Effect of Resident Water on Enhanced Coalbed Gas Recovery by Simultaneous CO2/N2 Injection

Majority of the coalbed methane reservoirs contain large amounts of water at initial reservoir conditions. Presence of water in the wet coal results from the chemical and physical bonding of coal with water, and influences the properties of coals. Frequently, dewatering is necessary prior to methane production from coal. However, some water still remains in the coal cleat and matrix after dewatering. The remaining water influences the coal gas isotherm, changes the coal gas phase diagram, and hence affects the performance of the enhanced coal gas recovery by simultaneous CO2/N2 injection. Experimental and theoretical are carried out to establish the multi-component gas, water, and coal isotherms under various pressures and coal grain sizes for both low and high rank coals. The obtained isotherms are used to describe the adsorption/desorption processes involving the system of CO2/N2/CH4/water mixtures in coals. It is determined that presence of water reduces the injected CO2/N2 adsorption rates on both low and high rank coals, and the methane production rate, depending on the composition of the gas and water phases, and the prevailing experimental conditions. It is demonstrated that a higher ratio of the injected nitrogen is required for wet coals than for dry coals to enhance the gas recovery by CO2/N2 injection. Introduction The cleats in coalbed methane reservoirs are usually saturated with water at initial reservoir conditions. The water may diffuse through the liquid and vapor phases, and adsorb on the internal surfaces of the coal matrix. The amount of the adsorbed water on the coal internal surface depends on the coal wettability to water and the prevailing reservoir conditions, such as the gas composition, temperature, and pressure. Gosiewska et al. (2002) show that the mineralogical nature of coal largely influences the wettability of the coal surface with water. At macroscopic scale, carbon surface is hydrophilic resulting in little water adsorption. However, if Hbonding sites are present at the surface, forming strong bonds between the water and the coal surface may alter the wettability and enhance the water adsorption. Therefore, various coals have different affinity to the water. However, coal properties are usually affected by water at elevated pressure and temperature reservoir conditions. Under those circumstances, the water diffuses into the coal and is adsorbed on the coal internal surfaces. The presence of water in the coal internal surfaces, where the gas phase is mostly adsorbed and stored, affects the ability of the coal to adsorb and hold various gases, and hence complicates the development of the coalbed methane isotherms. Kross et al. (2002) reported that moisture-equilibrated coals showed 20-25% lower methane adsorption capacity than dry coals. They also indicated that the moisture content of coal reduces the coal sorption capacity for carbon dioxide. The adsorbed water occupies some of the sorption sites and reduces the available surface for gas molecules adsorption on the coal internal surfaces. Some of the adsorbed water may block the gas path to the micropore system. There exists certain coal water content, beyond which the coal sorption gas content does not decrease by increasing the coal water content. Under this condition, all possible adsorption sites for water are occupied and hence maximum water adsorption occurs. Allardice and Evans (1971) modified the BET equation to correlate the equilibrium mono-layer water adsorption data of the Yallourn brown coals. They offered a number of conclusions as following. The number of functional groups present on the coal surface may affect the water sorption capacity of the coal. Most of the functional groups contains large amount of oxygen atoms that may make strong bonds with the hydrogen atoms available in water. Thus, the presence of such bonds may cause some water adsorption in coal. It is also presented that the capillary raise may be another major factor, affecting the water adsorption on the coal internal surface. Stamm (1956) measured the diffusion of water into uncoated cellophane at steady-state. Stamm measured liquid and vapor water adsorption on cellophane versus time. The results show that the frequency of the impact of the surface by water molecules controls the takeup of the water on the solid surface by vapor adsorption. The frequency of the impact of the surface by water molecules is a function of the vapor SPE 102634 Effect of Resident Water on Enhanced Coal Gas Recovery by Simultaneous CO2/N2 Injection H. Jahediesfanjani, SPE, and F. Civan, SPE, U. of Oklahoma