Geomechanical Performance Analysis of Potential Long-Term Tests of Gas Production from Hydrate Deposits in North Slope, Alaska

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.

Coupled flow and geomechanical analysis for gas production in the Prudhoe Bay Unit L-106 well Unit C gas hydrate deposit in Alaska

“…In the analyses, the horizontal production well was used for the D-unit, whereas the vertical production well was used for the C-unit. The results of Rutqvist et al (2011) show larger subsidence around the well in Unit C at the PBU-L 106 site than Unit D in Mount Elbert, although both of them indicate no significant interference with nearby wells in geological stability. But Rutqvist et al (2011) limited their analyses to one-way coupling, considering the effect from fluid flow to geomechanics only, but not the other way around.…”

“…The results of Rutqvist et al (2011) show larger subsidence around the well in Unit C at the PBU-L 106 site than Unit D in Mount Elbert, although both of them indicate no significant interference with nearby wells in geological stability. But Rutqvist et al (2011) limited their analyses to one-way coupling, considering the effect from fluid flow to geomechanics only, but not the other way around. Moridis et al (2010) investigated gas production in vertical and horizontal wells in the PBU-L 106 C unit, comparing them with the PBU-L 106 D unit.…”

“…The previous studies on coupled flow and geomechanics for the hydrate deposits were based on one-way coupling for fluid flow and geomechanics, where the effect from fluid flow to geomechanics was considered, while the effect from geomechanics to fluid flow was not fully considered Rutqvist et al, , 2011. Recently, Kim et al (2012) showed clear differences between one-way and two-way couplings for numerical test cases related to gas production from hydrate bearing sediments.…”

“…Reservoir compaction greater than 5% is a typical and consistent indicator for potential casing failure . Note that the simulation results in this study are based on the generalized reservoir model that contains several assumptions such as the failure model and geomechanical properties, which are approximated from other experimental and field data (Masui et al, 2005(Masui et al, , 2008Moridis et al, 2010;Rutqvist et al, 2011). Thus, a specific reservoir model for the PBU-L 106 unit C site with accurately measured geomechanical data and failure models is required for higher accuracy in geomechanical analysis.…”

“…In previous studies on gas hydrate reservoirs, Rutqvist et al (2011) performed geomechanical analyses on potential future long-term tests of gas production from hydrate deposits at Mount Elbert (Unit D) and Prudhoe…”