Hydrates in the Ocean beneath, around, and above Production Equipment

Abstract

Hydrates amass beneath and around production equipment and can form in hydrocarbon−seawater jets/plumes. The sources of hydrocarbons in these hydrates are natural seeps or temporary production system leaks. In this paper, some of (i) the formation parameters for these hydrates and (ii) the impacts on normal production operations and hydrocarbon-spill capture systems are discussed. ■ INTRODUCTIONChallenges in the design of deepwater production systems include management of hydrates external and internal to the production hardware. This paper focuses entirely on external hydrates. These include hydrates in which the forming-gas sources are near-mudline natural seeps or production fluid leaks. Management of external hydrates is significantly different from internal-hydrates management for two reasons: (1) The goals of external-hydrates management are often quite different from those of internal-hydrates management. For example, external-hydrates management of near-mudline reservoir hydrates seeks to maintain or increase the volume fractions of these hydrates as they contribute to mudline equipment and wellbore stability. (2) The parameter paths along which external and internal hydrates move into the hydrate region are usually different. For example, at a water depth of 5000 ft in the Gulf of Mexico, (a) in an internal steady-state flowing production line with free gas present, (i) liquids are close to gas saturated, (ii) the pressure and temperature decrease as the fluids flow along the line (in the 40°F seawater environment), which can move the system into the hydrate region (see Figure 1), and (iii) the system is far (>30°F) into the hydrate region at 2200 psi and 40°F (see Figure 1), which promotes relatively fast hydrate-formation kinetics. (b) In an external (to the production system) environment with seeps, (i) seawater is near 40°F in temperature and up to 6000 psi in pressure, (ii) seawater is typically under-saturated with gas, and (iii) elevating the seawater dissolved-gas concentration sufficiently to enter the hydrate region can be difficult in locations with ocean currents. (c) In an external (to the production system) environment with rare, temporary leaks, (i) leak−seawater plume temperature decreases from the leak hydrocarbon temperature (e.g., 200°F) to the seawater temperature (∼40°F ) with the distance up from the source, (ii) seawater is typically under-saturated with gas, and (iii) elevating the seawater dissolved-gas concentration sufficiently to enter the hydrate region can be difficult in locations with ocean currents.In other words, the evolution into the hydrate region for internal systems is usually dominated by a decrease in the Figure 1. Hydrate region boundary plot for a system below the bubble point with flowline fluid pressure−temperature trace. Note that the pressure−temperature point at the flowline outlet is more than 30°F inside the hydrate region, which promotes fast hydrate-formation kinetics.Article pubs.acs.org/EF

Integral models for bubble, droplet, and multiphase plume dynamics in stratification and crossflow

“…At 50 m above the release, the oil and gas have fully cooled to the temperature of the plume fluid, and the multiphase plume continues to rise until ultimately forming an intrusion layer. This rapid heat transfer at the source is similar to that reported in Anderson et al [72], who also evaluated the potential for methane hydrate formation in these deepwater plumes. For Case 2, the vertical scales over which heat transfer occurs are similar, but the maximum temperature in the plume reaches 14 C, due to the faster heat transfer from the smaller oil droplets, which have a greater surface-area-to-volume ratio.…”

Dynamic morphology of gas hydrate on a methane bubble in water: Observations and new insights for hydrate film models

“…The water in the HWTF contained dissolved methane at a concentration of 0.0021 mol fraction. Elevated dissolved gas concentrations are thermodynamically required for initial hydrate formation on a bubble surface [ Anderson et al ., ]. Thermodynamic calculations, based on experimental data by Lu et al .…”

Observations of bubbles in natural seep flares at MC 118 and GC 600 using in situ quantitative imaging

“…Direct in situ observations of natural seep bubbles in the water column are rare. These observations are important since both the laboratory and field observations of hydrate dynamics show that the surrounding water needs to have an elevated concentration of the hydrate‐forming gas before hydrates start to form [ Anderson et al ., ; Warzinski et al ., ; Maini and Bishnoi , ]. Rehder et al .…”

“…For example, Socolofsky et al [2011] estimated that on the order of 1000 m 3 /s of entrained water entered the 1100 m deep intrusion layer for the Deepwater Horizon spill. Anderson et al [2012] showed that for a wide range of potential blowout types in deep water, the plume quickly cools to near ambient temperatures (within a maximum of 50 m height above the release) and the dissolved gas concentration remains well below the saturation concentration needed for rapid hydrate growth. Chen and Yapa [2001] observed similar behavior for a different numerical blowout model with a hydrate kinetics module.…”

“…Direct in situ observations of natural seep bubbles in the water column are rare. These observations are important since both the laboratory and field observations of hydrate dynamics show that the surrounding water needs to have an elevated concentration of the hydrate-forming gas before hydrates start to form [Anderson et al, 2012;Warzinski et al, 2014;Maini and Bishnoi, 1981]. Rehder et al [2002Rehder et al [ , 2009 suggest that saturation concentration existing at the bubble-water interface is adequate to start hydrate formation, and Rehder et al [2009] observed hydrates to form on methane bubbles released artificially into the ocean in the mid water column.…”