Treated Sewage Effluent Injection — Microbial and Formation Damage Assessment for a Low Permeability Carbonate Reservoir

摘要

Large-scale seawater injection in two high permeability carbonate reservoirs within the Khurais field commenced with the onset of oil production in early 2008. Following the recent proposal for an incremental development plan that includes an underlying low permeability (2 millidarcies (mD)) reservoir with high calcium content (37,000 mg/L) formation water, it became necessary to examine alternative options to the seawater injection to avoid calcium sulfate scaling and microbial fouling. Secondary treated sewage effluent (TSE) from nearby urban treatment plants is abundant and presents an attractive option for injection in the high-risk, divalent ion formation brine (FB) environment. An initial feasibility study focused on geochemical and microbial compatibility to assess the benefits expected from substituting TSE for costly desulfated water was conducted. A formation damage risk evaluation was also considered critical for the planned low permeability reservoir development since the significantly high initial capital expenditure associated with a new injection water processing facility would further erode the economic value of the project. Laboratory coreflood experiments and conventional bottle tests were therefore conducted, in addition to software simulation to appraise the TSE interaction with the formation fluid, and the inorganic scale deposition tendency at in situ conditions. The microbial study looked at the potential impact on the reservoir for both microbial contamination and the nutritional load of TSE. The study confirmed that the sampled TSE had a relatively low level of contaminants, such as oxygen demanding substances (ODS), heavy metals and dissolved solids, and presented minimal formation damage risk compared to both seawater and field produced water. But it also revealed various total organic carbon (TOC) content, which may enhance troublesome microbial activities and impact the various systems' operational stages. This article discusses the laboratory experiments and simulation conducted to assess the impact of injected TSE on microbial growth and in situ scale deposition, and the associated formation damage risk. It also provides insight into the quality threshold required for effluent injection in a reservoir of high divalent salt connate water.