Gas in Place Identification from Mudlog Gas, Barnett Shale - Fort Worth Basin, Texas

Jeffrey E. Nunneley
Marathon Oil Company, Houston, TX

Total gas in place (GIP) within a shale is commonly identified by directly measuring the volume of gas given off from sections of full core sealed in canisters at the well site. It is usually measured in standard cubic feet per ton (SCF/ton) and includes free gas in pore spaces, adsorbed gas, plus gas dissolved in liquids in the pores. Gas that is liberated from the rock during the drilling process is frequently measured on a mudlog. The low permeability of a gas shale allows little migration of formation fluids into the well bore during drilling. The gas that is measured on a mudlog represents the total gas present in the volume of rock ground up and removed from the well bore. The mudlog gas can be normalized for penetration rate and mud flow volume to yield total gas from the mud system in parts per million. The volume of rock that is ground up while drilling and the density of the rock can be measured from wireline logging tools after drilling is completed. The combination of hole volume and rock density allow for calculation of the ground up rock mass responsible for the measured gas in the mud system.

GIP values from a canistered, Barnett Shale core have been compared to the GIP values calculated from a normalized mudlog. The two values have a favorable, relative relationship. The mudlog calculated data is more variable in magnitude than that of the canistered core data. A “calibration factor” must be applied to the mudlog gas calculation in order to match the magnitude of the conventional core data. This “calibration factor” is mainly related to the sampling process of the mudlog detector which only samples a portion of the gas present in the total volume of the mud system.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas