Abstract

Cleaning sand fill out of large diameter, deviated wellbores with low-bottom-hole pressure has proven to be a major challenge. During a conventional coiled tubing cleanout on these lower reservoir wells, either a low density fluid, such as diesel, or a nitrified system, is utilized to maintain an underbalanced condition during the cleanout process. It is common that many of these cleanout operations are conducted with limited annular fluid velocities. Sand vacuuming technology addresses these challenges by utilizing a jet pump powered by single phase liquid which carries all of the returns up the concentric coiled tubing (CCT) annulus. Previous field practices indicate that most of the sand/well vacuuming jobs were applied in low-pressure reservoirs, unconsolidated sands with relatively small size solids (less than 20/40 mesh) in horizontal slotted liner completions.

There are still some challenging issues for sand vacuuming operations. These include the use of heavy coil in coil strings, limited jet pump capacity for deep, extended reach wells, vacuuming large particulates through the narrow flow intake to the concentric coiled tubing annulus, and breaking consolidated sand bridges with limited forward flow jetting. This paper discusses the modification of a previous jet pump with a three-way switch tool (sand vacuum, well vacuum and jetting mode) to clean large size proppant (8/14 mesh). The modified bottom hole assemblies were tested in a full scale flow loop. The flow loop setup and test results will be discussed. The distance of the rear facing jets to the intake of the jet pump and the pull-out-of-hole speed of BHA are crucial to insure the sand can be completely removed. A few field cases are also discussed in this paper. Both flow loop tests and field operations indicate that a proper designed jet pump with a jetting switch tool could effectively vacuum the larger proppant in low pressure extended reach wellbores.

Introduction

Performing sand cleanouts remains one of the top uses for coiled tubing. Though sand cleanouts are very common, the actual processes are often very difficult. Wells with ultra low bottom hole pressure reservoirs are extremely challenging for conventional CT cleanout methods to succeed1. These low success rates lead to the development of concentric coiled tubing. The use of concentric coiled tubing (CCT) to power a modified downhole jet pump for solids removal has been utilized since the mid 1990s2. Hundreds of successful cleanouts have taken place3–16, primarily in horizontal, low reservoir pressure oil wells containing formation fines. The system incorporates coiled tubing inside of coiled tubing, with typical combinations consisting of 2–3/8" outer string with 1-¼" inner string, 2" × 1", or as small as 1- ½" by ¾". This system allows the cleanout to be performed with single-phase fluid only, significantly simplifying logistics and costs when compared to circulating nitrified fluids during a conventional cleanout11.

Since inception of the sand vacuuming technology, multiple iterations of the downhole tool system have been employed. Initial applications were conducted with a 3-¾" BHA2–8 and were focused mainly on horizontal, slotted liner completions with 2000 ft TVD /7200 ft measured depths. In the past several years, additional challenges have been faced, such as smaller completion sizes, deeper TVD/MD wells, lower reservoir pressures, tougher sand bridging, and larger particle sizes. These challenges led to enhanced design changes that broaden the operating envelope for the use of the sand vacuuming system. The modern versions of the BHA are available in 2–1/2", 2–1/8" and 1-¾" outside diameter9,12 and incorporate multiple operational modes to ensure cleanout efficiency. Additionally, the end of the sand vacuuming system may be equipped with a supplementary tool to broaden the work scope13. Such tools as lateral entry guidance systems, rotating jetting tools, etc., have been incorporated into the sand vacuuming system.

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