DESIGN AND INSTALLATION OF A COST-EFFECTIVE COMPLETION SYSTEM FOR HORIZONTAL CHALK WELLS WHERE MULTIPLE ZONES REQUIRE ACID STIMULATION

An innovative completion design that allows multiple acid fractures to be performed in horizontal subsea chalk formation wells with a single trip into the wellbore has recently been codeveloped by a major North Sea operator and an oilfield engineering/manufacturing/service compan y. The project was begun to develop a system that would allow multiple acid stimulations to be efficiently performed in the shortest possibl e time in the North Sea Joanne field. The system ultimately developed allows acid stimulation of up to 10 different zones in a single trip w ith no through-tubing intervention. The first well in which this new t echnique was used had seven zones, and three additional wells with 10 zones each were later completed. This paper presents the development o f this system and case histories of the first four subsea wells requir ing stimulation. The key element of the system is a multistage acid fr acture (MSAF) tool that is similar to a sliding sleeve circulating dev ice and is run in the closed position. Up to nine MSAF tools can be ru n in the completion with isolation of each zone being achieved try hyd raulic-set retrievable packers that are positioned on each side of an MSAF tool. Each sleeve contains a threaded ball seat with the smallest ball seat in the lowest sleeve and the largest ball seat in the highe st sleeve. With this system, stimulation of 10 separate zones is accom plished in 12 to 18 hours. A unique procedure that lubricates various sized, low specific gravity (SG) balls into the tubing and, then, pump s them to a mating seat in the appropriate MSAF tool to seal off the s timulated zone is used. This allows stimulation of the next zone, whic h is made accessible by opening the sleeve. This technique provided a substantial reduction in the operation time normally required to stimu late multiple zones and allowed the stimulations to be precisely targe ted within the reservoir. The case history data provides comparisons i n operation times between traditional stimulations and this new method , as well as the significant enhancements to cost efficiency that resu lted from its use. Additionally, this completion method allowed the st imulations io be designed and matched to the requirements of each rese rvoir zone, which provided the most cost-efficient treatments possible .