Investigation On Fracture Formation By Hydraulic Fracturing On Longmaxi Shale Reservoir In Sichuang Basin Under Different In-situ Stresses Conditions

Different buried depths of Longmaxi shale formations in Changqing and Sichuan,China have different horizontal stress differences and rock mechanics parameters such as strength and brittleness.The bedding planes and natural fractures have an impact on the effectiveness of horizontal well with multi-stage hydraulic fracturing,resulting in a large fracturing pressure and a great difference in the rule of fracture initiation and propagation for different shale formations.It is necessary to conduct the laboratory experimental research to provide the base for hydraulic fracturing operation parameters optimization.Hence,in this paper,a series of large true tri-axial hydraulic fracturing stimulation experiments are conducted and pressure respond and fracture features of fracturing curve at Fuling Jiaoshibai shale formation in Chongqing are analyzed,in order to investigate hydraulic fracture initiation mechanism and propagation behavior in horizontal well under natural factors(horizontal stress differences and natural fracture)and engineering factors(pump rate,fracturing fluid viscosity,fracturing method,and perforation parameters).The main conclusions of this paper are following(1)Hydraulic fracture propagation behavior and fracture morphology in shale formation under different horizontal stress differencesUnder a low horizontal stress difference,hydraulic fracture propagation behavior is affected by natural fracture easily,resulting in diversion or crossing and diversion Three main fracture morphologies are observed:simple fracture,complex fracture,and complex fracture network.However,a high horizontal stress difference control fracture propagation behavior,leading to four main fracture morphologies:transverse fracture,transverse fracture with bedding planes,natural fracture with bedding planes,and transverse fracture with bedding planes and natural fractures.It is difficult to create a complex fracture network.(2)Hydraulic fracture initiation method and propagation behavior in horizontal spiral perforated well.Hydraulic fracture initiates from the base of perforations and propagates along the direction of the maximum horizontal stress in the horizontal spiral perforated well.The different horizontal stress differences and different perforation parameters have an impact on fracture propagation behavior,resulting in three main fracture morphologies:single flat fracture,multiple-layered fracture,and spiral-shape fracture.A low perforation density and a small perforation phase create a single flat fracture,however,the a high creates a spiral-shape fracture.Furthermore,a large perforation phase generates a multiple-layered fracture.(3)The design and suggestion of hydraulic fracturing operation optimization for shale formation reformation.In order to reduce fracturing pressure and improve the complexity of fracture,hydraulic fracturing operation parameters should be optimized.A low perforation density and a small perforation phase should be used to create a single flat fracture which is good for fracturing fluid flowing and proppant immigration.A low viscosity of fracturing fluid with a low pump rate could enhance hydraulic fracture to open more natural fractures,resulting in complex fracture.But this method could not enlarge induced fracture length,causing a small SRV.A periodic variable pump rate fracturing is conducive to create complex fracture under a large horizontal pressure difference by low pump rate to increase the complexity of fracture morphology and high pump to enhance the size of complex fracture network.Moreover,temporary plugging fracturing and shut-in turnaround fracturing can stimulate the formation effectively,resulting in complex fracture morphology.