Establishment and application of material balance equation for deep coalbed methane reservoirs based on coal formation stimulation background

In recent years, the deep coalbed methane was developed rapidly, with breakthroughs in daily production of tens of thousands of cubic meters per well in multiple deep coalbed methane blocks. In order to develop deep coalbed methane more scientifically and efficiently, significantly improve the recovery rate of coalbed methane, establishing a reasonable, reliable, and comprehensive coalbed methane material balance equation, and studying the controlled reserves, average coal reservoir pressure, proportion of different types of gas production, productivity evaluation indicators, and EUR of deep coalbed methane reservoirs or gas wells have important theoretical and practical significance. At present, there are few reports on the material balance equation of coalbed methane considering the presence of free gas, coal matrix shrinkage, the presence of dissolved gas, the pressure difference between micropores enriched with adsorption gas and mesopores occupied by free gas and water, and the impact of coal formation stimulation. Firstly, based on the principle of material balance in deep coalbed methane reservoirs, a material balance equation for deep coalbed methane reservoirs under formation stimulation background was established by further considering the pressure difference between micropores enriched with adsorption gas and mesopores occupied by free gas and water, changes in coal reservoir physical parameters caused by formation stimulation, and dissolved gas, except considering pore compression caused by stress sensitivity, matrix shrinkage, water expansion, and water production in conventional material balance equation of coalbed methane reservoir. Then, a linear fitting method for evaluating the reserves of adsorbed gas, free gas, and dissolved gas, an explicit calculation method for average formation pressure, an evaluation method for the production proportion of adsorbed gas, free gas, and dissolved gas, a productivity evaluation method for deep coalbed methane wells, and an EUR prediction method were proposed. Finally, the proposed methods were applied in an example to evaluate the reserves of adsorbed gas, free gas, and dissolved gas controlled by the well. The variation law of the production proportions of adsorbed gas, free gas, and dissolved gas in the production process of deep coalbed methane well were revealed, and the characteristics of changes in productivity indicators were analyzed. The EUR and coalbed methane recovery under different abandoned pressures were predicted. Results show that: The proposed method for evaluating coalbed methane reserves only requires two or more actual measurements of the average coal reservoir pressure and corresponding cumulative gas and water production data, and can use linear fitting to evaluate the reserves of adsorbed gas, free gas, and dissolved gas controlled by deep coalbed methane wells. The proposed method for calculating the average formation pressure of deep coalbed methane is an explicit expression, which avoids the complex computer programming calculation of implicit solving methods; The proposed evaluation method for the production proportion of adsorbed gas, free gas, and dissolved gas in deep coalbed methane wells does not require the installation of carbon isotope monitoring devices at the wellhead, and can real-time evaluate the proportion of different gas production. The proposed productivity indexes of comprehensive fluid, free gas, adsorbed gas, dissolved gas, and water at bottomhole condition for deep coalbed methane wells can be used to identify the rationality of working systems of the deep coalbed methane well; The proposed EUR prediction method for deep coalbed methane wells can organize production data into a linear expression of apparent pressure p/Z * and Gp. Given a certain abandoned pressure, the EUR of deep coalbed methane wells can be predicted. In the initial stage of production of the example well, the production proportion of free gas is almost equivalent to that of adsorbed gas. During the production process, the proportion of free gas first rapidly increases, then slowly decreases, and finally gradually stabilizes at 29%. The production proportion of adsorbed gas first rapidly decreases, then slowly increases, and finally gradually stabilizes at 70%, while the production proportion of dissolved gas remains low. When the abandoned pressure is 4, 3, and 2 MPa, the coalbed methane recovery rates of the deep coalbed methane well are 37.8%, 44.2%, and 52.4%, respectively. Reducing the abandoned pressure is an effective way to improve the coalbed methane recovery rate of the deep coalbed methane reservoirs.