New generation sand control system optimizes Åsgard production

Historically, advances in drilling methods have spurred parallel innovations in completion technologies. For example, recent years have seen a proliferation of horizontal wells, subsea wells, and often, horizontal subsea wells. Many of these require sand control. - Enhanced engineering design maintains much needed hydrostatic pressure to the wellbore below the sealing packer, leaving the filter cake in place.

To meet tougher sand control challenges in longer horizontal intervals and from deepwater floating rigs, a new-generation, hydrostatically-balanced gravel pack system has been developed by Baker Oil Tools, improving completion efficiency in Statoil's Midgard field.

The Midgard gas field, located in approximately 300 meter water depths on Norway's Haltenbanken terrace, is one of three fields that comprise Åsgard, the North Sea's largest and most complex subsea development to date. Estimated reserves for the Åsgard development are 830 million bbl of oil and condensate and 7.5 tcf of natural gas. Gas from Midgard will be produced from as many as 12 high-rate subsea wells.

Specific challenges

An integrated team of Statoil and service company drilling and completion engineers determined that open-hole gravel packing would be the most efficient method of addressing the anticipated sand control problems in the Midgard wells. - The Midgard gas field is one of the three fields in the Åsgard development project, consisting of 12 high-rate subsea wells.

Operating gravel pack equipment in 300-meter water depths on the Haltenbanken terrace presented a significant challenge.

The challenge was heightened by a drilling schedule requiring gravel packing and completion of the first four wells take place in January and February 2000 - typically the stormiest months in Northern Europe. Additionally, the high permeability and production potential of the Midgard reservoir sands required full fluid control to ensure both a successful gravel pack and a secure rig operation.

A rigorous evaluation by Statoil of rig and marine logistics led to the decision to use the two drilling units dedicated to Åsgard to drill and complete the Midgard wells in drilling, gravel packing, and upper completion "batches" of four wells per batch. This approach required that all aspects of gravel pack equipment selection and procedures be thoroughly planned, qualified, and reviewed prior to start of operations. Discovery of a systematic error during gravel packing of the first well would have cascaded through all wells in the batch or could have led to costly rescheduling of the rigs.

The reservoir interval at Midgard is drilled with a specially formulated, non-damaging, calcium carbonate-based drill-in fluid supplied by Baker Hughes Inteq. The drill-in fluid forms a stable, thin filter cake against the borehole wall. Prior to installation of the gravel pack screen assemblies, the drill-in fluid is circulated out of the open hole and replaced with clear brine. The clear brine exerts a hydrostatic over-balance of approximately 435 psi on the filter cake. Maintaining this overbalance is essential for full fluid control of the well.

Conventional gravel packing

Traditional gravel packing system design has been based on wisdom gained from completing relatively short, low-angle intervals drilled from land or from fixed offshore platforms. Meeting the challenges of gravel packing longer intervals and in deepwater, from floating rigs, has caused completion companies to re-think the functionality of traditional tools.

Typically, gravel pack screen assemblies are installed in the open-hole interval with a running tool, often called a crossover tool. Manipulation of this crossover tool facilitates fluid circulation, gravel pack packer setting, testing, gravel placement, and pumping excess gravel back to surface.

Crossover tool manipulation has also been thought to affect open-hole filter cakes. An excellent opportunity to study these effects occurred when an operator acquired downhole gauge data during gravel packing of a deepwater well in the Gulf of Mexico.

Deepwater experience

Prior to this well, Baker Oil Tools suspected that the primary causes of loss of filter cake in open-hole gravel packs were premature shear-out of the ball seat when setting the packer, and/or surge effects when moving the crossover tool. Results of the Gulf of Mexico deepwater gravel packing study verified that removal of the hydrostatic head from the formation during packer setting operations was equally, if not more, problematic.

Baker Oil Tools' previous crossover tool required dropping a setting ball to temporarily plug the workstring and allow pressure application to the packer setting tool. The crossover tool is sealed in the packer bore. Therefore, as soon as the packing element seals, the open hole is isolated from hydrostatic pressure. With only minimal static fluid loss, the pressure below the packer quickly equalizes with reservoir pressure, causing the filter cake to loosen from the borehole wall.

Upon realizing that removal of the hydrostatic head during packer setting was one of the largest single sources of problems during open-hole gravel packing, Baker engineers initiated efforts to develop a fully hydrostatically balanced crossover tool that could eliminate this problem.

New packing system

Baker Oil Tools developed a new gravel packing system, the Cake-Saver (or CS)-300, based on a patented, fully hydrostatically balanced crossover tool. Additional components of the system include a gravel pack packer assembly, a gravel pack sliding sleeve, a multi-acting locating device with indicating collets to ensure positive down-weight location, and an anti-swab valve.

The cake-saver system maintains hydrostatic pressure on the filter cake by creating a flow path from the annulus above the packer via a special by-pass area. A soft shear, solid ball seat eliminates the "hammer" effect of prematurely blowing a ball seat. Surge effects are eliminated by a mechanically actuated, reversing flapper valve. Since the fluid column in the drill pipe/casing annulus above the packer is dedicated to maintaining a hydrostatic overbalance on the filter cake, release of the crossover tool from the gravel pack packer is facilitated by a tubing pressure-actuated hydraulic release mechanism.

Every operation of the crossover tool is carried out with the tool located with down-weight on an indicating collet. As a result, gravel packing can be carried out independent of wave-induced heave of the drilling rig and thermal - or hydraulically induced changes in drill pipe strain.

The new system has advanced horizontal open-hole gravel packing to intervals of record length (beyond 4,000 ft) in the British sector of the North Sea. An integrated team of Statoil and service company engineers, impressed with these results, began studying the feasibility of the CS-300 system for Midgard during early planning stages.

Midgard results

Prior to finalizing the gravel packing program for the first four-well batch at Midgard, Statoil initiated HAZOP and risk analysis processes. As part of the process, Baker Oil Tools organized a series of Midgard-specific gravel pack training seminars. Engineers and key offshore personnel from Statoil, Baker, and the rig contractor participated in classroom lectures and question-and-answer sessions and also in inspections and demonstrations of the crossover tool and screen assemblies.

To date, six Midgard wells have been successfully gravel packed using the new-generation system and are now ready for production. Operations were carried out without interruption, despite the fact that the first four wells were gravel packed during area's stormiest season. During the same period, other rig operations in the Haltenbanken area suffered many days of downtime due to poor weather.

All six Midgard wells were gravel packed with minimal lost time due to operational problems. Each operation was finished in less than the planned time. One well required a later re-entry to install a "piggy-back" gravel pack packer due to a problem not related to the gravel pack or crossover tool design.

For one of the wells, the gravel pack operation was timed to under 27 hours from the moment the first screen assemblies were prepared on the rig floor to the moment the crossover tool was laid down. This may constitute the fastest gravel pack operation from a floating drilling rig in the Norwegian sector to date.

Full fluid control was maintained in each well, with zero fluid loss to the formation recorded following packer setting, during gravel packing or tool manipulation. Clean-up and initial testing of the wells indicate that productivity for each is equal to or greater than what was expected.

Statoil has benchmarked all of its well completion activities from floating rigs and platforms during the first quarter of 2000. The top place in "the least rig time" category is occupied by a Midgard well, with four of the six wells completed to date ranking in the top 10 fastest completion operations for Statoil during Q1-2000.