The Fluid-structure Interaction Model And Permeability Prediction For Pore-fissure In Multi-scale In Coalbed Methane Reservoir

Our country is rich in reserves of coalbed methane resources. The exploitation and utilization of coal-bed methane have been considered as promising methods to satisfy the problem of energy supply, reduce coal mining accidents, diminish greenhouse gas emissions and ease the deterioration of the ecological environment, etc. Thus, making rational plan and layout coalbed methane extraction and improving the productivity are urgent to solved. In this study, based on the uniaxial compression test, CT scanning experiment and permeability test, the physical characteristics of coal samples, such as mechanical properties, pore structure, permeability and so on, are obtained. Coupling models of gas-solid two phases and gas-liquid-solid are established in the process of coalbed methane production through theoretical analysis. Furthermore, different factors influencing coalbed methane production are analyzed by numerical simulation, which provides a reference for the further research on coalbed methane production. The specific works and conclusions are as follows:(1) Experiment on measuring the pore structural characteristics of of coal sample with fractures was carried out. The elastic modulus and Poisson's ratio were obtained by uniaxial compression test. To capture the images of the fractured coal samples, CT scanning experiment was conducted. The impurities of high density are distributed unevenly. The main fractures and the branch cracks constitute the three-dimensional fracture network in the sample. The porosity of the coal sample was found to be around 6~8.5%.(2) Permeability inversion of coal samples with fractures was developed. MATLAB program was used to tackle the section images for generating the 3D model, which contains the data on structure characteristics of holes and cracks. Then, the permeability was calculated through importing the 3D model into the COMSOL Multiphysics software. The general permeability is a comprehensive reflection of various microelements in the model. The greater porosity of the sample is, the greater permeability is. The maximum permeability can reach up to 104 mD, and the minimum is only 10 m D. The distribution of porosity also affects the general permeability. Moreover, there are plenty of frequent fluid exchanges between the matrix and fissures in the percolation process.(3) The permeability test system of the standard sample was designed. The experiment on measuring the permeability of coal sample with fractures was carried out. The permeability of each sample is within the scope of 100 mD ~1000mD. The general permeability is approached by calculation. However, considering the distributions of fractures and joints vary with the samples, the permeability of each sample should be slightly different from the calculation result. Moreover, the permeability decreases with the increase of confining pressure, and increases with the enhancement of pore pressure. Compared with confining pressure, the effect of pore pressure on permeability is less. The effect of confining pressure on permeability is 1.3~1.4 times that of pore pressure.(4) A gas-solid coupling model of coalbed mathane flow was established. Considering the change of pore volume modulus and compressibility coefficient, the dynamic models of porosity to effective stress and permeability to effective stress were proposed. Variation of permeability caused by pore pressure and matrix shrinkage in the reservoir was analyzed. Compared with the current models, the model is more accurate and has a wider application.(5)The effects of different factors on production efficiency of coalbed methane were discussed. Firstly, the coupling model of gas-solid was calculated with COMSOL Multiphysics. Secondly, the variation of pore pressure and permeability with time and space were obtained. Finally, different factors such as solid adsorption deformation, deformation, elastic modulus, Poisson's ratio, permeability, porosity, gas content, reservoir pressure, wellhead pressure and so on, which affect the coalbed methane production rate and cumulative production, were analysed.(6) The coupling model of gas-liquid-solid was established. The accuracy of the finite element simulator COMSOL Multiphysics was verified by 1-D Terzaghi's consolidation problem. The simulation results fit well with the field data, the variations of stress with pore pressure change and strain of shrinkage have great influence on the productivity prediction of coalbed methane. In particular, the sensitivity analysis indicates that the stress of reservoir cannot be ignored in the exploitation of coalbed methane. Coal reservoirs with higher Young's modulus and smaller Poisson's ratio will generate the higher permeability and larger production rate, which should be more favorable for the exploitation.