Abstract
This paper examines alternatives to using core for gas content measurement of coal. The measurement of gas content from cuttings is presented in two forms, one involving the recovery of cuttings with air drilling and the second involving the collection of all released gas from the hole during overbalanced drilling. The latter approach is suitable for not only coal but all gas bearing formations such as shales which do not have major open pore space such as vugs.
Core desorption is the standard process for determining the gas content of cores. This process is described by McCulloch and Diamond (1976), and more recently Standards Australia (1999). The process generally involves using wireline coring to cut a core so that the core may be retrieved quickly. Once the core is retrieved to surface the core is placed in a canister and the released gas is monitored with respect to time. This should be undertaken at reservoir temperature. An example of gas release versus time is shown in Fig. 1.
Once the core has been further desorbed the canister is opened and the core is logged and weighed with density determination. Weighed sub sections of the core are then crushed to enable the remaining gas to diffuse out more quickly than from the core.
This process is relatively straight forward but the determination of the gas lost before the core is placed in the canister is not. The usual process adopted is to assume a time when the core begins to release gas and to plot the gas release with respect to the square root of time. An example of such a plot is shown in Fig. 2.
