Regional Variations in Water Quality - A Key Factor Influencing Water Injection

Abstract

Natural dumpflood injection is used for pressure maintenance in a good channel sand holding considerable reserves. The layer is on depletion drive with very weak aquifer support. Natural dumpflood injection was modeled based on success in a similar layer in an adjacent field. Due to the high mud weight used during drilling, the layer is usually damaged. Dumpflooding experience in the analogue field suggests that injection is initiated only after matrix acid stimulation. Therefore, it was not a surprise when the layer did not take in any water initially. However, once the usual stimulation attempts did not achieve the desired results, possible causes and alternates were evaluated. A comprehensive lab study was designed to assess all the potential problems associated with injection. Fluid-fluid interaction issues like scaling, possible formation of oil-in-water emulsion, fluid-rock interaction such as clay swelling, fine migration related to water incompatibility, all impacting efficiency of water injection on oil mobility and recovery were studied. To simulate every possible field condition all the major water sources were sampled and taken for study. For fluid rock interaction studies, core plugs from the layer was used. Scale tendency was studied using Jar test and predictions at reservoir conditions were made through ChemScale software. The scaling products from Jar test was analyzed with SEM confirming the results. Routine core analysis was performed to measure porosity and permeability in the coreplugs. XRD was done to understand the mineralogy of the layer. After measuring the baseline permeability, the plugs were subjected to flooding with source waters. One set of coreplugs were fully saturated with target water and flooded with source waters. Flooding was repeated on core plugs saturated with oil at irreducible water saturation until it reached residual oil saturation. An investigation of emulsion formation was conducted between oil and source waters. Study results gave an indication of many potential issues like self scaling tendency of source waters and permeability reduction due to fine migration and scale formation. Out of the many factors, the presence of considerable concentrations of sulphate forming cations was identified as the primary issue to focus on. Sulphate reducing chemicals are being evaluated for injection along with the source water. Injection water quality is one of the primary factors to consider for effective injection. This is even more crucial in case of dumpflooding, wherein water is sourced from sub-surface. Due to the wide extent of the field and possible diagenesis effect there could be regional variations in water quality. Due diligence needs to be paid to these regional variations in water quality and choose prudently among alternate source water reservoirs.