Feasibility Assessment of Through-Tubing Intervention for Sand Cleaning in A-Annulus

While drilling the reservoir section of a 6,000 ft sandstone well, the drill string became trapped and had to be abandoned. In this section, the annular space between drill pipes and the intermediate casing was left with a gas cap and 2,600 ft of sand. Above this, both the annulus and the drill string were filled with cement plugs as part of the abandonment procedure. Re-entry of this well in compliance with current standards required the design of a cleaning operation to remove the sand from A-Annulus while maintaining the trapped drillstring in place. To clean the sand in the A-Annulus, a through-tubing operation was devised, using an adapted cementing tool to jet the sand. Operational feasibility was evaluated by a series of computational fluid dynamics (CFD) simulations, considering a multi-phase flow of sand and pumped fluid. Results suggest that the operation is feasible, provided the sand remains movable and has not hardened due to interaction with inorganic salts, even after months of abandonment. Many wells that need to be abandoned suffer from the same problem, except for sand ingress traps the completion string, instead of the drillstring. Legislation for abandonment usually requires cement plugs to be placed right above the top of reservoirs, in both A-annulus and tubing. Sand contamination can decrease quality of cement plugs, increasing time for a satisfying abandonment operation. The through-tubing pumping cleaning operation simulated in the present work can guarantee less than 5% of cement residue, with approximately 1.3 hours of operation, allowing for a safe and cost-effective abandonment strategy for strings with sand entrapment. Results show that operational parameters comply with most of the coiled tubing limits. Strategic positioning of the pumping tool is essential, to avoid excessive operational time and guarantee effective sand removal.