Study On The Law Of Fracture Propagation In Multi Reservoir

The purpose of this paper is to study the rule of hydraulic crack of Linxing block which in Ordos basin's east margin,preliminary exploration showed that there are coalbed methane and tight sandstone gas,also sandstone and coal are interbeded.In this paper,firstly,the insitu stress of sandstone and coal is analyzed.Based on the combined spring model,the structural stress coefficient of tight sandstone is calculated by using the logging data and rock mechanics test.For soft coal rock,first calculate tectonic strain of adjacent hard sandstone formation,based on the principle of adjacent strata tectonic strain equivalent,build the finite element model of by analysis software ANSYS based on the structure,calculate the tectonic stress coefficient and horizontal stress created by overburden pressure.Taking joint layer fracturing process realizes production of coalbed methane and tight sandstone gas when sandstone and coal are interbeded,but the property of rock in vertical orientation and stress state is largely different,the morphology of crack initiated from sandstone layer or coal layer have very big difference.Three dimensional crack propagation model is used to simulate the two cases that perforates in coal layer and perforates in sandstone layer,considering the layered earth stress,differences in the lithology,stratigraphic interface,construction conditions,study on the shape of fracture in different conditions and influence factors.The results show that the fracture height is about 2 times of the length when the fracture initiated from sandstone layer,the height of fracture will be not under contral,which can not reach the purpose of Hydraulic fracture.The height of fracture initiated fromcoal seam is restricted by stress shadow effect,fracture length is long.And several influence factors are discussed,the 5MPa of minimum horizontal principal stress difference can limit the fracture in the coal seam.When stress difference is 4MPa,the critical displacement is 5.7m~3/min.Low elastic modulus,high tensile strength,high permeability sandstone beds effectively restrict the crack height expansion,high viscosity of fracturing fluid can help the wear layer behavior,but the effect is weak.The research of this paper provides some theoretical support for the construction of multi gas production in Linxing block,and it can be used as reference for the effective exploit of coalbed methane and tight sandstone gas.