Customer Challenge
Offshore Congo, an appraisal well encountered a prolific oil-bearing interval characterized by a complex mixed-lithology carbonate formation. However, due to the highly overpressured formation pressure, conventional well-killing methods after perforation were impractical. The challenge was to efficiently clean and stimulate the two lobes, situated 10 meters apart, to maximize production potential. The low matrix permeability and limestone nature of the formations made them suitable candidates for acid stimulation. However, various challenging operational constraints posed potential risks to the success and commercial viability of the discovery.
Solution
To address the challenges effectively, a cost-effective acid treatment was devised with the goal of creating wormhole channels within the heterogeneous formations and achieving negative skin values. The formation was perforated using TCP (Tubing Conveyed Perforator) hangers suspended below the test string, which was then dropped. Different shot densities were employed for the upper and middle lobes to balance proper pre-stimulation well clean-up and efficient acid diversion. The well was perforated under-balanced and cleaned under reservoir pressure. The acid treatment sequence included inhibited HCl followed by retarded emulsified acid to facilitate wormholing and deeper penetration. Ball sealers were also introduced to divert acid injection and ensure effective treatment of both lobes.
Outcome & Value Delivery
The successful acid stimulation was confirmed through well testing and Pre- and Post-Acidization Pressure Transient Analysis (PTA). The reservoir skin value significantly improved from slightly positive to close to -4, leading to a substantial increase in well production from 1200 barrels of oil per day (bopd) to 3000 bopd on a 32/64 » choke. At its peak, the well achieved a maximum rate of 4300 bopd on a 44/64 » choke. The test results demonstrated that acid stimulation was an essential element for the commercial development of the reservoir. The combination of customized gun perforation, acid diversion techniques, and the use of retarded acid proved to be a highly successful approach in stimulating the complex carbonate formation, unlocking its full potential for production. The outcome delivered significant value to the client by optimizing production and confirming the commercial viability of the discovery.
