Numerical Modeling of Induced Seismicity: Background, Code Development, and New Results for the 
Porosity-Pressure Constiutive Relationship

Human-caused earthquakes are becoming more common in Oklahoma and Colorado with the rise of wastewater 
injection deep underground.  Pore fluid pressure, in-situ stress state, fault orientation, and the 
properties of rocks and fractures all help dictate if induced seismicity will occur.  In the first part 
of the talk, we discuss how the code PFLOTRAN can be used to faithfully model key aspects of induced 
seismicity such as the coupled nature of fluid pressure and geomechanics, the ability for hydraulic 
diffusivity to change as a function of effective stress and shear failure history, and the importance 
of thousands of fractures and faults both mechanically and hydrologically in induced seismicity.  
PFLOTRAN is a massively-parallelized, coupled poromechanical flow code, but incorporation of fracture 
capability must be futher developed.  Once implemented, this tool should offer insights into the 
spatiotemporal patterns of induced seismicity, seismogenic permeability, and the effects of background 
stress state and fault orientation on induced seismicity likelihood.
In part two of the talk, we examine mathematically and numerically how subsurface flow codes that can 
accept a number of constitutive relations for porosity, fluid density, and fluid flux can be used to 
simulate saturated flow.  Examples of these codes include PFLOTRAN, FEHM, STOMP, TOUGH2, and the oil 
and gas simulator BOAST.  We show that the governing equations of these codes often differ from the 
groundwater flow equation by not properly accounting for the motion of liquid relative to solid.  We 
propose an alternate porosity-pressure relationship that brings the governing equations of these 
subsurface flow codes into agreement with the groundwater flow equation.  This alternative porosity 
model results in much better agreement with the Theis solution, and it should be incorporated into 
these codes.