This paper is not a technical paper about electric submersible pumps (ESP's), instead it traces the 20-year journey that has led to the development and successful application of multiple generations of rigless ESP conveyance systems in a commercial oilfield. The end result, after many lessons learned, is a success story in which over 300 rig interventions have been eliminated over a 20-year period, with savings of 100's of millions of dollars in intervention costs, reduced HSE exposure and many millions of barrels of additional production.
Utilization of ESP technology in the West Sak viscous oil field in Alaska is challenging. The unconsolidated nature of the West Sak sands impacts the performance and reliability of conventional ESP systems due to sand production. This challenging environment causes ESP pump erosion and accumulation of sand in the tubing above the pump and in the lower completion below the ESP.
The initial development of the West Sak formation was the basis for the original development of the through-tubing conveyed progressing cavity pump (TTCESPCP) in the mid 1990's. With time, the West Sak completions evolved from vertical wells to long horizontals, resulting in production capacity increasing beyond the capabilities of the 3.5 in. and 4 in. TTCESPCP systems. This led to the development of a 4.5 in. through-tubing ESP in the early 2000's. In this design, the PCP of the TTCESPCP system was replaced with a high capacity, centrifugal pump or through-tubing convenyed ESP (TTCESP).
With time and successful experience utilizing the TTC systems, it became evident that although the through-tubing technology resulted in significant savings and increased production, the design was lacking in one major aspect – the ability to remove sand accumulation in the 7-5/8 in. production casing below the end of tubing. The inability to perform interventions without pulling the tubing, was leading to expensive and avoidable rig workovers not related to the ESP equipment. The resulting economics drove the development of a through-tubing, slickline (SL) deployed ESP that, when all components are removed, leaves a minimum diameter of 3.80 in. for well interventions below the end of the tubing. The wireline retrievable ESP (WRESP) system was launched in 2005 and was fully commercialized in 2014.
Numerous papers have been written on this specific technology and references are provided at the end of this paper. This list does not represent a complete listing of all through-tubing technologies, as there are other systems with substantially different characteristics. This paper will focus only on the through-tubing technology development and evolution in Alaska. It will present the 20-year development history of the Alaskan through-tubing technology, how the system is deployed, answers to frequently ask questions, and as the title suggests – What went Right, What went Wrong, and What's Next?
The development and successful commercial deployment of through-tubing ESP systems in Alaska has been a long journey, with many lessons learned in the evolution from conventional ESPs, to through-tubing PCP's and ESP's (TTC or Generation 1 (Gen 1)), to the wireline retrievable ESP (WRESP or Generation 2 (Gen 2)). It should be recognized that both technologies had a development phase, followed by a commercial deployment phase. There were unexpected problems and benefits that were encountered as the technology matured. The technical difficulties significantly increased while advancing from the TTC (primarily mechanical changes) to the WR technologies (which adds the electrical component of a downhole wet connect).