Water injection is the most commonly used method for pressure maintenance in depletion drive reservoirs worldwide and there are numerous examples where the recovery has been enhanced significantly. However early breakthrough of injected water due to high permeability zones or thief zones mostly in heterogeneous carbonate reservoirs causes water recycling resulting in poor volumetric sweep and in turn adversely affects the oil recovery from the reservoir. A judicious mixture of proper reservoir management and development strategy may reduce the water recycling to a significant level.
Sabiriyah Mauddud is a giant carbonate reservoir in North Kuwait and has been under production since last 57 years. Historical performance indicated very little aquifer support for the reservoir with no water production. A water flood pilot was initiated in 1997 when the reservoir had produced only 1% of its oil in place and pressure declined from its initial value of 3700 psi to the level of 2600 psi. Currently, the reservoir is under water flood with inverted 9 spot patterns in the crestal part and peripheral injection mainly in the western flank area. The water cut has gone up to the level of 42% after only low current oil recovery. High water cut is attributed to poor sweep efficiency due to preferential movement of water from conventional injector to producer through high permeability streaks/thief zones in the crestal area.
As short term measure, a conformance plan to shut off the water contributing layers was prepared and has been introduced in the wells worked over during the period. Simulation run shows reduction in water cut by 5% by adopting the conformance plan. The new development strategy formulated for the field is to inject low and produce high from the reservoir and shifting the focus from crestal to peripheral injection. All the producers are planned to be completed with inflow control device to control the water production through the wells.
The recently drilled producers are showing positive results and it is expected that the measures taken will help to maximize the benefits of water injection. The learning and the water flooding experience gained will be a valuable input for other reservoirs under water flooding.