3D Simulation of Borehole Breakouts in High-Porosity Sandstone using the Discrete Element Method

Jensen, R.; Hide, Sakaguchi,*

Dependent upon the microstructure of a rock, different mechanical behavior and failure mechanisms have been noted in stress-induced borehole breakouts. Relatively recently, Haimson has identified a somewhat counter-intuitive material response for high-porosity rocks such as Berea Sandstone. Long, slot-like fractures emanating from the borehole, perpendicular to the maximum principal stress, were observed in laboratory samples that had been subject to far-field stresses while undergoing drilling. It has been proposed that that the failure mechanism for this phenomenon is compaction band formation in front of the crack-tip and is dependent upon the grain bonding characteristics. This paper discusses the numerical simulation of borehole breakouts in high porosity sandstones using the 3-D discrete element method (DEM) code, OpenDEM. DEM models use discrete particles that interact only with neighboring particles. Therefore, there is great advantage to using a DEM code for this type of analysis where there is dis-aggregation of material in the region of interest. This advantage stems from the formulation of assumptions inherent to DEM that are more closely aligned to the micro-structural behavior of the rock. The numerical model uses a bonded particle model where the rock is represented as a collection of randomly sized spherical particles that are densely packed and bonded together at the particle contact points. The drilling action is modeled by incrementally removing particles in the region of the borehole. Drilling fluid is not modeled. Therefore, in order to account for the flushing action of the drill fluid that carry the de-bonded sand particles from the fracture, as the DEM particles break free from the simulation matrix, they are removed from the simulation. Simulation results will be presented showing qualitative representation of the borehole breakouts.