for shale gas exploration in the Sichuan Basin and its peripheral areas, and commercial shale gas has been discovered from this interval in Jiaoshiba, Changning and Weiyuan shale gas fields in Sichuan Province. However, there is no significant discovery in other parts of the basin due to the different quality of black shale and the differences of tectonic evolution. Based on the progress of shale gas geological theory and exploration discoveries, as well as the theory of ＂source rock and cap rock controls on hydrocarbon accumulation＂, of the Upper Ordovician the main controlling factors Wufeng Formation-Lower Silurian Longmaxi Formation shale gas enrichment in the Sichuan Basin and its peripheral areas were analyzed, and the source rock and cap rock controls on the shale gas were also discussed. The results can provide new insights for the next shale gas exploration in this area.

A Strain Rate Dependent Constitutive Model for the Lower Silurian Longmaxi Formation Shale in the Fuling Gas Field of the Sichuan Basin,China

Shale,as a kind of brittle rock,often exhibits different nonlinear stress-strain behavior,failure and timedependent behavior under different strain rates.To capture these features,this work conducted triaxial compression tests under axial strain rates ranging from 5×10-6 s-1 to 1×10-3 s-1.The results show that both elastic modulus and peak strength have a positive correlation relationship with strain rates.These strain rate-dependent mechanical behaviors of shale are originated from damage growth,which is described by a damage parameter.When axial strain is the same,the damage parameter is positively correlated with strain rate.When strain rate is the same,with an increase of axial strain,the damage parameter decreases firstly from an initial value(about 0.1 to 0.2),soon reaches its minimum(about 0.1),and then increases to an asymptotic value of 0.8.Based on the experimental results,taking yield stress as the cut-off point and considering damage variable evolution,a new measure of micro-mechanical strength is proposed.Based on the Lemaitre’s equivalent strain assumption and the new measure of micro-mechanical strength,a statistical strain-rate dependent damage constitutive model for shale that couples physically meaningful model parameters was established.Numerical back-calculations of these triaxial compression tests results demonstrate the ability of the model to reproduce the primary features of the strain rate dependent mechanical behavior of shale.

Occurrence and influence of residual gas released by crush methods on pore structure in Longmaxi shale in Yangtze Plate, Southern China

The composition of gas released under vacuum by crushing from the gas shale of Longmaxi Formation in Upper Yangtze Plate,Southern China was systematically investigated in this study.The effect of residual gas release on pore structures was checked using low-pressure nitrogen adsorption techniques.The influence of particle size on the determination of pore structure characteristics was considered.Using the Frenkel-Halsey-Hill method from low-pressure nitrogen adsorption data,the fractal dimensions were identified at relative pressures of 0‒0.5 and 0.5‒1 as D1 and D2,respectively,and the evolution of fractal features related to gas release was also discussed.The results showed that a variety component of residual gas was released from all shale samples,containing hydrocarbon gas of CH4(29.58%‒92.53%),C2H6(0.97%‒2.89%),C3H8(0.01%‒0.65%),and also some non-hydrocarbon gas such as CO2(3.54%‒67.09%)and N2(1.88%‒8.07%).The total yield of residual gas was in a range from 6.1μL/g to 17.0μL/g related to rock weight.The geochemical and mineralogical analysis suggested that the residual gas yield was positively correlated with quartz(R^2=0.5480)content.The residual gas released shale sample has a higher surface area of 17.20‒25.03 m^2/g and the nitrogen adsorption capacity in a range of 27.32‒40.86 ml/g that is relatively higher than the original samples(with 9.22‒16.30 m^2/g and 10.84‒17.55 ml/g).Clearer hysteresis loop was observed for the original shale sample in nitrogen adsorption-desorption isotherms than residual gas released sample.Pore structure analysis showed that the proportions of micro-,meso-and macropores were changed as micropores decreased while meso-and macropores increased.The fractal dimensions D1 were in range from 2.5466 to 2.6117 and D2 from 2.6998 to 2.7119 for the residual gas released shale,which is smaller than the original shale.This factor may indicate that the pore in residual gas released shale was more homogeneous than the original shale.The results indicated that both residual gas and their pore space have few contributions to shale gas production and effective reservoir evaluation.The larger fragments samples of granular rather than powdery smaller than 60 mesh fraction of shale seem to be better for performing effective pore structure analysis to the Longmaxi shale.

