Fracture Propagation Law Of Shale Horizontal Wells Based On Expansion Finite Element Method

Because of low or ultralow porosity and permeability,Shale reservoir needs stimulation measures such as multi-stage cluster fracturing in horizontal wells to improve reservoir volume and obtain considerable economic production.Fracture growth pattern after hydraulic fracturing largely determines the stimulation effect after fracturing.Therefore,it is of great significance to control and predict fracture growth pattern.In the process of multi-cluster fracture fracturing in horizontal wells,the induced stress interaction caused by fractures disturbs the local geostress field constantly,and the fracturing propagation pattern and regularity after fracturing are extremely complex due to the influence of shale reservoir geology and construction factors.Therefore,rules of fracture propagation and morphological change after multi-cluster fracture fracturing still need to be further studied.Based on this background,this paper uses numerical simulation method to study the fracturing fracture propagation law of shale horizontal wells.The specific work and the understanding are as follows.(1)Based on fracture mechanics theory and extended finite element method,a mathematical model of hydraulic fracture propagation is established.The correctness and validity of extended finite element method in simulating hydraulic fracturing are verified by large physical model test results.(2)The perforation azimuth and fracturing fluid viscosity are positively correlated with the fracture width near the wellbore and negatively correlated with the fracture length when a single cluster fracture expands in a horizontal well;the higher the horizontal stress difference,the more likely it is to form "long,straight and narrow" fractures,and vice versa,"short and wide" fractures will be formed,and "long and wide"fractures will be formed if the injected fluid is increased.(3)When two clusters of fractures in horizontal wells expand at the same time,the fracture morphology is "dumbbell".The fracture spacing and horizontal stress difference are positively correlated with the fracture length and turning position.The longer the injection displacement is,the longest the fracture length is under the same conditions.When three or four clusters of cracks expand simultaneously,the induced stress will restrain the length of the middle crack while the two sides of the crack repel each other.Under the condition of sequential fracturing with three clusters of fractures,the longest fracture can be obtained compared with synchronous fracturing and two-step fracturing.(4)When a hydraulic crack encounters a single natural crack with 90 degrees dip,secondary cracks will occur at both ends of the natural crack,otherwise only one secondary crack will occur at the end of the natural crack with small dip;when two natural cracks with 90 degrees dip are encountered,the second natural crack will severely inhibit the length of secondary cracks produced at the end of the first natural crack.This paper further reveals the law of fracture propagation and shape change in shale horizontal wells.The method of hydraulic fracturing simulation based on extended finite element method can be used to analyze the law of fracture propagation under various factors.