The interfacial phenomena of wettability, adhesion, mass transfer, and phase behavior due to miscible hydrocarbon gas injection have been studied by the use of the sessile drop contact angle technique, qualitative adhesion tests, and visual phase behavior observations under reservoir conditions. This experimental study has been conducted using crude oils from three Canadian reservoirs, pure mineral crystals of quartz, calcite and dolomite to represent the reservoir rock, and ethane for the hydrocarbon solvent.

While one of the three reservoir crude oils, namely that of Beaverhill Lake, showed significant changes in contact angles upon miscible gas addition, the others did not. The changes observed in the Beaverhill Lake system constituted and shift from water-wet to intermediately-wet behavior. However, phase behavior effects such as solids separation and settling within the crude oil drop due to solvent injection were common to all three reservoir systems studied. This indicated that the observed phase behavior changes were not reflected by the contact angle measurements.

Qualitative tests for adhesion at the crude oil/mineral crystal interface were also conducted under reservoir conditions. In contrast to the response of contact angles, the adhesion behavior was affected both by the aging time of the interface and by the injection of a misciblegas. These experiments demonstrated the insensitively of the contact angles to the phenomenon of adhesion at the oil/crystal interface. The adhesion tests further demonstrated that an oil drop could adhere to the crystal surface even after the heavy ends (like asphaltenes) had precipitated within the oil drop away from the interface. This indicated that the heavy ends may not be solely responsible for the interfacial adhesion effects. Furthermore, it was observed that the solvent-mixed oil drop, when moved to a new spot on the crystal surface, could redevelop the adhesive bond with the solid surface.

This study brings forth the importance of including interfacial phenomena, which affect the reservoir flow characteristics, as an integral part of designing miscible gas floods.

1.1 The Need for This Study:

Substantial commercial interest lies in enhanced oil recovery by hydrocarbon flooding. In order to obtain maximum benefits, it is essential to understand the various physico-chemical effects that could result from injecting the hydrocarbon solvent into an oil bearing formation. These effects could include phase behavior changes in the crude oil (such as wax agglomeration and asphaltene deposition) due to the addition of light hydrocarbons and changes in the rockfluids interactions at the oil/brine/rock three-phase interface. Such changes within the bulk of the reservoir crude and at the interface could alter the rock wettability and also the flow behavior of oil and water in the porous medium.

Some of the important considerations to be made in the design and performance prediction of a miscible flood are adequate reservoir description, miscible sweepout, displacement efficiency, and injectivity. Changes in reservoir wettability and relative permeability during gas flooding (for example, due to permeability during gas flooding (for example, due to deposition and adsorption of heavy ends) would affect some of the above design factors, and would have to be taken into account for a reliable design and realistic prediction of performance and overall process prediction of performance and overall process economics.

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