Fracture characterization from seismic measurements in a borehole
Author(s)Bakku, Sudhish Kumar
Massachusetts Institute of Technology. Department of Earth, Atmospheric, and Planetary Sciences.
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Fracture characterization is important for optimal recovery of hydrocarbons. In this thesis, we develop techniques to characterize natural and hydraulic fractures using seismic measurements in a borehole. We first develop methods to characterize a fracture intersecting an open borehole by studying tubewave generation and attenuation at the fracture. By numerically studying the dispersion relation for fluid pressure in the fracture, we show that the tubewave measurements made in the transition regime from low to high frequency can constrain fracture compliance, aperture and length, while measurements made in the high-frequency regime can place a lower bound on fracture compliance. Analysis of field data suggest a large compliance value (10- 0m/Pa) for a meter-scale fracture and supports scaling of fracture compliance and applicability of scattering based methods for fracture characterization on a reservoir scale. We next study Distributed Acoustic Sensing (DAS), a novel Fiber Optic (FO) cable based seismic acquisition technology. We relate DAS measurements to traditional geophone measurements and make a comprehensive study of factors that influence DAS measurements. Using a layered borehole model, we analytically compare the sensitivity of DAS measurements to P- and S-wave incidence at arbitrary angles for the cases when the FO cable is installed in the borehole fluid or when cemented outside the casing. In addition, we study the azimuthal placement of the cable, the effect of cable design, and the effect of environmental conditions on time-lapse measurements. We show that DAS is a reliable tool for time-lapse monitoring. Finally, we analyze time-lapse DAS Vertical Seismic Profiling (VSP) data collected during a multi-stage hydraulic fracture treatment of a well drilled into a tight gas sandstone reservoir. We develop a processing workflow to mitigate the unique challenges posed by DAS data and propose methods for DAS depth calibration. We observe systematic and long-lived (over 10 days) time-lapse changes in the amplitudes of direct P-waves and nearly no phase changes due to stimulation. We argue that the time-lapse changes cannot be explained by measurement factors alone and that they may be correlated to the stimulated volume. Though the current geometry is not ideal, DAS is promising for hydraulic fracture monitoring.
Thesis: Ph. D., Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, 2015.Cataloged from PDF version of thesis.Includes bibliographical references (pages 219-227).
DepartmentMassachusetts Institute of Technology. Department of Earth, Atmospheric, and Planetary Sciences.
Massachusetts Institute of Technology
Earth, Atmospheric, and Planetary Sciences.