Study On The Mechanism Of Hydraulic Fracture Propagation In Complex Carbonate Reservoirs

With the continuous innovations of marine carbonate reservoir geological theory,the Ordovician carbonate fracture-cavity reservoir in Tarim basin has become one of the priorities and hot areas in oil and gas fields.The fracture-cavity reservoir in Tarim basin,is characterized by the reservoir deep burial depth,diversity of reservoir spaces and extremely strong heterogeneity.According to the current geophysical and drilling techniques,the bottom hole can only reach on the favorable area of karst development,a substantial part of wells are in need of reservoir stimulation to obtain production.The objective of reservoir stimulation for fracture-cavity reservoir is to form hydraulic fractures with enough conductivity to connect with the space filled with oil.Because of the presence of natural caves and fractures,reservoirs usually exhibit the features of complicated media.The mechanism of fracture propagation is a very complex problem and the path of fracture propagation is not deterministic.Using the classical theory of hydraulic fracturing has been unable to well explain the fracture propagation behavior in complex media conditions and model the nonplanar diverting behavior.The treatment design with poor pertinence restricted the efficient development of reservoir.Further research on the mechanism study of fracture propagation in fracture-cavity reservoir is needed.This paper uses extended finite element method(XFEM)as foundation theories,in the case of taking into full account of the reservoir geologic characteristics of fracture-cavity reservoir,emphasizing on the influence of caves and natural fractures on fracture propagation path,and completes the following exploration and research work.(1)Based on the basic principle of XFEM,taking into account of the particularity of hydraulic fracture propagation numerical simulation,the XFEM control equation,displacement pattern for dealing with fracture problem and the related mathematical formulas are derived.The mathematics form of numerical tool for applying this problem is clarified.(2)According to the relevant theories of fracture mechanics and rock mechanics,in view of the randomness of fracture propagation orientation,the maximum energy release rate criterion and the maximum circumferential stress criterion are used to judge the direction of fracture propagation,forming the hydraulic fracture extension criterion in fracture-cavity reservoir,providing the basic conditions for fracture dynamic propagation.(3)The physical model for fracture propagation in fracture-cavity reservoir is presented.The implement scheme and algorithm of fracture propagation are introduced in detail and the MATLAB program code is developed at last.(4)The numerical computing results are as follows:?The stress concentration around cave will lead to a deflection of fracture propagation orientation.The degree of deflection is related to the wellbore location,the size of cave,in-situ stress,injection rate and viscosity of treatment fluid.The larger the distance between wellbore and center line of cave is,the farther away the hydraulic fracture path is from the cave.The larger the horizontal differential principal stress is,the smaller the degree of deflection is.The larger the size of cave is,the larger the degree of deflection is.The higher the injection rate and viscosity is,the smaller the degree of deflection is.? A hydraulic fracture is more likely to divert into a natural fracture at low approach angle,and cross a natural fracture at high approach angle.When the ratio of critical energy release rate of natural fracture to the rock mass is very small(natural fractures weakly cemented),a hydraulic fracture may divert into a natural fracture at high approach angle,otherwise may cross a natural fracture at low approach angle.? The length,trend and position of natural fractures exert an important impact on the result of connecting with cave.Generally speaking,small distance between the natural fracture and the cave,low approach angle and long length of natural fractures will lead the hydraulic fracture to position near the cave and improve the probability of connecting with cave.The studies of this paper enrich the reservoir stimulation theory of fracture-cavity reservoir and have guiding significance for the design of treatment scheme and post-treatment evaluation.