Hydrate Control During Deepwater Drilling: Overview and New Drilling-Fluids Formulations
- Hege Ebeltoft (Statoil) | Yousif Majeed (IITRI Westport Technology Center Intl.) | Eirik Sœrgärd (Norsk Hydro ASA)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- March 2001
- Document Type
- Journal Paper
- 19 - 26
- 2001. Society of Petroleum Engineers
- 2.1.7 Deepwater Completions Design, 1.6 Drilling Operations, 1.7 Pressure Management, 1.6.1 Drilling Operation Management, 4.1.5 Processing Equipment, 2.5.2 Fracturing Materials (Fluids, Proppant), 1.11 Drilling Fluids and Materials, 3.4.1 Inhibition and Remediation of Hydrates, Scale, Paraffin / Wax and Asphaltene, 4.6 Natural Gas, 2.4.3 Sand/Solids Control, 4.3.1 Hydrates, 1.8 Formation Damage, 5.9.1 Gas Hydrates, 4.1.2 Separation and Treating, 1.7.5 Well Control, 1.11.2 Drilling Fluid Selection and Formulation (Chemistry, Properties)
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Gas-hydrate formation during deepwater offshore drilling is a well-recognized operational hazard. Plugging the blowout preventer (BOP) stack, choke, and kill lines with hydrates can cause a serious well-control problem. We conducted an up-to-date review of the drilling practices and mud formulations applied in deepwater drilling as related to gas-hydrate control and mitigation. The review indicated that salt/polymer mud systems are the most commonly used mud formulations in the Gulf of Mexico, North Sea, and offshore Brazil. Successful drilling with these systems has been achieved to water depths of up to 7,500 ft.
In the second part of this work, we measured the hydrate-phase equilibrium of 25 drilling-fluid formulations. Testing also included two new spotting-fluid formulations. Testing results indicated that, on weight basis, NaCl is the best thermodynamic inhibitor among the salts tested in this work, which are NaBr, Na-formate, KCl, and CaCl2. Although the high solubility of the Na-formate makes it possible to increase hydrate suppression beyond that of NaCl, the former is less effective on weight basis than NaCl. The glycols are considerably less effective inhibitors, on weight basis, than the salts. However, a greater degree of suppression can be achieved by using mixtures of salts and glycols. Among the tested glycols, ethylene glycol showed the best performance compared to Aqua-Col™S, Geo-Meo™ D207, and HP-100N™.
Gas-hydrate formation during deepwater offshore drilling and production is a well-recognized operational hazard. In water depths greater than 1,000 ft, seabed conditions of pressure and temperature become conducive to gas-hydrate formation. In a well-control situation, although kick fluid leaves the formation at a high temperature, with an extended shut-in period it can cool to seabed temperature. With high enough hydrostatic pressure at the mudline, hydrates could form in the BOP stack, choke, and kill lines, as observed in field operations.1
Record water depths are being set continuously by operators in search of promising reserves. The first deepwater well to be drilled in Norwegian deepwater licenses started in the summer of 1997. The extremely low mudline temperature of 28.4 to 30.2°F in this area brings the challenge of designing suitable drilling fluids that prevent hydrate formation and meet other drilling requirements. Current practice in deepwater drilling is to suppress hydrate-formation temperature by using highly saline drilling fluids formulated from NaCl or other salts. This solution is applicable for the Gulf of Mexico, but insufficient for conditions to be encountered in Norwegian deep waters. At extreme water depths or extremely low-mudline temperatures, thermodynamic inhibition alone may not be sufficient to prevent hydrate formation. Using kinetic inhibitors or crystal modifiers, in conjunction with thermodynamic inhibitors, may pave the way for successful operations in such an environment. The definition of kinetic inhibitors (to distinguish them from the classical thermodynamic inhibitors such as polar compounds and electrolytes) comes from the effect of chemicals on the nucleation and growth of natural gas hydrates which are time-dependent and stochastic processes.
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