Evaluation of the Hydrate Deposit at the PBU L-106 Site, North Slope, Alaska, for a Long-Term Test of Gas Production

Abstract

Abstract As part of the effort to investigate the technical feasibility of gas production from hydrate deposits, a long-term field test (lasting 18–24 months) is under consideration in a project led by the U.S. Department of Energy. We evaluate a candidate deposit involving the C-Unit in the vicinity of the PBU-L106 site in North Slope, Alaska. This deposit is stratigraphically bounded by impermeable shale top and bottom boundaries (Class 3), and is characterized by high intrinsic permeabilities, high porosity, and high hydrate saturation. The C-unit deposit is composed of two hydrate-bearing strata separated by a 30-ft-thick shale interlayer, and its temperatrure across its boundaries ranges between 5 and 6.5 oC. We investigate by means of numerical simulation the production potential of this deposit using both vertical and horizontal wells. We also explore the sensitivity of production to key parameters such as the hydrate saturation, the formation permeability and porosity, the system heterogeneity, the permeability of the bounding shale layers, and the initial temperature. The results indicate that production from horizontal wells may be orders of magnitude larger than that from vertical ones. Additionally, production increases with the formation permeability, and with a decreasing permeability of the boundaries. The effect of the hydrate saturation on production is complex and depends on the time frame of production. We show that hydrate dissociation declines very rapidly upon cessation of production, indicating that a production does not have long-term residual effects. In the same study, we investigate the potential of time-lapse seismic surveys to monitor gas production from hydrate deposits. The seismic properties, such as the elastic and shear moduli, are a function of the simulated time- and space-varying pressure and temperature, the aqueous-, gas-, and hydrate-phase saturation, and the porosity. We examine a variety of seismic measurement configurations and survey parameters to determine the optimal approach for detecting changes occurring in the hydrate deposit during production that can be used as the basis for monitoring hydrate dissociation, and the corresponding hydrate saturation and geomechanical status. Introduction Background. Gas hydrates (GH) are solid crystalline compounds of water and gaseous substances that are described by the general chemical formula G•NH 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 where 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 (Kvenvolden, 1988).