Preliminary Evaluation of the Production Potential of Recently Discovered Hydrate Deposits in the Gulf of Mexico

Abstract

Abstract In this numerical simulation study, we assess the production potential of three hydrate reservoirs that were discovered in the Gulf of Mexico in 2009 at the following locations: Walker Ridge Block 313 (deposits WR313-G and WR313-H), and Green Canyon Block 955 (deposit GC955-H). The three deposits involve clean depositional sands and high hydrate saturations. The most promising layers of the hydrate-bearing strata are used in the simulations. Based on preliminary data analysis, the deposits are treated as Class 3 deposits that appear to be confined by shale boundaries (overburden and overburden), which are considered nearly impermeable. The limited data availability necessitated assumptions about the flow properties of the porous media in the deposits. These were based on analogs of hydrate systems with similarly textured porous media of both the hydrate-bearing strata and of the overburden and underburden. Depending on the geologic models of each of the deposits, both vertical (WR313-H and WR313-G deposits) and horizontal wells (GC955-H deposit) are used. The simulation results provide an upper estimate of the expected production potential of the deposits, and demonstrate the significant benefits that horizontal wells confer in production from such Class 3 deposits. Using a constant bottomhole pressure regime, depressurization-induced production from all three deposits reaches high levels (well in excess of 10 MMSCFD), but decreases monotonically after reaching a maximum relatively early (i.e., after less than a year) in the production process. Introduction Background. Gas hydrates are solid crystalline compounds in which gas molecules (referred to as guests) occupy the lattices of ice crystal structures (called hosts). Under suitable conditions of low temperature T and high pressure P, the hydration reaction of a gas G is described by the general equationEq. 1 where NH is the hydration number. Hydrate deposits occur in two distinctly different geologic settings where the necessary conditions of low T and high P exist for their formation and stability: in the permafrost and in deep ocean sediments (Sloan and Koh, 2008). Methane is by far the predominant gas in natural gas hydrates. Pure CH4-hydrates concentrate methane volumetrically by a factor of 164 at standard P and T conditions (STP), and have a NH ranging from 5.77 to 7.4, with NH = 6 being the average hydration number and NH = 5.75 corresponding to complete hydration (Sloan and Koh, 2008). Although there has been no systematic effort to map and accurately assess the magnitude of the hydrate resource, and current estimates vary widely (ranging between 1015 to 1018 m3 STP), the consensus is that the worldwide quantity of hydrocarbon gas hydrates is vast (Collett, 1995; Milkov, 2004; Klauda and Sandler, 2005). Given the sheer magnitude of the resource, the ever increasing global energy demand, the dwindling conventional fossil fuel reserves, and the environmental desirability of natural gas over liquid and solid fuels, hydrates are emerging as a potential energy sourceeven if only a limited number of deposits might be suitable for production and only a fraction of the trapped gas can be recovered (Makogon, 1987; Dallimore and Collett, 2005; Boswell, 2007).