The anatomy of a step-change: A new perforating technique improves productivity by factors of three to six in several North Sea horizontal completions.

Using a methodical, scientific approach Statoil has made significant productivity improvements in the Visund Field, offshore Norway. The result is an excellent study in evaluation, analysis, innovation and field implementation. Throughout the process, a spirit of collaboration between the operator and service provider ensured objectivity and maintained focus on the common goal of finding the best solution for the situation they were addressing.

Visund is a subsea field originally brought into production by Norsk Hydro. In 2003, the field was taken over by Statoil. The reservoir is complex, highly faulted and characterized by fluvial sandstones with permeabilities ranging from 300 md to 3,000 md. Reservoir pressure is about 6,550 psi. Wells are drilled with long snakelike horizontal sections and target multiple traps.

The tough drilling conditions made problems during completions. Long exposure to high hydrostatic drilling fluids had resulted in deeply invaded zones. In addition, weak sandstones called for sand control measures. A couple of attempts were made to control sanding with screens, but the invasion damage prevented the wells from reaching their productivity potential. In addition, the screened completions did not provide the required degree of zonal isolation. An alternative method, oriented perforating in the direction of maximum formation stress, was proposed as a sanding prevention solution. If perforations could reach beyond the damaged zone and be oriented parallel to the prevailing stress field the wells should produce to full potential sand free.

But the problem was far more complex than originally envisaged. When the previously completed screened wells were recompleted using cemented liners and oriented perforators, results were no better. The wells had been perforated using a heavy CaCl2 /CaBr2 kill fluid. Subsequent analysis showed the perforations were plugged with large chunks of zinc oxide, debris from the perforating charges forced into the tunnels by the high overbalance. In addition, the analysis revealed that many of the perforations were not oriented in the intended direction, an artifact of the technology available at the time.

Systematically, Statoil and their service provider Schlumberger analyzed each problem and possible solution. The following areas were identified for improvement:

• Perforation penetration. Even though it was not possible to accurately determine the depth of the invasion damage, laboratory tests led the team to conclude that it would require an effective penetration of 40cm to 50cm (15.75-in. to 19.7-in.) to reach the virgin formation.
• Perforation debris. Ordinarily, zinc based charges do not cause plugging because the zinc disintegrates into a fine powder. However, in the presence of completion brines, the zinc reacts to form chunks of zinc oxide.
• Perforation orientation. At Visund, the maximum stress is provided by the overburden and therefore is vertical. Accordingly, guns should be oriented to fire upwards. This has several beneficial effects, including gravity-assisted clean-up. In addition, it was determined that filtrate invasion damage is deepest on the low side of the hole. Rock strength analyses determined that perforation tunnels could maintain their integrity as long as they were within 25? of the stress field orientation, thus it was decided to phase the charges from +10? to - 10? of vertical to increase the distance between adjacent tunnels while maintaining the shots within the ±25? envelope.
• Orientation verification. Because subsequent production decisions depended upon knowing with certainty the actual orientation of the perforations, a device was needed to verify perforation direction immediately after firing.
• Overbalance. Although normally preferable, perforating underbalanced was not practical at Visund. On the other hand, it was well known the effect overbalance could have on perforation damage and skin. It was proposed to investigate the instantaneous local underbalance created as the guns flood after they are fired. If this could be maximized, it was theorized that the resulting surge could clean the perforation tunnels.

• Completion fluid. An effective completion fluid was required that would neither react adversely with perforator debris, nor with the formation or formation waters.

The challenges involved both technology and technique. Since most of the technology problems involved the perforating gun systems, Statoil worked with Schlumberger to create new design specifications. A joint task force was formed to address the design of the perforating string. The completion fluid requirements were met by using a potassium formate brine supplied by a specialty company. Details of the engineering considerations and rationale involved in developing the swiveled orientation system can be found in SPE 84910. Final qualification tests were conducted based on parameters set by Statoil. Each special feature was tested individually and fully assembled as a total system. Design requirements and results achieved are summarized in Table 1.

Key to the project's success lay in designing the gun string orientation system. The basic principle involved heavy-sided swiveling cylinders containing the shaped charges. But in order for the swivels to work reliably, the gun carriers' natural bend preferences had to be neutralized. The swivels themselves had to be capable of sustaining loads of up to 250,000 lb to support the completion string in vertical well sections. Additionally, they had to operate properly while under up to 50,000 lb of tension in the horizontal well sections. Alignment hardware was developed to overcome potential orientation error build-up when multiple swivels were used in multizone completions.

Field testing commenced before all problems were completely solved. For example, the zero bend preference carriers were not available until the final well. In the interim, the task force used custom strings, constructed of tubulars whose bend preferences were known. These could be oriented in 30? increments to effectively neutralize torsional stresses caused by bending. Also, by the last well, 30-ft carriers were available (instead of the 20-ft [6-m] guns used previously) that allowed increased length between swivels, improving orientation stability of the string.

Satisfied that the gun design and field test performance met all specifications, it was time for the team to look at results. Previously completed wells had productivity indexes in the 60-90 Sm3/d/Bar range.
However, wells perforated using the new oriented perforation string with dynamic underbalance achieved productivity indexes of 300-900 Sm3/d/Bar. Post completion clean-up analysis revealed that production rates could be increased up to 8,500 Sm3/d. Wells cleaned up over the entire production interval and minimal debris was recovered. Sanding was carefully monitored and remained below acceptable limits.

Results of the team developed oriented perforating system and associated techniques indicated intervals of more than 1.2 miles (2 km) can be perforated within the specified orientation envelope. The new charges are able to shoot beyond the damaged zone with little debris and rapid cleanup. Orientation accuracy can be positively verified within 1? and sufficient localized underbalance can be generated to clean out perforation tunnels even though the entire well is under static overbalance of about 35 Bar. Most importantly productivity index improved by factors of 3 to 6 in wells completed using the new system. Schlumberger has commercialized the orientation system under the name OrientXact and the dynamic underbalance guns under the name PURE, an acronym for Perforating for Ultimate Reservoir Exploitation.