Strategies to Mitigate the Challenges of Short Circuiting in Waterflood Reservoirs with Tracers: A Case Study

Water short circuiting leading to early, sudden and massive water breakthroughs in producer wells has been a lingering concern to oil operators for many years. Unfavorable mobility ratio leading to viscous fingering, horizontal wells exhibiting ‘the heel-toe effect’ and fields with fracture-fault activities are more prone to these kinds of unwanted water breakthroughs, suffering from oil production losses and higher operational cost for management of the excessive produced water. A brown field in the south of the Sultanate of Oman was experiencing massive water short circuiting within two of its patterns. [MJO1]While conformance was well established and dynamically confirmed through production performance and artificial lift parameters in most patterns within the field, the complicated inverted nine spot injector-producer pattern scenario[MJO2] was making it difficult to ascertain the offending injectors or unexpected flow paths leading to the condition within the study area. The lower API oil and slightly fractured and faulted geology was exhibiting conditions for injection imbalance and the challenge was to bring the high water-cut wells back to full potential and increase oil output whilst reducing water flow. To investigate the breakthrough occurrences and mitigate the challenge, chemical water tracers were introduced in the reservoir as a part of Integrated Reservoir Management framework to identify flow directions and offending injectors. The Phase-1 of the two-phase study, discussed in this paper, was carried out to determine reservoir conformance that was contributing to short circuiting and once the cause was identified and treated, Phase-2 was carried out post well intervention to validate the success of the treatment. Phase-1 of the tracer study was initiated in October 2019 where two injectors and nineteen producers across two adjacent patterns were traced with two unique chemical water tracers. Massive tracer responses were obtained within the first few days in few wells, directly pointing out towards the offending injector(s). Sampling and analysis for Phase-1 was continued for about six months, after which, a zonal isolation was carried out in one the identified injectors in August 2020. Cement was pumped across all the perforation intervals and a new perforation was performed across the top and bottom of the reservoir avoiding the middle intervals that were taking about 70% of injection as per production logging. Phase-2 of the study was initiated in March 2021 and continued sampling and analyses are still being carried out. With about 15% reduction in water cut and a three-fold increase in oil rate at the target producer, the study validated that an integrated knowledge of reservoir geology and production behavior coupled with tracer studies was a very successful strategy for managing short circuiting in waterflood reservoirs. The study showed that this sequence and combination of methods can be useful in effective treatment for wells experiencing high water cut across the world.