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Densely fractured oil-wet carbonate fields pose a true challenge for oil recovery, which traditional primary and secondary processes fail to meet. The difficulty arises from the combination of two unfavourable characteristics: the dense fracturing frustrates an efficient water-flood; the negative capillary pressure retains the oil inside the matrix blocks because oil is the wetting phase. In the past decade, EOR researchers have studied options to chemically revert the wettability of carbonate rock while not decreasing the oil-water interfacial tension drastically, using a new class of surfactants. These chemicals termed "wettability modifiers" (WM) effectively reverse the sign of the capillary pressure function such that the capillary pressure becomes the driving force for oil expulsion from the matrix into the fracture system. Previous publications on chemical wettability modification focused on the different chemical properties of the rock/oil/WM-brine system and demonstrated up to almost full oil recovery from core plugs. Little attention, however, has been paid to the mechanism underlying the transport of the chemical into the matrix block and to proper scaling of laboratory results to reservoir size. The present study aims to demonstrate that imbibition after wettability modification is diffusion-limited. To this end the recovery profiles for spontaneous capillary imbibition as well as for imbibition after wettability modification are calculated. The results are then used to match the data of Amott cell imbibition experiments. It is confirmed that in either case the cumulative recovery is initially proportional to the square root of time. Imbibition after wettability modification, however, takes about 1000 times longer than spontaneous capillary imbibition into a water-wet medium. The slow recovery observed for the case of imbibition after wettability modification is in excellent agreement with the assumption that, in the absence of significant spontaneous imbibition, the wettability modifier must first diffuse into the porous medium to unfold its action. In any diffusion process the time scale is linked to the square of the length scale of the medium. Therefore, it would take up to 1000 times longer, equivalent to 200 years, before the same recovery is obtained from a metre-scale matrix block as is obtained from a centimetre scale plug in the laboratory in 100 days. Consequently, unless a significantly faster transport mechanism for the wettability modifier is identified, or unless viscous forces or buoyancy enable forced imbibition, chemical wettability modification of fractured oil-wet carbonate rock does not provide an economically interesting opportunity. |
