Gas Production From Oceanic Class 2 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 are lodged within the lattices of ice crystals. Because vast amounts of hydrocarbon gases are trapped in natural hydrate accumulations in geologic media in the permafrost and in deep ocean sediments, a significant effort has recently begun to evaluate these deposits as a potential energy source. Class 2 hydrate deposits are characterized by a hydrate-bearing layer (HBL) underlain by a saturated zone of mobile water. The base of the HBL in Class 2 deposits may occur at the edge of, or within, the zone of thermodynamic hydrate stability. In this numerical study of long-term gas production from such deposits, we determine that hydrate-originating gas can be produced from a single vertical well at high rates (several MMSCFD) for long periods of time using conventional technology. The production strategy involves a cyclic approach, with each cycle composed of two stages. The first stage involves depressurization-induced gas production, lasts 1–2 years, and proceeds until secondary hydrate formation in the vicinity of the wellbore drastically reduces the well pressure. The second stage involves destruction of the secondary hydrate near the wellbore by thermal stimulation or inhibitor injection and lasts less than a month. The presence of salt in oceanic HBL enhances hydrate dissociation, but the effect is weakened by the release of fresh water. Gas production from Class 2 hydrate accumulations continuously increases over time during each cycle, while the corresponding water production continuously decreases. The simulation results indicate that gas production is affected by the (a) intrinsic permeability; (b) initial pressure, temperature, and hydrate saturation; (c) fluid withdrawal rate; (d) thickness of the water zone; and (f) permeability of the confining boundaries. The effect of these parameters and conditions on gas production is not uniform over time and requires the definition of the time frame before conclusions can be stated. Fine discretization in the vicinity of the wellbore is very important for the description of the evolving flow-controlling secondary hydrate accumulations and of the corresponding dynamic phase changes.

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)