Experimental Measurement of Coal Matrix Shrinkage Due to Gas Desorption and Implications for Cleat Permeability Increases

Abstract

Abstract Coal is not an inert reservoir rock and reacts to gas desorbed from its surface. Coal matrix shrinks as gas is desorbed, increasing cleat width and, therefore, permeability. Very few coal matrix shrinkage data have been reported in the literature so a series of experiments was undertaken to measure such data at reservoir pressures, temperatures, and 100% relative humidity. Strain gages were affixed to the coal sample in the face and butt cleat directions as well as the vertical direction. This work reports measured deformations of a sample of high volatile C bituminous coal from the San Jan Basin during sorption and desorption of first methane then CO2. A pressure cycle was also run with helium, a nonsorbing gas, to determine mechanical compliance of the sample. Observed strain gage behaviors are discussed and shrinkage coefficients for both gases reported. Matrix shrinkage was found to correlate with gas content rather than pressure, confirming the work of a previous investigator. Shrinkage coefficients varied more among replicate gages aligned in the same direction than between gages in different directions. Anisotropic shrinkage effects are discussed. Using a matchstick geometry model, equations are derived for permeability change due to matrix shrinkage. Coefficients reported here are used in example calculations of absolute permeability and porosity increases during coalbed depletion. Introduction Coal matrix swells and shrinks as gas is first adsorbed then desorbed. The amount of swelling depends on coal rank and sorbed gas composition. During recovery of coalbed methane, coal matrix will shrink, increasing cleat width. As permeability depends on the cube of cleat width, a small increase in cleat width may significantly increase permeability. Such an increase will offset permeability losses due to increased stress during depletion. A search of the literature revealed eight previous studies of coal matrix shrinkage.14 These studies are summarized in Table I. The matrix shrinkage coefficients ranged from a low of 8.62E-7 psi-1 to a high of 6.55E-4 psi-1. Moffat and Weale investigated sorption induced swelling of low volatile bituminous and semi-anthracite coals. They determined a methane swell in coefficient of 1.70E-6 psi-1 at a pressure of 200 atm. Gun the investigated swelling due to methane and carbon dioxide on coals with ranks ranging from high volatile A to anthracite. Reported swelling coefficient ranged from 2.76 to 6.90E-6 psi-1 and carbon dioxide was reported to swell coal more than did methane. Vinokurova found low rank coals swelled more than high rank coals but no swelling coefficients were reported. Wubben, et al., investigated swelling of anthracite and bituminous coals and reported swelling coefficients of 1.4 to 6.9E-6 psi-1. Reucroft and Patel used coals from the Appalachian Basin of the USA to investigate swelling due to sorption of carbon dioxide, nitrogen, and xenon. They reported a carbon dioxide swelling coefficient of 6.55E-6 psi-1. Gray measured a methane swelling coefficient of 8.62E-7 psi-1 for a Japanese coal of unreported rank. No details of coal rank were provided by Juntgen when he reported a methane swelling coefficient of 2.57E-4 psi-1. Only one of the previous studies, Harpalani and Schraufnagel, dealt with a coal currently of interest to coalbed methane recovery. Using a sample of Piceance Basin coal, they reported a shrinkage coefficient of 6.2E-6 psi-1. Unfortunately, many of these studies were done at pressures below those typically encountered in CBM fields and none of them used reservoir temperatures or the high humidity conditions typical of in situ coals. Nor did any provide information on ash content (or mineral matter) which would intuitively have a strong influence on swelling behavior as it significantly affects the amount of gas which can be sorbed on a sample. Experimental Objectives of our work were to assemble experimental apparatus, develop experimental procedures, and then measure coal matrix shrinkage coefficients at reservoir conditions. Reservoir temperatures and pressures commonly encountered during coalbed methane recovery are not extreme conditions for strain gages. Many commercial gages would probably be quite adequate for this experiment. P. 575