| As clean and high-grade unconventional natural gas, coalbed methane (CBM) has attracted widespread attention of scholars from all over the world. With abundant CBM resource in China, efficient exploitation and utilization of CBM is of significant importance in terms of environmental protection, energy security and economic development. After drilling and completion, CBM wells, commonly having characteristics of low-pressure, low-porosity and low-permeability, need to be stimulated with some techniques for gas production. Unfortunately, existing stimulation techniques can not improve CBM production effectively and economically. Adsorption-gas dominated occurrence pattern, decompression-desorption dominated production feature and low-porosity and low-permeability geological characteristics result in the key method for production enhancement to be maximizing contact between fracture networks and coal matrix, and abundant face and butt cleat system provides necessary condition for complex fracture networks formation.Based on this issue, aiming at maximizing stimulated reservoir volume (SRV) for CBM gas reservoirs, this thesis, on the basis of existing research achievements, takes advantage of rock mechanics, mechanics of elasticity, fracture mechanics, hydromechanics, hydraulic fracturing mechanics, higher mathematics and computer programming to study deeply, having obtained the following achievements and cognitions:(1) Based on analysis of coalbed engineering geological characteristics and technical principles, technique features and shortcomings of existing stimulation methods for CBM gas reservoirs, hydraulically maximizing fracture networks fracturing technique of vertical coalbed methane wells has been proposed, aiming at maximizing stimulated reservoir volume by taking advantage of stress interference, and physical model for two vertical coalbed methane wells connecting line perpendicular to maximum principle stress has been built.(2) Numerical calculation Displacement Discontinuity Method (DDM), convenient for analysis of stress interference result from multiple fractures, and its computing procedure have been deduced. Difference between stress interference induced by single fracture and two fractures has been analyzed comparatively with MATLAB programming, based on which near-fracture complex fracture networks and far-fracture complex fracture networks is distinguished, and influence on complex fracture networks caused by stress interference result from hydraulically maximizing fracture networks fracturing is studied. Impact on stress interference caused by fracture length, net pressure, Poisson ratio and well spacing is discussed.(3) Relationship between maximum net pressure requirement for shear and dilation of cleat system and stress anisotropy has been analyzed, and branch fractures propagation direction outside the stress isotropic point of single well fracturing has been studied, which indicates that hydraulically maximizing fracture networks fracturing technique of vertical coalbed methane wells is beneficial for complex fracture networks formation.(4) Programmed by two dimensional Universal Distinct Element Code (UDEC) based on Discrete Element Method (DEM), fracture networks propagation configuration of single well fracturing and hydraulically maximizing fracture networks fracturing have been analyzed comparatively, proving that increasing stimulated reservoir volume by hydraulically maximizing fracture networks fracturing technique of vertical coalbed methane wells is feasible. Impact on fracture networks propagation configuration caused by initial horizontal principle stress anisotropy, Poisson ratio, well spacing, fracture half length and fluid viscosity has been discussed.This study indicates that hydraulically maximizing fracture networks fracturing of vertical coalbed methane wells can achieve more extensive stimulated reservoir volume compared with two wells fractured separately. Sensitive parameters analysis has significant guidance for fracturing treatment design and candidate selection of wells and formations, and distinction of near-fracture complex fracture networks and far-fracture complex fracture networks provides a guideline for next economic optimization. |
