Study On Hydraulic Fracture Propagation Mechanism Of Shunbei Fault-Karst Carbonate Reservoir

Shunbei fault-karst carbonate reservoir has a fracture-controlled fracture-cave system as their main reservoir space,and a large number of high-permeability zones and high-conductivity natural fractures are developed around them,so the distribution of reservoir seepage field has an important impact on fracture propagation during hydraulic fracturing of the Shunbei fault-karst reservoirs.However,most of the existing numerical simulation methods for fracture propagation in fracture-cave type reservoirs favor fluid flow in the fracture in terms of the hydro-mechanical coupling method without considering the influence of reservoir fluid flow,which results in a lack of accurate understanding of the fracture extension law of fault-karst reservoirs as well as the mobilizing mechanism of the cave,and leads to an unclear optimization method for fracture reforming of fault-karst carbonate reservoirs.Therefore,it is of great significance to carry out the research on fracture extension law of Shunbei fault-karst reservoir to improve the fault-karst reservoir reforming effect.In this paper,mathematical modeling,numerical simulation and theoretical analysis are integrated to carry out the research.Firstly,the basic rock mechanical parameters and the magnitude of the ground stress are obtained through indoor experiments in the studied area;based on geostatistical methods,the pore space construction method of the fault-karst reservoir is formed based on the characteristics of the fault-karst matrix,fault-karst and natural fractures;the inversion of the ground stress field of the fault-karst reservoir is carried out using finite element method,and the initial distribution characteristics of the ground stress field are analyzed;and the hydraulic fracturing simulation of the fault-karst reservoir is carried out to clarify the evolution law of the ground stress field in the process of fracture extension.The inversion of the ground stress field in the fault-karst carbonate reservoir is carried out by using the finite element method,and the initial distribution characteristics of the ground stress field in the fault-karst reservoir are analyzed;hydraulic fracturing simulation of the fault-karst reservoir is carried out to clarify the evolution law of the ground stress field in the process of fracture propagation.Based on the damage mechanics and seepage theory,a fracture propagation mathematical model and a reservoir seepage model of fault-karst carbonate rock are established,and the mathematical model is discretized by the finite element method and coupled to form a numerical model,which realizes the simulation of hydro-mechanical coupled fracture propagation under the complex medium condition of matrix-fault karst-natural fracture.Secondly,extensive numerical simulations were carried out based on the model to analyze the main controlling factors affecting the fracture morphology and the dynamic effect of the cave during the interaction between the hydraulic fracture and the fault-karst,and to study the influence of the distribution parameters of the fault-karst on the fracture propagation.Finally,on this basis,numerical simulations were carried out for the hydraulic fracture propagation in the carbonate reservoir with the distribution characteristics of the typical Shunbei fault-karst to make clear the fracture propagation rules of the fault-karst carbonate reservoir and optimize the construction parameters,and to propose the construction parameters,and then to simulate the fault-karst fracture propagation.Finally,numerical simulation of hydraulic fracture propagation in carbonate reservoir with typical Shunbei fracture cave distribution was carried out on the basis of which the fracture propagation law of fracture cave type carbonate reservoir was clarified,the construction parameters were optimized,and the corresponding fracture cave communication mode was proposed.The study shows that the high permeability zone and high conductivity natural fractures play a dominant role in the interaction between hydraulic fractures and the fault-karst.Hydraulic fractures can realize the hydraulic form of mobilization to the cave through the hyperseepage zone in the form of fracking fluid filtration loss without direct fracture connection,and even secondary fracture initiation occurs.Highly conductive natural fractures can be opened without tensile opening by directing the fluid flow in a way that the hydraulic fracture is turned to extend,and then connect the fault-karst in the non-primary stress direction.The holes in the non-dominant stress direction adopt the hydraulic form on the mobilization mode,so it needs longer fracturing fluid filtration time and smaller seepage resistance,and the appropriate injection displacement and fracturing fluid viscosity should be selected,and the high displacement and high viscosity fracturing fluid should not be pursued.For carbonate reservoirs with typical Shunbei fracture cave distribution characteristics,corresponding optimized communication modes are given,which are deep penetration fracturing mode,seam fracturing mode and multi-fracture fracturing mode.The research results provide theoretical basis and optimized design methods for fracturing in Shunbei fault-karst carbonate reservoirs,which can help to improve the development effect of reservoirs.