Oil recovery in fractured reservoirs by water flooding critically depends on the wetting properties of the matrix blocks between the fractures. The recovery from oil-wet reservoirs is small. In incompletely oil-wet systems, the presence of initial water may change the wettability characteristics so that imbibition and some oil recovery can occur. The hypothesis in this work is that water-soluble solvent (diethyl ether) improve the ultimate recovery and the imbibition rate in partially and completely water-wet cores. The main recovery mechanisms are the wettability change of the partially water-wet cores and oil swelling and the oil viscosity reduction in both partially and completely water-wet cores. This paper reports an experimental study concerning the recovery enhancement by water-soluble solvent (diethyl ether). We used an Amott imbibition cell studying oil saturated samples of various wettabilities, permeabilities using oils of different viscosities and two different diethyl ether (solvent) concentrations in the aqueous phase. In the first stage of the experiment, the completely water-wet core was exposed to brine without solvent. In a second stage, the core was put in a new Amott cell that was filled with solvent/ brine mixture. The extra recovery by solvent/brine mixture strongly depends on the residual oil saturation after brine imbibition and it is relatively insensitive to the permeability of the core or the oil viscosity. Therefore, larger residual oil saturation resulted in a higher extra recovery. For the partially water-wet samples, we also started with exposing the core to pure brine without solvent. Contrary to the completely water-wet samples, there was a significant increase in recovery rate when the sample is transferred to another Amott cell where it is exposed to a mixture of solvent and brine. In view of large values of the inverse Bond number in both partially and completely water-wet cores, the transfer between matrix and fracture capillary driven.

Dimethyl Ether (DME) - augmented brine injection is a novel Shell proprietary enhanced oil recovery (EOR) method. An experimental and numerical study of the enhancement and the acceleration of spontaneous imbibition using DME is carried out, both in sandstone and carbonate limestone cores. The experiments were performed under different boundary conditions using a modified high-pressure Amott cell. The primary recovery with brine from four (weakly water-wet to mixed-wet) sandstone cores, with the top-end, bottom-end, both-ends and all sides open to brine imbibition, was 38-46% of the OIIP. By adding DME, an additional oil recovery of 11-16% of the OIIP was obtained. The primary recovery with brine from the tight limestone cores was only 1-2% of the OIIP. By adding DME to the imbibing brine an additional oil recovery of 43-55% of the OIIP was obtained, which was much higher than the additional recovery from the sandstone cores. A workflow was considered to model DME-enhanced spontaneous imbibition experiments. The model comprises Darcy's law and the simplified phase behavior of the DME-brine-crude oil system. The model parameters were optimized to match the experimental results. Numerical simulation shows that the oil recovery is more sensitive to the molecular diffusion coefficient and to the partition coefficient, than to the relative permeability and capillary pressure. In the presence of a higher oil saturation in the core, a higher partition coefficient and a higher DME concentration in the aqueous phase, DME/brine imbibition leads to a higher oil recovery.

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