A computationally efficient numerical model based on the discrete-element method (DEM) is described and applied to simulate well-known borehole failure phenomena: hydraulic fracturing and borehole breakouts. Radially graded, two-dimensional DEM models of the near-wellbore region were created of bonded disk elements. Inspired by the molecular model of a fluid, source elements were used to simulate fluid pressurization of the model borehole subjected to far-field stresses.
 
The calibration and validation of such numerical tools will require extensive comparison against experimental and field data (Wawersik, 2000). To address this challenge, a joint experimental-numerical research effort has been undertaken to develop a robust numerical simulation capability for the exploration and prediction of near-wellbore mechanics. A true-triaxial vessel has been designed and constructed to enable the realistic laboratory simulation of the three-dimensional stress conditions present in the field (Wawersik et. al, 1997).
 
The structural damage in the DEM models was analyzed using histograms of the angular distribution of bond damage; results obtained for various stress states showed qualitative reproduction of the gross failure mechanisms associated with both hydraulic fracturing and borehole breakouts. The results from the laboratory simulation of near-wellbore failures demonstrate DEM's ability to capture the discontinuous failure processes under different stress conditions.

Collaboration with B.K. Cook, D.R. Bronowski, A.A. DiGiovanni, E.D. Perkins†, and J.R.Williams† (†-MIT).