Experimental and Numerical Study of Gravity Effects on Oil Recovery in Fractured Carbonates

Abstract

Recovering oil from fractured resources is very challenging. If waterflood is implemented, most of the injected water channels through the fracture network leaving much oil behind in the matrix. One option is to increase the viscosity of the injectant by using foams or polymers. Another option is the objective of this work. In this work, the use of surfactant floods as a means of improving oil recovery in fractured systems. Our claim is that surfactants reduce the interfacial tension (IFT) between oil and water allowing gravity to segregate oil and water by buoyancy. The mobilized oil moves towards the fracture network then to producers. This claim was investigated by conducting surfactant-flood experiments on fractured carbonates. Experiments were monitored by an X-ray CT scanner to quantify fluid saturations in situ. One experiment was conducted horizontally as a base case; another experiment was conducted vertically to maximize gravity effects. A special core holder was used to mimic aspects fractured systems for a whole core. This paper discusses experiment details and results along with simulation attempts that were made using two commercial simulators. The surfactant used in these experiments reduced 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 an incremental recovery of about 7% over the horizontal experiment. This work has shown that surfactant floods improve oil recovery in fractured systems not necessarily due to wettability alteration but mainly due to reduction of capillary effects. Consequently, gravity segregates oil by buoyancy mobilizing trapped oil to the fracture network and then to producers.