RSS saves time, money in deepwater GoM

The operator had pipe-conveyed logging tools on standby as a contingency. Hole cleaning also presented a challenge because the well trajectory fell in the 45-55° zone known for problems with poor cuttings transport and potential pack-offs.

The well was constrained by the narrow pore pressure/fracture gradient margins typically associated with deepwater drilling. Several offset wells in shallower water depths with greater margins had experienced losses. Therefore, managing equivalent circulating densities (ECD) would be critical to avoiding non-productive time.

Working closely with the operator, the drilling fluids service company used highly accurate hydraulics modeling software to evaluate the proposed well plan. Changes in drilling parameters could significantly affect cuttings loading and ECDs. The software provided infinite "what-if" scenarios based on drilling fluid properties, temperature and compressibility, pump output, hole and drillstring geometries, and estimated penetration rates.

Fast "Gunbarrel" wellbore

Fluid properties used for hydraulics modeling were based on the expected performance of the clay-free SBF system chosen to drill both the 17 1/2-in. and 12 1/4-in. intervals. The system has a track record for reducing and often eliminating whole mud losses while drilling, running casing, and cementing on high-angle wells. The absence of losses results from the system's unique fragile gel structure, which helps minimize circulation initiation pressure spikes and surge pressures while tripping and running casing.

Originally designed for deepwater applications, the clay-free system can provide flat rheology at both mudline and downhole temperatures, without sacrificing cuttings carrying capacity. The ester content in the blended ester/internal olefin base fluid provides exceptional lubricity, which improves weight transfer and penetration rates.

The well plan called for the entire curve to be built in the 12 1/4-in. interval. The operator specified an RSS to achieve the target angle of 56°. The RSS can be continuously rotated, improving cuttings agitation and removal, as well as providing effective weight transfer. The system selected for this well used point-the-bit rather than push-the-bit technology.

The point-the-bit design maintains direction by flexing a driveshaft between two bearings, causing the end of the shaft across the lower bearing to deflect off the centerline of the tool. This design does not require external pads or an aggressive cutting structure on the sides of the bit. Side-cutting bits have shown a tendency to produce spiraling that detracts from borehole quality.

Bit selection played a key role in the operation's success. The operator chose a six-bladed, extended-gauge PDC bit to help improve penetration rates, achieve a "gunbarrel" wellbore, and dramatically reduce vibration. The extended gauge allows the bit to remain centered in hole, eliminating non-constructive bit motion.

Because the rotary steerable system is based on a point-the-bit concept, it allows the use of extended-gauge bits, which are recognized for producing outstanding hole quality. The bit is held in the center of the hole and rotates smoothly so energy is focused at the face of the bit instead of being consumed by bit whirl. Because impact damage is dramatically reduced, the bit can produce high doglegs while achieving excellent hole quality and rate of penetration. An at-bit inclination sensor 3 ft from the end of the RSS tool enables the operator to react quickly to changes in wellbore trajectory.

Ten days under plan

The 26-in., 17 1/2-in., and 12 1/4-in. intervals each required only one bit run. Zero whole mud losses occurred while drilling the 17 1/2-in. and 12 1/4-in. intervals with the clay-free SBF. Continuous mud weight monitoring in and out every 15 min. showed that no barite sag occurred while drilling the high-angle 12 1/4-in. interval with the SBF system. The mud weight was relatively light at 10.6 ppg, making cuttings-carrying capacity a critical issue.

To drill the 12 1/4-in. hole size at an optimal ROP while building angle, superior hole cleaning was a must. Historically, the best way to ensure proper hole-cleaning is to run the mud system with elevated rheology, particularly in the low shear rate range. However, elevated rheological properties have also created problems with excessively elevated ECDs in conventional SBF systems.

With the clay-free SBF, ECD increases can be minimized, even with relatively high yield points. Monofilament fiber sweeps were pumped through the riser every 500 ft. Below 11,600 ft, weighted sweeps (1.5 ppg over the active mud weight) were pumped downhole. These sweeps were built using a proprietary coarse ground barite that was screened out at the shakers, allowing the mud weight to remain steady throughout the interval and eliminating the need for costly dilution. No hole cleaning issues occurred.

PWD logs show a minimal pressure spike with clay-free SBF.

The 12 1/4-in. interval, where all directional work took place, was drilled at a controlled rate of 164 ft/hr while the LWD tools captured the high quality ALD data needed for understanding the regional structure. No anomalous responses were evident. Due to its ability to group individual density measurements according to which direction the sensors are facing at the time of acquisition, the ALD sensor can produce a 360° image of borehole density. This process is effective only while the drillstring is rotated. Conventional directional drilling techniques produce gaps in the image during periods of nonrotation for oriented drilling. The RSS, on the other hand, enables a continuous imaging log to be acquired by allowing string rotation 100% of the time.

This image reveals significant information beyond traditional density measurements. Structural dip can be calculated very precisely unless the borehole geometry affects the data. Borehole spiraling can cause considerable "noise" in both density and neutron data because all nuclear sensors are limited in their depth of investigation and are particularly sensitive to borehole effects. A spiraled borehole can totally obscure critical data and prevent any interpretation of the dip information. Extremely high quality borehole results in outstanding log quality.

The main purpose of this well was to gain a better understanding of the extent of the reservoir, so the operator also planned a comprehensive wireline suite. In most conventionally drilled wells, a final inclination of 56° introduces considerable risk that wireline tools fail to reach TD and require the use of pipe-conveyed logging services. This can add significantly to the overall wireline expense and rig cost.

On Perseus, all three wireline logging runs, including a pressure and fluid sample run, went straight to bottom. Drag values while running the wireline logs were noticeably low. The line tension was a scant 300 lb over the weight of the tools.

ChevronTexaco completed all logging operations as planned within a single day, without a single trip in the hole to condition the well. The operator representative on the rig called this operation, "The smoothest well I have drilled in five years with any rotary steerable."

Authors

Kevin Lambert graduated from Marietta College with a BS in petroleum engineering. He is a drilling engineer in the GoM deepwater business unit at ChevronTexaco.

Blaine Comeaux is the global product champion for the Geo-Pilot RSS. He has been with Halliburton Sperry-Sun for 20 years. Comeaux has a BS in petroleum engineering from Louisiana State University.

Scott Costner graduated from the University of Southwestern Louisiana. In 2002, he became a technical representative and account manager for Baroid in New Orleans.

Paul Felker joined Baroid in 1975 after attending Louisiana State University. He has extensive deepwater experience and participated in fluid system design and development at Baroid . He has served as a technical representative since 1997.