During acidization of multilayered wells, major portion of the acids preferably enter into high permeable layers bypassing the low permeable layers. To combat the challenge a self-diverting acid (SDA) was developed with conventional chemicals for effective stimulation of all the layers. The technique involves a stable retarded acid system, a plain acid, a viscous diversion system and a chemical formulation for taking care of wettability issue of the formation rock. Descaling and formation preconditioning were also included for effective treatment and flow back. Two polymers were evaluated in the laboratory for customization of diversion system. The concentration of polymer was optimized to achieve the optimum viscosity required to divert the acid based on the reservoir properties and the flow back after the treatment.
The technique has been implemented in 31 multilayered wells of Mumbai High, a major offshore oil field in India, consists of multilayered carbonate reservoirs, with substantial heterogeneities among the layers. All the candidates were selected with MDT approach. Well specific treatment designs were carried out considering the damage mechanism, reservoir properties and well completions. The treatments were designed in multiple stages of preflush, acids, diversion systems and postflush. The chemical formulation for wettability alteration was used in selected wells.
The placement was carried out by bull-heading and real time treatment plots were recorded in each job. The treatment plots indicate effective acid exposure and uniform stimulation among the layers. Post treatment analysis indicated significant oil gain. This paper will cover the critical areas of stimulation in multilayered wells, details of the SDA technique, treatment methodology, design and post treatment evaluation.