The new multistage water adsorption model of Longmaxi Formation shale considering the spatial configuration relationship between organic matter and clay minerals

The water adsorption by shale significantly affects shale gas content and its seepage capacity.However,the mechanism of water adsorption by shale is still unclear due to its strong heterogeneity and complicated pore structure.The relationship between the adsorbed water content at different relative humidities(RHs)and shale compositions,as well as shale pore structure and the spatial configuration relationship between organic matter(OM)and clay minerals,was investigated to clarify the controlling factors and mechanisms of water adsorption by Longmaxi Formation shale from the Southern Sichuan Basin in China.Consequently,the water adsorption process could be generally divided into three different stages from 0%RH to 99%RH.Furthermore,the Johnston’s clay mine ral interlayer pore structure model(JCM),the Freundlich model(FM)and the Dubinin-Astakhov model(DAM)were tested to fit the three water adsorption stages from low RH to high RH,respectively.The fitting results of the JCM and FM at lower RHs were far from good,while the fitting results of DAM at higher RHs were acceptable.Accordingly,two revised models(LRHM and MRHM)considering the spatial configuration relationship between OM and clay minerals were proposed for the two stages with lower RHs,and performed better fitting results indicating the pronounced effect of the spatial configuration relationship between OM and clay minerals on the water adsorption process of Longmaxi Formation shale.The outcomes of this study will contribute to clarifying the water distribution characteristics in the pore network of shale samples with variable water contents.

Study on brittleness templates for shale gas reservoirs——A case study of Longmaxi shale in Sichuan Basin,southern China

Differentiating brittle zones from ductile zones in low permeability shale formations is imperative for efficient hydraulic fracturing stimulation.The brittleness index(BI) is used to describe the rock resistance to hydraulic fracture initiation and propagation and measures the ease at which complex fracture networks can be created.In this study,we constructed brittleness templates through the correlation of fundamental rock properties and geomechanical characterization.We then employed the templates to distinguish the brittle,ductile,and brittle-ductile transition zones in the Longmaxi shale gas reservoir,Sichuan Basin of southern China.The approach works in two steps.First,we suggest a new expression for the mineralogical BI by their respective weights based on the analysis of correlation coefficients between mechanical testing and XRD results.Second,we correlate TOC,porosity,pore fluid,natural fractures,and improved BI model with multiple elastic properties to define the brittle,ductile,and transitional zones in the Longmaxi shale gas reservoir of China.Compared with the traditional mineralogy-based BI definition,the improved BI model differentiates the brittle and ductile zones and provides a better sense of the most suitable fracturing regions.Our results show that the brittleness templates,which combine fundamental rock properties,improved BI model,and geomechanical characterization led to identifying favorable zones for hydraulic fracturing and enhanced shale characterization.The proposed brittleness templates’ effectiveness was verified using data from horizontal wells,offset wells,shale gas wells from different origins,laboratory core testing,and seismic inversion of BI across the studied wells.

Logging prediction and evaluation of fracture toughness for the shales in the Longmaxi Formation, Southern Sichuan Basin

The mode-I fracture toughness is of great significance for evaluating the fracturing ability of shale reservoirs.In this study,the mode-I fracture toughness of the shale in the Lower Silurian Longmaxi Formation,Southern Sichuan Basin was determined with the Cracked Chevron Notched Brazilian Disc(CCNBD).Based on the experimental data,the relationships among the mode-I fracture toughness,the density,the acoustic time and the clay mineral content were analyzed.The shale samples for fracture toughness test from cores were commonly limited,the investigation of fracture toughness using well logs was necessary.Therefore,a prediction model was proposed by correlating the fracture toughness with well logs responses.The results indicate that the fracture toughness of shale samples is from 0.4744 MPa$m1/2 to 1.0607 MPa$m1/2 with an average of 0.7817 MPa$m1/2,indicating that the anisotropy of fracture toughness of the Longmaxi Formation shale.The clay mineral content and the density have a positive effect on the fracture toughness,whereas the acoustic time plays a negative role on the fracture toughness.The clay mineral content has an important effect on the relationships among fracture toughness,acoustic time and density.The prediction model can provide continuous data of mode-I fracture toughness along the wellbore for field hydraulic fracturing operation,and it has certain guiding significance in the exploration and development of oil and gas reservoirs.

Lithofacies and sedimentary characteristics of the Silurian Longmaxi Shale in the southeastern Sichuan Basin,China

The Silurian Longmaxi Shale (SLS) is the target for shale gas exploration and exploitation in the southeastern Sichuan Basin.Based on detailed description of cores and outcrops,analysis of Gamma Ray (GR) logs,thin sections and mineralogical data,the SLS can be subdivided into six lithofacies:(1) clayey shale,(2) siliceous calcareous shale,(3) siliceous dolomitic shale,(4) silty-clayey interlaminated shale,(5) muddy siltstone,and (6) graptolite shale.Three facies associations are recognized in the study area,i.e.,inner shelf,outer shelf,and bathyal plain (including turbidite) facies.Outer shelf and bathyal plain facies dominate the lower interval of the SLS,while inner shelf facies dominates the upper part of the SLS.The thickness of the black shale in the northern part of the study area is the greatest,where siliceous calcareous shale,siliceous dolomitic shale,and graptolite shale are predominant.These three kinds of shale are rich in total organic carbon (TOC) content and brittle minerals,which is favourable for the accumulation of shale gas and its future exploitation.

