Identifying and understanding the important mechanisms which control recovery and retention in a given reservoir is important for successful oil production.

Most carbonate reservoirs have a dual character of matrix and fractures. The matrix provides the main oil storage capacity, whilst the fractures provide the principal flow paths.

This dual nature makes carbonate reservoirs notoriously difficult to produce efficiently and oil recovery is often poor.

The emphasis in Statoil has been on understanding these mechanisms in fractured carbonate reservoirs when gas or water is injected to serve as the displacing fluid. Factors which control both microscopic and macroscopic sweep efficiency must be considered.

Effects of depletion, repressurisation by gas or water injection, and full pressure maintenance have been studied by the group.

In many naturally fractured carbonate fields, wettability is found to favour gas rather water injection. In other words, oil not water wets the reservoir rock.

Remaining matrix oil saturations in a cross-section of a dual porosity reservoir model at the end of history. The purpose of the work is investigate the effect of a given model concept and drainage strategy. Red colour indicates high oil saturation. The vertical lines indicate well locations.

Studies have investigated ways of improving oil recovery through gas or water injection, and have yielded expertise in simulation and upscaling of multiphase flow in naturally fractured carbonate reservoirs.

Gas injection can be performed using a gas which is equilibrium or non-equilibrium with respect to the reservoir oil.

Studies performed by Statoil include small-scale sensitivities with different injection gas compositions, the effect of diffusion, and equilibrium and non-equilibrium gas injection at different pressures.

The displacement process is much more complicated for a non-equilibrium gas than with an equilibrium one. Compositional effects affecting ultimate recovery include vaporisation of the oil, density variations, and gas-oil interfacial tension (IFT) changes.

Several non-equilibrium injection gases have been applied in simulation studies, ranging from nitrogen dioxide to various hydrocarbon and carbon dioxide mixtures.

It has been found that such gases may have a big potential for improved oil recovery, and these results have been supported by experimental laboratory work.