• Simulation and analysis of wellbore stability in permafrost formation with FLAC

      Wang, Kai; Patil, Shirish; Chen, Gang (2015-07)
      Permafrost underlies approximately 80% of Alaska. Permafrost's high sensitivity to temperature variations plays a significant role in the stability of wellbores drilled through permafrost formations. Wellbore instability may cause stuck pipes, lost circulation, and/or collapse of the wellbore, resulting in extra cost and time loss. In order to minimize the influence of the heat produced during drilling, a vertical well is the only choice to penetrate permafrost formation. Fast Lagrangian Analysis of Continua (FLAC) was used in this simulation to test the minimum wellbore pressure to maintain stability in a permafrost formation. Three layers were set in the simulation model: clay, silt, and sand. With the drilling fluid temperature set at 343K and a 267K initial formation temperature, four different thermal times, i.e. 1 week, 1 month, 1 year, and 5 years, were tested to determine the minimum stable pressure. Pore pressure of the formation has the strongest effect on this pressure. And in a short operation period, drilling fluid temperature will not influence the minimum mud pressure value significantly. A regression analysis was conducted on the simulation results, and the minimum wellbore stable pressure was found to be a function of pore pressure, cohesion, frictional angle, temperature difference, conductivity difference, thermal time, and wellbore radius. With the help of this function, engineers could calculate stable pressure for wells in arctic area before drilling based on drilling fluid temperature.