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Abstract

At present, coalbed methane is being recovered by means of reservoir pressure depletion. While this method is simple and effective, it is not efficient. Reduction in reservoir pressure deprives fluids of the energy necessary to flow to the well bore. Furthermore, there is a practical and economic limit on the extent to which reservoir pressure can be reduced. It is estimated that pressure can be reduced. It is estimated that reservoir pressure depletion strategy of coalbed methane production will permit the recovery of 50% or less of the gas-in-place.

This paper discusses an alternate method of coalbed methane recovery which consists of nitrogen flooding a coal seam. Lab research has shown that essentially all methane sorbed on coal can be stripped by nitrogen without necessarily reducing the total system pressure. That is, methane desorption from coal is achieved by reducing the partial pressure of methane rather than merely the total pressure of methane rather than merely the total pressure. Lab and modeling results are presented to pressure. Lab and modeling results are presented to demonstrate how nitrogen injection in coal can accelerate methane production rates and enhance reserves. Scoping process economics of enhanced coalbed methane production also appear to be attractive.

Introduction

Coalbed methane gas-in-place is estimated to be 400 TCF, of which about 95 TCF is considered to be economically recoverable with current technology'. This represents a significant resource since the total U.S. proved gas reserves are estimated to be 187 TCF. Efforts to commercially recover coalbed methane have begun with the drilling of over 2000 wells, primarily in the San Juan and Warrior Basins.

METHANE IS ADSORBED TO COAL SURFACE

Unlike conventional gas reservoirs, methane in coal is not stored as a free gas but rather as sorbed gas, at near liquid densities, on the internal surface area of the microporous coal. As shown in Figure 1, the methane storage capacity of coal is given in terms of a sorption isotherm where the gas content in SCF/ton is plotted verses pressure at a constant temperature. For example, if the reservoir pressure of a coal seam were 500 psia at discovery, pressure of a coal seam were 500 psia at discovery, the coal would be capable of holding a maximum of about 270 SCF of methane gas per ton of coal.

LOW GAS RECOVERY ANTICIPATED WITH CURRENT TECHNOLOGY

At present, coalbed methane is recovered using a reservoir pressure depletion strategy. That is, reservoir pressure is reduced by removing water which causes some gas to be desorbed from coal. Methane diffuses through the microporous matrix blocks and upon reaching the cleat system, a network of natural fractures, flows to the wellbore along with water as per Darcy's Law. Figure 2 shows a schematic view of the coalbed methane recovery mechanism.

While the pressure depletion method of coalbed methane production is simple and effective, it is not efficient. Loss in pressure deprives the reservoir fluids of the energy necessary to flow to the wellbore. Consequently, gas production rates suffer and the ultimate methane recovery by means of pressure depletion is generally not expected to be pressure depletion is generally not expected to be greater than 50% of the gas-in-place, even after several decades of production. For example, as shown in Figure 3, to recover 50% of the gas-in-place from coal, the reservoir pressure would have to be reduced from 500 psia to less than 150 psia.

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