Critical Crack Densities Due to Interactions of Tensile Stress Fields - Basis for a Practical Tool of Fracture Prediction

In previous work correlations between the fluctuations in flowrates at pairs of wells were shown to have characteristics consistent with the concept that the lithosphere is generally in a near-critical mechanical state. This concept has previously assumed spatial intersection of fractures at the critical point. Elastic tensile stress fields around dilated, stress-aligned fractures have been invoked by several authors as the basis for the genesis of faults: in this paper they are examined as the agents whereby a critical threshold of crack density can occur, a density which is lower than that required for crack coalescence. This is demonstrated, firstly on a regular 2D hexagonal grid, and then in 3D by estimating the percolation threshold of cracks and their associated tensile stress fields. This leads to a critical crack density in 3D of 0.035, within the range of crack densities interpreted from observations of shear-wave splitting in many different rocks. It provides a more satisfactory basis for the explanation and exploitation of flowrate correlations for determining at low cost the all-important fracture characteristics between wells. There are profound and commercial implications for reservoir engineering and geophysical aspects of rock-fluid behaviour from stress-sensitive cracks and permeability within a critical system.