Strategies for Gas Production From Oceanic Class 3 Hydrate Accumulations

Authors: George J. Moridis, Matt T. Reagan, Lawrence Berkeley National Laboratory.

Venue: 2007 Offshore Technology Conference, Houston, TX, April 30–May 1, 2007 (http://www.otcnet.org/ [external site]).

Abstract: Gas hydrates are solid crystalline compounds in which gas molecules (referred to as guests) are lodged within the lattices of ice crystals (called hosts). Vast amounts of hydrocarbon gases (mainly methane) are trapped in hydrates, and a significant effort has recently begun to evaluate hydrate deposits as a potential energy source. 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, and are encountered in the permafrost and in deep ocean sediments. The base of the HBL in Class 3 deposits may occur at the edge of, or within, the zone of thermodynamic hydrate stability. Depressurization may be used in deposits with low hydrate saturation (usually <50%) and higher intrinsic permeability, while thermal stimulation may be a viable option in deposits with high hydrate saturation in formations with low intrinsic permeability. In this numerical study of long-term gas production from typical representatives of unfractured Class 3 deposits, we determine that (a) simple thermal stimulation appears to be an inefficient production method, and (b) depressurization under constant-rate fluid withdrawal is superior to pure thermal stimulation but suffers from low production rates (<500 MSCFD). However, an alternative approach involving a staged application of depressurization (constant pressure or constant rate) and thermal stimulation can lead to very large volumes of hydrate-originating gas produced at high rates (>15 MMSCFD) for long times using conventional technology. Gas production from hydrates is accompanied by a significant production of fresh water. However, the water production rate declines with time, and the disposal of the hydrate-originating water may not pose significant environmental problems because of its superior quality and the oceanic environment. The simulation results indicate that gas production from oceanic Class 3 deposits is affected by the initial pressure, temperature, and hydrate saturation and by the intrinsic permeability of the HBL. Additionally, gas production by means of thermal stimulation is affected by the rate of heat addition, while depressurization-induced production is affected by the fluid withdrawal rate and the design of the constant-pressure regime imposed at the well.

Related NETL Project: The goal of the related NETL project entitled “Numerical Studies for the Characterization of Recoverable Resources from Methane Hydrate Deposits.” (FWPG308) is to develop and maintain a reservoir model that simulates the behavior of hydrate-bearing geologic systems and evaluates appropriate hydrate production strategies for both permafrost and marine environments, including thermal stimulation, depressurization, and dissociation induced and/or enhanced by inhibitors (such as brines and alcohols).

NETL Project Contacts:
NETL – Kelly Rose (Kelly.Rose@netl.doe.gov or 304-285-4157)
NETL – Rick Baker (richard.baker@netl.doe.gov or 304-285-4714)
LBNL – George J. Moridis (GJMoridis@lbl.gov or 510-486-4746)