The standard well-completion method in the Valhall field has evolved since 1996 to include multiple proppant fractures along the horizontal laterals. Coiled tubing (CT) has been used since then to perform over 200 post-fracture proppant cleanout operations. Field practices were developed from these jobs but the process in different stages of proppant cleanout operations had not been systematically optimized to realize best performance until the launch of a new process to optimize CT proppant cleanouts.
In May 2004, a study of proppant cleanout process optimization was completed by analyzing historical cleanout performance and incorporation of a new, integrated wellbore-cleanout system to improve operational efficiency through optimized processes, improve safety and reliability by implementing standardized procedures, and reduce stuck-pipe potential through improved design. This study analyzed the whole process of proppant cleanout, captured best practices through validation of field experience, introduced innovations to routine operations, and engineered a new, more efficient cleanout procedure for field operations.
The field implementation of this new process optimization on the first three wells, a total of 22 zones, indicated a significant performance improvement. The optimized process saved a total of 6 days' working time and encountered zero incidents that might have led to stuck pipe. The new cleanout procedure was followed by different operational crews on the job and has been adopted as a best practice for future wells.
This paper details the engineering and implementation of CT post-fracture proppant cleanout process optimization in the Valhall field and demonstrates a step-change improvement in Valhall CT cleanout performance during the past 9 years.
The Valhall field is an Upper Cretaceous, asymmetric chalk anticline that forms an over pressured, under saturated oil reservoir located in the Norwegian sector of the North Sea (Fig. 1). Throughout the producing life of Valhall field, the completion methodology demanded strategies to manage poor chalk stability, which leads to formation influx and tubular collapse in both the reservoir and the overburden. Since 1996, proppant fractures along the horizontal wellbores have become the standard and most successful completion technique; CT has been used to convey perforating guns, to clean out excess proppant left at the end of each zone stimulation, and to establish proppant plugs between zones to ensure isolation while performing individual fracture treatments.
The planned operational sequence of a typical well stimulation work is summarized below:
Drift and clean out liner with CT.
Run a tubing-conveyed perforation (TCP) gun on CT to shoot lowest Zone 1.
Proppant fracture Zone 1 with stimulation vessel.
Run pump over gun (POG) on CT to clean out excess proppant left in wellbore; establish proppant plug between next upper zone; shoot next upper Zone 2.
Proppant fracture Zone 2 with stimulation vessel.
Repeat Steps 4 and 5 for the remaining zones.
Perform final cleanout with CT and hand over to production.
While CT remains an important component in completion of well stimulation work, the CT size has evolved from early attempts in 1996 that used only 2-in. CT. The 2-in. was not successful due to hydraulic limitations, so 23/8-in. tubing became the conventional work string.This was used successfully from 1996 to 2003 to complete 20 well stimulations in the Valhall field. When the Valhall Flank development began in 2003, the well program posed more challenging trajectories, with 2,000-m-long laterals at up to 97º inclines. To handle these more challenging wells, 2–7/8-in. CT has become the dominant work string for developing long horizontal wells throughout the whole Valhalla Flank.