Summary

With the increasing number of deep offshore drilling operations, operators and service companies are confronted by technical challenges ever growing in complexity. Extreme conditions encountered at these depths require an adaptation of the drilling muds. In particular, the range of temperatures and pressures encountered (as low as -1°C and up to 400 bar) is favorable for the formation of gas hydrates. Gas hydrates are solid structures formed from water and gas; water contained in drilling muds under certain temperature and pressure conditions will form solid cages that entrap the gas molecules. Formation of these solid gas hydrates is liable to plug kill and chokelines as well as the annular and may cause interruption of the drilling operation and even destruction of rig equipment. Deep offshore drilling operators are aware of this problem, and some operational solutions and drilling mud formulations have been proposed and utilized, but when extreme water depths are attained, classical inhibitor solutions alone may be ineffective. The usual way to determine the thermodynamic conditions of the formation of gas hydrates in drilling mud formulations is to use a pressure/volume/temperature (PVT) cell. This technique requires heavy instrumentation and often does not permit work with a whole formulation (especially in the presence of solids). Moreover, PVT cells do not give a quantitative evaluation of the kinetic properties of gas hydrate formation.

In this work, we present experiments using differential scanning calorimetry (DSC) to determine the thermodynamic equilibrium properties of methane hydrate in calcium chloride solutions. Additionally, thermograms of an emulsion and a complete drilling mud are presented that show the possibility of characterizing hydrate thermodynamics in complex systems as synthetic mud formulations. This technique is easy to use, and, when correctly interpreted, it enables the dangerous zones for hydrate formation to be determined.

Introduction

In recent years, deep offshore drilling operations have continuously set new depth records. This should continue in the near future, particularly in the Gulf of Mexico or Gulf of Guinea areas, where numerous exploitation licenses have been taken to extreme depths. A water depth approaching 3000 m is now a reality. These depths are characterized by extreme conditions (poor consolidation of the surface formation, narrowness of the mud-weight window, and shallow gas zones); temperatures down to -1°C and pressures of up to 400 bar are not uncommon at the mudline. With these harsh conditions, more and more complex technical challenges have to be overcome with adapted solutions. In particular, these ranges of temperatures and pressures are favorable conditions for gas hydrates formation into the drilling muds. Gas hydrates are solid structures formed from water and gas; these solid crystals may block the lines or the annular and cause serious damage to the rig equipment and even threaten the security of the crew.1 Operators are aware of this problem, as shown by the number of publications related to this topic, and a certain number of operational solutions exist. These solutions are classically based on the use of thermodynamic inhibitor additives (mainly salt and glycol additives), which displace the equilibrium point of hydrate formation. In deep offshore conditions (deeper than 2,000 ft), classical inhibitors may be ineffective, and other solutions may have to be proposed. The first step is to define precisely the conditions of hydrate formation in drillings muds in order to identify the drilling phases in which hydrate problems may occur. In this work, we present experiments using DSC to determine the thermodynamic equilibrium properties of methane hydrate in various media, from simple solutions to multiple-phase dispersed systems. After a review of the operational procedures and the existing data related to hydrate formation in drilling muds, the DSC technique will be described. In the third part of this paper, we will present the experimental procedure and the first experimental results showing the possibility of characterizing gas hydrate formation in drilling muds with this technique.

State of the Art
Conditions of Gas Hydrate Formation and the Consequences of This Formation.

Favorable conditions for gas hydrates formation are often encountered when drilling in deep offshore areas; very low temperatures are rapidly reached. For example, in the Gulf of Mexico, it is common to have 8°C at 500 m depth and 4.4°C after 900 m depth.2 As for the pressure values, 400 bar are not uncommon when approaching extreme depths. If a certain quantity of gas penetrates into the well (during a gas kick situation, for example), the drilling mud aqueous phase (continuous phase for water-based muds or dispersed phase for oil-based muds) comes in contact with the gas, leading to gas hydrate formation. This risk is even higher when circulation is stopped. In well controlled situations, even if the gas leaves the reservoir at high temperature, it may cool down to sea temperature when circulation is stopped for an extended period.3

Consequences can be catastrophic; gas hydrates may plug surface equipment at the blowout preventer (BOP) level and/or in the kill and chokelines, where there is little fluid circulation. But it is also possible to form hydrates in the annular, which will block the drillstring, forcing the drilling operation to stop. When hydrates are formed, part of the water present in the mud is consumed, which can lead to barite sedimentation or viscosity increase problems. Hydrate formation is, therefore, dangerous for several reasons and can halt operation. Safety problems must not be forgotten: 1 m3 of hydrates can liberate 170 m3 of gas very rapidly. Propulsion of gas hydrate plugs at very high velocity is also a hazard.

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