Gas Zones Detection Using the Dual-Spaced Neutron Tool Responses

Abstract

Abstract Gas zones are routinely identified with the neutron-density crossover. Many a times however, this crossover is not present or misleading because the lithology used in the tool's calibration is not representative of the formation of interest. Also the crossover could be completely obliterated by the shale effect. Furthermore, in cases where the well has been cased before logging only the neutron tool can be run. We have developed a simple algorithm that determines three apparent neutron porosities. The porosities derived from the raw counts of the dual-spaced neutron tool have three different radii of investigation. All three neutron porosities read the true porosity in water-bearing zones. In gas-bearing zones the mud filtrate invasion profile will cause the three apparent neutron porosities to be different. The resulting neutron porosities crossover can be used in a way similar to the neutron-density response in detecting the presence of gas. The validity of the algorithm has been tested using a numerical model and field examples. Introduction The detection of gas zones using the density-neutron crossover relies on the assumption that the lithology selected to calibrate the tool is representative of the lithology of the formation of interest. The absence of this condition leads to a complex interpretation problem as the neutron-density combination must be used to both determine the lithology and to detect the presence of gas. The shaliness of the formation can also obliterate the neutron-density crossover, since shale and gas have the opposite effect on these logging tools. The presence of gas may be overlooked in cases where the shale effect offsets the gas effect. In highly rugose boreholes, the reading of the density tool is much more affected than that of the compensated neutron. In these conditions, the crossover pattern may be misleading. Also, being a padded device, the density tool cannot be run through casing. In these scenarios the only porosity indicator is the dual-spaced neutron sonde.