Stability Analysis for a Borehole to be Drilled into the Tohoku Fault Zone, Japan
Physical and Biological Sciences
Undergraduate Senior Thesis project, Earth Science
To study mechanisms that produce great subduction zone earthquakes such as the March 11, 2011 Tohoku-Oki earthquake, the Integrated Ocean Drilling Program (IODP) expedition 343 will engage in rapid response drilling into the recently ruptured fault. This expedition is based on the Japan Trench Fast Earthquake Drilling Project (JFAST) proposal 787, and is scheduled aboard the D/V Chikyu from April 1, 2012 through May 24, 2012. Prior to drilling, models are utilized to predict borehole stability under the intense pressure and stress conditions at the proposed drilling depth. In this study, the stresses acting within the proposed borehole were calculated to assess failure conditions for three possible farfield stress regimes: a reverse fault, a critically stressed shallowly dipping (5°) reverse fault, and a normal fault. The intermediate principle stress in each farfield stress model is allowed to vary from the value of the minimum to the maximum principle stress. The stress concentration around the circumference of the borehole is determined by the principle stresses acting within the horizontal plane of the vertical borehole. Failure by borehole wall breakouts are expected in this horizontal plane, as determined by the value of the intermediate principle stress and the rock strength of the borehole wall. To achieve a greater range in our stability analysis, the coefficient of friction and cohesion of the local rock is varied according to reasonably high and low values for an accretionary prism. To test the sensitivity of the stress concentrations to depth, stability analyses are also examined at two possible drill depths. This study shows that failure of the borehole depends strongly on the farfield stress state and cohesion, and stability decreases with depth. Both catastrophic collapse and successful completion are possible.
