Experimental Study On The Interaction Mechanism Between Spercritical CO₂ And Shale

Supercritical carbon dioxide（CO₂）fracturing shale gas reservoir and enhanced shale gas recovery has attracted more attention all over the world,which can effectively avoid a series of damage and environmental hazards of hydraulic fracturing to the shale formations.In addition,CO₂ injection into deep shale gas reservoir will replace CH₄molecules and increase the production of shale gas,and effectively sequestrate CO₂ in shale formations.After injecting supercritical CO₂ into shale gas reservoir,the interac-tion of supercritical CO₂-shale and its effects on the mineral compositions,surface properties,pore structure,and mechanical properties of shale will signifacantly affect the shale gas recovery and the security of CO₂ sequestration.To address these problems,based on the shale samples collected from Changning（CN）area of Sichuan basin and Fuling（FL）region of Chongqing,a specially designed supercritical CO₂-shale reactor was used to conduct supercritical CO₂ exposure experiments（T=40℃,P=16 MPa）on CN and FL shales.X-ray diffraction（XRD）,fourier transform infrared spectroscopy（FTIR）,scanning electron microscopy（SEM）,and low temperature nitrogen adsorption were used to comparatively analyze the mineral compositions,surface properties,and pore structure of shale samples before and after exposure to supercritical CO₂ to invest-gate the interaction mechanism of supercritical CO₂-shale on the physical properties and pore structure of shale.And then,under the temperature condition of 38℃,we con-ducted the exposure experiments of CO₂ on shale under the pressures of 4,6,8,12,and16 MPa.The raw and CO₂-treated shale samples under differents pressures were used to conduct normal mechanical testing to study the uniaxial compressive strength（UCS）and Young’s modulus（E）of shale.During the loading process,the acoustic emission monitor technology was applied to analyze the crack iniation,propagation and damage of raw and CO₂-treated shale samples.The main fingdings and conclusions are obtained,as follows:（1）Supercritical CO₂ can dissolve organic matters and minerals（e.g.montmorillonite,kaolinite and calcite）in shale,but it does not react with quartz.After exposing to Supercritical CO₂,the increase of quartz content in shale is mainly due to the dissolution of other minerals in supercritical CO₂.The increase quartz content per-centage after exposure indicates greater amount of other minerals were dissolved by supercritical CO₂ in shale.In addition,the interaction of supercritical CO₂-shale will cause a certain degree of corrosion to the surface morphology of shale,but it will not influence the surface functional group structure of shale.After supercritical CO₂treatment,the specific surface area and total pore volume of shale decreased,while the average pore size increased.This is mainly caused by the dissolution effect of the micropores developed in organic matter by supercritical CO₂,and a portion of micropores are transformed into mesopores or macropores.During the CO₂-ESGR and CO₂ geological sequestration,the effects of supercritical CO₂-shale interaction on the pore structure of shale will impact the potential of CO₂ sequestration and the migration mechanism of gas in shale.（2）After exposuring to supercritical CO₂,the pore surface fractal dimension（D₁）and the pore structure fractal dimension（D₂）of shale are decreased,indicates the decrease of surface roughness of shale pores,and the complexity degree of shale pore structure also decreases.There are two main mechanisms can be used to interpret above phenomena,as follows:1)the dissolution of supercritical CO₂ to the organic matters of shale leads to the reduction of micropores,thus reducing the roughness of the pore surface,as a result,the D₁ shale decreases as well.Therefore,the dissolution effect of supercritical CO₂ can transform the micropores into mesopores or macropores of shale;2)The CO₂ adsorption-induced swelling of shale makes the larger pores trandform into smaller pores,which makes the complexitity of shale pore structure simpler than before.Therefore,the D₂ of shale decreases.（3）The reduction in unixial compressive strength（UCS）and elastic modulus（E）of shale caused by subcritical CO₂ treatment（4 and 6 MPa）is up to 22.86%and 23.10%,respectively.While Supercritical CO₂ treatment（8,12 and 16 MPa）causes more signif-icant reduction in UCS and E compared to subcritical CO₂ treatment.It results in up to33.89%reduction in UCS and up to 33.97%reduction in E.This greater reduction may be related to two combined effects.In addition to the CO₂ adsorption-induced swelling effect,the extraction and dissolution effects of supercritical CO₂ induced microstructur-al alterations could cause additional damages.Moreover,the UCS of shale and E de-creases with the increase of CO₂ exposure pressure,and reach their minimums at the pressure of 12 MPa.When pressure increases from 12 MPa to 16 MPa,the UCS and E of shale increase slightly.Therefore,we can conclude that the effects of supercritical CO₂-shale interaction on the mechanical properties of shale is highly dependent on the reaction pressure and phase state of CO₂.In the process of supercritical CO₂ fracturing shale gas reservoir,CO₂-ESGR,and geological storage of CO₂ in shale,the interaction of supercritical CO₂-shale plays an important role in the initiation and propagation of cracks in shale.In addition,the CO₂adsorption-induced swelling and dissolution effect of supercritical CO₂ on organic matter and other minerals can affect gas flow mechanism in fractured shale reservoirs,thus affecting the ultimate recovery of CO₂-ESGR.In actual CO₂ sequestration project,shale foramtions are considered as reservoir stratum,instead of cap rock.With the degradation effect of shale mechanics,there exist the risk of CO₂ leakage.Therefore,we should consider these issuees during the actual engineering situation.