The amount of hydrocarbons that remains in the oil reservoir at the final depletion phase of field production largely depends on the efficiency of the technologies and recovery techniques implemented during field development. Despite modern advances in Enhanced oil recovery techniques, it is estimated that about 30 to 70% of oil, on average, remains in the reservoir worldwide at the end of production. The structural and chemical nature of residual oil varies widely. In some reservoirs only a fraction of it is mobile. Studies showed that residual oil immobility is not that as much governed by the in situ pressure gradients (i.e. the influence of the hydrodynamic and capillary forces) or pore-throats restrictions, as by the differences in the chemical composition and density of the immobile and mobile oil. This article briefly discusses methods of determining immobile residual oil saturation in carbonate cores and its effects on fluid flow in the reservoir and field production prediction results.
The methods generally used for determining the residual oil saturation of a reservoir are very many, among which are both direct (on core samples) and indirect (well-log analyses). Direct methods generally imply carrying out tests on fresh cores extracted with clay mud from wash-out zones of the reservoir. This article futher describes a laboratory method for determining the immobile residual oil saturation on fresh core samples by alternating core-cleaning methods. A combination of mild and harsh cleaning methods were used to determine reservoir immobile residual oil saturation and several tests were carried out on carbonate cores to study its effects on basic reservoir properties and flow characteristics.
The results obtained from both laboratory flow tests and the hydrodynamic modeling of a carbonate reservoir show that the presence of immobile oil has a significant influence, not only on the nature of fluid flow in the reservoir, but also on field production prediction results.
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