Heavy oil trapped in fractured carbonate reservoirs possesses a great challenge. Previous experience has shown that heating the matrix to drain heavy-oil by gravity is a slow and inefficient process due to low matrix recovery caused by unfavorable matrix properties (oil wetness and low permeability). Acceleration of matrix-fracture interaction by changing matrix and oil properties could be possible, however, by injecting proper hydrocarbon solvent alternately with steam as reported by our earlier work (SPE 117626). This process called Solvent-Over-Steam injection in Fractured Reservoirs (SOS-FR) consists of several cycles, each having three phases: (1) steam/hot water injection, (2) solvent injection, and (3) steam/hot water injection.

This paper reports a detailed experimental analysis of this new method. Static and dynamic tests were conducted by exposing (or injecting) heavy-oil saturated fractured rocks into steam/hot water and solvent alternately. Oil-wet sandstone and carbonate samples were saturated with heavy-crude. Different solvents were tested including heptane, kerosene, decane and light crude. In addition to the core experiments, the same procedure was repeated on "Hele-Shaw like" glass models to visually identify the mechanics of the process.

Specific observations and conclusions as to how to apply this technique in field conditions, which mechanisms control the rate of recovery and residual matrix oil saturation, ideal solvent type, and optimal operational conditions (optimal soaking time for huff-and-puff and injection rate for continuous injection) were reported. It is hoped the proposed new technique will be an alternative for tapping heavy matrix oil from oil-wet, fractured, deep, carbonate fields.

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