Controls on fracture density and size: Insights from dynamic modelling

We have developed a new technique to generate more realistic discrete fracture network (DFN) models by dynamically simulating the propagation of fractures with models that use the equations of motion and conserve mass and momentum, and include the interactions between different fractures. We use this to simulate the propagation of layer-bound fractures in a thin, homogeneous reservoir, and compare the resulting cumulative fracture density functions (P30 and P32) generated under different conditions. A critical factor in the models is the subcritical fracture propagation index b. Subcritical fracture propagation, which can occur at low stress but is characterised by low propagation rates controlled by chemical and thermal processes at the fracture tip, results in a large number of relatively short fractures. By contrast critical fracture propagation, characterised by rapid rupture at high stress, generates a smaller number of fractures with a greater range of lengths, including some very long fractures, although the total fracture area is similar to that generated by subcritical propagation. Intersection with orthogonal fractures under biaxial strain conditions, and stress shadow interactions between parallel fractures, can also affect the resulting fracture area and lengths.