BAKER HUGHES - Drilling Fluids Reference Manual

TABLE OF CONTENTS
HYDRATES
INTRODUCTION
Gas hydrates are crystalline solids formed by low molecular weight hydrocarbon gas molecules
combined with water. The interaction between the water and the gas is physical in nature and is not a
chemical bond. They are formed and remain stable over a limited range of temperatures and pressures
and exist in a number of structure types depending upon the gas composition.
Gas hydrate concerns cover two distinctly different situations, i.e. naturally occurring gas hydrates
and the formation of gas hydrates in drilling fluids. The formation of hydrates in drilling fluids during
a shut-in is the most likely hydrate-associated hazard in deepwater drilling.
GAS HYDRATES
A number of unusual problems encountered during drilling for hydrocarbons in deep, cold marine
environments have been attributed to gas hydrates. Gas hydrates are small molecules formed when
water and the components of natural gas combine thermodynamically to form crystalline, clathrate
compounds. A total of seven different structures have been postulated. However, to date only three
different structures have been seen (Structure I, Structure II, Structure H) when hydrocarbons combine.
The basic unit of each structure is the pentagonal dodecahedron.
Figure 13 - 2
Gas Hydrate
Structure I
Figure 13 - 3
Basic Unit of
Gas Hydrate
Clathrates are complexes formed between two chemicals in which one type of molecule completely
encloses the other molecule in a crystal lattice. In the case of gas hydrates, hydrogen bonded water
molecules form a cage-like structure that surrounds gas molecules, forming a solid substance with a
high gas density. Agglomeration of these structures can form naturally occurring accumulations or
blockages in lines and valves in drilling equipment.
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 13-11