HR: 08:45h
AN: H51B-04    [PDF]
TI: Seismic Monitoring of Mineral Precipitation in a Single Fracture
AU: Gilbert, Z
EM: gilbertz@purdue.edu
AF: Purdue University, Department of Physics 525 Northwestern Avenue, West Lafayette, IN 47907-2036 United States
AU: * Pyrak-Nolte, L J
EM: ljpn@physics.purdue.edu
AF: Purdue University, Department of Physics 525 Northwestern Avenue, West Lafayette, IN 47907-2036 United States
AU: * Pyrak-Nolte, L J
EM: ljpn@physics.purdue.edu
AF: Purdue University, Department of Earth and Atmospheric Sciences 550 Stadium Mall Drive, West Lafayette, IN 47907-2051 United States
AB: Natural fractures and faults may be subjected to chemical dissolution and/or chemical precipitation of minerals that alter the fracture properties by changing the size and strength of the contact area and/or filling-in the void space. In this study, we used seismic measurements to monitor mineral deposition and its effect on hydraulic properties of single fractures. Hydraulic and seismic measurements were made prior to and after the deposition of CaCO$_{3}$ in initially water-saturated fractures in granitic samples (110 mm x 104 mm x 70 mm). The transmitted compressional waves were recorded in 1 mm increments within a 64 mm by 64 mm region of the sample to determine the spatial variation in fracture properties. Plane wave 1 MHz transducers were used to send and receive the signals. Four pairs of ports were distributed around the perimeter of the fracture to measure the variation in flow as a function of position. During mineral deposition, the flow rate at each port was monitored and the acoustic response was monitored at the center of the sample. Over a period of a month after mineral deposition, acoustic measurements were recorded over the two-dimensional region. Fluid flow measurements were repeated one month after chemical invasion. We observed that largest increases in seismic amplitudes after mineral precipitation occurred for fractures that were initially more compliant. The initial aperture of the fracture controls the amount and time-rate of mixing of the chemicals used to induce precipitation because mixing of the two chemical species is slower for smaller aperture fractures. The flow rate through the sample decreased by an order of magnitude and certain ports no longer supported flow, i.e., the flow paths to certain ports were blocked by mineral deposition. The most reliable seismic indicator that the fracture has been altered is the shift in the most probable frequency in the received signal. The most probable frequency shifted to higher frequency after chemical invasion indicating a stiffening of the contact regions of the fracture and possibly a reduction in fracture aperture through mineral deposition. The frequency shift may provide a seismic diagnostic method for remote assessment of alteration of a fracture by mineral deposition. Acknowledgments: The Authors acknowledge support of this research by the Geosciences Research Program, Office of Basic Energy Sciences, US Department of Energy. LJPN wishes to acknowledge Purdue University Faculty Scholar.
DE: 1829 Groundwater hydrology
DE: 1899 General or miscellaneous
DE: 5102 Acoustic properties
DE: 5104 Fracture and flow
DE: 5114 Permeability and porosity
SC: Hydrology [H]
MN: 2003 Fall Meeting