Gas-Water Capillary Pressure in Coal at Various Overburden Pressures
Authors: Dabbous M.K. , Reznik A.A. , Mody B.G. , Fulton P.F. , Taber J.J. 
Published: December 2007

Abstract
Drainage air-water capillary-pressure curves were obtained for Pittsburgh and Pocahontas coals at various overburden pressures. Capillary-pressure data were used to investigate pore-size characteristics. Results were indicative of the complex pore structure of coal, consisting primarily of a network of macro- and microfractures. In most cases, however, displacement pressure and residual water saturation increased at higher overburden pressure. Reasonable agreement between measured relative permeabilities and those calculated from capillary-pressure data with Purcell's model was obtained for only a few samples. Fracture permeabilities computed from pore-size distribution were lower than permeabilities pore-size distribution were lower than permeabilities actually measured at the same overburden pressure. Helium porosity was considerably higher than porosity determined by water saturation, indicating porosity determined by water saturation, indicating inaccessible pore volume to water. Pore compressibility was determined under triaxial stress-loading conditions. Changes in porosity with overburden pressure were more significant at pressures below 1,500 psig. Above this pressure, pore compressibility appeared to approach a pressure, pore compressibility appeared to approach a constant value averaging about 0.5 × 10(−4) psi(−1) for the coal samples studied.

Introduction
Increased interest in underground coal gasification and coal-seam degasification for the purpose of producing clean energy stimulated fundamental producing clean energy stimulated fundamental research into the phenomena of multiphase fluid flow through coal. Two previous papers presented results of investigation of the air- and water-permeability and relative-permeability characteristics at various overburden pressures for two different types of coal. However, to understand the mechanisms of two-phase flow (usually gas and water) through a complex porous system such as coal, one needs a clear insight into the internal pore structure of coal and the interaction between pore structure of coal and the interaction between this structure and the associated fluids. Such knowledge of the make-up of the pore structure helps in modeling fluid flow through the system and in interpreting permeability and relative-permeability data. Interaction between the pore structure and fluids results in the capillary-pressure phenomena. Capillary-pressure data have been used extensively to determine the pore characteristics of many petroleum reservoir rocks and to relate these petroleum reservoir rocks and to relate these characteristics to the single- and two-phase flow behavior in the rock.
It is also known that natural fracture systems are the principal source of flow capacity of many petroleum reservoir rocks and contribute materially petroleum reservoir rocks and contribute materially to the storage capacity of some. Changes in fracture capacity resulting from changes in net overburden pressure have an important influence on the flow pressure have an important influence on the flow properties of the rock, as reported by Jones. In our properties of the rock, as reported by Jones. In our previous work with coal, which is a naturally previous work with coal, which is a naturally fractured system, absolute and effective permeabilities were found to be highly sensitive to overburden pressure (pov). Thus, it would be expected that the pressure (pov). Thus, it would be expected that the effect of Pov on the fracture flow capacity, capillary pressure, and pore compressibility is more dramatic pressure, and pore compressibility is more dramatic for coal.
The internal structure of coal has been studied by microscopic methods, gas sorption measurements, and by mercury porosimetry. Data on helium porosity of different types of coal also can be porosity of different types of coal also can be found in Ref. 5. However, we are not aware of any determinations of capillary pressure in coal at different overburden pressures.
In this paper gas-liquid capillary-pressure relationships for coal at different overburden pressures are presented. pressures are presented.
SPEJ
P. 261