Abstract
X-ray computerised tomography (CT) has been increasingly applied in the study of rock properties and the in-situ monitoring of fluid displacements. A series of oil-water drainage-imbibition floods in laminated rock slabs have been performed in this laboratory. X-ray CT imaging techniques were used to evaluate the rock characterisation and to monitor the fluid saturation processes.
This paper describes significant technical advances in X-ray measurement of fluid saturations in very heterogeneous rock slabs. Approaches to minimising the beam hardening effect and suppressing high-frequency artefacts are also described. A number of typical experimental results indicate that our improved CT techniques can be used to accurately evaluate the rock heterogeneity and its effects on fluid displacements. These include the distribution of porosity and permeability in a laminated aeolian sandstone, the distributions of irreducible water and remaining oil saturation, as well as oil recovery. CT measurements were checked and calibrated against probe permeameter and fluid mass balance measurements and excellent agreements has been obtained.
At capillary-dominated conditions (low flow rate and unit ratio of water to oil viscosity), the average remaining oil saturation is around 35–40 % of OOIP. For flow across lamina, about 30–55% of the original oil was trapped in the upstream regions of low permeability layers due to the interaction of capillary forces with the lamination. For flow along lamina, the capillary trapping is reduced but flow bypassing results in similar levels of oil retention. The distributions of remaining oil saturation and oil recovery across lamination shows clear variations related to the local permeability and the initial oil saturation. By incorporating the detailed petrophysical analyses into a simulation model, an excellent match of experiments with numerical simulation was obtained both for cross-lamina and along-lamina flows.
These findings have important implications for the determination of field-wide residual oil saturation from core plug measurements.
