Gas Production From Class 2 Hydrate Accumulations in the Permafrost

Authors: Moridis, George (speaker) and Reagan, Matthew, Lawrence Berkeley National Laboratory.

Venue: Society of Petroleum Engineers’ Annual Technical Conference & Exhibition in Anaheim, CA, November 11–14, 2007 (http://www.spe.org/atce/2007/ [external site]).

Abstract: Gas hydrates are solid crystalline compounds in which gas molecules are lodged within the lattices of ice crystals. Natural hydrates in geological systems are composed mainly of methane. The amounts of hydrocarbon gases trapped in natural hydrate accumulations are enormous, leading to a recent interest in the evaluation of their potential as an energy source. Class 2 hydrate deposits are characterized by a Hydrate-Bearing Layer (HBL) underlain by a saturated zone of mobile water, and are encountered in the permafrost and in deep ocean sediments. The base of the HBL in Class 2 deposits may occur at the edge of—or within—the zone of thermodynamic hydrate stability. Because of the manner of their formation from pre-existing gas reservoirs, permafrost hydrate deposits are generally characterized by high hydrate saturations and are bounded by relatively impermeable strata. In this numerical study of long-term gas production from permafrost Class 2 deposits, project researchers investigated three different configurations that involve varying production intervals and combinations of depressurization (the main dissociation-inducing mechanism) with localized thermal stimulation. Using high-definition grids and realistic production scenarios, the researchers determined that large volumes of gas can be produced at high rates (several million standard cubic feet per day) for long times using conventional technology. The production approach involves initial fluid withdrawal from the water zone underneath the HBL. The production process follows a cyclical pattern, with each cycle composed of two stages: a long stage (months to years) of increasing gas production and a short stage (days to weeks) that involves destruction of the secondary hydrate (mainly through warm water injection) that evolves during the first stage. A well configuration that initially involves heating of the outer surface of the wellbore and later continuous injection of warm water at low rates appears to yield the highest average rates over the period it takes to exhaust the hydrate deposit. Researchers determined that gas production is affected by intrinsic permeability, initial hydrate saturation, fluid withdrawal rate, thickness of the water zone, and initial pressure and temperature of the hydrates.

Related NETL Project
The proposed research of the related NETL project FWP-G308, “Numerical Studies for the Characterization of Recoverable Resources from Methane Hydrate Deposits,” 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)
LBNL – George Moridis (gjmoridis@lbl.gov or 510-486-4746)