Abstract

Sorption-induced strain and permeability were measured as a function of pore pressure using subbituminous coal from the Powder River basin of Wyoming, U.S.A. and high-volatile bituminous coal from the Uinta-Piceance basin of Utah, U.S.A.We found that for these coal samples, cleat compressibility was not constant, but variable.Calculated variable cleat-compressibility constants were found to correlate well with previously published data for other coals.Sorption-induced matrix strain (shrinkage/swelling) was measured on unconstrained samples for different gases: carbon dioxide, methane, and nitrogen.During permeability tests, sorption-induced matrix shrinkage was clearly demonstrated by higher permeability values at lower pore pressures while holding overburden pressure constant; this effect was more pronounced when gases with higher adsorption isotherms such as carbon dioxide were used.Measured permeability data were modeled using three different permeability models that take into account sorption-induced matrix strain.We found that when the measured strain data were applied, all three models poorly matched the measured permeability results.However, by applying an experimentally derived expression to the strain data that accounts for the constraining stress of overburden pressure, pore pressure, coal type, and gas type; two of the models were greatly improved.

Introduction

Coal seams have the capacity to adsorb large amounts of gases because of their typically large internal surface area (30 m2/g to 300 m2/g)[1]. Some gases, such as carbon dioxide, have a higher affinity for the coal surfaces than others, such as nitrogen.Knowledge of how the adsorption or desorption of gases affects coal permeability is important not only to operations involving the production of natural gas from coal beds, but is also important to the design and operation of projects to sequester greenhouse gases in coal beds. [2]

As reservoir pressure is lowered, gas molecules are desorbed from the matrix and travel to the cleat (natural fracture) system where they are conveyed to producing wells.Fluid movement in coal is controlled by diffusion in the coal matrix and described by Darcy flow in the fracture (cleat) system.Because diffusion of gases through the matrix is a much slower process than Darcy flow through the fracture (cleat) system, coal seams are treated as fractured reservoirs with respect to fluid flow.However, coalbeds are more complex than other fractured reservoirs because of their ability to adsorb (or desorb) large amounts of gas.

Adsorption of gases by the internal surfaces of coal causes the coal matrix to swell and desorption of gases causes the coal matrix to shrink.The swelling or shrinkage of coal as gas is adsorbed or desorbed is referred to as sorption-induced strain.Sorption-induced strain of the coal matrix causes a change in the width of the cleats or fractures that must be accounted for when modeling permeability changes in the system.A number of permeability-change models [3,4,5,6,7,8]for coal have been proposed that attempt to account for the effect of sorption-induced strain.Accurate measurement of sorption-induced strain becomes important when modeling the effect of gas sorption on coal permeability.

For this work, laboratory measurements of sorption-induced strain were made for two different coals and three gases.Permeability measurements were also made using the same coals and gases under different pressure and stress regimes.The objective of this current work is to present these data and to model the laboratory-generated permeability data using a number of permeability-change models that have been described by other researchers.This work should be of value to those who model coalbed methane fields using reservoir simulators as these results could be incorporated into these reservoir models to improve their accuracy.

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