Produced Water Re-Injection: An Integrated Subsurface Approach to Planning and Execution for Downhole Produced Water Disposal in the Niger Delta

Low oil price and increased environmental regulations presents a new frontier for many indigenous oil and gas companies in Nigeria. In mature fields with significant water production, produced water treatment and handling could easily account for up to a third of OPEX. Underground produced water disposal is a tested approach that has been used worldwide with mixed results. Studies have been published on the subject; however, it was observed that there were no Niger Delta case studies. This paper presents SEPLAT's subsurface approach to in-field water disposal, drawing upon geological and petroleum engineering analysis coupled with learnings from over 6 years of produced water re-injection experience. Some of the areas that will be discussed include reservoir selection/screening methodology, water quality impact on permeability, produced water disposal well selection/completion, operating philosophy, general surveillance, and basic separation requirements. Thirteen reservoirs located within 2 proximal fields were screened for suitability and ranked as possible candidates for water disposal based on 8 criteria. The best 2 were then high-graded and detailed studies carried out, spanning detailed geological characterization for reservoir quality and connectivity (including quantitative interpretation), to dynamic simulation, injection well location optimization and performance prediction (for clean water). The results of core flood tests were incorporated. It is recommended that total suspended solids should not exceed 5 mg/L, with a maximum of 5 microns particle size, under matrix injection conditions while oil content should be limited to below 30-50 ppm. Tolerance for TSS can be relaxed to 10ppm – 50ppm at fracturing conditions, depending on the reservoir parameters and process systems. The knowledge of these parameters should drive the technology selection for optimum water treatment and injection.