Study Of Microstructure And Mechanical Properties On Shale Under Different Fluid Effect

As a new type of unconventional energy,the realization of large-scale commercial development of shale gas is of great significance to alleviate the energy supply and demand contradiction,and ensure energy security in China.At present,horizontal drilling and hydraulic fracturing techniques are commonly used in shale gas development.However,this technology has some drawbacks:1)gas flow channels in shale can be blocked by the aqueous fracturing fluids induced clay mineral swelling,which leads to the poor shale gas production);2)hydraulic fracturing requires large volumes of water,limited water availability may restrict shale gas production in the water-scarce regions.3)additives in water-based fracturing fluids may cause the pollution of groundwater and surface water.Supercritical carbon dioxide（ScCO₂）,with unique physical and chemical properties such as low viscosity and high diffusion capacity close to gas and high density like liquid,has a wide application prospect in unconventional oil and gas development.In addition to the enhancement of the shale gas recovery,ScCO₂-based fracturing technology simultaneously offers the opportunity for CO₂ sequestration in shale gas reservoirs.Based on the idea of the integration of ScCO₂ enhanced shale gas recovery and CO₂ sequestration in shale gas reservoir in this study,the research of the influence of CO₂ injection on the microstructure and mechanical properties of shale were conducted.Using scanning electron microscope（SEM）,low temperature nitrogen adsorption equipment,mercury porosimeter,X-ray fluorescence（XRF）,X-ray diffraction（XRD）,Mechanical Testing System（MTS）,and Acoustic Emission（AEs）collection system,the changes of pore structure,elemental composition,mineral composition and mechanical properties in shale were obtained.Shale samples before and after the saturation by different fluid media（Water,gaseous CO₂,Sc-CO₂,mixture of gaseous CO₂ + brine,mixture of Sc-CO₂ + brine）were characterized.The main results of the study are as follows:（1）After the saturation of different fluids on the shale,the surface morphology and microstructure in shale samples were changed,the main conclusion are as follows:1)The surface morphology in shale samples were changed by the dissolution effects of different fluids,and the influence degree is related to the phase state of CO₂,The erosion of Sc-CO₂ on shale surface is more significant than that of gaseous CO₂,which is related to the dissolution and extraction characteristics of Sc-CO₂.2)The N₂ adsorption-desorption curves of all the shale samples before and after fluid saturation have similar shapes,which indicate that the pore shape of shale was not changed by the fluid saturation.After saturated by different fluids,the specific surface area（SSA）of all the samples are decreased,The degree of the reductions of SSA are as follows: 70.2% of Sc-CO₂ > 46.2% of gaseous CO₂ > 36.2% of Sc-CO₂ + brine > 3.1% of gaseous CO₂ + brine respectively.The pore volume of shale samples are also decreased,the magnitude of the declines of the pore volume are as follows: 4.6% of gaseous CO₂,30.1% of Sc-CO₂,2.7% of gaseous CO₂ +brine,5.8% of Sc-CO₂ + brine respectively.The average pore diameter of shale samples were increased,the magnitude of the increase are as follows: 26.6% of gaseous CO₂),45.1% of Sc-CO₂,3.2% of gaseous CO₂ + brine,8.2% of Sc-CO₂ + brine respectively.The changes for each type of pore（micro-,meso-and macropores）in all the shale samples have similar trend: the proportion of micropores were decreased,and the proportion of transitional pores and mesopores were increased.And Sc-CO₂ has the most significantly influence on the microstructure in shale,which is related to the unique physical and chemical properties of Sc-CO₂ such as its strong dissolution,extraction and high diffusion capacity.3)After the saturation of different fluids on the shale,the fractal dimensions of all the samples were found to decrease,suggesting that the roughness degree of the shale geometrical surface declines,and the morphology of the shale pore structure is transformed gradually from complex to regular.the magnitude of the decline in fractal dimensions is as follows: 2.38% of gaseous CO₂,7.87% of Sc-CO₂,0.25% of gaseous CO₂ + brine and 2.42% of Sc-CO₂ + brine respectively.The fractal dimension calculated from the N₂ adsorption data of shale are decreased after fluid saturation,and is related to the specific surface area and pore volume.The decrease of fractal dimension reflects the transition trend of shale pore structure from complex to simple to a certain extent.（2）The XRD and XRF results show that after the saturation of different fluids,the shale mineral element content and mineral composition have a certain degree of change due to dissolution,precipitate of some of the shale minerals.The elements content of O,Si are increased,while the content of metal elements such as Ca,Fe are decreased.The contents of quartz,dolomite are increased significantly,the content of calcite,clay minerals are decreased.After the saturation of the mixture of Sc-CO₂ + brine,the element content and mineral composition in shale has the most significantly change.And compared with gaseous CO₂,Sc-CO₂ has a greater influence on the changes in shale mineral composition.