Unexpected behavior of logging-while-drilling neutron porosity logs in water-wet, sand-shale sequences

Abstract

When logging through certain water-wet sand-shale sequences offshore Malaysia, the thermal neutron porosity measurement from an 8-in logging-while-drilling (LWD) tool exhibits an apparent lack of dynamic range in its sandstone porosity response. Petrophysicists using these data to determine sandstone-silt-shale points were finding that the shale points fell too near the sandstone line to be usable. An extensive nuclear Monte Carlo modeling campaign was performed to address this issue, which indicated that the tool response, while unexpected, was accurate. Modeling verified that neutron logs can exhibit a poor dynamic range just by happenstance, but an accurate assessment of the shale volume fraction [Formula: see text] should still be feasible. Surprisingly, the shale responses for this LWD tool, a smaller LWD tool, and a wireline tool are quite similar, invalidating a common expectation that LWD tools should be noticeably more epithermal in their response, and thus be less sensitive to absorbers in the clays. Modeling the actual shales, based on X-ray diffraction (XRD) analyses of cuttings, revealed the answer. In the wells with a poor dynamic range in the shale response, the quartz content was very high. Simulating the actual XRD-determined shales reproduced the actual log crossplot very nicely. Points in the shale crossplot fall near the sandstone line because the formations are primarily [Formula: see text], with only a small amount of clay. This result explains the response of the LWD tool.