Injection or production of fluids from subsurface reservoirs lead to
stress changes affecting both reservoir and surrounding rocks. For
low-permeable caprocks overlying such reservoirs, the movement of pore
fluids to or from the formation is restricted and the immediate and
short-term response to changes in the stress field will be undrained.
Consequently, stress changes transfer partly into pore pressure changes.
The aim of the current study is to investigate theoretical means of
forecasting the undrained pore pressure generation in the Draupne
Formation shale and to compare predictions with experimental results.
Predictions are based on measurements from a single undrained triaxial
test on a sample with known orientation, and a combination of Skempton's
classical formulation and anisotropic poroelastic theory. The predicted
pore pressures are compared to measured pore pressures from a series of
triaxial tests on samples with various orientations exposed to different
total stress paths. First, it is confirmed that the normalized undrained
pore pressure measured is linearly connected to the total stress path.
Then it is demonstrated that a tensorial pore pressure parameter can be
used to accurately predict the influence of stress orientation on
generated pore pressure. Lastly, it is experimentally confirmed that the
two predictions can be combined to predict the pore pressure arising
from stress changes along any compressional stress path and orientation.
The observations herein may contribute significantly to the
understanding of induced pore pressure in low-permeable materials and
provide valuable input to geomechanical modeling of various field
operations.