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Heavy-oil recovery from tight naturally fractured carbonate reservoirs is a great challenge to the oil industry, yet no well-established recovery technique has been adopted by the oil industry. Steam-over-solvent injection in fractured reservoirs (SOS-FR) is a new technology proposed as an alternative method to the sole injection of steam or solvent, and this paper introduces this method through extensive experimental evidence and analysis. The method consists of three phases: (1) steam (or hot water), (2) solvent, and (3) steam (hot water) injection. Phase 1 produces heavy oil by thermal expansion and conditions the oil for phase 2, which is solvent injection. Phase 3 is applied mainly to retrieve the solvent. Several static experiments were conducted mainly to test four critical parameters: (1) wettability of the matrix, (2) solvent type, (3) initial water saturation, and (4) matrix boundary conditions. This was followed by several dynamic experiments, which were aimed at testing the effect of the solvent injection rate on the process. All three phases yielded above 90% recovery, with 85–90% solvent retrieval if the matrix boundary conditions were favorable (large surface area per volume for effective solvent diffusion), when heptane was used as the solvent, regardless of the wettability of the matrix. After provision of the experimental results, we discuss the up- and downsides of the technology and suggest ways to improve it. The importance of this work is that it provides a novel perspective on the interaction between steam/solvent and heavy oil in the matrix and presents an alternative technique for heavy-oil recovery from deep naturally fractured reservoirs with a tight and oil-wet matrix.

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