（3）Uniaxial compression experiment,Brazil splitting experiment and acoustic emission experiment show that:1)After the saturation of different fluids,the uniaxial compressive strength（UCS）of all the shale samples are decreased,the magnitude of the UCS values are as follows: unsaturated specimen> gaseous CO₂ saturated specimen> Sc-CO₂ saturated specimen > water saturated specimen > Sc-CO₂+ brine saturated specimen > gaseous CO₂ +brine saturated specimen.The decreases of all the samples are related to the changes of microstructure in shale caused by the saturation of different fluids,the change of microstructure leads to the change of macroscopic mechanical properties in shale.The reduction of UCS in shale caused by the pure CO₂ fluid saturation is greater than the water,which is due to the swelling of the shale caused by water absorption is greater than the the adsorption of CO₂;The effect of CO₂ interaction with shale on the mechanical properties of shale is related to phase state of CO₂,the decrease of shale strength after Sc-CO₂ saturation is more than that of gaseous CO₂,which is related to the extraction and dissolution characteristics of Sc-CO₂.Sc-CO₂ causes the organic matter in the shale to be extracted and the minerals in shale are dissolved,leading to the generation of secondary pores and fissures in shale,then weaken the mechanical properties of shale;With the saturation of the mixture of CO₂ and brine,the reduction of UCS in shale is greater than that the saturation of pure CO₂ fluid.2)After the saturation of different fluids,the elastic modulus of all the shale samples are decreased with different degrees,under the effect of Sc-CO₂ and gaseous CO₂,the reduction of the elastic modulus of shale caused by Sc-CO₂ saturation is more higher than that of gaseous;after the saturation of water,gaseous CO₂ + brine and Sc-CO₂ + brine,the elastic modulus of shale decreases by 15.07%,14.55% and 13.24% respectively.Compared with the pure CO₂ fluid,the reduction of elastic modulus of the shale samples caused by water and mixture of CO₂+brine saturation are more significant,which indicates that the swelling effect of water plays a dominant role in the reduction of elastic modulus.3)After the saturation of different fluid,the characteristic of AE data of all the shale specimens during the uniaxial compression failure process are similar,and three crack development periods were identified for all of the analyzed shale samples: crack closure,stable crack propagation,and unstable crack propagation.After the saturation of different fluids,the shale samples display a broader crack closure stage width compared to the raw shale sample,which indicates that there is a delay in the crack initiation.The percentage of the crack initiation stress to the peak stress is also increased.Compared with pure CO₂ fluid（gaseous CO₂,Sc-CO₂）saturated samples,the delay in the crack initiation of water saturated specimen is more prominent.and the delay of the crack initiation caused by the saturation of Sc-CO₂ is moresignificant than that of gaseous CO₂.All the shale samples also show an reduction in the damage stress to some extent,while the percentage of the crack damage stress to peak stress are increased,which indicates that the ductility of shale is enhanced after the saturation of different fluids.In addition,after the saturation of different fluids,the stable crack propagation and unstable crack propagation stage of all the shale samples become shorter.4)The tensile strength values of shale after different fluid saturation are as follows: raw specimen> gaseous CO₂ saturated specimen> water saturated specimen > Sc-CO₂ saturated specimen > Sc-CO₂ + brine saturated specimen > gaseous CO₂ + brine saturated specimen.This is consistent with the overall varition trend of UCS of shale after different fluid saturation.After saturated by water,gaseous CO₂,Sc-CO₂,gaseous CO₂ + brine and Sc-CO₂ + brine,the tensile strength of shale decreased by 29.15%,21.39%,36.65%,60.35% and 46.73%,respectively.It can be seen that the reduction of the tensile strength of shale caused by the saturation of mixture fluid of CO₂ + brine is the most among all the samples.5)After the saturation of different fluides,the brittleness index of shale are decreased.The brittleness index values of all the shale samples are ranked as follows: raw specimen> gaseous CO₂ saturated specimen > Sc-CO₂ saturated specimen > water saturated specimen > Sc-CO₂+ brine saturated specimen > gaseous CO₂ + brine saturated specimen.This is consistent with the varition trend of UCS of shale caused by the different fluid saturation.Under the saturation of the mixture fluid of CO₂ + brine,the transition trend of shale from brittleness to ductility is more obvious,which indicated that due to the combined effect of hydration effect of water,dissolution and extraction effects of CO₂,the brittleness index of the CO₂ + brine saturated shale samples is the lowest.The research results can provide scientific basis and guidance for the application of integrated technology of Sc-CO₂ enhanced shale gas recovery and CO₂ storage.