Sensitivity Analysis of Gas Production from Class 2 and Class 3 Hydrate Deposits

Abstract

Sensitivity Analysis of Gas Production from Class 2 and Class 3 Hydrate Deposits Matthew T. Reagan, SPE, George J. Moridis, SPE, Keni Zhang, SPE, Lawrence Berkeley National Laboratory Abstract Gas hydrates are solid crystalline compounds in which gas molecules are lodged within the lattices of an ice-like crystalline solid. The vast quantities of hydrocarbon gases trapped in hydrate formations in the permafrost and in deep ocean sediments may constitute a new and promising energy source. Class 2 hydrate deposits are characterized by a Hydrate-Bearing Layer (HBL) that is underlain by a saturated zone of mobile water. Class 3 hydrate deposits are characterized by an isolated Hydrate-Bearing Layer (HBL) that is not in contact with any hydrate-free zone of mobile fluids. Both classes of deposits have been shown to be good candidates for exploitation in earlier studies of gas production via vertical well designs—in this study we extend the analysis to include systems with varying porosity, anisotropy, well spacing, and the presence of permeable boundaries. For Class 2 deposits, the results show that production rate and efficiency depend strongly on formation porosity, have a mild dependence on formation anisotropy, and that tighter well spacing produces gas at higher rates over shorter time periods. For Class 3 deposits, production rates and efficiency also depend significantly on formation porosity, are impacted negatively by anisotropy, and production rates may be larger, over longer times, for well configurations that use a greater well spacing. Finally, we performed preliminary calculations to assess a worst-case scenario for permeable system boundaries, and found that the efficiency of depressurization-based production strategies are compromised by migration of fluids from outside the system. 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 equation G + N H H 2 O = G•N H H 2 O,……………… …(1) where N H is the hydration number. Natural hydrates in geological systems usually contain hydrocarbons (mainly CH 4 and other alkanes), but may also contain CO 2 , H 2 S or N 2 . 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. Although there has been no systematic effort to map and evaluate this resource and current estimates vary widely 1,2,3 (ranging between 10 15 to 10 18 m 3 ), the consensus is that the worldwide quantity of hydrocarbon gas hydrates is vast. Even the most conservative estimate exceeds the total energy content of the known conventional fossil fuel resources. The sheer magnitude of the resource, ever increasing global energy demand, and dwindling conventional fossil fuel reserves, point to hydrates a promising energy source 4,5 even if only a limited number of deposits might be suitable for production and/or only a fraction of the trapped gas can be recovered. The attractiveness of hydrates is further enhanced by the environmental desirability of natural gas (as opposed to solid or liquid) fuels. The production potential of gas hydrate accumulations demands technical and economic evaluation. Gas can be produced from hydrates by inducing dissociation, which also releases large amounts of H 2 O (Eq. 1). The three main methods of hydrate dissociation are 6 : (1) depressurization, in which the pressure P is lowered to a level lower than the hydration pressure P e at the prevailing temperature T, (2) thermal stimulation, in which T is raised above the hydration temperature T e at the prevailing P, and (3) the use of inhibitors (such as salts and alcohols), which shifts the P e -T e equilibrium through competition with the hydrate for guest and host molecules. Classification of hydrate deposits. Natural hydrate accumulations are divided into three main classes. 7,8 Class 1 accumulations are composed of two layers: a hydrate-bearing layer (HBL) and an underlying two-phase fluid zone containing mobile gas and liquid water. The bottom of the hydrate stability zone (the location above which hydrates are stable) coincides with the bottom of the hydrate interval. Production from such deposits was discussed in detail by Moridis et al. 8 Class 2 deposits consist of two zones: a HBL, overlying a zone of mobile water (WZ). Class 3 accumulations are composed of a single zone, the hydrate interval (HBL), and have no underlying zone of mobile fluids. In Classes 2 and 3, the entire HBL may be well within the hydrate stability zone and can exist under stable equilibrium conditions. Production from Class 2 and