Structure H Hydrates: Implications for the Petroleum Industry
Authors: Mehta A.P. , Sloan E.D. 
Published: December 2007

Structure H (sH) hydrates were discovered nearly a decade ago (1987) and are now recognized as a potential problem in the petroleum industry. sH hydrates are unique since their formation requires both a light gas such as methane and molecules typically present in oil and condensates. Several sH formers such as methylcyclopentane, methylcyclohexane, neohexane, and adamantane, are indigenous to petroleum. Phase equilibrium measurements and a comprehensive thermodynamic model indicate that the temperature and pressure conditions under which sH hydrates form as a stable phase are consistent with those in hydrocarbon production, processing and transportation facilities. The stable occurrence of sH hydrates calls into question existing hydrate prediction programs and suggests that the hydrate phase itself should be measured, in contrast to previous experimental practice. In this work, we provide a brief overview of the current state-of-the-art on sH hydrates, with an emphasis on its implications for the petroleum industry.

Introduction

Clathrate hydrates are ice-like crystalline compounds formed by a hydrogen-bonded network of water molecules. The hydrate lattice is composed of several cage-like structures which incorporate guest molecules such as methane, ethane, and propane. Since several hydrate formers are constituents of natural gas, clathrate hydrates are commonly referred to as 'gas hydrates'. Hydrate formation is favored at low temperatures and high pressures and can occur in any natural or artificial environment where free water exists in the presence of hydrocarbon molecules. Hydrates are a known nuisance in the oil industry since they plug flow channels from drilling fluids and jeopardize foundations of deepwater installations and pipelines.

Typically hydrates crystallize in two distinct structures denoted as Structure I (sI) and Structure II (sII). Both sI and sII contain a basic 'building block' water cavity referred to as the 512 cage since it is composed of twelve pentagonal faces. The 512 cage can accommodate guests such as methane and hydrogen-sulfide. In addition to the 512 cage there are two other commonly found cavities referred to as the 51262 (twelve pentagonal and six hexagonal faces) cage which can fit slightly larger guests such as ethane and carbon-dioxide, and the 51264 (twelve pentagonal and four hexagonal faces) cage which can fit guests such as propane and n-butane. sI hydrates are typically found in situ in deep oceans with biogenic gases such as methane and hydrogen-sulfide. sII hydrates are found predominantly in natural gas pipelines due to the presence of molecules such as propane and butane.

sI and sII hydrates have been studied extensively and their phase equilibrium conditions are well characterized. Sloan and Englezos have provided a comprehensive summary of both these hydrate structures. Traditionally, thermodynamic inhibitors such as methanol have been used to prevent hydrate blockages. While the thermodynamics of hydrates are well understood, the kinetics of hydrate formation are currently at the forefront of hydrate research. In the past five years researchers in both academia as well as major oil industries have focused heavily on means of kinetically inhibiting the growth of hydrate crystals by the addition of chemical additives as a substitute for methanol. Sloan and coworkers (Long et al.; Lederhos et al.) have recently provided examples of such 'kinetic inhibitors'.

Structure H Hydrate

Until 1987, molecules larger than n-butane were assumed to be non-hydrate formers. Several molecules like isopentane and methylcyclopentane were cited in literature as specific examples of non-hydrate formers by Katz et al., or were erroneously assumed to be sII hydrate formers. P. 607