Gadolinium tracers for enhancement of Sigma-log contrast measurements

Borehole neutron measurements are routinely used for in situ rock assessment in hydrocarbon reservoirs. We have used gadolinium oxide nanoparticles for enhancing the sensitivity of macroscopic thermal neutron absorption cross-section (Sigma) measurements of rocks. The gadolinium-based doping agent is used due to its exceptionally high neutron absorption cross section, low cost, and availability; it is also shown to preserve or enhance the differentiation between pore fluids. Injected from a pilot well, the doping agent could thus substantially improve the precision of Sigma-derived saturation measurements. Computational modeling verifies that modest gadolinium concentrations in the rock’s pore volume give rise to significant enhancement of reported Sigma: A concentration of 1750 weight ppm is shown to offer superior contrast enhancement across a wide range of rock solid and fluid compositions. Preliminary experimental work confirms the reported effects obtained with computational modeling of gadolinium doping in simulated porous rocks.