This new process enables the quantitative determination of the gas content of coals and other strata, such as shales or conventional reservoir rocks during overbalanced open hole drilling. This paper presents the results of a field trial through coal seams in Central Queensland where the system was tested and showed favourably comparable results to those obtained from gas content measurement by conventional core desorption in coals.
The system involves drilling in overbalanced mode through so that no reservoir fluids enter the well bore. Under these conditions the only material produced is that cut from the hole. This comprises cuttings and may also include gas and liquids contained therein. This material rises up the annulus and is diverted by a rotary seal to a cyclonic separator. This separates the free gas while the mud and cuttings overflow from the base of the separator and over a shale shaker. The free gas volume is measured directly while the cuttings are separated from the mud on the shaker and are sampled into canisters so as to permit measurement of any gas that may desorb from them freely or further after crushing.
The system thus enables the capture and measurement of all gas contained in the drilled strata with the exception of that desorbed during the brief period that the cuttings are on the shaker. This lost gas is small and may be readily estimated. The system is direct and simple and has the potential to replace core sampling and other more indirect and complex geophysical techniques to measure the gas contained in strata.