Study On The Interaction Mechanism Of Shale With Different CO₂ Pressure And Its Effects On The Properties Of Shale

The huge demand for energy has contributed to the tense the supply-demand relationship of natural gas in China.And the long-period dependence on importing from other countries may has had an adverse influence in sustainable development of domestic economy and energy security.Therefore,shale gas,as a new energy,confimed as a practicable alternative to traditional energy,has attracted increasing attention from China and even the whole world.Exploration of effective technology for shale gas exploitating has become the focus of energy research.The current main method of shale gas development is the horizontal well and hydraulic fracturing technology,which requires a large amount of water.At the same time,water based fracturing fluid may induce the swelling of clay minerals in shale,which bringing a blockage of seepage channels in rock formation and reducing gas recovery.Differing from hydraulic fracturing technology,supercritical CO₂ would not bring such a great damage to rock or huge water consumption,and having an advantage in environmental protection and efficiency on process of gas shale exploitation.It has become a promising technology in shale gas exploration.The main idea of this technology path is using supercritical CO₂ as fracturing fluid,and displace the shale gas by the injection of supercritical CO₂ into shale gas reservoir,based on the difference of adsorption between CO₂ and CH₄ in certain condition.By fully utilizing the fluid characteristics of supercritical CO₂,water consumption and subsequent sewage treatment can be successfully avoided.Simultaneously,it also alleviates the damage of greenhouse effect through sequestrating CO₂ in shale gas reservoir.The fracturing effect of reservoir has a direct impact on shale gas production and the effectiveness of CO₂storage after injection of itself.Therefore,it is of great significance for the productivity of shale gas and the storage of CO₂ to study the internal modification of shale under different treatmentsThe paper,through experimental studying,the variation of mineral compositions,pore structure and mechanical properties of shale under different CO₂ saturation pressures were explored.All samples were selected from the Wufeng Formation shale collected in Changning area of Sichuan basin.Through XRD,XRF,NMR,cryogenic nitrogen adsorption instrument,acoustic emission detector and MTS mechanical measurement system respectively,the mineral composition,pore structure and mechanical properties variation of shale before and after CO₂ saturation were obtained.Jade was then introduced to analyze the parameters of mineral composition of shale,the main conclusions are as follows.?1?The results of XRD analysis showed that the main mineral components of the Wufeng Formation shale samples were quartz,carbonate,clay minerals,etc.Quartz was orinally 53.71%,increased to 54.85%,56.97%and 59.52%with the increase of pressure from 6 MPa to 12 MPa.The percentages of carbonate and clay minerals reduced at the same time.The reason was that quartz did not react with CO₂,but carbonate and clay minerals were interacted with and dissolved by CO₂.XRF data showed that the proportion of SiO₂ which is related to quartz increased,and the contents of Al as well as other elements corresponding to carbonate and plagioclase decreased.?2?The test result of NMR showed that the porosity of the shale was 2.7395%before CO₂ treatment.It increased to 2.88%,3.09%and 3.15%respectively when the pressure increased to 6 MPa,9 MPa and 12MPa.This phenomenon was related to the pore dissolution of shale by CO₂ and the extraction of mineral particles in the pore.The micropores were transformed to mesoporous under the action of supercritical CO₂ and dissolution while the transformation to mesoporous of macropores was owing to the matrix expansion.?3?The nitrogen adsorption results told that the shape of adsorption and desorption curves and hysteresis loops of shale were the same under different CO₂ pressures.Therefore,the basic physical structure of shale pore remained unchanged.The N₂adsorption capacity of shale decreased at low pressure stage,indicating that the number of micropore and corresponding adsorption capacity decreased.?4?MTS rock mechanics test results showed that the uniaxial compressive strength and elastic modulus of the original shale were 251.13 MPa and 20.30 GPa,respectively.After the saturation of CO₂ under different pressures,the compressive strength?UCS?and elastic modulus?E?of the shale decreased in varying degrees.Compared with the sub-critical state,the deterioration of mechanical properties of shale was larger when saturated by supercritical CO₂.Shale was more likely to produce secondary pore and fracture due to heterogeneous expansion after CO₂ saturation,so the strain and time of shale compaction increased in the stage of elastic deformation.?5?The failure mode of original shale was relatively simple,mostly shear failure of weak structural planes.After CO₂ saturation,the microstructure of shale was changed,It brought new cracks,and the number of weak structural planes increased.During the loading process,the weak surface of the structure was subjected to load at the first time,and it appeared a variety of failure modes,such as shear,tension and splitting.The time from crack generation to specimen failure decreased.