Recent developments in oil and gas production from very low permeability reservoirs involving hydraulic fracturing indicate that much of the hydraulic energy is dissipated in shear failure reactivation of preexisting planes of weakness. Not all preexisting planes of weakness will reactivate for a given hydraulic fracturing treatment and present day state of stress. Thus, it is useful to have the ability to predict the existence and orientation of planes of weakness that can be reactivated in an area.
Typically, planes of weakness result from rock rupture when the stress on the rock exceeds the strength of the rock. How frequently rock failure occurs is a function of the structural and burial history of the rock and the strength changes over time. Here, we propose a method that generates a stress history including estimates of stress magnitude and direction and evaluates the history of rock strength. Thus, we can determine when the stresses exceeded the rock strength and what planes of weakness in what orientations should have formed.
In our approach we first derive the stress history by combining the structural history with the burial history through a series of stress polygons. Next the rock strength history is developed through an iterative process using standard correlations with porosity and the stress history. We are using a modified p-q approach that is constrained by internal friction angle of the rock at the time of initial shear rupture as recorded in the dip of nearby fault planes. One of the initial base case assumptions in our approach in both the stress and rock strength histories is a normal pore pressure model.
To illustrate the method, we will present an example using readily available information for the Barnett Shale in the Fort Worth basin that supports the existence of the two known fracture sets. With our approach we will constrain whether these fracture sets formed during contractional deformation related to the late Paleozoic Ouachita orogeny, rifting in the Triassic-Jurassic, a short phase of additional burial in the Cretaceous, or subsequent minor exhumation.