In situ characterization of chalk cores using a nuclear tracer imaging technique

Abstract

Core-flood experiments using chalk outcrop cores with petrophysical properties similar to North Sea chalk reservoirs have been conducted and history-matched. The objectives were to enhance the understanding of the mechanisms of fluid displacement in permeable chalk and to evaluate the imaging technique with respect to measuring three-phase saturations in chalk cores. The displacements included miscible brine/brine, drainage, miscible oil floods and water floods. The brine and oil phases were respectively labelled with gamma-emitting 22 Na and 59 Fe tracers. Fluid saturations were measured by a Germanium detector moved along the length of the core. In this investigation, emphasis has been placed on local in situ saturation development, recovery efficiency, fluid-front behaviour and tracer adsorption. The benefits from knowing the local in situ saturations, the identification of rock heterogeneity and the reproducibility of the saturation imaging have been emphasized. A good comparison was obtained between in situ fluid saturation information and that from effluent fluid measurements. The additional information on local fluid saturations obtained from the in situ measurements, coupled with petrophysical measurements, has enhanced the characterization of chalk cores and thus will contribute to a better understanding of displacement mechanisms during hydrocarbon recovery from chalk reservoirs. The nuclear tracer imaging technique was found suitable for measurements of three-phase fluid saturations.