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Paper 1981 EE :: Add Item
AN IMPROVED GAS CORRECTION METHOD FOR DENSITY AND NEUTRON LOGS
Services Techniques Schlumberger
ABSTRACT The Density and Neutron logs are strongly affected by the presence of light hydrocarbons, particularly gas, near the borehole. Present methods assume a rather deep invasion and make a correction proportional to the amount of hydrocarbon in the invaded zone. This type of correction may be very inaccurate in the case of shallow invasion, particularly in very porous and permeable gas sands with the result that porosity can be underestimated. An improved correction method is proposed taking into account the depth of invasion obtained from a combination of Resistivity logs; at the present time from the Dual Laterolog - Rxo. An approximate radial geometrical factor is used for the Density log and a different one for the Neutron log. Gas corrections are thus computed independently for each tool. These corrections depend on the gas saturation both in the invaded zone and in the virgin formation. Log examples illustrate the application of this new correction, and show the improvement in Density-Neutron interpretation.