Numerical Investigation On The Propagation Of Hydraulic Fractures In Fractured Tight Reservoirs

In the recent years,due to the decline of reserves and production of conventional oil and gas reservoirs,unconventional oil and gas reservoirs,such as shale gas,shale oil,tight gas,tight oil and so on,has attracted much attention.A large amount of oil and gas are stored in the low permeability fractured tight reservoirs,so it is very important to research the fractured tight reservoir,which with the generality of natural fracture development and the low permeability.There is a great difference between the expansion path of hydraulic fractures in fractured reservoirs and conventional reservoirs because of the existence of natural fracture.In order to get good fracturing result,it is necessary to study the law of hydraulic fracture propagation in fractured tight reservoir.On the basis of a large number of theoretical analysis,experiment and numerical simulation,two dimensional and three-dimensional model are built by using RFPA software which is based on the finite element method to study on the interference of natural fracture to hydraulic fracture.The main content of this paper is as follows:(1)To investigate the law of hydraulic fracture propagating of fractured reservoirs,a two-dimensional model of hydraulic fracture intersecting natural fractures was built and explored by applying theoretical analysis and numerical simulation.The research suggests that the approaching angle,principal stress ratio and cement strength of natural fracture are the three main factors that affect the directions of fracture propagation.In the case of low principal stress ratio and low approaching angle,the propagation of hydraulic fracture is likely along the natural fractures;On the contrary,in the case of high principal stress ratio and high approaching angle,the propagation of hydraulic fracture is likely crossing the natural fractures.In the case of low cement strength of natural fractures,the propagating fracture is likely turning into the natural fractures and along the natural fractures.(2)In practical engineering,the hydraulic fracture has extended behavior in the direction of length,width and height.Based on RFPA3 D software,three-dimensional calculation method of the strike of hydraulic fracture interacted by natural fracture was established to investigate effect of approaching angle and cement strength of natural fracture on hydraulic fracture propagation,and the results are compared with the two-dimensional simulation.The research suggests that the results of 3D and 2D simulation are consistent.And the results show that,in the case of high approaching angle,the hydraulic fracture may be attracted by natural fracture if cement strength is very low;In addition,when the hydraulic fracture extends to the natural fracture,if the tensile stress zone appears along the direction of the natural fracture,the hydraulic fracture is more likely to extend along the natural fracture.(3)To investigate the law and impacts of formation of hydraulic fracture network,a two-dimensional model of hydraulic fracture intersecting natural fractures was built and explored by applying theoretical analysis and numerical simulation.And fractal theory is introduced to evaluate the fracturing effect.The research suggests that the developing degree of natural fracture system affects the fracturing effect.The hgh-density and long trace length of natural fracture can make the fractal dimension of fracture bigger,and the effect of fractured well will be improved.