Study On Percolation Mechanism And Deliverability Evaluation Of Coalbed Methane

Coalbed methane (CBM) is a kind of self-generating and self-preserving, unconventional natural gas which stores in the coal seams and its surrounding rocks. It is a new type of clean energy and high-quality chemical raw materials. Mining and utilization of CBM can not only improve the safety of coal production, but also fully utilize the non-renewable resources to alleviate the shortage of conventional oil and gas, and to protect atmospheric environment, which are of great significance.In this thesis, based on the literature research, the reservoir capabilities, migration and production mechanisms of coalbed methane are analyzed; the unsteady state sorption model and the pseudo-steady state sorption model of single-phase flow for CBM are established. Analytical solutions are also carried out for these two models. According to the results obtained from the above models, the desorption, diffusion and percolation mechanisms are further studied. A Two-phase, three-dimensional mathematical model along with a numerical model for dual porosity media is established, and productivity formulas are derived both for the vertical well and the horizontal well. The Langmuir adsorption isotherm law and Fick's law are introduced to describe the CBM desorption and diffusion behavior in these models. With the aid of the computer technology, solutions for the mathematical model are carried out using finite difference method in the numerical way. At the same time, a computer calculation program and CBM numerical simulation software are developed with a friendly user interface.Main research results:(1) The single-phase migration equations which are established for the unsteady state sorption model and the pseudo-steady state sorption model are calculated using analytical methods. By analyzing the results of CBM production and coalbed pressure changes, big difference can be seen during the production of the first 30 days. The unsteady state sorption model can describe the pressure transient better. During the later production time, the results obtained from the two models tend to be consistent.(2) Under the vertical well production condition, by analyzing the effectiveness of 9 sensitive parameters, such as coalbed thickness, permeability, porosity, diffusion coefficient, and adsorption isotherm constants, we find that coalbed thickness, permeability and adsorption isotherm constants are of great importance for CBM production.(3) Under the horizontal well production condition, by analyzing the effectiveness of 11 sensitive parameters, including the length and the mining layer of the horizontal well, and all the parameters analyzed in the vertical well, we find that parameters that affect vertical well's production also have great effects on horizontal wells. In the same geological conditions, the horizontal wells'production is 20% higher than that of vertical wells. Moreover the different length and mining layer of the horizontal well can also have remarkable influence.(4) Through contrasting the vertical well and the horizontal well production curves, we discover that the horizontal well has big superiority in CBM production. Therefore under the same condition, the horizontal well should be considered first.