Wintershall, BJ Services pump warm frac off Holland

Jan. 10, 2005
Wintershall Noordzee BV and BJ Services Co. used heated cross-linked fluids in a hydraulic fracturing project off the Netherlands.

Wintershall Noordzee BV and BJ Services Co. used heated cross-linked fluids in a hydraulic fracturing project off the Netherlands. BJ Services performed the propped fracture treatment using the MV Vestfonn, an offshore stimulation vessel, on Well L8-A2 in the Dutch sector of the North Sea. The operation took place over 3 days, with onshore support provided by BJ Services' marine base in Great Yarmouth, England.

Special modifications

During the past 5 years, BJ Services has carried out a number of propped/acid frac stimulation services using MV Vestfonn for Wintershall/Clyde in this sector (Wintershall Nederland BV, a BASF subsidiary, acquired Clyde Netherlands BV in October 2002). 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. At that time, the short polished bore receptacle, which allows tubing movement of just 15 ft, was not an issue.

The operational envelope (e.g. producing, injecting) is dictated by the physical conditions during the installation of the completion. Because the bottomhole temperature of this well was moderate to high, engineers were concerned about the cooldown effect during treatment. The ambient water temperature in the storage tanks dictates the frac fluid temperature on the Vestfonn.

Treatment simulation with an injection temperature of 50° F. indicated that the reduction in temperature gradient of the completion string would cause a contraction of more than 15 ft, resulting in unseating of the upper completion string. Further calculations showed that a frac fluid temperature of 70º F. or higher would provide an acceptable contraction of the string. Unfortunately, the Vestfonn was not capable of heating the frac fluid to this temperature.

After considering alternatives, BJ Services decided the safest and most efficient plan was to modify the vessel to heat the water directly in the storage tanks, and it completed the modifications in 1 week.

The primary treatment could require more than 100,000 gal of fluid and the injections necessary before this, such as the breakdown, step-rate test, and minifrac, would require additional fluid. Due to the time required to heat these large volumes, a number of water tanks would have to be individually heated. BJ decided to install heaters in six of the vessel water tanks (140,000 gal) based on the design criteria of heating the water from 50-70º F. in a 24-hr period.

The Vestfonn is equipped with two process generators to provide power to run the hydraulic power packs and various electric motors, but it would not have sufficient capacity to power the additional water tank heaters. Two additional temporary generators were therefore installed on the cargo deck of the Vestfonn and cabled to the heater in each water tank.

An operator manages the hydraulic fracture process from the control room on BJ Services Co.'s MV Vestfonn, the offshore stimulation vessel used for Wintershall's Well L8-A2 (Fig 1; photo from BJ Services Co.).
Click here to enlarge image

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The two-stage configuration of the elements reduced the start load on the generators and a special control panel was made for each element. The control panels were connected to the elements, and wired back with 31 and 40 mm diameter cables to the generators approximately 50 m from the elements.

Downhole

The reservoir temperature was 284º F. To meet the required stability criteria, Spectra Frac G5000 was chosen as the cross-linked fluid. Fann 50 rheometer testing to assess temperature stability at 100 sec-1 (R1 rotor, B5 combination) gave a 2-hr stability greater than 400 cp at 284º F. 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 (90º F.) still resulted in a fast lip of less than 1 min.

Wintershall was concerned about excessive viscosity and, therefore, a limited pump rate led to testing 40 lb and 45 lb Spectra Frac. Each of these fluids still gave 1-1/2 hr stability on the Fann 50 at 284º F., 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 before the main treatment led to additional 25% caustic soda being added to the cross-linker (XLW-56/C10) to delay the cross-link to 4-5 min with 85º F. 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 before the operation.

The operation proved to be a success in a number of ways. First, the heaters worked as planned to raise 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 fortunate because there was insufficient time before sailing to test the heating system.

In addition, higher than anticipated temperatures (about 80º F.) contributed to faster than expected cross-link times, which resulted in higher friction pressures. Engineers solved the friction issue by delaying the cross-link and using a lower polymer loading. The main fracture treatment was successfully pumped at 25 bbl/min 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, BJ Services. "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."

MV Vestfonn

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 to more than 140 wells in the UK, Danish, Norwegian, and Dutch sectors of the North Sea during its 20-year history—more than 900 fracture operations.

The MV Vestfonn is a dynamically positioned offshore stimulation vessel (Fig. 2; photo from BJ Services Co.).
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As operators have progressed to highly deviated, multi-zone and horizontal well configurations, BJ has upgraded vessel capabilities accordingly.