Geophysical characterization of the effects of fractures and stress on subsurface reservoirs

Abstract

We study the effect of fractures on reservoir characterization and subsurface rock property measurements using seismic data. Based on the scale of a fracture relative to seismic wavelength, we divide the dissertation into two parts: larger scale fractures and microcracks. In the first part, we study the sensitivity of seismic waves and their time-lapse changes in hydraulic fracturing to the geometrical and mechanical properties of fractures that have dimensions comparable to the seismic wavelength. Through our analysis, we give the general seismic response of a fracture with a linear slip boundary and introduce the fracture sensitivity wave equation for optimal time-lapse survey design. Based on the characteristics of scattering from fractures, we develop an approach to determine the fracture properties using scattered seismic waves. The applicability and accuracy of our method is validated through both numerical simulations and laboratory experiments. Application of our approach to the Emilio Field shows that two orthogonal fracture systems exist and the field data results are consistent with well data. In the second part, we study the effects of microcracks and in situ stress on the formation properties measured from borehole sonic logging. Formation property measurements in a borehole could be biased by the borehole stress concentration, which alters the near wellbore formation properties from their original state. To study this problem, we first develop an iterative approach, which combines a rock physics model and a finite-element method, to calculate the stress-dependent elastic properties of the rock around a borehole when it is subjected to an anisotropic stress loading. The validity of this approach is demonstrated through a laboratory experiment on a Berea sandstone sample. We then use the model obtained from the first step and a finite-difference method to simulate the acoustic response in a borehole. We compare our numerical results with published laboratory acoustic wave measurements of the azimuthal velocity variations along a borehole under uniaxial loading and find very good agreement. Our results show that the variation of P-wave velocity versus azimuth is different from the presumed cosine behavior due to the preference of the wavefield to propagate through a higher velocity region.