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Drilling operations in water depths of between 5,000 and 10,000 ft take place all over the world, and their success underscores the adaptability of oilfield technology and the industry’s capacity to overcome significant technical challenges.
Drilling fluid considerations
The unique conditions presented by deepwater drilling require certain drilling-fluid characteristics related to:
- Temperature variation
- Overpressured shallow water-bearing sands
- Narrow pore pressure/fracture gradient margins resulting from the extra fluid weight in the long drilling riser
- The potential for hydrates at the mudline
The seafloor temperature in deepwater locations is approximately 40°F, but it can approach 32°F. The temperature downhole can exceed 300°F. The drilling fluid should exhibit the appropriate rheological properties throughout this wide range. In the riser near the mudline, the fluid is apt to thicken excessively from exposure to the cold seafloor temperature. Downhole, the fluid might become too thin as it heats up, and problems with hole cleaning and barite sag might develop. SBFs that contain little or no commercial clay appear to remain the most stable under these conditions. These clay-free and low-clay systems rely on emulsion characteristics to achieve the desired rheological properties and provide sufficient barite suspension.
Shallow-water flow (SWF)
Seismic data can help operators to predict and evaluate the risk of encountering an SWF on a given well. These water-bearing sands typically are located in the first 2,000 ft below the mudline, and are often encountered while drilling in riserless mode. Stopping the flow under these circumstances is difficult. Pumping weighted mud that is cut with seawater on the fly, generally, is successful, but it requires pumping thousands of barrels of weighted WBF that returns to the seafloor, because the riser is not yet connected to the wellbore. If the SWF is not brought under control or cased off successfully, its continued flow can undermine the structural integrity of the well, and even affect neighboring wells.
Pore pressure/fracture gradient(PP/FG)
Because of the long riser that is required in deepwater operations, the hydrostatic pressure from the column of drilling fluid can approach or exceed the fracture gradient, especially when breaking circulation after a static period, tripping in the hole, or running casing. Significant loss of whole mud can occur, and might lead to well-control problems. Control of the ECD, as verified by pressure-while-drilling (PWD) data, is critical to maintaining wellbore stability. The drilling fluid that is selected should be evaluated for its demonstrated capacity to minimize or eliminate whole-mud losses. A suitable fluid will be characterized in part by a comparatively small pressure spike on PWD logs when circulation is resumed after a long static period.
When a water-based fluids(WBF) is used, the cold seafloor temperatures coupled with high pressures can cause the formation of hydrates, or “dirty ice.” Hydrates form from hydrogen bonding between water molecules and low-molecular-weight gas. The water actually forms a crystalline cage structure around the gas, and creates the risk of blocking the choke and kill lines at the blowout preventers. The four conditions required for hydrate formation are:
- The presence of gas
- The presence of water
- Low temperature
- High pressure
Shutting in a gas influx on a deepwater well that is drilled with WBF makes a likely scenario for hydrate formation.
Maintaining the appropriate salinity level in the WBF suppresses hydrate formation. For extreme situations, glycerine, polyglycerine, and polyglycol products might be needed to further suppress the hydrate-formation temperature.
- Burrows, K., Carbajal, D., Kirsner, J. et al. 2004. Benchmark Performance: Zero Barite Sag and Significantly Reduced Downhole Losses with the Industry's First Clay-Free Synthetic-Based Fluid. Presented at the IADC/SPE Drilling Conference, Dallas, Texas, 2-4 March 2004. SPE-87138-MS. http://dx.doi.org/10.2118/87138-MS.
- Eaton, L.F. 1999. Drilling Through Deepwater Shallow Water Flow Zones at Ursa. Presented at the SPE/IADC Drilling Conference, Amsterdam, Netherlands, 9-11 March. SPE-52780-MS. http://dx.doi.org/10.2118/52780-MS.
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