Carbonate reservoirs are estimated to contain around half of the total oil and gas reserves in the world. Exploitation of these reservoirs is specifically challenging and their recovery factor is generally lower than clastic reservoirs, due to their structural complexity, local heterogeneities, fracture porosity and the oil-wet-nature of the carbonate rocks.

The principle objective of this study was to investigate through laboratory experimentation, the feasibility of improving oil recovery from a fractured tight carbonate reservoir by spontaneous and forced imbibition of a compatible low salinity water (LSW), with and without a surfactant. To facilitate this objective, core material and reservoir crude oil from an active field were combined with reservoir temperature and wettability restoration, in a series of complementary tests, supported by compelling photographic images. Wettability screening of the restored core samples confirmed an oil-wet system with small tendency for water imbibition, which is typical behavior of such low permeability carbonates. In spontaneous imbibition tests, the samples were exposed to resident formation brine, followed by a LSW (2253ppm), with and without surfactant. The start point for the two-stage imbibition sequence was a residual oil saturation (~ 32%PV), which was representative of the target reservoir, established by centrifuge displacement. Exposure to the formation brine resulted in no additional recovery. In contrast the LSW prompted a reduction in the residual oil saturation of 20.47% (9%OOIP). With the addition of a surfactant to the LSW, there was an apparent improvement in the effectiveness of the displacement process, which lowered the residual oil saturation by 27.02% (13.14%OOIP).

To assess the benefits of forced imbibition of the LSW, a combined “soak-and-drive” sequence was deployed. For a core sample with a restored wettability and an established residual oil saturation of ~ 32% PV, the sequencing almost doubled the additional oil production when compared with spontaneous imbibition tests using the same fluid.

Wettability modification has often been cited as a possible mechanism for the success of LSW, particularly in clastic lithologies. An alternative mechanism for improving oil production has recently been introduced in the technical literature, described as an osmosis like phenomenon. This paper explores the possibility of this type of oil displacement in the context of a carbonate reservoir, with the movement of the LSW from the fracture network into the matrix blocks. The data generated by the experimentation, coupled with the progressive series of photographic images, are presented to give credence to the suggested mechanism.


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