Accurate Prediction of Gas Hydrate Suppression Capability of Water-Based Drilling Fluids in Deepwater Drilling
Authors: Hemphill T. 
Published: October 2008

As drilling proceeds into increasingly deeper offshore basins, the need to quickly determine the gas hydrate suppression capability of drilling fluids becomes more critical. Published papers have documented the occurrence of gas hydrates in the field and have described the negative impact their formation has had on well control situations. Because deepwater drilling applications typically occur under conditions quite favorable to gas hydrate formation, operators need to know whether their drilling fluid has sufficient gas hydrate suppression capability for their particular well conditions. Recently, a computer model has been constructed to determine suppression capability of water-based drilling fluids that contain a wide variety of salts and alcohols that are commonly used in drilling fluids as gas hydrate suppressants. To increase its application range, the computer program predicts the formation temperatures of methane gas hydrates and heavier gas hydrates. With this information, the user can then quickly determine whether the drilling fluid has sufficient gas hydrate suppression capability for the particular well conditions, and adjustments in fluid composition can be quickly determined. In pre-well planning the model has been used to determine appropriate drilling fluid formulations that achieve the necessary level of suppression capability, thereby eliminating unnecessary expense involved in "overdoing" hydrate suppression capability. In addition, given a wide choice of fluid formulations that can adequately do the job, the user can then decide the final formulation based on economics and fluid performance.

Formation of Gas Hydrates

Natural gas hydrates result from the injection of natural gas into a water medium under favorable conditions of temperature and pressure - usually low temperature and high pressure. Under such conditions, natural gas and water form cage-like structures that are commonly called "clathrates". Clathrates usually have two basic structure types and can contain several parts of ice for each part of gas. Once formed, clathrates can remain stable for short periods at temperatures much higher than those at which they were formed. The bulk of study of natural gas hydrates over the past 60 years has resulted largely from the problems they present to the natural gas transmission industry: blocked transmission lines and the resulting pressure fluctuations. For the drilling industry, their importance lies in the problems that can result from their formation: blocked choke and kill lines during well control operations. The occurrence of natural gas hydrates in offshore drilling has been described in the literature!. In recent years, the subject of natural gas hydrates has received much attention in pre-well planning of deepwater drilling projects, where the pressure and temperature conditions at the sea floor are favorable for their formation. This attention has largely focused on identifying techniques to suppress the formation of gas hydrates under conditions existing at the sea floor. This paper addresses the issue of gas hydrate suppression and thermodynamics and hence does not include items pertinent to gas hydrates stabilization and kinetic inhibition.

Laboratory Investigation

The conditions under which gas hydrates form has been studied extensively in the laboratory. Investigators have determined the effect of fluid chemistry and which inhibitors work best to suppress their formation.