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

Abstract

We perform numerical simulation for the gas hydrate reservoir, in the vicinity of Prudhoe Bay Unit L-Pad on the North Slope (i.e., Unit C in the PBU-L 106 site), considering vertical and horizontal well production scenarios. In order to analyze coupled flow and geomechanics more rigorously we employ two-way coupling between fluid flow and geomechanics, and compare the results with those from one-way coupling used in previous studies, where two-way coupling accounts for changes in pore volume induced by geomechanics, while one-way coupling does not. We find clear differences in the variables of flow and geomechanics between one-way and two-way couplings in this field case (e.g., pressure and effective stress). Using geomechanical properties used previously for the PBU-L 106 C unit, we find that the effective stresses are within the elastic region, located away from the Mohr–Coulomb yield function for both vertical and horizontal well production scenarios. This indicates that there is little danger in geomechanical instability and failure. We also investigate vertical displacement to assess well stability, using two-way coupling. The results from the vertical well scenario show small vertical displacement, from which we anticipate that the vertical well will be stable and safe. On the other hand, the horizontal well scenario causes larger subsidence for a given simulation time because of higher production rates. Even in the case that the hydrates are completely dissociated and the aqueous phase pressure is equilibrated with the constant bottom hole pressure, the estimates of the maximum vertical displacement and strain are 73cm and 2%, respectively, which do not appear to be a danger of potential well failure. Based on the results and analyses, the horizontal well production is feasible for gas production from the hydrate layers of Unit C in the PBU-L 106 site. But the reservoir model used in this study is relatively generalized. Thus, a specific reservoir model for the site will be required for higher accuracy in the future, after we obtain accurately measured geomechanical data and failure models.