Sorption Irreversibilities and Mixture Compositional Behavior During Enhanced Coal Bed Methane Recovery Processes

Abstract

Abstract Enhanced coalbed methane (ECBM) recovery processes rely on the sorption of a stripping or displacing gas to recover the sorbed methane. Both the adsorption and desorption of methane and the injected gases are important to the success of the process. Oxygen, which can chemisorb to coal, is expected to show different adsorption than desorption behavior. However, adsorption and desorption isotherms for mixtures of physisorbed gases such as nitrogen and methane appear similar in form to those for oxygen. Moreover, results from a field ECBM recovery project also suggest that in the mixed state physisorbed gases may show different behavior during desorption than during adsorption. Because reservoir simulators typically use a reversible equilibrium sorption model such as the Langmuir isotherm, the presence of hysteresis due to sorption irreversibilities could affect the reliability of the simulations. Constant composition expansion and constant volume depletion sorption results were analyzed. This analysis demonstrates that although mixtures of physisorbed ECBM recovery gases (no oxygen) become enriched in the more strongly held component during desorption, the process is one of reversible equilibrium sorption and isotherm models such as the extended Langmuir isotherm effectively capture this adsorption/desorption equilibrium. No additional changes to the simulator are needed because of this compositional effect. However, for oxygen-bearing gases modification of the Langmuir isotherm will be needed to quantitatively represent adsorption/desorption behavior during an ECBM recovery process. Introduction Coalbed methane (CBM) reservoirs hold gas primarily as a sorbed phase at liquid-like densities within the microporous matrix of the coal, not as a free gas as in conventional gas reservoirs. In CBM some free gas exists in the natural fractures or cleats of the coal, but this gas represents only a small fraction of the total gas. CBM recovery is, therefore, primarily recovery of desorbing gas. Desorption may be accomplished by lowering the overall pressure of the reservoir, as in conventional recovery, or by lowering the partial pressure of methane in the free gas by injecting a second gas, as in enhanced recovery. Puri et al. describe an air huff 'n puff technique for the rapid evaluation of enhanced coalbed methane (ECBM) recovery, prospects. They refer to this methodology as a "mirco-pilot." In this technique air is injected for a period of time, the well shut-in for a period of time, and then flowed back. The present study into irreversibilities in the adsorption/desorption process derives in part from the results of that mirco-pilot. Simulation of the micro-pilot process with Amoco's Generalized Compositional Model (GCOMP) led to the conclusion that there should be no methane in the initial gas production on flowback. The near wellbore area should be swept of methane with no methane in the initial production. However, a thirty mol % methane composition was observed in the field upon the start of flow back. Furthermore, as shown in Fig. 9 of Puri et al., this lag in predicted methane composition persists throughout the flow back period. Puri et al. suggest that discrepancies between simulation results and field observations may be attributed to ignoring the effect of diffusion of the desorbed gas through the coal matrix to the fractures or cleats. The shut-in period was of approximately the same length of time as the characteristic diffusion times of coals from the test area; the characteristic time is that time needed for only approximately 63% of the gas to diffuse out of the coal. Thus, desorbed gas may still have been diffusing out of the coal at the start of flowback It has also been suggested that the difference in compositional response between the simulation and the field observations may be influenced by a compositional hysteresis or lag in nitrogen desorption on flowback. Such a lag in desorption, if due to an irreversible effect, would lead the mixed sorbed phase of nitrogen and methane to preferentially release methane in higher amounts than predicted by a reversible sorption isotherm model such as the extended Langmuir isotherm (described in a subsequent section) and used in GCOMP. Such a compositional effect would then lead to produced gases richer in methane than predicted by the Langmuir isotherm and consequently to simulated produced gases leaner in methane than observed. P. 431