Controls on Fracture Evolution

Abstract

In this chapter, we use the new method to study some of the controls on fracture network evolution, with a particular emphasis on the properties that can be used to calibrate the models against outcrop or subsurface fracture networks. Thus, we show that the main controls on mean linear fracture density (P32) are the layer thickness and the stress shadows, but not the duration of deformation, since fracture networks tend to grow very rapidly (tens of thousands of years) and reach saturation early in the deformation episode. We show that lateral variation in the mechanical properties or the horizontal strain rate can control the fracture distribution and can result in a sharp “deformation front” sweeping across the structure, dividing it into unfractured and fully fractured zones. We show that the principal controls on fracture length and volumetric fracture density are the subcritical fracture propagation index (which controls the fracture propagation rate) and the initial microfracture density. However, because the fracture lengths do not scale linearly with layer thickness, this will affect the aspect ratio of the fractures. The subcritical fracture propagation index can also control the fracture mode (Mode 1 dilatant fractures or Mode 2 shear fractures), as well as the fluid overpressure and depth of burial. We then examine methods of quantifying the connectivity and anisotropy of the fracture network and look at the controls on these attributes. We discuss the controls on fracture orientation and different methods of calculating the horizontal strain depending on the cause of the deformation. Finally, we discuss how the data from these models can be used to calculate fracture porosity and permeability.