Petrobras used an innovative horizontal sand control completion system from Schlumberger to execute a successful openhole gravel pack (OHGP) using the alpha/beta (A/B) placement technique from a drillship on a well in 3,700 ft (1,130 m) of water.

The well, in the Campos Basin, was drilled to 11,650 ft (3,551 m) measured depth by the Leo Segerius drillship, with 1,998 ft (609 m) of 8½-in horizontal openhole section. The completion, likewise performed from this drillship, employed the Schlumberger QUANTUM OH System to convey and set the QUANTUM Packer and successfully gravel pack the open section. This was the first Schlumberger horizontal openhole gravel-pack treatment performed from a drillship using the alpha/beta wave technique.

The QUANTUM OH System tool operated normally, experiencing no difficulties due to heave, and enabled completion of the treatment with 100% pack efficiency. The Full Bore Set Down module of the QUANTUM OH service tool was successful in minimizing any tool movement during these operations.
Project execution demonstrated that performing safe, successful gravel-pack treatments on long horizontal openhole completions from a drillship operating at these depths is possible - notwithstanding some increased difficulties - provided the proper service tool capabilities and the appropriate job planning.

Openhole gravel-pack treatment issues

Most horizontal wells requiring sand control are openhole completions for both productivity and economic reasons. Although stand-alone screen completions used to predominate among these wells, openhole gravel packing has gained wider acceptance in various parts of the world - reflecting uncertainties over maintaining long-term sand control integrity when screens alone are used and the unwillingness to sacrifice productivity by casing the producing section. In deepwater, where completion reliability is of utmost importance due to the high cost of intervention, openhole gravel packing is increasingly becoming standard practice. There are two techniques for gravel packing openhole horizontal wells - water packing and viscous-fluid packing using Alternate Path technology. These techniques have completely different packing mechanisms, each having its own advantages and limitations.

Gravel packing using the A/B wave technique, otherwise referred to as water packing, involves gravel packing with low-viscosity fluids - usually completion brine. The low side of an annulus is packed first until gravel reaches the toe of the well, or until gravel packs off and forms a bridge due to high fluid leak off or geometric variation in the open hole. Gravitational forces dominate this "alpha" wave, so gravel settles until reaching an equilibrium height. If fluid flow remains above the crucial velocity for particle transport, gravel will move down a horizontal section toward the toe. After the alpha wave stops, a second, "beta" wave packs the annulus topside back towards the heel of the horizontal section. The beta wave requires enough fluid velocity to maintain turbulent flow and move gravel along the top of the wellbore annulus.

Viscous fluid packing uses viscous fluids and Alternate Path technology, in which the pumped packing slurry can be diverted to flow through shunt tubes on the outside of the screen assembly, should a bridge form due to an area of high leakoff. The shunts provide an alternative pathway for the slurry. Horizontal Alternate Path screens feature shunt transport and packing tubes attached eccentrically to the screen joint. The transport tubes feed the packing tubes with sand slurry and the slurry exits from nozzles on the packing tubes. These screens also include an outer shroud that protects the gravel-pack screen and shunt tubes during running in hole and provides centralization of the assembly in the openhole section.
In conditions where the risks of fluid leakoff and pack integrity problems due to sandface bridging are not high, a gravel pack using the alpha/beta placement technique is often the more economical sand control method. This is mainly due to its use of low-viscosity carrier fluids, such as brines, and a regular screen assembly without shunts. With no need for shunts, the effective screen diameter - and wellbore producing section - is larger.

An effective horizontal openhole gravel pack using the A/B placement technique is highly dependent on the selection of drilling fluid. The fluid needs to be non-invasive and capable of providing a thin effective filter-cake layer that will resist erosion and prevent premature losses during high-rate water packing. In addition, careful management of fluid filtration, rate control, lubricity, formation overbalance and especially surge and swab effects (sudden increases or decreases in pressure) is necessary.

Where treatments are less effective or fail, the general causes include excessive fluid leakoff; erratic gravel loadings, poor control of blending and pumping equipment, pump failure; formation fracturing and contamination of carrier fluids by shales. Proper treatment design must address all of these factors and minimize their risk levels.

