Method for determining critical desorption pressure of coal bed gas

Abstract

The invention discloses a method for determining critical desorption pressure of coal bed gas. The method comprises the steps that basic parameters for calculating the critical desorption pressure ofthe coal bed gas and original stratigraphic data of a coal-bed gas well are collected and tested; the influence of liquid water in a coal bed is considered, and a coal-bed-gas isothermal adsorption and desorption curve is calculated and drawn by utilizing a Gibbs free energy theory and a reservoir pore throat distribution curve; and the critical desorption pressure corresponding to the coal-bed gas content is obtained according to the coal-bed-gas isothermal adsorption and desorption curve, and then the obtained desorption pressure is corrected by utilizing the calculated pressure drop from amatrix pore to a cleat capillary and / or a Darcy flowing pressure drop from the cleat to a fracturing crack. According to the method, the influence of the liquid-phase water in the reservoir on the calculation result and the additional pressure drop consumption generated between the desorption gas-producing gas and the bottom-hole gas are introduced, and the problem of a large difference between the numerical value obtained through a currently-used method for determining the critical desorption pressure and that of an actual site is solved.

Description

technical field [0001] The invention relates to the field of gas reservoir engineering, in particular to a method for determining the critical desorption pressure of coal bed gas. Background technique [0002] Coalbed methane refers to hydrocarbon gas stored in coal seams with methane as the main component, mainly adsorbed on the surface of coal matrix particles, and partly free in coal pores or dissolved in coal seam water. It is an associated mineral resource of coal. Coalbed methane is very clean after burning, and hardly produces any waste gas. It is a good fuel for industry, chemical industry, power generation and residential life. Determination of critical desorption pressure of coalbed methane well is the most important link in coalbed methane exploitation. In principle, the critical desorption pressure of coalbed methane refers to the corresponding pressure when desorption and adsorption reach equilibrium, that is, the pressure when the gas adsorbed on the surface o...

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Problems solved by technology

The chart method can easily obtain the specific value of the critical desorption pressure, but there are two main problems in terms of mechanism: one is that for the isothermal adsorption and desorption curve, the measurement of the curve does not take into account the influence of water in the actual reservoir to cause errors; It is aimed at the coupling method of the desorption curve and the gas content of the reservoir, without considering the additional pressure drop consumption caused by diffusion and seepage between nanopore desorption gas production and bottom hole gas penetration

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