Abstract
Gas hydrate formation during the drilling and completion phase can add significant operational costs in a deepwater environment. In an ultra-deepwater well off the east coast of India with a water depth of 2830 mMD, a hydrate plug was discovered during well-killing operations after a well test. Multiple attempts to remove the plug using cyclic pressurization/depressurization failed, and a Coiled Tubing (CT) intervention was required to mill out the hydrate plug.
A Coiled Tubing Lift Frame (CTLF) was rigged up to accommodate the CT stack inside the derrick then CT was deployed with a milling Bottom-Hole Assembly (BHA). This BHA comprised of an even-walled stator assembly motor and Hurricane mill bit. Heated brine with 6%-30% glycol was the fluid recipe for the job with the glycol concentration at different pressure and temperature conditions calculated based on data derived from the methane hydrate formation curve and Hammerschmidt's equation. The engineering plan also incorporated the backpressure and fluid temperature selection criteria based on available reservoir and well test data.
Milling was started and operating parameters were maintained to achieve a controlled rate of penetration. Since hydrate formation is not homogeneous, backpressure was maintained throughout the job to regulate the expansion of any gas pockets between the plug and prevent solid hydrates from being propelled to the surface during milling. The "less-aggressive" Hurricane mill bit was selected to prevent any large chunks of solid hydrate from getting dislodged from the plug. An even-walled stator was selected as it provides higher operating limits and is more resistant to deformation and degradation. The temperature of pumping and return fluids was constantly monitored to ensure sufficient thermal energy downhole to prevent hydrate re-formation. Since bottom-hole temperatures were close to the freezing point of water, fluid was continuously pumped at a minimum rate during CT trip-in to keep the stator elastomer lubricated and "heated". The 600m hydrate plug inside the riser string was milled-out in less than 24 hours, following which the CT did not encounter any obstructions to the depth of the sub-sea test tree (SSTT) at 2830 mMD. Pressure communication was established with the well and the job was concluded.
Removal of gas hydrate plugs is a technically challenging and operationally complex job. Furthermore, available literature and case histories on this topic are sparse. This paper presents key learnings from both the planning and execution stages which made this challenging job a success. The paper aims to serve as a reference for operators and service companies to plan, develop and execute similar CT well intervention solutions.
