Abstract

In this study, we analyzed geomechanical responses during potential future long-term tests of gas production from permafrost associated hydrate deposits at Mount Elbert and Prudhoe Bay L-Pad vicinity in North Slope, Alaska. At Mount Elbert we investigated a potential production test from a single 37 ft thick hydrate layer (the Unit D formation) that extends from a depth of 2023 to 2060 ft. At PBU-L106 we analyzed a potential production test within the C-layer, which extends 148 ft from a depth of 2226 to 2374 ft, and is characterized by two hydrate-bearing strata separated by a 30 ft shale interlayer. We use a coupled hydraulic, thermodynamic and geomechanical model simulating 3 years of production from single vertical or horizontal wells (constant pressure Pw = 3 MPa). The analysis shows that most stress changes occurs rapidly (within tens of days) near the wellbore and then progressively away from the wellbore along with the reservoir pressure depletion. Consequently, a few years long test would be sufficient for the stress changes to fully develop and would not change much further around the well bore region, even if the production would be continued for tens of years. The reservoir deformations and surface subsidence on the other hand progresses more gradually with time and is restricted by an arching effect of the overburden. The simulation results show larger subsidence for a production test at Prudhoe Bay L-Pad vicinity, because of a larger hydrate thickness. However, the results indicated about 5 cm compaction of reservoir intervals, which corresponds to a vertical strain of only about 0.2%, and a negligible ground surface subsidence suggesting little danger for the production well and no significant interference with nearby wells and facilities. The results also indicate limited shear failure in the formation around the well perforation which may require appropriate sand screens to prevent solid production.

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