PROCEEDINGS, INDONESIAN PETROLEUM ASSOCIATION Thirty-Fifth Annual Convention & Exhibition, May 2011 EVALUATION OF COAL BED METHANE RESERVOIRS (CBM) USING A DOWN-HOLE GEO- CHEMICAL TOOL

Evaluation of coalbed methane (CBM) reservoirs and wells differs from that of conventional oil and gas producers. The search for conventional reserves involves identifying source rocks underlying permeable reservoir rocks that have sufficient storage volume (porosity) to contain commercial quantities of hydrocarbon. By contrast, coals are source, trap and storage media. So a different approach must be taken to evaluate them as gas reservoirs. Coals are characterized as having low density, low gamma ray and a high resistivity. The solid hydrocarbon matrix and water in the cleat structures and pore spaces are the primary reason for the high hydrogen index in a coal layer. This high hydrogen index causes a higher than normal thermal neutron porosity reading of the reservoirs. The type and the rank of the coal would cause an exception to these log properties. Identification of coal layers based on density logs is difficult in washed out holes. Also the resistivity log can be affected based on the moisture content and mineralogical assemblages of the ash. The primary uncertainty lies in the response parameters of ash since it may contain varying amount of quartz, clay, calcite, pyrite and other minerals. Because of these reasons it is important that an alternative direct measurement is available for identifying the coal seams and fully characterizing these coals in terms of the mineralogy. A down-hole geo-chemical tool can identify minerals based on neutron induced gamma ray spectroscopy. The coal is identified from the hydrogen measurement. The coal rank and the gas content can be estimated from a parametric analysis. This method is immune to borehole artifacts and complements other open hole logs.