Measurement and Evaluation of Coal Sorption Isotherm Data

Abstract

Abstract The proper measurement and interpretation of sorption isotherm data is of primary importance in understanding and predicting the gas production primary importance in understanding and predicting the gas production performance of a coalbed methane reservoir. These data are used to predict performance of a coalbed methane reservoir. These data are used to predict the release of gas from the reservoir as the pressure is reduced during production and the eventual gas recovery (reserves). A sorption isotherm production and the eventual gas recovery (reserves). A sorption isotherm is a laboratory measurement performed on a representative coal sample to determine the methane storage capacity as a function of pressure. The term "isotherm" results as the experiment is performed at a constant temperature which should be equal to that of the reservoir. This paper presents the use of the data, procedures for performing the measurements, a methodology for interpreting the experimental data for estimates of gas storage capacity and diffusion coefficients of the coal and isotherms obtained from a variety of coal samples. Isotherm Utility A sorption isotherm relates the gas storage capacity of a coal sample to pressure. This information is required to predict the volume of gas that pressure. This information is required to predict the volume of gas that will be released from the coal as the reservoir pressure is reduced during continued production. The gas storage capacity is a function of the rank of the coal, the temperature, the moisture content of the coal matrix, and pressure. pressure. Figure 1 illustrates a typical sorption isotherm relationship measured on a sample collected from the Northeast Blanco Unit #403 well which is completed in a Fruitland Formation coal seam of the San Juan Basin, New Mexico. More specific location and experimental details are listed in Table 1. The measured isotherm data are listed in Table 2. As an example of the use of the isotherm consider the following example. The gas content of the coal was determined to be 355 SCF/Ton by desorption canister tests performed on whole core samples at the well location. The gas content is performed on whole core samples at the well location. The gas content is less than the sorption isotherm gas storage capacity of 440 SCF/Ton at the initial reservoir pressure of 1,620 psia. During production, the pressure of the coal near the well must be reduced to 648 psia at the point pressure of the coal near the well must be reduced to 648 psia at the point where the isotherm gas content is equal to 355 SCF/Ton. This point is known as the "critical desorption pressure." During initial production, at bottomhole pressures greater than this value, the produced fluid will be water and solution gas originating from the water. Ideally, once pressures fall below the critical desorption pressure, gas will diffuse through the coal matrix, desorb at the cleat / coal matrix boundary, and flow through the cleat system to the well. In the event that the average pressure of the drainage area of the well can be reduced to 100 psia, the remaining gas content would be 128 SCF/Ton, which results in a recovery of 227 SCF/Ton of coal in place or 64% of the gas in place. This estimate of the recoverable reserves is generally optimistic as reservoir heterogeneity, deliverability, end development economics are not taken into account. A common method of utilizing sorption isotherm data is to assume that the relationship between gas storage capacity and pressure can be described be an equation originally presented by Langmuir. Data which fit this relationship are known as a "Langmuir Isotherm." The form of the equation is as follows. P. 157