Methods for Shale Gas Play Assessment: A Comparison between Silurian Longmaxi Shale and Mississippian Barnett Shale

Based on field work, organic geochemical analyses and experimental testing, a six-property assessment method for shale gas is proposed. These six properties include organic matter properies, lithofacies, petrophysical properties, gas content, brittleness and local stress field. Due to the features of continuous distribution over a large area and low resource abundance in shale plays, a sweet spot should have these following properties： （a） TOC〉2%; （b） brittle minerals content （〉40%） and clay minerals （〈30%）; （c） Ro （〉1.1%）; （d） porosity （〉2%） and permeability （〉0.000 1 mD）, and （e） effective thickness （30-50 m）. Applying these criteria in the Sichuan Basin, the Silurian Longmaxi shale consists of four prospecting sweet spots, including blocks of Changning, Weiyuan, Zhaotong and Fushnn-Yongchuan. Although these four blocks have some similarities, different features were usually observed. After comprehensive analyses using the six-property assessment method, the Fushun-Yongan Block ranks the most favorable sweet spot, followed by the Weiyuan Block. For the other two blocks, the Changning Block is better than the Zbaotong Block. By comparing with the Mississippian Barnett shale, characteristics that are crucial for a high-yielding in the Sichuan Basin include a high content of organic matter （TOC〉2.5%）, a moderate thermal maturity （Ro=0.4%-2%）, a high content of brittle minerals （quartz： 30%-45%）, a high gas content （〉2.5 m^3·t^-1）, and types I and II1 kerogen.

Methane Adsorption Capacity Reduction Process of Water-Bearing Shale Samples and Its Influencing Factors: One Example of Silurian Longmaxi Formation Shale from the Southern Sichuan Basin in China

Due to the existence of water content in shale reservoir,it is quite meaningful to clarify the effect of water content on the methane adsorption capacity(MAC)of shale.However,the role of spatial configuration relationship between organic matter(OM)and clay minerals in the MAC reduction process is still unclear.The Silurian Longmaxi Formation shale samples from the Southern Sichuan Basin in China were prepared at five relative humidity(RH)conditions(0%,16%,41%,76%,99%)and the methane adsorption experiments were conducted on these water-bearing shale samples to clarify the MAC reduction process considering the spatial configuration relationship between clay minerals and OM and establish the empirical model to fit the stages.Total organic carbon(TOC)content and mineral compositions were analyzed and the pore structures of these shale samples were characterized by field-emission scanning electron microscopy(FE-SEM),N2 adsorption and high-pressure mercury intrusion porosimetry(HPMIP).The results showed that the MAC reduction of clay minerals in OM occurred at different RH conditions from that of clay minerals outside OM.Furthermore,the amount of MAC reduction of shale samples prepared at the same RH condition was negatively related with clay content,which indicated the protection role of clay minerals for the MAC of water-bearing shale samples.The MAC reduction process was generally divided into three stages for siliceous and clayey shale samples.And the MAC of OM started to decline during stage(1)for calcareous shale sample mainly because water could enter OM pores more smoothly through hydrophobic pathway provided by carbonate minerals than through hydrophilic clay mineral pores.Overall,this study will contribute to improving the evaluation method of shale gas reserve.

Insights into the Formation Mechanism of Low Water Saturation in Longmaxi Shale in the Jiaoshiba Area, Eastern Sichuan Basin

Investigating the variation of water content in shale reservoir is important to understand shale gas enrichment and evaluate shale gas resource potential.Low water saturation is widely spread in Longmaxi marine organic-rich shale.To illustrate the formation mechanism of low water saturation,this paper analyzed water saturation of Longmaxi shale reservoir,restored the history of natural gas carrying water capacity combining homogenization temperature and trapping pressure of fluid inclusion with simulated thermal history,and established a model to explain pore water displaced by natural gas during the thermal evolution.Results show that the gas-rich Longmaxi shale reservoir is characterized by low water saturation with measured values ranging from 9.81%to 48.21%and an average value of 28.22%.TOC in high-mature to over-high-mature Longmaxi organic-rich shale is negatively correlated with water saturation,indicating that well-connected organic pores are not available for water.However,quartz and clay mineral content are positively correlated with water saturation,which suggests that inorganic-matter-hosted pores are the main storage space for water formation.The water carrying capacity of natural gas varies as a function of gas generation and expulsion history,which displaces bound and movable water in organic pores that are part of bound and movable water from inorganic pores.The process can be divided into two phases.The first phase occurred due to the kerogen degradation into gas at Ro of 1.2%–1.6%with a water carrying capacity of natural gas ranging from 5632.57–7838.73 g/km3.The second phase occurred during the crude oil cracking into gas at Ro>1.6%with a water carrying capacity of natural gas ranging from 10620.04 and 19480.18 g/km3.The water displacement associated with natural gas generation and migration resulted in gas filling organic pores and gas-water coexisting in the brittle-mineral-hosted pores and clay-mineral-hosted pores.

