Cleaning 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.
Fill cleanouts with CT have been performed for many years. They account for approx 30% of the services performed with CT. Different cleanout methods have been developed over the decades, employing a number of different techniques and approaches. CT or conventional jointed pipe provides two circulation modes to remove solids: either forward or reverse circulation mode. Using conventional water-based fluids, a conventional sand cleanout method may apply excess hydrostatic pressure on the formation, resulting in some lost circulation to a low formation pressure reservoir 3. If the losses are significant this makes sand removal impossible. Also such losses can seriously damage the formation. Nitrogen can be used to reduce hydrostatics, but this necessitates a very specific job design and execution, and in larger diameter wellbores and especially in horizontal wells can result in using large amounts of nitrogen with corresponding logistical and economic consequences
The evolution of CT technology offers a unique solution for this problem. To mitigate challenges associated with large nitrogen requirements a sand vacuuming technology has been developed and proven in field operations worldwide 34. The vacuuming system consists of a specialized downhole jet pump connected to a CCT string. The tool can be operated in three modes: sand vacuuming, well vacuuming and high pressure jetting. The tool provides a localized drawdown wherever it is positioned in the wellbore and is effective in removing sand in the sand vacuuming mode and removing localized mud damage in the well vacuuming mode.
The selection of sand cleanout methods has to be based on both logistical and technical issues. Things like equipment cost, reel weight (string length and diameter), and availability/cost of N2 will be deciding issues in many sand cleanout operations. Technical issues include the consideration of formation damage potential, low BHP, small completion tubular and particle size/type of debris.