Investigation Of Hydraulic Fracture Propagation Mechanism In Laminated Shale Gas Reservoirs

Multi-stage and multi-cluster fracturing of horizontal wells has been widely used as an essential technology to form complex fracture networks in the shale gas reservoirs,which are featured by ultra-low porosity and permeability.Different from those conventional gas reservoirs,shale gas reservoirs are characterized by significant heterogeneity in different directions,namely the inherent anisotropy.Meanwhile,numerous local discontinuities are present in shale formations,such as faults,bedding planes and natural fractures.These discontinuities result in the structural anisotropy of shale reservoirs.The obvious anisotropic characteristics of laminated shale reservoirs exert a certain influence on hydraulic fracture morphology.Understanding of the complex fracture network propagation mechanism and application of various fracturing techniques in laminated shale gas reservoirs under different physical characteristics can significantly contribute to the high efficiency development.Taking Sichuan Longmaxi Formation shales as the subject,this study conducts a series of experiments to evaluate the anisotropic characteristics,including mineral composition analysis,SEM analysis,porosity analysis,permeability analysis,mechanic property analysis,cemented natural fracture property analysis,spontaneous imbibition analysis,water saturation front analysis,and so on.Based on the complex function solution of generalized plane strain elasticity problems,analytical solutions for stress distribution around the wall of perforated wells are derived,and a new calculation modelis built.Then,a parametric study is carried outto investigate hydraulic fracture initiation pressure in the transverse isotropic formation in combination with the small hydraulic fracturing physical simulation.A new fracture extension model near the wellbore for non-uniform perforation of horizontal wells is established,and the coupling effect between multiple fractures is solved by asymptotic solution,to simulatethe interaction between multiple fractures under different perforating processes.In order to reveal the relationship between spatial distribution of hydraulic fractures and the stimulated reservoir volume(SRV)monitoredby micro-seismic(acoustic emission),a series of large scale tri-axial fracturing simulation experiments,combining the CT scanning technology and acoustic emission monitoring technology,areper formed on samples mined from an outcrop of Longmaxi Formation shales.A 3D complex fracture propagation model based on FDEM is established.Transverse isotropy constitutive relations are primarily considered to characterize layered shales.Thereafter,a series of sensitivity analysis areperformed to investigate controlling factors of the complex fracture geometry,such as anisotropic characteristics,bedding planes,and natural fractures.The long-term fracture conductivity experiments,which are performed by gas and water alternately,are carried out to analyze the influence of water-rock interaction and fracture surface softeningon fracture conductivity.The effects of filling minerals,fracture offset and fracture surface morphology on the permeability and stress sensitivity of self-supported natural fractures with different fracture patterns are studied.Through theoretical analyses,experimental tests and numerical simulation studies,the effect of typical characteristics of laminated shale reservoirs on fracture initiation andcomplex fracture propagation geometries are discussed.The research results can provide theoretical basis and technicalguidance for the optimization of fracturing treatment in shale gas reservoirs.