CHARACTERISTICS OF COALBED GAS COMPOSITION SHIFTS: FROM FIELD WELL PRODUCTIONS TO FIELD AND LABORATORY DESORPTION TESTS - A NUMERICAL INVESTIGATION

The variation in production rates and gas composition are critical in determining the economic success of coalbed methane wells. Many coalbed methane reservoirs include significant CO₂ which is co-produced with CH₄ and requires stripping prior to marketing. It has been documented that in producing coalbed methane fields such as parts of the San Juan Basin, the production gas becomes progressively more enriched in CO₂ through the production life of a reservoir. During field and laboratory desorption testing of cores however the opposite occurs, the ratio of CO₂:CH₄ declines with time. To understand these seemingly contradictory phenomena, we have developed a numerical model that provides insight into how and why gas composition likely evolves. We recognize three contrasting gas composition trends during well production from a coal seam containing mixed CH₄-CO₂ gas. An initial rapid enrichment of CO₂ in the production gas is followed by a depletion of CO₂ during the very early production stage. Thereafter, CO₂ becomes constantly enriched again. Our model indicates that the initial water production/pumping which causes the coal cleat and matrix pressures near the well bore to be out of equilibrium, leads to gas production controlled by differential gas diffusion near the well bore. Thus, the higher diffusion coefficient of CO₂ than CH₄ in microporous coals results in the initial enrichment of CO₂ and a subsequent depletion of CO₂. However, during the major stage of gas production, the cleat and matrix pressures approach equilibrium, resulting in gas production dominated by equilibrium gas desorption. The stronger adsorption of CO₂ than CH₄ in coals retards the release of CO₂ relative to CH₄ from the internal surface of the coals, consequently causing a steady enrichment of CO₂ during the late stages of production. During coal core desorption tests, however, gas production is mainly controlled by diffusion, and thus the gas composition shift is similar to the early stages observed during well production. Our analyses also suggest that the cleat/fracture spacing and aperture width in the coal seam have a marked effect on the variation in production rate and gas composition with time. Overall, the composition shift of coalbed gas is strongly controlled by the different adsorption and transfer characteristics of gases in coals.