Inhibition of Gas Hydrates in Deepwater Drilling

Abstract

Summary. With the movement of offshore rigs into deep water, the problem of gas hydrates has become an important issue in drilling. If a kick is taken, gas hydrates can form in the blowout preventer (BOP) or chokelines while the kick is circulated out. The water-based pill presented here significantly improves gas-hydrate inhibition. This pig, which can be spotted in the BOP and weighted up, is environmentally safe and easily adaptable to offshore operations. Compatible with commonly used drilling fluids, the pig can be mixed directly into the mud system without any adverse effects after the danger of hydrate formation diminishes. This technology is an important safety consideration for deepwater drilling well control and hydrate-free operations above the mudline. Introduction Shell Oil Co.'s drilling operations have recently moved into record-setting water depths >7,000 ft) in the Gulf of Mexico, where seabed temperatures are near the freezing point of water. In deeper waters, the ambient conditions at the seabed (38deg.F and 4,000-psia hydrostatic pressure) are conducive to hydrate formation. Hydrate formation in shallow-water and onshore wells usually results from the presence of produced water. Several adverse effects may result. Barker and Gomez reported the formation of natural-gas hydrates during drilling operations at two domestic offshore sites. In both instances, the hydrates plugged subsea equipment, causing considerable difficulties in subsequent operations. Potential difficulties in subsea operations include (1) formation of gas-hydrate plugs in the ram cavity, (2) formation of annular plugs between the drillstring and the BOP, (3) plugging of the chokelines and kill lines, and (4) plugging at or below the BOP. We defined the following criteria for the development of a successful pill formulation. 1. The spotting fluid must inhibit the formation of gas hydrates for anticipated well conditions in 7,000 ft of water in the Gulf of Mexico (hydrostatic pressure=4,000 psia, well pressure=6,000 psia, and temperature=38deg.F). 2. The fluid must be compatible with a 20 % sodium chloride/ partially hydrolyzed polyacrylamide polymer mud system and/or other water-based mud systems.3. The fluid should be capable of supporting mud weights up to 14 lbm/gal. 4. The fluid should have low toxicity, with an LC. bioassay value greater than 30,000 ppm. 5.The fluid must be nonflammable. We developed a salt/glycerol-based spotting fluid that meets an these requirements. This paper describes our R and D. Gas-Hydrate Theory Gas hydrates in drilling and production operations consist of cagelike water lattices that entrap gas molecules. Methane, ethane, propane, n-butane, i-butane, hydrogen sulfide, nitrogen, and carbon dioxide are well-known hydrate-forming components. Hydrocarbons that are heavier than butanes do not form hydrates. Natural-gas hydrates form two distinct, mutually exclusive structures. Structure hydrates form a body-centered cubic cell; Structure 2 hydrates form a diamond lattice. Structure formation depends on the components in the gas stream. Light gases-e.g., methane, hydrogen sulfide, carbon dioxide, and ethane-form Structure 1 hydrates. Heavier components, propane, n-butane, and i-butane-form Structure 2 hydrates. Nitrogen was initially thought to form Structure I hydrates but now is reported to form Structure 2 hydrates. The literature shows that even. small amounts of propane and heavier components produce Structure 2 hydrates. In our study, we focused on typical Gulf of Mexico gas compositions that cause Structure 2 hydrate formation. Another important consideration is the amount of gas trapped in the hydrate structure. Just 1 ft of hydrate can contain as much as 170 scf of gas. When hydrates dissociate, this gas is liberated. If it is released in a closed environment, pressure can increase con-siderably.he phase equilibria of gas hydrates have been studied extensively, and results are reported in the literature. The formation of hydrates depends on the gas composition (i.e., the gas specific gravity). As the gas becomes heavier, the temperature and pressure requirements for hydrate-free operations become more severe. At an isothermal condition, a hydrate-forming heavy component (e.g., n-butane) forms hydrates at pressures lower than those at which lighter components (e.g., methane) form hydrates. Similarly, at an isoboric condition, a hydrate-forming heavy component forms hydrates at temperatures higher than those required by lighter components. The composition of the aqueous phase and the presence (or absence) of a condensed hydrocarbon phase can affect the hydrate formation's pressure and temperature. Because temperature and pressure conditions are predefined and because water content cannot be reduced economically at mudline, the aqueous-phase composition is adjusted to suppress hydrate formation. Solutes within the water phase (e.g., salts, alcohols, and glycols) provide good hydrate inhibition. Also, such solutes often depress the freezing point of water. Freezing-point measurements are more readily made experimentally than gas-hydrate-formation measurements. In our study, we obtained the freezing points experimentally and used them to determine gas-hydrate formation/inhibition with our computer-modeling study. Computer Model We developed a computer model based on freezing-point data to study the formation and inhibition of gas hydrates in the presence of salt/glycerol/water mixtures. The solvent activity (i.e., water activity) may be calculated from freezing-point-depression data. For our model, we used the Moelwyn-Hughes equation: ...............................................(1) where aw, = water activity, Tf = mixture freezing point, and Twf = water freezing point (273.16 K). Eq. I follows from the general thermodynamic relationship. ...............................................(2) where as = solvent activity, Lf= solvent latent heat of fusion at Tsf, C = CpL - Cpcr = difference m constant-pressure heat capacities of liquid solvent and crystal, Tsf = freezing point of pure solvent, and R = gas constant. To derive Eq. 1 from Eq. 2, we must assume that C is constant over the temperature range of freezing-point depression. In a strict thermodynamic sense, one may do so for only a limited composition range of the solute in water. We relaxed this constraint and assumed that this relationship would apply to concentrations approaching 60 wt% glycerol in water. Note that the nature Of the inhibitor does not affect functional relationship between water activity and freezing point. SPEDE P. 109⁁