Application of Acid Diversion System for Effective Stimulation of Multilayered Wells in Sandstone Reservoir: A Case Study

Abstract

Abstract During acidization of wells completed with long perforation interval or in multiple layers, major portion of the acids preferably enter into high permeable layers/streaks bypassing the low permeable layers. Exposure of acid solutions in entire perforation clusters or long interval is necessary for effective stimulation of such wells. To address the issue, an acid diversion system was customized based on laboratory evaluations and field requirement. The technique involves a mineralogy-based acid system and a viscous diversion system. The acid system was designed based on the formation clay mineralogy. Two polymers were evaluated for customization of the diversion system. The compatibility of the identified viscous gel with acid formulation was evaluated. The concentration of polymer was optimized to achieve the optimum viscosity required to divert the acid based on the reservoir properties and effective flow back after the treatment. The technique has been implemented in numbers of water injection and effluent disposal wells completed in multiple layers of sandstone reservoirs, having substantial heterogeneities among the layers. The distribution of injected fluid was highly uneven among the perforation clusters as indicated by PLT. Well specific treatment designs were carried out considering the damage mechanism, reservoir properties and well completions. The treatments were designed in multiple stages of pre- flush, acids, diversion systems and post-flush. The placement was carried out by bull-heading through tubing. Post treatment analysis indicated significant improvement of injection rate through effective stimulation. Well A of Nawagam field of Western Onshore, ONGC is completed in five perforation clusters, 02 in upper pay and 03 in lower pay. The position of upper pay is almost 500m above the lower pay. The target injection was 50 m3/day in lower pay. However, PLT indicated that the total injection rate was 18 m3/day in upper pay intervals and lower pay was accepting the negligible volume of water. Specific treatment was planned with suitable acid formulation for lower pay reservoir and diversion system. The treatment was executed as per plan and resulted improvement of injection rate from18 m3/day to 55 m3/day, mainly injected into lower pay. Well B of Jotana field is an effluent disposal well completed in long perforation interval of 22m. Treatment with mineralogy-based acid formulation and diversion system was executed, which resulted improvement of injection rate from 55 m3/day to 144 m3/day. Similar treatments have been executed in numbers of wells completed in multiple perforation clusters and observed highly encouraging results. The customized technique is applicable for effective stimulation of wells completed in multiple layers or with long perforation intervals. This paper will cover the critical areas of stimulation in multilayered wells, details of the mineralogy-based acid formulations, acid diversion systems, laboratory evaluations, treatment methodology, execution and post treatment results.