An Innovative Offshore Fracturing Campaign for a Mature Oil Field: Case Study Offshore West Africa

Abstract

Abstract Many of the offshore oil reservoirs along the West African coast are maturing with substantial reserves still in place; solid technical and operational solutions are required to maximize their value. The field was initially developed in the late 1960's and has since been under water injection and gas lift. The reservoir is composed of laminated sand, silt, and shale sequences, all of varying reservoir quality. As a result of the widely ranging permeability and poor vertical permeability, hydraulic fracturing was the technology chosen to unlock the remaining reserves. Operationally, "hard rock" hydraulic fracturing is not commonplace in offshore West Africa. Therefore, procuring the proper equipment, materials, and skilled personnel was the one of the biggest challenges. To alleviate the high cost of functioning with a drilling rig, an innovative rig-less setup was devised for a 9 well campaign. The hydraulic fracturing method employed was conventional multi-zone fracturing using composite bridge plugs. Although very common in onshore, tight gas developments, the challenge was to properly apply it in an offshore project. Milling composite bridge plugs in under-pressured oil wells initially proved to be extremely challenging, while scale deposition made slickline well interventions very problematic. The project has yielded numerous lessons learned and technical contributions: ▪ While the learning curve was steep, rigless "hard rock" fracturing techniques were successfully applied in offshore West Africa, thus being one of the few examples. This could have direct impacts on the future of the many offshore oil and gas fields. ▪ Consistent successful well interventions with slickline and coiled tubing proved to be the highest risk operation and often the root of most operational problems. ▪ The HSE risks using a floating barge and catenary coiled tubing system are key points which need to be considered when planning a rig-less fracturing campaign.