The Formation Mechanism Of Supercritical CO₂-induced Complex Fractures In Shale And The Equivalent Seepage Model

Supercritical CO₂ fracturing,as a new type of non-aqueous fracturing method,is applied to the development of shale gas,which can effectively solve the water shortage problems in China shale gas rich areas.In addition,compared with conventional hydraulic fracturing technology,it can effectively avoid clay hydration and swelling in shale gas reservoirs and reduce reservoirs’damage.Fracture morphology distribution and its hydraulic parameters have a decisive influence on the regularity of shale gas seepage and its output in reservoirs.Supercritical CO₂ has the features of low viscosity and strong diffusibility,combined with the effects of discontinuous surfaces in shale reservoirs,which makes the fracture morphology and formation mechanism complicated.The morphology,surface roughness and seepage path of supercritical CO₂-induced fractures in shale are different from those of single fracture with hydraulic fracturing,so traditional models of closure deformation and seepage rules for rough fractures no longer apply.Research on the stress and seepage coupling mechanism of supercritical CO₂-induced fractures in shale is necessary for the supercritical CO₂fracturing design and productivity prediction.This paper carried out the simulation experiments of supercritical CO₂ fracturing shale.The synchronous acoustic emission detection,DR scanning and CT scanning were adopted to investigate the fracture morphology of supercritical CO₂ fracturing shale,and to reveal the mechanisms of fracture propagation and complex fractures’formation of supercritical CO₂ fracturing.Numerical simulation of gas route selection and seepage in complex fractures using Fluent was conducted in order to establish the equivalent seepage model of supercritical CO₂-induced complex fractures in shale.Finally,the experiments of gas seepage in complex fractures in shale after supercritical CO₂ fracturing under true triaxial stress were carried out,and the deformation rules and the equivalent seepage and stress coupling laws of supercritical CO₂-induced complex fractures in shale were studied.The main achievements are as follows.（1）The formation mechanism of complex fractures in shale with supercritical CO₂fracturing was revealed.When using supercritical CO₂ for fracturing,its percolation effect is strong,and it can increase the circumferential stress caused by fluid percolation.At the same time,it leads to the increment of pore pressure in shale matrix.Before shale fractured,supercritical CO₂ can flow into micro pore of shale,and it can cause shale damage and uneven fracture surface.Supercritical CO₂ fracturing shale has a long stage of micro fracture germination.Supercritical CO₂ fracturing shale can induce complex fracture networks that contain multiple traverse main fractures,secondary fractures and fractures intersecting with natural beddings.The fracture width of supercritical CO₂-induced fracture is bigger,the fracture surface is more rough,and the fracture distribution density is bigger（than the hydraulic fracture）.（2）The equivalent seepage model of supercritical CO₂-induced complex fractures in shale was established.Based on the high-precision CT scanning images of complex fractures,the adaptive binary method was adopted to extract the fracture contour curve data and get the geometry model of complex fractures.The numerical calculation of gas seepage flow in supercritical CO₂-induced fractures was carried out.The correction coefficient of hydraulic fracture width?was introduced to establish the calculation model of the friction factor f of supercritical CO₂-induced fractures.The effects of secondary fractures and natural beddings on the gas seepage in traverse main fracture were analyzed.The correction coefficient of fracture morphology?was introduced to build the equivalent seepage model of supercritical CO₂-induced complex fractures in shale.The permeability of complex fractures was directly related to the hydraulic fracture width and fracture morphology distribution.The correction coefficients?and?for supercritical CO₂-induced complex fractures in shale were both larger than those for fractures in sandstone.（3）The closure rule and gas seepage law of complex fractures under triaxial stress was determined.The experimental results of gas seepage in complex fractures verified the correctness of the above equivalent seepage model established by numerical simulation.The relationship between each stress and the normal deformation of complex fractures was fitted to investigate the closure deformation law of supercritical CO₂-induced complex fractures in shale.Combining the experimental results of gas seepage,the seepage-stress coupling relationship was fitted to build the seepage and stress coupling model of supercritical CO₂-induced complex fractures in shale under the action of triaxial stress.The normal stress sensitive coefficient K_n of supercritical CO₂-induced complex fractures in shale was small,and the complex fractures remained high permeability under triaxial stress.The lateral stress influence factors?（17）,?（18）were different,and the lateral stress parallel to bedding plane was more helpful to improve the fractures’permeability.Under the effects of largest triaxial stress,the permeability of shale and sandstone with supercritical CO₂ fracturing were respectively 731.2 and12.16 times that with hydraulic fracturing.The research results in this article can make some contributions to the improvement of the fracture propagation mechanism of supercritical CO₂ fracturing shale and the gas seepage theory.The research can provide a theory support for productivity prediction of field supercritical CO₂ fracturing shale,and it is helpful for the application and popularization of supercritical CO₂ fracturing technology.