Structure H Hydrates: Implications for the Petroleum Industry
- A.P. Mehta (Shell E&P Technology Co.) | E.D. Sloan (Colorado School of Mines)
- Document ID
- Society of Petroleum Engineers
- SPE Journal
- Publication Date
- March 1999
- Document Type
- Journal Paper
- 3 - 8
- 1999. Society of Petroleum Engineers
- 5.2.1 Phase Behavior and PVT Measurements, 4.3.1 Hydrates, 5.3.2 Multiphase Flow, 4.6 Natural Gas, 4.2 Pipelines, Flowlines and Risers, 5.9.1 Gas Hydrates, 4.5.5 Installation Equipment and Techniques, 5.2 Reservoir Fluid Dynamics
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Structure H (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 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.
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 recognized nuisance in the oil industry since they plug flowlines and could 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 12 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 (12 pentagonal and 2 hexagonal faces) cage which can fit slightly larger guests such as ethane and carbon-dioxide, and the 51264 (12 pentagonal and 4 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. Sloan1 and Englezos2 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 1990s, researchers in both academia as well as industry have focused on means of kinetically inhibiting the growth of hydrate crystals by the addition of chemical additives as a substitute for methanol. Sloan and co-workers (Long et al.3 and Lederhos et al.4) have recently provided examples of such "kinetic inhibitors." Anti-agglomerant type low dosage hydrate inhibitors have also been patented by Klomp et al.,5 and demonstrated to be effective in preventing the formation of hydrate blockages.
Structure H Hydrate
Until 1987, molecules larger than n-butane were assumed to be nonhydrate formers. Several molecules like isopentane and methylcyclopentane were cited in literature as specific examples of nonhydrate formers by Katz et al.6 or Lippert et al.7 were erroneously assumed to be sII hydrate formers. The discovery of sH hydrate by Ripmeester et al.8 lead to a dramatic shift in hydrate perspectives since sH could encapsulate much larger guest molecules such as methylcyclopentane, methylcyclohexane, and adamantane.
At the time of their discovery, single-crystal analysis of sH hydrates had not been done, but due to its analogy with dodecasil 1-H (a clathrasil9 wherein the host molecules are SiO2 instead of H2O) Ripmeester et al. assumed sH hydrate to be composed of similar cages. Recently, Ripmeester et al.10 have confirmed the crystal features of sH hydrate. Similar to sI and sII hydrates, sH is composed of the basic 512 cage and two other cages: a 435663 cage (three strained square faces, six pentagonal, and three hexagonal faces) and a bulky 51268 cage (12 pentagonal and 8 hexagonal faces). The 435663 cage is similar in size to the 512 cage, while the 51268 cage can fit large (=9 Å in diameter) guests such as neohexane and methylcyclohexane. Fig. 1 shows the constituent cages and an orthogonal view of sH hydrate.
sH is a double hydrate: it requires the presence of two types of guest molecules to be stable. Small guest molecules (referred to as help guests) such as methane, xenon, or hydrogen-sulfide occupy the two small sH cages, while the intermediate hydrocarbon molecule resides in the large sH cage. Ripmeester and Ratcliffe11 used 129Xe and X-ray diffraction techniques to identify 24 hydrocarbon molecules which could stabilize sH hydrate. Since many of these molecules constitute a small fraction of petroleum, they suggested the possible occurrence of sH hydrate in natural and artificial environments such as crude oil reservoirs. Ripmeester et al.12 also demonstrated that constituents of gasoline and naptha contained molecules which could stabilize sH hydrate. A list of sH formers which could be potentially encountered in industrial situations is given in Table 1. Note that each of these components requires the presence of a small help guest such as methane, nitrogen, or xenon, for sH hydrates to crystallize.
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