VISCOPLASTIC DEFORMATION OF THE BAKKEN AND ADJACENT
FORMATIONS AND ITS RELATION TO HYDRAULIC FRACTURE GROWTH
*Yi Yang
Dept. of Geophysics, Stanford University
California, USA
(now at Shell, USA)
(*Corresponding author: yyangpku@gmail.com)
Mark Zoback
Dept. of Geophysics, Stanford University
California, USA
ABSTRACT
We report laboratory studies of the time-dependent deformation of core samples from
four different formations in the Williston Basin - the Lodgepole formation, the Middle
and Lower Bakken formations and Three Forks formation. The laboratory tests reveal
varying amounts of viscoplastic deformation in response to applied differential stress.
The time-dependent deformation is generally greater in rocks with higher clay and
organic content, and can be described by a power-law function. Because the magnitude of
the creep strain is linearly proportional to the applied differential stress, we can utilize
viscoelastic theory and geophysical logs to estimate the degree to which tectonic stress is
affected by viscoplastic stress relaxation. We suggest that viscoplastic stress relaxation
results in the Upper and Lower Bakken Formations act as frac barriers during hydraulic
fracture stimulation in the Middle Bakken, but the Lodgepole and the Three Forks
Formations are not frac barriers.