Introduction of Stimulation with Single-Phase Retarded Acid Help to Achieve Superior Results in Highly Depleted Reservoir Newly Drilled Wells Offshore Republic of Congo

Abstract Redevelopment of the Tchibeli main field in offshore Republic of Congo began with the drilling of five new wells targeting the Upper Albian formation. Being carbonate in nature, these reservoirs benefit from initial hydrochloric (HCl) acid stimulation to enable and promote economic production. A project was devised with a stimulation vessel pumping single-phase retarded acid and a particulate/fiber-based diverter. The wells were drilled and completed with a 9 ⅝-in. cemented casing. This casing was perforated using 7-in. tubing-conveyed perforation (TCP) guns with a shot density of one to six shots per meter. A single well was completed using a ball-actuated fracture port completion with the target of completing three acid fracturing stages in a single well. For this well, acid fracturing modeling software was used to predict fracture coverage and ideal placement of fracturing ports. Optimization of stimulation fluids was carried out using core flow testing with formation cores. Analysis of the cores showed a variation in permeability to gas (10 md to 2,000 md) and carbonate content across the pay zone. By investigating different fluids and injection rates, engineers were able to optimize treatment design and react during operations without compromising on quality. Degradation tests of the particulate diverter were conducted and evaluated to ensure the correct product used. Well stimulation operations were carried out using a stimulated vessel to enable efficient use of time and achieve large stimulation design volumes in one bullheading pumping operation. During the process, three stages were completed in a single load out for the acid fracturing candidate at rates up to 40 bbl/min. This was followed by the completion of three matrix wells in a single load out, with the final well being completed at a later date. All these stimulations were performed offline with the rig performing the flexible drilling and completion sequence. Post-stimulation production results were analyzed and matched with the stimulation software production model to further improve the accuracy of the simulation. This campaign used a combination of single-phase retarded acid for matrix and acid fracturing operations, plus degradable particulates and fiber diverters to achieve complete zonal coverage of matrix treatment. Core testing showed pore volume to breakthrough (PVBT) results in high-permeability cores above 345 md. Plus, cores were used with high calcite content instead of the traditional dolomite/limestone lithology that is often presented in PVBT results. The combination of core data and production prediction was able to further develop stimulation fluid placement in modeling software.