Sand Cleanout with Coiled Tubing: Choice of Process, Tools, or Fluids?

Abstract

AbstractCleaning sand from wellbores is a major part of the coiled tubing (CT) industry. The process is a function of multiple variables including fluid properties, flow velocities, hole size and deviation, pipe eccentricity, particle properties and fill penetration rate and wiper trip speed. It becomes even more challenging to successfully remove fill from wells with low bottom hole pressure (BHP) especially if the well is completed with smaller diameter production tubulars which reduce circulation flow cross-section and choke flow. The problem is further compounded by high deviation or horizontal well trajectories especially in large diameter wellbores.Several cleanout options have been developed in the past, employing a variety of different approaches. CT or conventional jointed pipe often incorporates high circulation rates, special fluids, wiper trip, or reverse circulation mode to remove solids. Many of these conventional sand cleanout methods often apply excess hydrostatic pressure on the formation, resulting in lost circulation in low formation pressure reservoirs. The conventional solution to overcome excess hydrostatic has been to include nitrogen to reduce fluid density and thus lessen the hydrostatic head. Sand vacuuming technology combines a concentric coiled tubing (CCT) string with a downhole jet pump to remove fills without placing hydrostatic loads on the reservoir.This paper reviews the individual sand cleanout systems and discusses the advantages and limitations related to each method. Compared with other processes, cleaning sand using a wiper trip has become the most common and efficient mehtod. However, it needs an appropriate pump rate and reservoir pressure to maintain a proper return flow rate to carry the sands to the surface. For a depleted reservoir and especially for horizontal heavy oil wells, a sand vacuuming system can be used to efficiently remove the debris without circulating nitrogen and without high pump rates. The main application for the reverse circulation technique is cleaning sand from large diameter wellbores and when the necessary pump rates for conventional "forward" circulation are not achievable. Fluids with high solids suspension capability (under shear conditions) may be an economic option when the debris cannot be removed from a large diameter deviated wellbores using more conventional, but lower cost, cleanout fluids. The venturi junk bailer is often used to retrieve larger material or junk which is typically not circulated out by any other methods. Field cases are provided, demonstrating how to select the proper cleaning method and how to efficiently remove sands from a wellbore based on both operational and logistical conditions.