| Coalbed methane resource is very abundant in China,so the exploitation of coalbed methane has very important strategic significance for improving China's energy structure. In this paper, the gas-water flow characteristics in low rank coalbed methane reservoirs, as well as that coupling the flow in coalbed methane reservoir and variable mass flow in pinnate horizontal wellbore, are investigated by experimental study, theoretical analysis and numerical simulation, and the research has made some progress.Firstly, this paper investigated the gas seepage characteristics in block raw coal samples, and emphatically discussed the effects of effective stress and moisture content on the seepage flow velocity. Considering the interaction between coal deformation and methane seepage flow, a motion equation was presented to describe the nonlinear seepage characteristics of coalbed methane in coal. According to the above experimental research and the different characteristics of low rank coal seams, that is fractures are hardly developed and matrix porosity is relatively larger than high rank coal, the low rank coal seams were considered as a porous media composed of matrix micropore system, matrix macropore system and fracture system and a new mathematical model for the gas-water seepage flow in low rank coalbed methane reservoirs was established. Furthermore, not only gas-water two phases flow in matrix macropores and fractures but also coal deformation effects on the fracture permeability was taken into consideration in this model.Secondly, linear difference equations of the gas-water flow model in low rank coal seams were obtained by the fully implicit finite difference method and solved by the OrthoMin iterative method with block incomplete LU factorization preconditioner. Moreover, a calculation program was developed to analyze influence parameters of the model and the model reliability was validated by production history matching and dynamic forecasting of three coalbed methane test wells in Liujia prospecting district. The results showed that the permeability, porosity and water saturation of matrix macropore have large influence on the water drainage performance, and then on the gas production. Comparing with the non-equilibrium adsorption model, the forecasting gas production of this model in initial and stable production periods is relatively low, while the forecasting water production is high.Thirdly, based on rock mechanics properties and elastic constitutive relation, new theoretical formulations for stress-dependent permeability and porosity were presented, which include both stress effects and matrix shrinkage. On the basis of the theoretical formulations, a mathematical model of gas-water seepage flow in coalbed methane reservoirs was given. Besides that, a pressure drop model of variable mass flow in pinnate horizontal wellbore was established in consideration of the mixing loss resulted from the multilateral aggregate flow as well as friction loss and acceleration loss. The mathematical and numerical models of pinnate horizontal wells for coalbed methane recovery were obtained by coupling the gas-water seepage flow model in coalbed methane reservoirs with the pressure drop model of variable mass flow in pinnate horizontal wellbore. And then various factors influencing the pinnate horizontal well productivity were analyzed, which provided a scientific theoretical basis for the optimal design of lateral parameters. The analysis of influencing factors indicates that the contribution of matrix shrinkage to the fracture permeability can't be neglected and higher gas recovery factor could be obtained when the main horizontal wellbore is drilled perpendicular to the maximum permeability orientation.Finally, the influence of wellbore structural parameters on the corresponding forecast model of economic net present value(NPV) was analyzed to achieve optimize design in view of the wellbore structural features of pinnate horizontal wells.
|
PDF Full Text Request