Improving Water Injection Vertical Conformance in the Mishrif Carbonate Reservoir

Abstract Injection logging in Mishrif water injection wells has historically revealed very uneven inflow profiles, which can result in poor sweep efficiency and early water breakthrough at producers. This is due to the highly heterogeneous nature of this carbonate formation, and the compounding effects of non-selective acid stimulations. This paper reviews the vertical conformance improvements that have been achieved through selective acid stimulation of under-performing zones, and selective suppression of high-permeability zones. Operational details are presented which illuminate technical enhancements, and potential pitfalls, of these treatments. Coiled-tubing deployed mechanical diversion methods were used in an extensive selective stimulation campaign. This began with the use of through-tubing inflatable/retrievable packers for targeted zonal stimulation. Casing corrosion logging and conditioning were performed to increase success rates of zonal isolation. Focussed acid jetting was also conducted prior to setting packers for bullhead treatments. Techniques have evolved to include suppression of high-permeability vuggy zones, trialling injection of water-swellable polymers and calcium carbonate particles. Surveillance data was gathered to appraise the treatments, through pre/post job injection tests and logging. Downhole gauges were deployed on coiled tubing to monitor stimulations during execution. Water injection was initiated or enhanced in multiple under-performing Mishrif sub-zones. In some early cases conformance objectives were not achieved despite successful execution of selective zonal stimulation; this can be attributed to inadvertent over-stimulation of vuggy zones during well "conditioning" operations. Analysis of downhole gauge data during pumping operations revealed that industry norms for acid volumes (gal/ft) were sub-optimal for lower-permeability zones and dosages have now been increased. Where maximum pumping rates for acid were heavily restricted (through coiled-tubing pumping) it was concluded that focussed acid jetting across perforations achieved greater stimulation gains than in the mechanical diversion phase itself. Through coiled-tubing treatments (rate-constrained) were improved by using retarded acid to attain deeper formation penetration. Trials for deliberately impairing injectivity of vuggy zones started with non-selective bullheading of water-swellable polymers. The polymer was successfully pumped away into the formation and target injectivity reduction achieved. Questions remained regarding achievement of conformance modification objectives, so the suppression method was refined to include selective zonal placement of suppressant, in combination with selective stimulation of low-permeability zones. Operational complexity was reduced by pumping sized calcium carbonate in place of swellable polymers. Post-job appraisal (Injection Logging Tool - ILT) showed that the desired injection profile outcome had been achieved. The large number of carefully executed selective stimulations, combined with a wealth of high-quality surveillance data, offer many insights and learnings from the giant Mishrif carbonate reservoir. The novel application of "combination" treatments (selective stimulation and suppression in the same intervention) promises rigless solutions for maximizing water injection vertical sweep efficiency in this multi-zone reservoir, without the requirement for costly rig-workovers and installation of complex downhole flow control completions (DHFC).