Hydrate control during deepwater drilling:: Overview and new drilling-fluids formulations

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(TM)S, Geo-Meo(TM) D207, and HP-100N(TM).