Abstract
There are lots of cleats, fractures and many other structure weak planes in coal seams, which make the bullet holes and cleats intersecting. During the hydraulic fracturing process in coal seams, fractures will initiate at coal rock body of borehole wall, and cleats or fractures are different from conventional reservoirs. Thus a new model for initiation pressure calculation during coal seams fracturing should be established. Based on the rock mechanics and elasticity mechanics, and also on network distribution characteristics of coal seam cleats and the space position relationships between the intersected bullet holes and cleats, stress distribution around the bullet holes and at the cleats wall were deducted. The model was established in tensile failure condition. The calculated initiation fracture pressure of Well HX-3 was 10.71MPa. The pressure obtained from bottom hole pressure gauge was 11.24MPa. The relative error was 4.72%. The model could be applied for initiation pressure calculation during hydraulic fracturing process in coal seams. The fractures would initiate at the cleats during fracturing.
Highlights
Hydraulic fracturing is an important technology to ensure the effective development of coalbed methane
Z = rz = 0 where: r is the radial stress in borehole area, MPa; is circumferential stress, MPa; z is the vertical stress, MPa; r, z and rz are the shear stress, MPa; H is maximum principal stress; h is minimum principal stress, MPa; V is the overburden stress, MPa; r is the wellbore polar radius, m; rw is the radius of wellbore, m; μ is the Poisson’s ratio, dimensionless variable; pw is the fluid pressure in the wellbore, MPa; is the sinistral polar angle of maximum principal stress in radial direction
Where: pr is the radial stress around the perforation tunnels, MPa; p is the circumferential stress around the perforation tunnels, MPa; pz is the vertical stress around the perforation tunnels, MPa; pr, prz and pz are the shear stress, MPa; is the Biot factor; is the porosity, dimensionless variable; pp is the pore pressure, MPa; rp is the radius of perforation tunnel, m; Lp is the distance from perforation tunnel axis, m; is the sinistral polar angle of circumferential stress in radial direction of perforation tunnel
Summary
Hydraulic fracturing is an important technology to ensure the effective development of coalbed methane. The accurate calculation and prediction of initiation pressure are important for the successful implementation of hydraulic fracturing. It has become a research hotspot and difficulty in CBM hydraulic fracturing. Jinzhou Zhao [8,9,10] established the initiation pressure calculation model of fractured reservoir under the condition of perforated completion, and analyzed the influence of different initiation patterns on initiation pressure. The calculation of initiation pressure in perforated coal seams mainly referred to the existed sandstone reservoir initiation pressure calculation model, or did simulation using finite element program [11], and there had not been a relatively perfect theory to analyze the influence of cleats and fractures on the initiation pressure of coal seams. The paper considers the differences in mechanical properties of face cleats and butt cleats in coal rock body, and establishes calculation model of initiation pressure in perforated coal seams, which can be used to analyze and determine the different initiation patterns and calculate the initiation pressure, and provides a new method for the initiation pressure calculation during hydraulic fracturing process of perforated coal seams
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