Experimental Study On Degradation Mechanism Of The Mechanical Properties Of Shale Treated By CO₂

The characteristics of shale gas reservoir,such as low permeability,high adsorption provide the possibility of geological storage of CO₂.However,in the process of sequestration,interaction between CO₂ and shale can change the original pore structure of shale,affecting the macroscopic mechanical properties of the shale,thereby affecting the sequestration effect of carbon dioxide and the long-term stability of the formation.There are currently relevant studies on the mechanical properties of shale treated by CO₂,and the intrinsic reasons for CO₂ changing the mechanical properties of shale are still in a simple qualitative analysis stage,and its mechanism of action is still unclear.This paper takes the Longmaxi formation shale in Fuling area of Chongqing as the research object,and adopts a self-developed high temperature and high pressure rock immersion device to treat shale under different pressure conditions.Relying on the uniaxial compression test and acoustic emission test,studying the changes in the mechanical properties of the shale before and after treatment.The pore structure parameters of the shale before and after the treatment were measured by using a nuclear magnetic resonance analyzer?NMR?.Combined with SEM,XRD and adsorption deformation tests of shale,to study the meso-porous structure characteristics and microscopic mechanism of shale after CO₂ treatment.Then,the relationship between microscopic pore structure characteristics and macroscopic mechanical properties was analyzed to reveal the mechanism of CO₂ deteriorating mechanical properties of shale.The main research results obtained are as follows:?1?The law of mechanical properties of shale with CO₂ pressure were obtained.Shale mechanical parameters?uniaxial compressive strength and elastic modulus?decrease first and then increase slightly with increasing pressure,and reach a minimum near 12 MPa.The UCS and E of shale after gaseous CO₂ treatment were reduced by11.4%and 3.3%,respectively.Supercritical CO₂ had a higher impact on shale mechanical parameters than gaseous CO₂,and UCS and E were reduced by 32.9%and16.8%,respectively.After CO₂ treatment,the initial compaction stage of shale is prolonged,the elastic deformation stage is shortened,the distribution of acoustic emission energy probability density changes,the power exponent increases,and the probability of occurrence of larger energy events increases.?2?The effect of carbon dioxide on the mesoporous structure of shale was revealed.Shale porosity increased from 1.18%to 1.23 to 1.36%after CO₂ treatment.With the increase of pressure,the porosity increases first,reaches the maximum value near12MPa,and decreases when the pressure is 16MPa.After CO₂ treatment,the shale pore size distribution changed significantly,in which micropores?0-2 nm?slightly decreased;mesopores?2-50 nm?decreased from 70.61%to 66.23-67.15%,and macropores?>50nm?consisted of 29.39%increased to 32.84-35.83%,and the pore size distribution with increasing pressure of action showed three patterns of change.?3?It reveals the microscopic mechanism of the influence of carbon dioxide on the pore structure of shale.When the pressure is less than the critical pressure of 7.38 MPa,CO₂ is gaseous?4 and 6 MPa?,and the change of pore structure is mainly affected by the adsorption and deformation characteristics of shale.When the pressure is higher than the critical pressure?8MPa and 12MPa?,CO₂ is in a supercritical state,and the change of pore structure is also affected by the adsorption and deformation characteristics of the shale and the supercritical CO₂ dissolution performance.Continuing to increase pressure?12-16 MPa?,changes in pore structure are also affected by gas pressure.?4?It reveals the mechanism of action of carbon dioxide deteriorating mechanical properties of shale.The interaction between CO2 and shale will change the original pore or fracture structure in the shale,which will change the mesoporous structure of the shale,promote the initiation and expansion of the compression crack,increase the macroscopic cracking,change the failure mode,and deteriorate the mechanical properties of the shale.The anisotropy of shale deformation caused by CO₂ can cause local stress concentration,resulting in plastic deformation and destroy the original pore structure,and induce 50-300 nm micro-crack.With the increase of pressure,shale deformation will increase first and then decrease.The damage degree changes accordingly.When the pressure exceeds 8¹0MPa,the pore shrinkage effect of shale is significant under the influence of air pressure,which affects the pore structure.When the pressure exceeds the critical pressure,CO₂ changes from gaseous state to supercritical state,and the dissolving capacity is greatly improved.Dissolve part of the carbonate minerals,and the native microstructure is eroded to produce 0.3-5?m pores.