Gas Hydrates and Offshore Drilling: Predicting the Hydrate Free Zone

A well-recognized hazard in offshore drilling is the formation of gas hydrates in the event of a hydrocarbon flow into the well bore from the reservoir (e.g., a kick). This could potentially block the BOP stack, kill lines and chokes, obstruct the movement of the drill string, and cause serious operational and safety concerns. Currently, salts are added to the drilling fluids to inhibit hydrate formation in offshore and arctic drilling. For deep water drilling, even saturated saline solutions may not provide the required protection, unless combined with chemical inhibitors. The reported experimental data on gas hydrate formation in drilling fluids are very limited and in some cases inconsistent. The available predictive methods are generally empirical correlations based on limited data and with limited application. In this presentation, a thermodynamic model capable of predicting the hydrate free zone in the presence of salts (NaCl, KCl, CaCl₂, NaBr, Na-formate, etc.) and chemical inhibitors (methanol, ethanol, ethylene glycol, glycerol, etc.) is presented. The model developed has been employed to predict the hydrate free zone in drilling fluids designed for offshore and deep water applications. The predictions are compared with experimental data and an empirical correlation, demonstrating the reliability of the thermodynamic model.