CONTROL OF THE PERMEABILITY OF FRACTURES IN GEOTHERMAL ROCKS

Open Access
Author:
Faoro, Igor
Graduate Program:
Energy and Geo-Environmental Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
October 21, 2009
Committee Members:
  • Derek Elsworth, Dissertation Advisor
  • Derek Elsworth, Committee Chair
  • Chris Marone, Committee Chair
  • Charles James Ammon, Committee Member
  • Demian Saffer, Committee Member
Keywords:
  • TCHM properties of fractures
  • permeability
Abstract:
This thesis comprises three journal articles that will be submitted for publication (Journal of Geophysical Research-Solid Earth). Their respective titles are:”Undrained through Drained Evolution of Permeability in Dual Permeability Media” by Igor Faoro, Derek Elsworth and Chris Marone, “Evolution of Stiffness and Permeability in Fractures Subject to Thermally-and Mechanically-Activated Dissolution” by Igor Faoro, Derek Elsworth Chris Marone; “Linking permeability and mechanical damage for basalt from Mt. Etna volcano (Italy)” by Igor Faoro, Sergio Vinciguerra, Chris Marone and Derek Elsworth. Undrained through Drained Evolution of Permeability in Dual Permeability Media: temporary permeability changes of fractured aquifers subject to earthquakes have been observed and recorded worldwide, but their comprehension still remains a complex issue. In this study we report on flow-through fracture experiments on cracked westerly cores that reproduce, at laboratory scale, those (steps like) permeability changes that have been recorded when earthquakes occur. In particular our experiments show that under specific test boundary conditions, rapid increments of pore pressure induce transient variations of flow rate of the fracture whose peak magnitudes decrease as the variations of the effective stresses increase. We identify that the observed hydraulic behavior of the fracture is due to two principal mechanisms of origin; respectively mechanical (shortening of core) and poro–elastic (radial diffusion of the pore fluid into the matrix of the sample) whose interaction cause respectively an instantaneous opening and then a progressive closure of the fracture. Evolution of Stiffness and Permeability in Fractures Subject to Thermally-and Mechanically-Activated Dissolution: we report the results of radial flow-through experiments conducted on heated samples of Westerly granite. These experiments are performed to examine the influence of thermally and mechanically activated dissolution on the mechanical (stiffness) and transport (stress-permeability) characteristics of fractures. The sample is thermally stressed to 80C and measurements of the constrained axial stress acting on the sample and of the flow rate of the fracture are recorded with time. Net efflux of dissolved mineral mass is also measured periodically to provide a record of rates of net mass removal. During the experiment the fracture permeability shows high sensitivity to the changing conditions of stress and temperature but no significant permanent variation of permeability have been recorded once the thermal cycle ends. Linking permeability and mechanical damage for basalt from Mt. Etna volcano (Italy): volcanic edifices, such as Mt. Etna volcano (Italy), are affected from repeated episodes of pressurization due to magma emplacement from deep reservoirs to shallow depths. This mechanism pressurizes the large aquifers within the edifice and increases the level of crack damage within the rocks of the edifice over extended periods of times. In order to improve our understanding of the complex coupling between circulating fluids and the development of crack damage we performed flow-through tests using cylindrical cores of Etna Basalt (Etna, Italy) cyclically loaded either by constant increments of the principal stress: sigma1 (deviatoric condition), or by increments of the effective confining pressure: sigma1 =sigma2 = sigma3 (isostatic conditions). Under hydrostatic stresses, the permeability values of the intact sample decrease linearly with the increments of pressure and range between 5.2*10-17 m^2 and 1.5*10-17 m^2. At deviatoric stresses (up to 60 MPa) the permeability from the initial value of 5*10-17 m^2 slightly decays to the minimum value of 2*10-17 m^2 observed when the axial deviatoric stresses range between 40 MPa and 60 MPa. For higher deviatoric stresses, increases to 10-16 m^2 are then observed up to the peak stress at 92 MPa. After failure the permeability persisted steady at the value of 8*10-16 m^2 for the whole duration of the test, independently from the applied stress. We interpreted the decrease observed as due to the progressive closure of the voids space, as the axial load is incremented.