Study On Fracture Propagation Law Of Shale Gas Reservoir Of Longmaxi Formation Subject To Alternating Fluid Injection Hydraulic Fracturing Treatment

The shale gas reservoirs of Longmaxi formation with vertical depth of more than 3500 m are characterized by high in-situ stresses,high horizontal stress difference,development of bedding seams and natural fractures,as well as stronger plasticity than shallow shale.All of these will hinder the extension of hydraulic fractures and the formation of complex fracture networks.In view of the problems in deep shale development,such as difficult fracturing construction,low production,difficult jointmaking and so on,injecting acid solution to depressurize pressure,closely cut perforation,reducing perforation hole resistance and alternating injection will be often used in the project to improve the efficiency of joint-making and the stimulated reservoir volume.In order to improve the fracturing process and to increase the scale of the expansion of the fractures,true triaxial hydraulic fracturing tests were conducted on shale outcrop specimens excavated from the Shallow Silurian Longmaxi Formation in order to study the initiation and propagation of hydraulic fractures when the specimens were subjected to alternating fluid injection with guar fluid and slick water.The initiation and propagation of fractures in the specimens were monitored using an acoustic emission system connected to a visual display.The results revealed that the guar fluid and slick water each plays a different role in hydraulic fracturing.At a high in-situ stress difference,the guar fluid tends to open the transverse fractures whereas the slick water tends to activate the bedding planes as a result of the temporary blocking effect of the guar fluid.Based on the development of fractures around the initiation point,the initiation patterns can be classified into three categories: transverse fracture initiation,bedding seam initiation,and natural fracture initiation,each of these fracture initiation patterns has a different propagation mode.The complex fracture system can be formed in shale gas reservoir by making full use of alternating fluid injection.According to the distribution of natural fractures in the black shale of Longmaxi formation,the PFC numerical models with developed bedding and natural fractures are established.The numerical simulation enriches the comparison of the experimental conditions and complements the defects in the number of laboratory tests,and validates the experimental results furtherly.The alternating fluid injection treatment can exploit the advantages of two fracturing fluids to form a large complex fracture network in deep shale gas reservoirs.The experimental basis is provided for the optimization of pumping program for shale gas fracturing.