Role of Pore Fluid Pressure on Fracture Propagation
University of Maryland, USA
Wen-lu Zhu is professor of Geology at the University of Maryland. She received her PhD at Stony Brook University in New York. Upon graduation, she worked as a research scientist at the Woods Hole Oceanographic Institution before joining the faculty at the University of Maryland. Wen-lu’s research focuses on the relationships between deformation and fluid flow. She conducts laboratory experiments and quantifies the change of the microstructure of deforming rocks in 3-D at in-situ pressure and temperature conditions. For her work in the field of rock deformation, she was awarded the 2020 Louis Néel Medal by the European Geosciences Union.
Introduction of the Lecture
Recent experimental studies show that high pore fluid pressure causes a transition from rapid and dynamic to quasi-stable faulting in compact rocks such as granites. The stabilizing effect of pore fluid pressure on faulting can be explained by dilatant hardening—fault nucleation leads to creation of new void space, resulting in a decrease in pore fluid pressure and an increase in effective normal stress, which impedes further fault growth. It has been shown is that dilatant failure stabilization requires the deformation to be undrained, i.e., the rate of pore fluid pressure re-equilibration must be slower compared to the rate of deformation. Under laboratory loading rates, undrained conditions can be readily achieved in low permeability compact rocks. However, tectonic strain rates can be 6-10 orders of magnitude slower than laboratory strain rates. Thus, the stabilizing effect of pore fluid pressure observed in compact rocks may not be directly applicable in modeling rupture processes in nature. To circumvent the obvious physical limitation of conducting experiments at tectonic strain rates, we deformed porous sandstones with permeability 6-10 orders of magnitude higher than that of compact rocks at typical laboratory strain rates. Our experimental results show that porous sandstones subjected to high pore fluid pressures fail by slow faulting under fully drained conditions. We conducted quantitative microstructural analysis on deformed samples. Based on our findings, we proposed that the stress corrosion cracking played an important role in the pore fluid pressure stabilizing effect on fault propagation in porous rocks.