Successfully Adapting and Overcoming Challenges for Coiled Tubing Applications in High-Pressure Sour Environment and Horizontal Unconventional Tight Gas Wells in UAE
Authors: Sorensen Ole , Wu Yang , Caillon Didier , Viguerie Bjorn , Khan Rao Shafin Ali , Molero Nestor , Franco Delgado Ernesto , Ahmed Mohamed Fahim , Mattos Rafael Fernando , Gilash Anna , Khan Muhammad Talha , Galvan Irma , McClelland Garry 
Published: March 2022

During the completion phase of four unconventional wells in the United Arab Emirates (UAE), a detailed engineering approach enabled overcoming challenges presented by the extreme conditions of pressure, temperature, and sour environment across long horizontal sections to successfully carry out cleanout activities. The methods implemented to address those conditions prioritized personnel safety and asset integrity.

The unconventional wells in this campaign were characterized by a reservoir pressure of approximately 13,000 psi and a bottomhole temperature of approximately 325°F. Gas was expected as reservoir fluid. The environment was sour, with 5% mol of H2S and 5% mol of CO2. After fracturing, the shut-in pressure was approximately 8,000 psi. Those conditions triggered a new engineering approach to design a customized coiled tubing (CT) pipe that could reach depths of 20,700 ft and perform efficient cleanouts. Other considerations included the selection of the appropriate pressure control equipment (PCE), downhole tools, and intervention fluids that could withstand the harsh working conditions.

The wells were completed with 5 1/2-in. fracturing strings; horizontal sections ranged from 5,000 to 8,200 ft. Each of the 95 fracturing stages was isolated by dissolvable plugs. The CT pipe design considered well trajectories, working pressures, and downhole environment. The highly engineered 22,000 ft, 2 3/8-in. CT string was manufactured with quench and tempered (Q&T) material with yield strength of 110,000 psi. A custom-fit CT string integrity program was implemented to ensure safe intervention in high-pressure sour environment. Pipe life was then monitored in real time using magnetic flux leakage. Downhole tools (CT connector and check valves) and the PCE stack were designed to hold pressures up to 15,000 psi. The PCE configuration was thoroughly reviewed, and since the wellhead stack exhibited a maximum height of approximately 120 ft, a stack stress analysis through finite-element modeling software was conducted to validate structure stability, to confirm forces and bending moments remained within safe working limits, and to determine the optimal setting for guy wires. Job execution highlighted challenges linked to the high-pressure cycling and the need to optimize the number of achievable sweeps in the horizontal section. Well trajectories also proved challenging for the cleanout, with solids accumulation in specific intervals.

This operation, the first of its kind in the country for the operator, yielded lessons for design considerations and the execution process and recommendations for CT intervention in similar working environments. It also confirms that 110,000-psi yield strength Q&T CT strings can be safely deployed in a high-pressure sour environment by implementing proper risk mitigation strategies.