Water injection is an enhanced oil recovery method which is applied for number of reservoirs around the world especially offshore fields. Sea-water injection process is usually associated with important concerns affecting the efficiency, safety, and economy of the operation like formation damage, mineral scaling, early breakthrough and corrosion. Incompatibility between injected and formation waters may result in inorganic scale precipitation in the reservoir and then reduction of oil production rate (productivity loss) and water injection rate (injectivity loss). A comprehensive study supported by reliable modeling and experimental investigation will therefore, significantly improve the success of the operation.
In this paper, mineral scale deposition phenomenon in one of Iranian offshore carbonate oil reservoir under water injection is studied and its influence on reservoir performance and production rate is investigated. For this purpose, an extensive experimental work has been conducted on compatibility of sea and formation waters and the amount and type of scale formed as a result of mixing different portions of these waters. Experimental results have been applied and incorporated with simulation studies. STARS simulator (from CMG 2005) that is capable of evaluating the effect of scale formation on reservoir performance allowing for alteration of permeability and the flow path has been used in simulation study. One of the major contributions of this work is to incorporate the equilibrium conditions, reaction rates and solubility parameters for various scale types in reservoir simulation, so the results can be more realistic.
The simulation results show that in oil reservoirs under water injection, scale formation undermines the productivity index and has a major impact on reservoir performance. These results have been confirmed by production history of the field.