The QUANTUM OH sand control system

Addressing multiple requirements for horizontal completions, the QUANTUM OH system provides:
• Hydrostatic communication to the openhole at all times;
• Anti-swab and surge features;
• Setdown capability during gravel-pack operations;
• Washdown capability during installation;
• Tool positioning systems to support deepwater applications; and
• Single-trip cleanup of gravel pack and filter cake.

The OH service tool performs a key function by preventing premature setting of the packer while running in hole using the non-pressure sensitive (NPS) module. It maintains hydrostatic pressure on the formation at all times by using a series of passive bypass ports, avoiding the need for a complex valve/channeling mechanism, and eliminating any swabbing effects due to tool movement. The MudSOLV module of the service tool converts back to washdown mode following the successful gravel pack.

The OH service tool can place the appropriate sand-control treatment and subsequent filter cake removal treatments when the job is done. Unlike conventional tools, it can do this in a single trip. The MudSOLV module allows circulation to the bottom of the sand-control assembly after the gravel slurry is pumped, increasing operational flexibility and saving rig time.

Running the A/B gravel pack completion in deepwater

Horizontal water-pack completion procedures began on March 5, 2004, with the arrival of the Schlumberger wellsite co-coordinator onboard the drillship.

The screens and bottomhole assembly running operations began on March 13. A total of 51 joints of premium screen and 24 acid diverter subs were run without incident. Once the sand control assembly was at depth, the QUANTUM GP packer was set by dropping a ball and applying pressure to the tubing. The NPS module was disabled during this process, enabling the packer to be set. The OH service tool was then released, the packer tested and the positions located and marked.

Circulation tests were then performed in the reverse and circulate positions to identify the friction pressures and return rates. The data gathered from these tests were then used to confirm the calibration of the SandCADE software, and the pumping program was redesigned, based on the results.
The water-pack treatment was carried out successfully with 42,892 lbs of sand displaced into the openhole section and screenout achieved. The excess slurry in the drill pipe was then reversed out rapidly and safely, and the well was observed for losses. No fluid loss was observed during this period, signifying the mud filter cake was still in place.

The QUANTUM OH tool was then converted to the washdown mode by dropping the MudSOLV ball and applying pressure to shift the tool into the MudSOLV position. A selective acid treatment was then performed to remove the mud filter cake from the openhole section. Acid diverter subs placed at regular intervals along the screened interval provided a sealing bore for the service tool, allowing the selective treatment of each openhole section with the acid filter-cake removal fluid. Post-acid fluid loss was recorded at 580 bbl/hr.

Following the selective acid treatment, the drillpipe, service tool and washpipe were pulled out of hole until the formation isolation valve (FIV) shifting shifted the valve closed and tested to 1,000 psi. The completion fluid was then replaced with packer fluid.

To summarize the key aspects of this completion:

• The screen and wash pipe running operations, gravel setting and water pack pumping proceeded successfully with no lost time;
• Bottom recorder data indicated no swabbing occurred, even with the reciprocation of the string required to set and test the QUANTUM GP packer. This proves the QUANTUM OH tool was successful in maintaining hydrostatic pressure on the formation at all times;
• The FIV acted as a barrier to control losses after acid operations; and
• Using the QUANTUM OH tool in the MudSOLV mode to change out the completion brine to packer fluid reduces the fluid swap out time from 5 hours to 1 hour .

Recommendations for future treatments

Several provisions of the job design merit emphasis as recommended practices for similar future jobs from drillships. These include:

• Perform torque and drag analysis to ensure application of sufficient weight on the service tool so that the ball valve will activate;

• Ensure proper working of the onboard compensator, as positioning the service tool in water depths such as these depends mainly on the compensator's ability to adjust for heave;
• Job design should call exclusively for drillpipe connections (IF, FH, VAM, EIS, etc.) rather than premium connections (Acme or tubing) above the service tool, in case of an unplanned event; and
• The packer pressure test module should incorporate the higher 40,000-lb shear ring.

Conclusion

The execution of an A/B horizontal gravel pack completion this deepwater well showed that with the right service tool capabilities and sufficient job planning, it is possible to manage and minimize the risks stemming from drillship heave on service tool positioning during gravel pack placement in deep water.

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