Study On The Action Mechanism Of Anhydrous Carbon Dioxide Fracturing Fluid On Shale Reservoir

Shale oil and gas as an important unconventional oil and gas resources,its efficient development mode has become the focus of research.Conventional hydraulic fracturing has many problems,such as large consumption of water resources and severe water sensitivity effect,which will affect the efficiency of shale oil and gas exploitation.Anhydrous carbon dioxide fracturing fluid has shown a good application prospect because of its advantages such as low reservoir damage,strong dispersibility and storage of carbon dioxide.There are different phase states in the field application of anhydrous carbon dioxide fracturing fluid.Before injecting into the formation,it is a low-temperature liquid state,and after reaching the reservoir,it turns into a supercritical state.The influence mechanism of different phase states of carbon dioxide on shale reservoir is also different.Comprehensive evaluation of the overall mechanism of anhydrous carbon dioxide fracturing fluid in the fracturing process of shale reservoirs is of great significance to improve the exploitation efficiency of anhydrous carbon dioxide fracturing fluid in shale reservoirs.In this paper,the effects of anhydrous carbon dioxide fracturing fluid in liquid state and anhydrous carbon dioxide fracturing fluid in supercritical state on shale reservoir are simulated respectively.The cooling damage of low temperature liquid carbon dioxide to high temperature shale was studied by characterizing the porosity and permeability properties and mechanical properties of shale.The mathematical model of fracture propagation in liquid carbon dioxide fracturing is established,and the numerical simulation of fracture propagation in liquid carbon dioxide fracturing shale is carried out.Through the characterization of shale surface properties,mineral composition,microscopic pores and wettability,the effects of supercritical carbon dioxide on pore and permeability properties and physicochemical properties of shale were studied.The research results show that when-10 ℃ low temperature liquid carbon dioxide flows through 90 ℃ high temperature shale reservoir,it will cause cooling damage to shale reservoir,increase shale porosity and permeability,and degrade the mechanical properties of shale,decrease the compressive strength and elastic modulus.The higher the fracturing fluid injection pressure,the greater the influence on the properties of shale samples.When the injection pressure is 30 MPa and the fracturing fluid is injected for 3 h,the porosity of shale core increases by 21.1%,permeability increases by 40.6%,tensile strength decreases by 31.8% and elastic modulus decreases by 11%.The increase of porosity and permeability and the degradation of mechanical properties can improve the exploitation efficiency of shale reservoir.The variation rules of stress field,displacement field,temperature field and pressure field in the process of fracture propagation were determined by the numerical simulation of liquid carbon dioxide fracturing.Compared with the simulation results of slickwater fracturing fluid,the fracture width generated by liquid carbon dioxide fracturing can be increased by 5.52% at most,and the pumping pressure required can be reduced by 15.29% at most.The results show that liquid carbon dioxide fracturing has better fracture formation effect and requires lower pumping pressure.Under the condition of 90 ℃ and 25 MPa reservoir,the number of micropores in the shale decreases after soaking by supercritical carbon dioxide,and the micropores gradually change into mesopores and macropores.After soaking for 48 h,the specific surface area of shale decreases by 53.0%,the total pore volume decreases by 48.2%,and the average pore size increases by 18.8%,and the gas adsorption capacity of shale weakens.This is mainly because the supercritical carbon dioxide will dissolve the organic matter in the shale micropores,and the carbon-dioxide-water-rock reaction will also dissolve the mineral components such as kaolinite and calcite inside.After soaking with supercritical carbon dioxide,the wettability of shale reservoir changes from aqueous wettability to neutral wettability.The longer the soaking time is,the more obvious the wettability changes will be,which can prevent the formation of water film on the surface of shale internal pores,ensure the smooth flow of internal pores,and help to improve the production efficiency of shale gas reservoir.