Three-Phase Flow in Fractured Porous Media: Experimental Investigation of Matrix-Fracture Interactions Using X-Ray Microtomograph

In this work, we used a high-resolution micro-CT scanner integrated with a three-phase core-flooding apparatus to perform two- and three-phase flow experiments on fractured sandstone and carbonate rock samples. The study aimed at investigating how the fluids in the matrix interact with those in the fracture conduit when oil spreadability plays a key role. In-situ fluid distribution maps are compared under spreading and nonspreading conditions during secondary and tertiary gas injection processes. In the case of spreading fluid system and secondary gas injection process, the oil phase in the matrix is not trapped. The injected gas invades large oil-filled pores in the matrix and displaces oil into the fracture while leaving relatively thick spreading layers of the defending fluid in the crevices. These layers play a pivotal role in maintaining the hydraulic connectivity of oil and ultimately lower remaining oil saturation significantly. In case where the initial oil saturation in the matrix is hydraulically trapped due to a preceding waterflood, gas first displaces brine before accessing oil. When gas reaches trapped oil globules, the oil spreads between brine in the corners and gas in the center of the elements and becomes reconnected. In both scenarios, the oil phase maintains its hydraulic conductivity through spreading layers and drains into the fracture. This phenomenon has a significant impact on oil production from matrix in fractured oil reservoirs. However, infrequent oil layers observed under nonspreading condition are not stable and collapse easily as gas/oil capillary pressure is increased during gas injection. This means that oil has poor connectivity that hinders its movement from the matrix to the fracture. In another group of experiments, the effect of wettability on matrix-fracture interactions is investigated by performing tertiary gas injection on an oil-wet fractured limestone sample. The distribution of the measured in-situ contact angles indicated that the wettability alteration in the matrix surrounding the fracture was relatively less uniform and lower than that in the intact matrix. This was attributed to the presence of fracture, which reduced the invasion of crude oil into the neighboring matrix during drainage.;The insight developed in this work using different rock types, wettability conditions, and three-phase saturation histories may have direct implications for the design of EOR schemes involving gas injection.