Synthetics reduce trouble time in ultra-deepwater borehole

The Discoverer 534 successfully used synthetics on an ultra-deepwater well in the US Gulf of Mexico.

Radically improved operating efficiencies, in tandem with re-engineered mud systems, reduced Amoco Production Company's drilling costs $4.6 million on an ultra-deepwater Gulf of Mexico well.

Overall, between 1996 and year-end 1997, Amoco's drilling performance, as measured by days versus depth profiles, improved by more than 46% for wells drilled in water depths 5,000 ft or greater (Figure 1 [25,461 bytes]). A principal reason for the enhanced drilling proficiency was a remarkable 69% decline in so-called "trouble time," defined as downhole problems that had to be eliminated before the operator could continue to make operational progress (Figure 2 [21,301 bytes]).

In deepwater exploration, where daily rig operating costs run upwards of US$200,000, unnecessary trips and other measures to deal with "trouble" materially increase well costs.

Amoco's latest well, drilled in 6,403 ft of water in Mississippi Canyon, further redefined the already aggressive benchmark established by the operator for deepwater projects. The rig was moved, the well drilled, logged, and abandoned in 57 days (Fig. 3 [31,714 bytes]). The well recorded a spud-to-total depth time of 34 days to 11,347 ft below mud line (BML) measured depth.

The nearest offset in water depths greater than 5,000 ft had recorded spud-to-TD time of 57 days to 10,000 ft BML (Figure 1). The 23-day improvement, which on the basis of US$200,000/day rig operating costs, translates to a savings of $4.6 million on this well. As measured by the significant improvement in days versus depth below the mudline in water depths greater than 5,000 ft, the drilling of this Mississippi Canyon well from spud to TD was rated "best in class. "

Historical problems

Drilling problems common to the deepwater environment above and below the mud line have been well documented in the literature. Downhole, the typical formations are young and fragile with a low pore pressure to fracture gradient differential. Further, throughout the Gulf of Mexico, the presence of reactive, gumbo clays in the upper and intermediate sections give rise to the majority of the difficulties restricting operators' performance.

While gumbo clays predominate in those intervals in the shallow Gulf, their costly after-effects are compounded greatly in Mississippi Canyon, Viosca Knoll, Green Canyon, DeSoto Canyon, and other deepwater blocks. Bit and bottom hole assembly (BHA) balling and gumbo accumulations in the riser and in the wellhead area have severely limited operators' attempts to optimize effective drilling rates.

Stuck pipe, lost circulation, inability to drill and underream simultaneously, difficulties in cleaning the hole, poor core recovery, and out-of-gauge wellbores were but some of the difficulties that have escalated drilling costs in the deep and ultra-deepwater Gulf of Mexico.

Downhole aside, the water depth itself presents a dilemma, particularly for the drilling fluids engineer. Constant water temperature of 38°F at the mud line and pressures in the 3,000-psi range thicken standard oil-base muds, rendering them sluggish. Conversely, standard water-base muds cannot repress the potentially dangerous build-up of shallow gas hydrates that occur all too frequently at such low water temperatures.

New generation synthetic-base muds appear to hold the best of both worlds, but in permeable zones where lost circulation is a real possibility, accurate determinations of downhole conditions are critical. Proper steps can be taken to understand and control equivalent circulating densities (ECD). Otherwise, the risk of losing expensive fluids to the formation could put the economic feasibility of synthetics into doubt.

That dilemma apparently was laid to rest in another ultra-deepwater well drilled successfully with the latest generation synthetic fluid. Prior to spudding its DeSoto Canyon well in 6,740 ft of water, Amoco used the results acquired through a proprietary software package to calculate ECD, static fluid density, frictional pressure losses and pump pressures.

Then, while drilling the well, the operator ran a pressure-while-drilling (PWD) tool, which verified in real-time that the ECD estimates were within 0.07 ppg of actual measurements. The DeSoto Canyon well was drilled with only 11.6% trouble time, which remains Amoco's lowest level to date.

Even through the synthetic-base drilling fluid contributed heavily to the DeSoto Canyon well being drilled five days ahead of schedule, problems were still recorded in the upper hole intervals drilled with water-base mud. Prior to spudding its most recent well in Mississippi Canyon, the operator made the elimination of gumbo problems a priority.

Mississippi Canyon well

The upper intervals were drilled uneventfully to the 36-, 26- and 20-in. casing intervals, using seawater with viscous gel sweeps to clean the hole. Prior to drilling out the 20-in. casing, the seawater was displaced with a reformulated calcium chloride (CaCl₂)/polymer mud system. The then-experimental mud was designed specifically to minimize gumbo-related problems, and in so doing, reduce the associated "trouble time" required to deal with the assorted difficulties encountered when drilling these highly reactive clays.

While drilling the gumbo-laden interval, the CaCl₂/polymer mud system exhibited superior solids encapsulation and wellbore inhibition. These properties eliminated the typical gumbo-related problems that commonly occurred on offset wells drilled with standard sodium chloride (NaCl)/PHPA polymer muds. Drilling of the interval proceeded with no indication of bit/BHA balling or gumbo accumulation and build-up in the riser and on the wellhead, respectively.

