Management of Water Injection Quality in a Giant Carbonate Onshore Oil Field, Abu Dhabi, U.A.E

Abstract

This paper reviewed the management of injection water quality in a super-giant carbonate oil field operated by ADCO onshore Abu Dhabi since 1973. This field was subjected to peripheral water flooding in order to maintain reservoir pressure and provide a mechanism to sweep the oil. Injected water was sourced from 23 water supply wells completed into deep hyper-saline aquifers with total suspended solids (TSS) of 1.5 mg per liter (mg/l). Each water supply was connected to a cluster of 4–5 water injectors. Clusters were interconnected. Produced water having average TSS of 100 mg/l and oil in water (OIW) content of 260 parts per million (ppm) was being reinjected into the most permeable reservoir through five well peripheral pilots injector. No significant operational problem was reported apart from occasional injectivity degradation which was restored with a maximum of one acid stimulation per well since 2002. New facilities under construction were designed for a maximum OIW of 100 ppm with plans being made to reduce it to 50 ppm as the volume of produced water was expected to rise with the field wide implementation of gas lift. After more than 30 years of production from two major high-quality reservoirs, ADCO recently started the development of a third oil-bearing Reservoir X classified as low permeability. Accepted assumptions based on the extensive water injection experience proved questionable considering the low median pore size diameter of the reservoir under development. The common water supply for the peripheral water injectors assigned to the three different oil-bearing reservoir zones posed an additional challenge as the water injectors drilled in the low permeability reservoir needed to be shielded from temporary degradation of water quality typically experienced during start-up operations after maintenance or water supply well workover. The issue of temporary high solid loading gained importance as the practice of discharging water loaded with solids to a pit was discontinued after 1998 for environmental reasons. Several studies related to water quality were recently performed including the onsite cycling of aquifer water through reservoir core plugs of Reservoir X in order to predict the potential degradation in matrix injectivity over time and evaluate the resulting operational cost and timing of future acid stimulations. Material selection for the surface pipeline network carrying the aquifer water was also reviewed with non-metallic internals being recommended. In the event of a temporary degradation of water quality following system start-up or a workover in a water supply well, disposal of such water into the source aquifer was determined to be the best solution to avoid injecting water loaded with solids in the low permeability reservoir, also fulfilling all regulatory requirements.