The calculation method based on the equivalent continuum for the fracture initiation pressure of fracturing of coalbed methane well

Abstract

Coal rock is a type of fractured rock masses in which lots of cleats, fractures and weak planes develop, and these structures make the coal rock mechanics properties largely different from those of conventional sandstone rock. Previously established calculation models for the fracture initiation pressure of fracturing of conventional reservoirs cause large deviations when used to calculate the fracture initiation pressure of coalbed methane fracturing. In order to accurately calculate the fracture initiation pressure of coalbed methane fracturing, a new calculation model needs to be established while taking coal rock mechanical properties into consideration. In this paper, first, we carry out the macroscopic measurements and scanning electron microscopy (SEM) observation experiments on coal rock samples to describe the cleats distribution characteristics of coal rock samples. Second, we have carried out tests on rock mechanical parameters of different coring directions to obtain the anisotropy of coal rock mechanical parameters. Third, a calculating model has been established to determine the elastic modulus and Poisson’s ratio of coal rock based on the equivalent continuum mechanical methods, and then the model gets validated using the results of mechanical parameters tests. Finally, we have established the calculation models for borehole wall stress and fracture initiation pressure of hydraulic fracturing, which is based on the calculation model for equivalent coal rock mechanical parameters, to analyze the influence of mechanics properties of coal cleats on initiation pressure. The research result provides a new kind of research idea and approach for the calculation of fracture initiation pressure of coal hydraulic fracturing.