Design of a potential long-term test of gas production from a hydrate deposit at the PBU-L106 site in North Slope, Alaska: Production predictions and sensitivity analysis

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. A potential site for the test is the hydrate deposit of the C-Layer at the PBU L106 site in North Slope, Alaska. This layer extends from a depth of 2226 to 2374 ft, and is characterized by two hydrate-bearing strata (the deeper C1 and the shallower C2, 62 ft and 56 ft thick, respectively) that are separated by a 30 ft shale interlayer. This is a deposit bounded by impermeable shale top and bottom boundaries (Class 3), and is characterized by high intrinsic permeability (1-5 Darcys), high porosity (40%), high hydrate saturation (75%), a hydrostatic pressure distribution, and a temperature ranging between 5 and 6.5 oC. This numerical study is the first part of the analysis involved in the design of such a long-term field test, and focuses on the coupled processes (flow, thermal, thermodynamical, chemical) that occur during the depressurization-induced production from hydrate deposits. We investigate the production potential of this deposit using both vertical and horizontal wells, and we analyze the system behavior after the cessation of production (well “shut-in”). To adequately describe the system behavior, we use very fine grids in the simulations. We also explore the sensitivity of production to a number of key parameters such as the hydrate saturation, the formation permeability and porosity, the heterogeneity in these conditions and properties, the permeability of the bounding shale layers, and the initial temperature. The results of this study 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 porosity, with the temperature of the deposit, with heterogeneity along the vertical axis (layered systems) and with a decreasing permeability of the boundaries. The effect of the hydrate saturation on production is not monotonic, i.e., it appears to be optimal at medium levels (about 50%) and depends on the time frame of production.