Optimizing production with hydraulic propped fracturing technology is the goal of North Sea workhorse.

With over 900 frac stimulation operations in its log book, the dynamically positioned offshore stimulation vessel MV Vestfonn is celebrating its 20-year anniversary this year. Designed to meet the technological and environmental challenges posed by the demanding conditions of the North Sea, the vessel's primary objective is to maximize fracture penetration and conductivity. Throughout its history, the Vestfonn has successfully delivered acid and propped fracturing treatments in the British, Danish, Norwegian and Dutch sectors of the North Sea.

From pipe carrier to state-of-the-art stimulation vessel

In response to offshore market demand for vessel-based stimulation services, BJ began the initial design and engineering work for the conversion of the workboat MV Vestfonn into a world-class North Sea stimulation vessel in 1983. Originally built in Norway as a workboat and pipe carrier, the vessel could also be used for diving and remotely operated vehicle (ROV) operations. Commencing in late 1984, the conversion took eight months, with final commissioning in Stavanger, Norway, in 1985. Vestfonn carried out its first pumping operations soon thereafter for Phillips Petroleum Company Norway. Since then, the vessel has delivered stimulation services on approximately 130 wells in the North Sea on behalf of Maersk, BP Norge AS, Amerada Hess AS, Phillips Petroleum, Shell, Statoil and Clyde Petroleum, among others.

Protecting the environment: Valhall Field, Norway

Currently, BJ is carrying out an extensive 2-year hydraulic fracturing stimulation campaign on behalf of BP Norge AS on as many as 28 new and old wells located in the Valhall Field development in the Norwegian sector of the North Sea. The company estimates it will treat 120 propped fractures and 30 acid fractures using about 30 million pounds of proppant delivered and pumped by the MV Vestfonn.

Operating within Norway's strict environmental regulations in the North Sea, the Vestfonn maintains stringent control on the use of all chemical products and additives. To achieve this, the company continually reviews its technologies with the goal towards developing more environmentally friendly products for everyday use. To further minimize the impact on the environment, BJ is working in close co-operation with BP Norge AS and its sub-contractors to use innovative proppant recycling technology.

Maximum fracture penetration achieved offshore Denmark for Amerada Hess

One of the highlights of the Vestfonn story in recent years is the stimulation program successfully completed on behalf of Amerada Hess A/S offshore Denmark. The vessel serviced three wells - SA-06C, SA-07 and SA-08 - in the South Arne Field located in the Danish sector of the North Sea. The three wells are horizontal completions, involving multi-zone hydraulic fracture treatments. While wells SA-06 and SA-08 were completed using acid fracturing technology, well SA-07 was completed using hydraulic propped fracturing technology.

Propped fracture treatments in the North Sea often require large volumes of proppant at high concentrations to provide the necessary conductivity compatible with high production rates. In the case of SA-07, the well was successfully completed by placing a total of 12.7 million pounds of proppant into 13 zones, a record for the amount of proppant used on a single well. In this well, premium borate-crosslinked guar fluid called SpectraFrac G was pumped. This versatile fracturing fluid helped to optimize fracture properties in the horizontal wells by maximizing proppant placement and minimizing fluid damage. To do this, a tapered polymer loading from 40 lb/Mgal down to 30 lb/Mgal was used while placing a large mesh 16/30 proppant at concentrations from 1 ppa to 12 ppa. Since then, the company has continued to refine these earlier treatments and further optimized its fluids and operations to the extent that SA-12 was one of the most successful wells treated.

Ongoing improvements keep pace with the times

To keep up with ever-changing technology, and market demands, the Vestfonn regularly undergoes major upgrades, improvements and modifications. Some of these include:

• Increasing the diameter of the acid tank manifold system to speed up delivery rates.
• Enlarging proppant tank volumes to 1.2 million-pound capacity.
• Increasing the hydraulic horsepower (HHP) by and fitting an additional two 2,000 HHP units, giving the Vestfonn a total of 13,400 HHP.
• Adding "Liquid Frac Concentrate" to allow "on-the-fly" mixing of gel systems, to reduce customer cost and further minimize the impact on the environment.
• Fitting new blender control systems to allow more accurate control of base fluids liquid/dry additives and proppants.
• Installing "Fracman" remote control system for the high-pressure pumps, allowing one operator to simultaneously control all eight HP pumps
• Replacing the complete high-pressure manifold. The original 3-in. manifold was replaced with larger diameter 4-in. discharge lines to allow for higher pump rates.
• Equipping the vessel with a fully integrated satellite communications system for worldwide communication and remote data transfer. The real-time treatment monitoring software was upgraded to take advantage of the latest IT developments and to implement real-time remote job monitoring at customer offices. This also allows personnel aboard the Vestfonn to access to all necessary company HSE and QA databases.
• Providing dedicated tanks and equipment fitted to allow the possibility of pumping reclaimed proppant to reduce environmental impact.
• Upgrading Vestfonn to DP-2 status for improved safety and redundancy.

Evolving Technologies

Stimulations of highly deviated, multi-zone, horizontal wells requiring millions of pounds of proppant pumped at rates up to 57 BPM (9 m3/Min), are now performed on a routine basis. In addition, the vessel often carries out large volume, high rate (60BPM) acid frac stimulation treatments (186 Mgal of acid storage). All treatments are controlled with the aid of BJ's JobMaster data acquisition and monitoring instrumentation. The monitoring software displays real-time data to remote locations, such as the platform or onshore company offices during a treatment, an added efficiency for any operation. The monitoring package is also equipped with fracture modeling software interfaces that provides a vital real-time analysis capability, critical to the decision-making process during the treatment.

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