Gas Production From Oceanic Class 2 Hydrate Accumulations
Authors: Bybee Karen 
Published: February 2015

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper OTC 18866, "Gas Production From Oceanic Class 2 Hydrate Accumulations," by G.J. Moridis, SPE, and M.T. Reagan, Lawrence Berkeley National Laboratory, prepared for the 2007 Offshore Technology Conference, Houston, 30 April-3 May.

Gas hydrates are solid crystalline compounds in which gas molecules are lodged within the lattices of ice crystals. The vast amounts of hydrocarbon gases trapped in hydrate deposits in the permafrost and in deep ocean sediments constitute a 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 (WZ). This study investigated three methods of gas production that used vertical-well designs.

Introduction

Gas hydrates are solid crystalline compounds in which gas molecules occupy the lattices of ice-crystal structures. Natural hydrates in geological systems usually contain hydrocarbons [mainly methane (CH4) and other alkanes] but also may contain CO2, H2S, or N2. Hydrate deposits occur in two distinctly different geologic settings, permafrost and deep ocean sediments, where the necessary conditions of low temperature and high pressure exist for their formation and stability.

Although there has been no systematic effort to map and evaluate this resource and current estimates vary widely (ranging from 1015 to 1018 m3), the consensus is that the worldwide quantity of hydrocarbon-gas hydrates is vast. Even the most conservative estimate exceeds by a factor of two the total energy content of the known conventional fossil-fuel resources. Hydrates are emerging as a promising energy source 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.

Gas can be produced from hydrates by inducing dissociation, which also releases large amounts of H2O. The three main methods of hydrate dissociation are:depressurization, in which the pressure is lowered to a level lower than the hydration pressure at the prevailing temperature;thermal stimulation, in which temperature is raised above the hydration temperature at the prevailing pressure; andthe use of inhibitors (such as salts and alcohols), which shift the equilibrium pressure and temperature.

Geological System

The geological system in this study is based on the Tigershark area in the Alaminos Canyon Block 818 of the Gulf of Mexico. Log data from a specially designed exploration well in approximately 2750 m of water at the site indicated the presence of an 18.25-m-thick sandy HBL (3210- to 3228-m drilling depth) with a porosity of approximately 0.30 and Darcy-range intrinsic permeability. Initial estimates of gas-hydrate saturation derived from analyses of the resistivity and p-wave-velocity data indicate a range from 0.6 to more than 0.8. The Tigershark data are particularly valuable because they describe a promising target for gas production and because of the paucity of data on marine hydrate deposits.