Effects of overpressure variations on fracture apertures and fluid transport

• Published in: Tectonophysics Vol. 581; pp. 220 - 230
• Main Authors: Gudmundsson, Agust, Kusumoto, Shigekazu, Simmenes, Trine H., Philipp, Sonja L., Larsen, Belinda, Lotveit, Ingrid F.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 18.12.2012
• Subjects: Apertures; Crustal fluids; Crustal stresses; Dykes; Flow rate; Fracture mechanics; Fractures and faults; Hydrogeology; Mathematical models; Overpressure; Petroleum geology; Rock; Veins; Volcanology; Volcanology; Petroleum geology; Crustal fluids; Crustal stresses; Hydrogeology; Fractures and faults
• ISSN: 0040-1951; 1879-3266; 1879-3266
• DOI: 10.1016/j.tecto.2012.05.003

For an isolated rock fracture in a homogeneous, isotropic rock subject to constant internal fluid overpressure or constant external driving stress, the ideal opening-displacement (aperture) profile or variation is that of a flat ellipse. For many mineral veins, dykes, tension fractures, normal faults, and other rock fractures, however, the opening-displacements show irregular aperture (thickness) variations very different from that of a flat ellipse. Here we present field data on typical fracture-aperture variations, as well as new numerical and analytical models to explain these data. We present the overpressure variation by Fourier cosine series, a very flexible method that can be used to model abrupt overpressure and driving-stress variations in vertical and lateral sections for fractures of various sizes and types. We calculate the opening-displacements of typical hydrofractures, and discuss the results with reference to mineral veins and dykes. We also present numerical models showing that when a fracture dissects layer, or parts of a single layer (such as a lava flow), with different stiffnesses (Young's moduli), the opening-displacement may show irregular variation even when the overpressure is constant. From the cubic law, the volumetric flow rates in the large-aperture segments of the mineral veins and dykes discussed in the paper may have been as much as 3–5-times the flow rates in the small-aperture segments. We propose that differences in volumetric flow rates are related to irregular opening-displacement variations in feeder-dykes is one principal reason for the development of crater cones, a universal feature of volcanic fissures. ► Presents new data on abrupt, irregular variation in apertures of rock fractures of various types. ► Presents analytical solutions, using Fourier series, for overpressure variation in hydrofractures. ► Demonstrates the importance of flow channelling for fluid transport in rock fractures in general. ► Suggests that flow channelling is one main reason for crater–cone formation during fissure eruptions.