Wellbore cleanouts represent the main application of coiled-tubing (CT) services. Despite a long history of utilizing CT to remove sand and other fill material from oil and gas wells, advancement in the technology, and a growing body of experience, many wells are still not cleaned adequately, some wells cannot be cleaned at all, and a downhole stuck CT or other serious problems are encountered too often. Based on gathered experience and extensive research and development of new tools and techniques, a new, highly engineered and integrated system for wellbore cleanouts was developed. This paper presents the new integrated system approach, which can eliminate wellbore fill-removal problems and provide effective wellbore cleanouts under virtually any wellbore conditions. Case studies are included that show the performance of the new system in difficult field environments.
The removal of fill material (well-produced sand and fines, proppant, etc.) from producing wells has been the most common application of CT services. The primary reason for wellbore fill removal is, generally, to restore the production of the well. Additionally, fill removal may be necessary to permit the free passage of wireline or service tools, or to remove material that may interfere with subsequent well service or completion operations.
Wellbore fill removal is frequently considered inadequate, leaving large quantities of fill material in the well, which often requires repeating well cleanouts within relatively short time intervals. Additionally, wellbore cleanouts are extremely time-consuming, preventing timely return of wells to production and increasing the cost of well maintenance. Many wells, such as large-casing, higher-temperature or very deep wells, cannot be cleaned at all with older technology.
In this study, theoretical and experimental work was performed to understand the behavior of particles (sand, bauxite, etc.) that are transported by a cleanout fluid. A wide range of cleanout conditions were studied. The study included comprehensive experiments in 3.5-in. and 7.0-in. transparent flow loops at various deviation angles, and a range of cleanout nozzles, fluids, and procedures at realistic flow rates. A long flow loop was used to determine the ability of various fluids to transport solids over long distances.
In addition to a better understanding of particle transport phenomena, the main result of the study was development of a highly engineered, integrated system that can effectively clean the entire wellbore fill from most difficult wells. The new system consists of improved cleanout nozzles, specialized fluids, an enhanced analytical model and software for simulating the cleanout process under specific well conditions, and a system for monitoring particle returns at surface in real time.
This paper presents the main theoretical and experimental work along with major findings from this study. Some major misconceptions about wellbore cleanouts are presented and explained. Additionally, case studies are provided that illustrate results from the integrated system in a multitude of field applications.
Removing wellbore fill, such as formation-produced sand and fines and/or proppant left over from fracturing operations, was one of the first applications of CT services. It also has been the largest CT application, approaching 50% of all CT operations industrywide. CT cleanout technology has gradually evolved in an effort to keep up with developments in well design and the increased complexity of wellbore conditions, such as higher well deviations, deeper wells, larger completions, and higher bottomhole temperatures. New equipment, tools, fluids, and cleanout methods have been developed to keep up with a constantly increasing degree of cleanout difficulty.