Producer Acid Stimulation to Remove Near Wellbore Damage in Unconsolidated Sandstone Reservoirs: Lessons Learned, Best Practices and Challenges in Penglai Oilfield, Bohai Bay

Abstract

Abstract This study investigates the evolution of acid stimulation to remove the near wellbore damage in producers at Penglai Oilfield which is the largest offshore oilfield in China. The near-wellbore plugging has been observed in certain wells within the unconsolidated sandstone reservoir. So far, producer acid stimulation has been performed in over 200 wells. This paper will review the design change, lessons learned, current best practices and challenges in acid stimulations. Producer acid stimulation in Penglai Oilfield was first executed in two wells through coiled tubing in 2007 with positive outcomes. Since then, acid stimulation designs have improved as more studies were performed and field trials were made, such as larger acid volume to increase the treated radius, customized acid recipe for effective removal of solids, modified procedures for flowback into FPSO, and improved execution methods through selective production string, coiled tubing, sliding sleeve, or ESP-bypass, etc. An acid stimulation database is created to aggregate well parameters, formation properties, stimulation design variables, and productivity matrices. Various designs were evaluated using the integrated database to guide the future improvements. This study shows that both hydrofluoric (HF) based acid system and a new chelant fluid system prove to be effective in removing the near wellbore plugging for the Penglai field. Laboratory evaluation of acid systems serves its purpose for initial benchmarking; but it loses fidelity in testing conditions that are not realistic to reflect the complicated environment of rapidly varying lithology, scales, waterflooding, etc. Therefore, continuous evaluation, summarization, and optimization of the results of the previous treatments are vital factors contributing to the ongoing improvement of acidization effectiveness in the Penglai Oilfield. This study also highlights the continuous optimization of handling emulsion issues after acidizing. The flowback stream right after stimulation is depressurized and filtered, then directed into the injectors. This practice mitigates the issues in FPSO caused by the acid stimulation and has no impact to the designated injectors. This study showcases significant advancements in acid diversion techniques, specifically emulsion diversion, resulting in improved acid distribution across intervals exceeding 500 feet in thickness. These advancements have been implemented since 2022 and have yielded highly encouraging production outcomes. This study presents an innovative method that solved the emulsion challenges after acid stimulation through flowing back into water injectors after processing the stream.