Experimental study on hydration damage mechanism of shale from the Longmaxi Formation in southern Sichuan Basin, China

As a serious problem in drilling operation,wellbore instability restricts efficient development of shale gas.The interaction between the drilling fluid and shale with hydration swelling property would have impact on the generation and propagation mechanism of cracks in shale formation,leading to wellbore instability.In order to investigate the influence of the hydration swelling on the crack propagation,mineral components and physicochemical properties of shale from the Lower Silurian Longmaxi Formation(LF)were investigated by using the XRD analysis,cation exchange capabilities(CEC)analysis,and SEM observation,and we researched the hydration mechanism of LF shale.Results show that quartz and clay mineral are dominated in mineral composition,and illite content averaged 67%in clay mineral.Meanwhile,CEC of the LF shale are 94.4 mmol/kg.The process of water intruding inside shale along microcracks was able to be observed through high power microscope,meanwhile,the hydration swelling stress would concentrate at the crack tip.The microcracks would propagate,bifurcate and connect with each other,with increase of water immersing time,and it would ultimately develop into macro-fracture.Moreover,the macrocracks extend and coalesce along the bedding,resulting in the rock failure into blocks.Hydration swelling is one of the major causes that lead to wellbore instability of the LF shale,and therefore improving sealing capacity and inhibition of drilling fluid system is an effective measure to stabilize a borehole.

Study on anisotropy of Longmaxi shale using hydraulic fracturing experiment

Hydraulic fracturing reservoir reconstruction technology is crucial in the development of shale gas exploitation techniques.Large quantities of high-pressure fluids injected into shale reservoirs significantly alter compressional(P)and shear(S)wave velocities,rock mechanical parameters,and anisotropic characteristics.In this study,differentiated hydraulic fracturing petrophysical experiments were carried out on Longmaxi Formation shale under pseudo-triaxial stress loading conditions.The effects of stress loading methods,and water-rock physical and chemical reactions on P-and S-wave velocities and rock mechanical parameters were compared.The experimental results showed that isotropic stress loading may increase the P-and Swave velocities and Young’s modulus of dry shale kldnsample.Furthermore,it may lead to a weakening of the corresponding anisotropy.In contrast,differential stress loading was able to improve the anisotropy of Young’s modulus and accelerate the decrease in the compressive strength of shale in the vertical bedding direction.The water-rock physical and chemical reactions prompted by hydraulic fracturing was found to"soften"shale samples and reduce Young’s modulus.The influence of this"soften"effect on the compressional and shear wave velocities of shale was negligible,whilst there was a significant decrease in the anisotropy characteristics of Thomsen parameters,Young’s modulus,and Poisson’s ratio.The negative linear relationship between the Poisson’s ratios of the shale samples was also observed to lose sensitivity to stress loading,as a result of the"soften"effect of fracturing fluid on shale.The results of this study provide a reliable reference point and data support for future research on the mechanical properties of Longmaxi shale rocks.

Numerical modeling of the dynamic variation in multiphase CH_(4) during CO_(2) enhanced gas recovery from depleted shale reservoirs

Regarding CO_(2)enhanced shale gas recovery,this work focuses on changes in the multiphase(free/adsorbed)CH_(4)in the process of CO_(2)enhanced shale gas recovery,by utilizing a rigorous numerical model with real geological parameters.This work studies nine injection well(IW)and CH_(4)production well(PW)combinations of CO_(2)to determine the influence of IW and PW locations on the dynamic interaction of multiphase CH_(4)during 10000 d of CO_(2)injection.The results indicate that the content of both the adsorbed CH_(4)and free CH_(4)is strongly variable before(and during)the CO_(2)-CH_(4)displacement.In addition,during the simulation process,the proportion of the adsorbed CH_(4)among all extracted CH_(4)phases dynamically increases first and then tends to stabilize at 70%-80%.Moreover,the IW-PWs combinations signifi-cantly affect the outcomes of CO_(2)enhanced shale gas recovery-for both the proportion of adsorbed/free CH_(4)and the recovery efficiency.A longer IW-PW distance enables more adsorbed CH_(4)to be recovered but results in a lower efficiency of shale gas recovery.Basically,a shorter IW-PWs distance helps recover CH_(4)via CO_(2)injection if the IW targets the bottom layer of the Wufeng-Longmaxi shale formation.This numerical work expands the knowl-edge of CO_(2)enhanced gas recovery from depleted shale reservoirs.