On the Norwegian continental shelf (NCS), coiled tubing (CT) cleanout requires small bites and frequent wiper trips to the surface due to potential sand bedding in a large and deviated completion. A real-time CT downhole measurement system is used to optimize the operation, following a dynamic workflow. Conventionally, the system is powered by downhole lithium battery, which limits CT downhole operating time. A continuous surface-powered system was needed to promote further optimization for such operation.
A new hybrid electro-optical cable was introduced to enable continuous power supply from surface to the real-time downhole tool sensors. The system consists of a surface power module that sends power through a layer of low-DC-resistance conductors and optical fibers that enable data telemetry. Conventionally, only three to four trips can be completed before replacement of the downhole battery is required. Battery replacement can take up to 8 hours due to the complexity of that offshore environment. With the continuous power supply, the CT cleanout operation can continue for days without interruption of data from the downhole tool sensors.
A three-well CT cleanout campaign in the NCS demonstrated the benefits of this new real-time downhole measurement system by using accurate downhole weight and torque readings to control the penetration through scale and avoid motor stalls. Sections of scale bridges were identified during the cleanout by monitoring fluctuations of downhole torque of the mill. The monitoring allows CT operators to control penetration rate and bite length during the cleanout. When the milled debris are swept, downhole weight is used to detect early signs of solids plugging around the mill. Downhole pressures complement surveillance of the sweeping of solids to the surface by giving a qualitative measurement of solids loading through conversion of the real-time bottomhole pressure reading into equivalent circulating density with changing CT depth. The process of optimizing bite length and sweeping speed is repeated without interruption thanks to continuous power supply from the surface, eventually leading to time reduction. In one of the wells, downhole tools uninterruptedly acquired data for 10 days straight. The CT managed to clean out a total of 40 908 kg of a mixture of scale and sand, with an estimated average time reduction of 25% when compared to CT cleanout without real-time downhole data.
Delivery of continuous high-voltage power to downhole tools not only enables reduction in operating time, it also paves the way for extending the capabilities of CT interventions by enabling the operation of more electrically activated application tools. It allows combining multiple work scopes in a single CT run, which reduces operating cost and provides greater operational flexibility. Finally, eliminating the dependency on lithium batteries reduces the carbon footprint for a more sustainable operation.