Furthermore, unlike in previous wells, the wear bushing was retrieved without doing extensive washing prior to pulling out of the hole or making an additional trip to wash gumbo from the wellhead area.

The only notable problem with the previously untried CaCl₂/polymer system was shaker-screen blinding, resulting in excessive mud losses over the shakers. The blinding problem, which was a direct result of the strong elastic properties of the polymer, is under investigation in a series of tests to determine if the formulation can be modified slightly to permit flow through the screens without sacrificing the encapsulating or inhibiting properties of the systems. Early results of the studies have been encouraging.

The last two intervals - the 14 3/4-in. by 17 1/2-in. and the 12 1/4-in. hole sections - were drilled with the latest-generation synthetic-base mud system. The 14 3/4-in. by 17 1/2-in. interval was drilled and then under-reamed in two passes. The 14 3/4-in. assembly included a steerable drilling assembly to reach an extended casing point with a maximum hole inclination of 38°. The hole was then under-reamed to 17 1/2 in. without problems.

Although directional, the interval was drilled with a perfectly in-gauge wellbore, resulting in a few tight spots that were reamed out easily. Further, no mud was lost while drilling the section. However, some losses occurred while cementing the 13 5/8-in. casing, where close tolerances between the casing and wellbore allowed for significant increases in ECD, which equaled the formation fracture gradient.

Nonetheless, the 13 5/8-in. casing was successfully run and cemented, after which the cement was drilled out and the shoe tested. Upon drill-out of the 13 5/8-in. casing, a leak-off test was performed with enough margin to eliminate the need for a contingency drilling liner.

The 12 1/4-in. hole interval (4,809 ft) was drilled at an average rate of penetration (ROP) of 90 ft/hr, dropping and building angle with two rotary assemblies to a maximum inclination of 63° at TD. A 90-ft, 4-in. diameter conventional core was cut in one hour with 99% recovery. Furthermore, the well was logged for 112.5 consecutive hours with no problems (Fig. 4 [25,100 bytes]).

Performance review

In addition to the efficiency achieved with the reformulated CaCl₂/polymer and latest-generation synthetic-mud systems, a number of inter-related factors, some initiated well before the spudding of the latest well in Mississippi Canyon, contributed to the "Best of Class" performance on this project.

Over the years, Amoco heightened its emphasis on training and improving technical communication between all members of the deepwater drilling teams, including operator, drilling contractor and service company personnel.

Several other key factors have contributed to this well and the 1997 drilling success of Amoco's deepwater drilling efforts. To insure application of deepwater drilling "best practices," the rig team implemented the use of "efficiency worksheets." The efficiency worksheets are used to document and define actual operational time versus predicted time with an emphasis on unproductive operations.

The worksheets are then referred to in subsequent planning and implementation efforts to continue making improvements to drilling efficiency. Rig-site supervisors' project-management efforts continue to contribute to effective preplanning. Through the efficient use of all available planning tools, each phase of rig operations proceeded relatively trouble-free.

Use of a continuous drilling program with a "world class" contractor continued to pay dividends with near flawless rig operations. Transocean's dynamically positioned drillship, Discoverer 534, performed admirably in all phases of operations. Rig repair time was limited with a comprehensive preventative-maintenance program.

Service company tools and personnel performed at peak levels during a major upswing of activity for the Gulf of Mexico. Rig-site management of continued shortages of equipment and personnel, resulted in very few operational delays or problems.

It is also important to note that the Mississippi Canyon well was drilled during a relatively light hurricane season in the Gulf, during which only 34.5 hours were spent waiting on weather due to Hurricane Danny. Historically, operations are suspended an average of seven days per hurricane event with at least one event per well.

Acknowledgements

Amoco would like to acknowledge the following contractors, services, and personnel for contributions to this paper and Amoco's 1997 drilling successes: John Pantaleo, Gene Riddle, Jimmy Reed, Randy Ditmore, Jim Spizale, Tommy Patterson, Martin Breazeale, Paul Richard, and Jack Droste of Amoco EPTG Drilling; M-I L.L.C., Anadrill, Halliburton, Schlumberger, Smith International, Franks Casing, Inteq Mud Logging & Conventional Coring and Transocean Offshore.

Authors

Toby Woodis a drilling engineer for Amoco EPTG, based in New Orleans. He joined Amoco in 1992 and is currently assigned to WEBG deepwater exploration drilling projects utilizing Transocean's DP drillship Discoverer 534. He holds a BS in natural gas engineering from Texas A&I University.

Brad Billon is Project Engineer for M-I L.L.C., based in New Orleans, where he oversees and coordinates M-I's business with Amoco's Central and Offshore Business units. He has worked for M-I for 18 years and has extensive drilling fluids experience throughout the Louisiana Gulf coast area, both offshore and onshore.

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