The Influencing Factors Of Permeability In Shale During The Process Of Supercritical Carbon Dioxide Enhanced Shale Gas

At present,the contradiction between the supply and demand of natural gas in China has become increasingly prominent,and the supply of natural gas is heavily depended on foreign imports.It has long been a serious problem affecting domestic economic development and energy security.The efficient exploration and development of unconventional oil and gas resources,represented by shale gas,is of great significance for improving China's energy structure,ensuring energy security and mitigating the“gas shortage”phenomenon.Currently,shale gas development technologies mainly based on horizontal wells and hydraulic fracturing technologies which not only consume a lot of water resources,but also the expansion of clay minerals in shale,which leads to blockage of percolation channels in pore fissures,and affect the shale gas recovery.The usage of supercritical CO₂ for shale gas development shows good application prospect.Supercritical CO₂ is used as a drilling fluid and fracturing fluid to enter shale gas reservoirs.It has strong seepage and transmission performance,and the difference in the adsorption capacity of CO₂ and CH₄ promotes shale gas to be displaced from the pores of the shale by CO₂ under certain conditions,and the geological storage of CO₂ in shale reservoirs can be achieved at the same time.After CO₂ is injected into the shale reservoir,the dynamic changes of reservoir permeability and its influencing factors directly determine the shale gas production and the geological storage effect of CO₂.Now,the variation of shale permeability after supercritical CO₂ fracturing under various factors is not yet known.On the basis of a large number of domestic and foreign relevant literature data,this paper uses a combination of theoretical and experimental research to characterize the microscopic pore fracture structure of marine shales in the Longmaxi Formation,Sichuan Basin,using a variety of experimental methods.With 2D and 3D perspectives reveal the distribution of pore fractures in the fractured shale.Further research was conducted on the influencing factors of permeability change of fractured shale,and the following major results were obtained:?1?The average TOC content of the Longmaxi Formation shale in the Sichuan Basin is 3.99%.The organic matter abundance is rated well,and the average organic matter maturity?R₀?is 2.59%,which meets the geological evaluation conditions of shale gas reservoirs.The shale samples are more complex in mineral composition,mainly include quartz,carbonate,clay,the content of brittle mineral quartz in excess of50%,very favorable for the reform of supercritical CO₂ fracturing reservoir,preliminary judgment,Longmaxi group shale reservoir has good gas potential and continuous gas production capabilities.It is suitable for exploration and development and commercial mining under existing technological conditions.?2?Shale microscopic pores have complex structures and are widely distributed.Their scales are generally nanometer and micrometer.By analyzing the results of SEM test,low-temperature N₂ adsorption test and mercury porosimetry test,it was found that the Longmaxi Formation shale has a rich nanoporous system,and the shale pore structure is dominated by mesopores and macropores,the average surface area and pore volume is 21.04m²/g,and the shale has very low initial porosity and permeability.The pore space distribution of fractured shale was analyzed using CT scanning technology and Avizo software.The three-dimensional perspective revealed the main crack that penetrated the direction of the maximum principal stress in the fractured shale,and there was a local crack development.The main cracks are open and shear cracks.The main cracks and local cracks after fracturing transformation constitute the main percolation channels and reservoir space of shale gas.?3?When the pore pressure is constant,the permeability of fractured shale after low fracturing pressure stage has the highest sensitivity to the confining pressure change;The change law of permeability of fractured shale body is closely related to the loading path of effective stress,there is a negative exponential relationship between permeability and effective stress;Under the conditions of low pore pressure,the slippage effect dominates,and the effective stress-permeability relationship shows an"abnormal"phenomenon;Under the same pore pressure conditions,with the confining pressure gradually increasing,the pressure point of the gas slippage effect may retrusion,there is a confining pressure threshold in the range of 5⁹MPa,and the contribution of the gas slippage effect to the increase of the permeability is weakened.After the threshold,the influence of confining pressure on the fracture structure of the fracture shale gradually becomes smaller.?4?At the same temperature,the permeability of He is larger than the permeability of CH₄ and CO₂,respectively.He permeability of fractured shale decreases with increasing temperature,and the sensibility of He permeability becomes smaller with increasing temperature in the range of 30-60°C.CH₄ permeability is relatively tiny sensitive to temperature,and the volumetric expansion and thermal stress of shale solids are greater than matrix shrinkage after CH₄ desorption.As a result,CH₄ permeability of fractured shale also increases with temperature.The CO₂ permeability of cracked shale measured as a"positive U-shape"with temperature,and when it is less than 50°C,thermal expansion causes thermal expansion and shale matrix expansion,and the pore volume decreases,so that the permeability decreases.After 50°C,the increase of temperature will increase the viscosity of liquid CO₂,and the change of CO₂ from gas to liquid will reduce the gas adsorption,increase the flowability and increase the permeability.In addition,the increase of the kinetic energy of CO₂ molecules with increasing temperature also promotes the permeability.?5?The temperature sensitivity coefficients?C_T?for different gas permeability measurements vary in different temperature ranges.The specific size relationship is as follows:30-40°C,C_T?He?>C_T?CO₂?>C_T?CH₄?;40-50°C,C_T?CH₄?>C_T?He?>C_T?CO₂?;50-60°C,C_T?CO₂?>C_T?CH₄?>C_T?He?.?6?The CO₂ adsorption time of different phases has a significant effect on the permeability of fractured shale bodies.When the CO₂ adsorption pressure is 6 MPa,the permeability decreases significantly with the increase of CO₂ adsorption time when the adsorption time is less than 12 h.After the adsorption time exceeds 12 h,the change of rock permeability is smaller and the shale permeability tends to be stable.Because,before the critical point of time,the effect of adsorption and expansion of shale is significant,and the permeability declines greatly.After the critical point,the adsorption of CO₂ by shale gradually increases,the adsorption swelling effect decreases,and the decrease in permeability slows down.When the CO₂ adsorption pressure is 8 MPa,the time for the shale adsorption to supercritical CO₂ to reach saturation is shortened,making the change trend of the permeability slow after 10 h.