Investigation on Hydraulic Fracture Initiation and Propagation with LPG Fracturing in Shale Formation based on True Tri-Axial Laboratory Experiments

摘要

Traditional hydraulic fracturing requires lots of water and sand resulting in short fracture length and small SRV with a low production.However,a new waterless fracturing,called Liquefied Petroleum Gas(LPG)fracturing,is applied to stimulate shale formation effectively.In order to figure out the mechanism of fracture initiation and propagation in LPG fracturing,four largescale true tri-axial fracturing simulation experiments have been conducted on shale outcrops.Meanwhile,the effects of engineering factors,pump rate and fluid viscosity,on fracture propagation behavior in the shale formation are discussed.The experimental results indicate that LPG fracturing not only activates discontinuities to form a complex fracture network,but also enhances induced fracture length to form a large SRV.Induced fractures have two initiation points,open-hole section and stress concentration point of wellbore wall,and have three main propagation behaviors,crossing,shear and arrest,dilation and crossing in shale formation.A low viscosity fracturing fluid activates discontinuities resulting in complex fractures,whereas,a high viscosity fluid would like to create some main fractures without opening discontinuities.Moreover,a high pump rate offers more energy for induced fractures to cross the discontinuities resulting in a long fracture length and large SRV.In addition,the anisotropic of shale formation and the existence of discontinuities cause signals attenuation,which increases the arrival time,resulting in location deviation of acoustic emission(AE)events in the AE monitoring.The pressure-time-energy curve,however,shows that the fracture initiation is earlier than the sample ruptured.That is,the initiation pressure is smaller than the ruptured pressure.The experiments conducted in this paper prove that the LPG fracturing indeed has some advantages than traditional hydraulic fracturing,such as long fracture length and large SRV.And then,the research results provide the theoretical basis for the LPG fracturing operation in shale formation.