BJ Services performed the propped fracture treatment using the MV Vestfonn, an offshore stimulation vessel, on Well L8-A2 located in the Dutch sector of the North Sea offshore Holland. The operation took place over a three-day period, with onshore support provided by BJ Services staff at the company's Marine Base in Great Yarmouth, England.
Vestfonn Specially Modified to Carry Out Operation
During the past five years, BJ Services has carried out a number of propped/acid frac stimulation services using MV Vestfonn for Wintershall/Clyde in this sector. This particular operation was especially challenging because the production string through which the treatment was to be pumped contained a travel joint that allowed for limited movement. In 1988 when the well was completed, fracturing was not considered. Therefore, the short polished bore receptacle – which allows tubing movement of just 15 feet – was not an issue.
The operational envelope (e.g. producing, injecting) is dictated by the physical conditions during the installation of the completion. Since the BHT of this well was moderate to high, there was concern about the cooldown effect during treatment. The frac fluid temperature on the Vestfonn is dictated by the ambient water temperature in the storage tanks. Treatment simulation with an injection temperature of 50ºF indicated that the reduction in temperature gradient of the completion string would cause contraction in excess of 15 feet, resulting in unseating of the upper completion string. Further calculations showed that a frac fluid temperature of 70ºF or higher would provide an acceptable contraction of the string. Unfortunately, the Vestfonn was not capable of heating the frac fluid to this temperature.
. Following considerable analysis of alternative methods, BJ Services decided the safest and most efficient alternative was to modify the vessel to heat the water directly in the storage tanks. Over a one-week period, BJ worked with great care to properly modify the Vestfonn to carry out this procedure.
The primary treatment could require in excess of 100,000 gallons fluid and the injections necessary prior to this, such as the breakdown, step-rate test and Minifrac, would require additional fluid. Also, due to the time required to heat these large volumes, it meant that a number of water tanks would have to be individually heated. BJ decided to install heaters in six of the vessel water tanks (e.g. 140,000 gallons) based on the design criteria of heating the water from 50ºF to 70ºF in a 24-hour period. The Vestfonn is equipped with two process generators to provide power to run the hydraulic power packs and various electric motors, but would not have sufficient capacity to power the additional water tank heaters. Therefore, two additional temporary generators were required which had to be situated on the cargo deck of the Vestfonn and accordingly cabled to the heater in each water tank. The elements were of a two-stage configuration to reduce the start load on the generators, a special control panel was made for each element. The control panels were connected to the elements, and wired back with 31 and 40mm diameter cables to the generators approximately 50 meters from the elements. The reservoir temperature was 284ºF, so in order to meet the required stability criteria, Spectra Frac® G5000 was to be pumped as the cross-linked fluid. Fann 50 testing at 100 reciprocal sec-1 (R1, B5 combination) gave two-hour stability greater than 400 cp at 284ºF. The initial breakdown-slug was pumped with this fluid, but the high fluid viscosity and fast cross-link caused by the heated water resulted in high friction and a low injection rate. For the minifrac, the XLW-56 cross-linker was cut with 25% caustic soda (XLW-56/C2.5 mix) to try to reduce the cross-link time, but the high fluid temperature (90ºF still resulted in a fast lip of less than one minute. Wintershall's concern about excessive viscosity and, therefore, limited pump rate led to a 40# and 45# Spectra Frac being tested. Each of these fluids still gave 1-1/2 hour stability on the Fann 50 at 284ºF, which was deemed adequate stability. As a second slug was required, the SFG4500 fluid was pumped and, despite a fast cross-link, an acceptable pump rate was achieved.
Further testing prior to the main treatment led to additional 25% caustic soda being added to the cross-linker (XLW-56/C10) to delay the cross-link to four to five minutes with 85ºF fluid. The main fracture treatment was successfully pumped with the delayed SFG4500 at the desired pump rate. Typical of sampled fluid on the Vestfonn during pumping, the cross-link time was faster than that observed in the vessel laboratory prior to the operation
The operation proved to be a success in a number of ways. First, the heaters did their job by raising the water temperature in the vessel tanks so that the fluid pumped down-hole did not cause excessive contraction of the tubing. This was particularly satisfying considering that there had been insufficient time prior to sailing to test the heating system, due to time constraints. Plus, the high than anticipated temperatures of about 80ºF contributed to faster than expected cross-link times, which resulted in higher friction pressures. However, by delaying the cross-link and using a lower polymer loading, we solved the friction issue. The main fracture treatment was successfully pumped at 25bpm with total proppant – extended from the original design – of 174,000 lbs 16/20 Carbolite +OptiProp. "Wintershall was extremely satisfied with both the overall operation, and the resulting main fracture treatment," said Brian Cooper, Vestfonn District Engineer – Europe & Africa for Well Services, BJ Services. "It was a considerable team effort made possible by the hard work and expertise of the Vestfonn staff, both on and offshore. It was especially challenging due to the modifications we had to make to the Vestfonn to heat the water directly in the storage tanks onboard the vessel," he added.
MV Vestfonn: Record of Success
BJ Services' MV Vestfonn is a dynamically positioned offshore stimulation vessel that was designed to meet the technological and environmental challenges posed by the demanding conditions of the North Sea. It has delivered stimulation services on over 140 wells in the British, Danish, Norwegian and Dutch sectors of the North Sea over its 20-year history – which translates to more than 900 frac operations. As operators have progressed to highly deviated, multi-zone and horizontal well configurations, BJ has upgraded her capabilities accordingly.
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