In-Situ Visualization of Multidimensional Imbibition in Dual Porosity Carbonates

Abstract

AbstractWell-performing waterflood usually recovers 50% of oil in place. This number is even lower in fractured carbonates where the injected water selectively channels through fractures towards producers leaving much of the oil trapped in the matrix. To mitigate the low recovery, one option is to reduce fracture uptake by increasing the viscosity of the injected fluids using polymers or foam. Another option, which is the objective of this work, is to inject surfactant solutions to reduce capillary effects responsible for trapping oil and allow gravity to segregate oil by buoyancy. Analysis of gravity and capillary forces suggests that such segregation is achievable in the laboratory provided that cores are moderately long and oriented vertically. Besides investigating gravity effects on oil recovery, the effect of surfactant flood mode (secondary flood mode and tertiary flood mode) on the ultimate recovery (UR) was also investigated.To investigate the predictions of this analysis, core-flood experiments are conducted using carbonate cores and monitored by an X-ray computed tomography (CT) scanner to quantify fluid saturation history in situ. Novel aspects of this work include cores that are oriented both horizontally and vertically to maximize gravitational effects as well as a special core holder that mimics aspects of fractured systems using whole core. This paper discusses the contrast in experimental results in vertical and horizontal orientation with and without surfactant.To study gravity effect, the used surfactant reduced interfacial tension (IFT) from 40 mN/m to 3 mN/m. For this mode of recovery, ultralow IFT is not preferred because some capillary action is needed to aid injectant transport into the matrix. The vertical experiment showed that gravity has the potential of improving oil recovery at low IFT. Although the IFT reduction was not large (one order of magnitude reduction), recovery factor (RF) increased by more than 4% of original oil in place (OOIP) leading to an incremental recovery factor (IRF) of more than 7%. Another surfactant was used to study the flood mode effect; the used surfactant reduced IFT from 40 mN/m to 0.001 mN/m (ultralow IFT). In this study, two experiments were conducted: a tertiary surfactant flood experiment and a secondary surfactant flood experiment. The secondary flood experiment showed an increase of 7% in the ultimate recovery factor (URF) compared to the tertiary flood experiment.This work highlights the importance of gravity at low IFT in terms of mobilizing trapped oil and also the impact of flood mode on UR. Moreover, this work emphasizes on the use of surfactant solutions as a method of enhancing oil recovery in fractured resources not necessarily due to wettability alteration but mainly due to gravity effects. Results are presented primarily as 3-dimensional (3D) reconstructions of in-situ oil and water phase saturation.