Mechanism Researches Of Stimulated Reservoir Volume In Coal Seams

Coalbed methane (CBM) that occurs in coal seams is formed in the coalification process. In general, the CBM exploitation experiences depressurization, desorption, diffusion and seepage. CBM reservoirs have the characteristics of high rank, low permeability, low porosity and high gas content in the southern of China. In order to increase CBM production in this region, hydraulic fracturing is the key technology. However, the existing conventional fracturing methods just improve the seepage channel of CBM, not make significant contributions to CBM desorption and diffusion so that the single well production of CBM is low and the economic benefit is poor.Stimulated Reservoir Volume (SRV) that originates from the shale gas exploitation is rarely applied to the CBM field. This paper draws lessons from the successful experience of the shale SRV and proposes the SRV thought of the CBM horizontal well according to the geological characteristics of Qinshui basin. Coal seams can be breaked into pieces by the SRV, which contributes to increasing the modified volume. This technology can not only improve the seepage channel of CBM, but also reduce the distance of CBM diffusion and seepage to speed up CBM output velocity and achieve the aim of increasing the single well production.However, the research about SRV is less at home and abroad. Understanding the ability of the coal reservoir forming fracture networks is insufficient, and the simulation method aiming at the fracture propagation in coal seams after being fractured is absent. Therefore, based on the research of the horizontal well site, the study of the mechanism of SRV in coal seams is carried out in the ways of experiment, theoretical study and field technology. The main content are summed up in the following aspects:(1) The stimulation technology of CBM is investigated, and the factors influencing the single well production of CBM is analyzed. Aiming at the existing problems of the conventional technologies, the SRV thought of the horizontal well in coal seams is put forward. The characteristics of permeability, porosity, microstructure, fracture development and coal mechanics are measured in the experiment. Unconventional features about CBM reservoirs in the aspects of geology and exploitation are deeply analyzed.(2) By considering the effect of coal mechanics, in-site stress, density of coal cleats and net pressure of the fracturing construction on multiple fractures of coal, the comprehensive evaluation method about the compressibility of SRV in coal seams is put forward based on the comprehensive compressible index. This method that has a good economy can evaluate the geological condition of the block before fracturing, which can provide some suggestions for the fracturing design.(3) Adopting the method of fracture mechanics, the stress fields of tip and lateral regions of the hydraulic fracture are calculated, and the interactive relationship between hydraulic fracture and coal cleat is built. Shear and tension degrees of different locations are calculated, and the effect of cleat cohesion, internal friction angle, net pressure, included angle between cleat and hydraulic fracture and differential stress coefficient on the cleat initiation is analyzed.(4) Considering the developmental characteristics of coal cleats, four models of body initiation, open initiation of cleats, shear initiation of cleats and perforation tip initiation in the condition of open hole completion and perforation completion of the horizontal well are built respectively, and fracture pressures relative to the different initiation patterns are calculated by the means of the VB programmer. In the four initiation patterns, the pressure of the cleat open initiation is the lowest, while that of the body initiation is the highest; When drilling along the minimum horizontal major stress, pressures of body initiation, open initiation and shear initiation are the highest, while that of the shear initiation is the lowest; the fracture pressure under the condition of pressure building and deflection can promote forming the fracture network near the wellbore area.(5) Based on the knowledge of fracture mechanics, the influence relationship between fracture disturbance and initiation pressure is built. The stress shadow of the pre-fracture can affect the fracture pressure of the hydraulic fracture afterwards. With the increase of the fracture net pressure, the initiation pressure of the hydraulic fracture increases.(6) Based on the feature of the orthogonal cross between face cleat and butt cleat, the simulation method of the fracture propagation is built. This method adopts 3DEC software and FISH language programmer to build models and set up conditions. Relative to finite element and boundary element, the discrete element can preferably consider the characteristic of the continuous structure in coal, which makes the simulation results more in line with the reality in order to guide the fracturing design.(7) The numerical simulation results fit well with the experimental results of the true triaxial tests, and the unity of physical and numerical simulation is realized to some extent. Based on different construction parameters and geological parameters, the results show: ① With the increase of the displacement, hydraulic fractures affecting areas are increasing; ② The low differential stress coefficient is in favor of forming branching fractures; ③The higher the density of the cleats is, the more complex the fracture network is; ④When the orientation angle between face cleat and maximum horizontal major stress is small, the branching fractures are not formed easily, but the branching fractures are easy to produce in 30°～50°; ⑤The smaller the cluster spacing of the staged fracturing is, the more complex the fracture network is.