This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper OTC 22152, ’Evaluation of the Hydrate Deposit at the PBU-L106 Site, North Slope, Alaska, for a Long-Term Test of Gas Production,’ by George J. Moridis, SPE, and Matthew T. Reagan, SPE, Lawrence Berkeley National Laboratory; Heidi Anderson-Kuzma, SPE, East Donner Research; Yang Zhao, University of California at Berkeley; Katie Boyle, Lawrence Berkeley National Laboratory; and James W. Rector, University of California at Berkeley, prepared for the 2011 Arctic Technology Conference, Houston, 7-9 February. The paper has not been peer reviewed.
To investigate the technical feasibility of gas production from hydrate deposits, a long-term field test (lasting 18 to 24 months) is under consideration in a project led by the US Department of Energy. A candidate deposit involving the C-Unit in the vicinity of the PBU-L106 site on the North Slope, Alaska, was evaluated. The results indicate that production from horizontal wells may be orders of magnitude larger than that from vertical wells.
Gas hydrates (GHs) are solid crystalline compounds of water and gaseous sub-stances that are described by the general chemical formula GsNH H2O. In the GH clathrates, the molecules of gas, G (referred to as guests), occupy voids within the lattices of ice-like crystal structures. If gas and H2O availability is not a limitation, hydrate deposits can occur in two distinctly different geographic settings in which the necessary conditions of low temperature, T, and high pressure, P, exist for their formation and stability: in the Arctic (typically in association with permafrost) and in deep ocean sediments. CH4 is the dominant GH-forming hydrocarbon gas in natural hydrates that occur in geologic media. Under standard T and P (STP) conditions, each cubic meter of simple CH4 hydrate releases, upon dissociation, approximately 164 m3 of methane and 0.8 m3 of H2O.
Classification. Natural GH accumulations are divided into three main classes on the basis of simple geologic features and initial reservoir conditions. Class-1 settings have two layers: a hydrate- bearing layer (HBL) and an underlying two-phase-fluid zone containing mobile gas and liquid water. A distinct feature of Class-1 settings is that the base of the GH-stability zone coincides with the bottom of the HBL. Class 1 is the most desirable target because it is the easiest to destabilize to release gas. In Class-2 settings, an HBL overlies a zone of mobile water. Class-3 accumulations have a single HBL with no underlying zone of mobile fluids.