Numerical Simulation Of Three Phase Gas Content Dynamic Changes In Recovery Of Coal Reservoirs

According to relative theories, the geological and mathematical model ofcalculating three phase (adsorptive, free and dissolved) gas content in coal reservoirare constructed within the coalbed methane (CBM) well drainage area, and typicalexamples are calculated. In single drainage well, characteristics of three-phase gasand their dynamic change during drainage are studied. Characteristics of three-phasestate gas and change regulation during drainage are understood. The works provide abasis for fine descriptions of reservoir dynamic process and a guide to the coalbedmethane well reasonablely drainaging.An algorithm of calculating the adsorptive, free and dissolved gas content areestablished. The algorithm is based on gas-water phase and two-dimensional kineticequations and analytical. It can calculate the gas content in condition of saturate,under and over saturate of coal reservoir. Three typical wells which belongs togas-pressure, hydraulic-pressure and mixed coal reservoirs respectively are selectedfor the reservoir modeling study. And the simulation results were used to study thedynamic variations of three phase gas content during CBM well drainaging. Theresults indicate that in general, gas output from the CBM wells, adsorptive gas isdominant, while free and solubel gas is the minor. Three phase gas content decreasedto different extent during CBM well drainaging. Water content in gas-pressurereservoir is extremely low, the outputing gas are mostly adsorptive gas, adsorptive gasdecreased at a speed corresponded with gas production rate. Free gas content changesin the similar way of adsorptive one, but its value is very low. The difficulty ofpressure dropping in hydraulic-pressure reservoir leads to difficult desorption ofadsorptive gas. Very low gas production of CBM well is mainly supplied by free andsoluble gas and gas content of them decreased at a speed corresponded with gasproduction rate. During drainage of the mixed reservoir, all three-phase gas contributeto the gas production, but the adsorbptive gas is still dominant, three-phase gas stilldecreased at a speed corresponded with gas production rate. In addition, the reducingrange of three-phase gas content are bigger than the other two types of reservoirs, itcan get high gas outputing in general.