The Effect of Diagenesis on Fracture Aperture Development

Fracture mechanics models are a tool for predicting and analyzing the nature of natural fracture attributes. An important parameter that can be readily measured in natural fracture populations is the opening or aperture, so it is a good attribute to use for constraining model results. The nature of preserved aperture distributions in veins suggest that mineral precipitation and fracture growth often occur simultaneously. In addition to mineral precipitation in the fractures, there will also be concurrent mineral precipitation in the matrix rock around the fracture. This mineral precipitation, in both the fracture and the host rock around it will act to preserve fracture opening. A fracture mechanics model is used to examine the feedback between these aperture propping mechanisms and fracture network growth. Field observations from many authors indicate that veins or partially mineralized fractures often have power-law aperture distributions, and the explanation is sometimes given that the power-law is a consequence of a process that involves interactive feedback, such as the mechanical interaction between fractures. The modeling work to be presented suggests that only certain degrees of interactive feedback will generate a power-law distribution. Mechanical interaction without diagenetic propping of apertures does not result in a power-law aperture distribution, but power-laws do results with the diagenetic effects active during fracture propagation. An analogy will be drawn to the development of fracture length distributions, where numerical modeling can show that weak crack arrest mechanisms (non-intersecting) generate negative exponential length distributions while stronger crack arrest mechanisms (T-intersections) result in power-law distributions.