Abstract
The Abu Attiffel oilfield was discovered by Agip in 1967. This field is a giant reservoir located in the Sirt Basin of the Libyan desert. It is producing a volatile oil (41° API) from sandstones of continental environment at an average depth of 4,200 m b.s.l. (13,800 feet). Water injection process started after two years of primary oil production, involving 9 injector wells for pressure maintenance purposes. Current field (main area) oil production rate is 91,000 BOPD (14,500 m³) with a water cut-off of about 37%. The final oil recovery is expected to be around 50%.
Since the water cut-off has dramatically increased in the last years, an EOR approach was evaluated in order to enhance the final oil recovery. Among the conventional EOR processes, only gas injection was suitable for Abu Attiffel field because of its very severe reservoir conditions. The performance of two different gases (CO₂ and separator gas) was studied in the laboratory at the average reservoir temperature of 145°C (292°F) and pressure of 47.6 MPa (6,897 psia). CO₂ achieved miscibility (which assures the best conditions for an improvement of the final oil recovery) with the reservoir oil below the original reservoir pressure but paraffinic precipitation and corrosion phenomena occurred in all the tests performed.
Lean gas, besides causing paraffinic deposition in the porous media and corrosion problems, was not miscible with the oil under the current reservoir conditions. A significant improvement of the lean gas performance was achieved by solubilizing a volume of LPG (liquefied petroleum gas) which permitted the miscibility to be reached under the current reservoir conditions (i.e., one MScf of lean gas enriched with about 52 bbl of LPG).
In order to confirm the improvement of the microscopic efficiency of the oil displacement, several experiments were carried out using enriched gas under current reservoir conditions. Core plugs representative of the reservoir formation were selected on the basis of Nuclear Magnetic Resonance Imaging analysis. Three different reservoir conditions (residual oil saturation, intermediate oil saturation, and irreducible water saturation) were simulated. Most of the tests demonstrated the very high displacement efficiency of the enriched gas as regards final oil recovery. Gravitational forces slightly influenced oil recovery, while an important role was played by rock texture when samples at residual oil saturation were considered. The results confirmed that the enriched gas was more effective than water in the final oil recovery at core scale.