Effect of Viscous Fluids on P-to-S Converted Waves from Fractures with Oriented Micro Cracks

Abstract:

Oriented micro cracks often occur during the early stage of formation of large faults. Previous work (Nakagawa et al., 2000) demonstrated that compressional to shear (P-S) wave conversions occur even at normal incidence from fractures with oriented micro cracks because of cross-coupling fracture compliances. Here, we examine, experimentally, the effect of fluid-filled oriented micro cracks on the existence and behavior of P-S converted modes using 3D printed sample with oriented micro cracks that are saturated with a fluid. In this study, water and silicon oils with different viscosities (1, 10, 100, 1000, 10k, 100k cSt) were used to study energy partitioning through the fractures.

A FormLabs 2 3D printer was used to fabricate samples (50 mm x 50 mm x 100 mm) with a linear array of micro cracks oriented at 45⁰ (the angle that produces the largest amplitude P-S wave) with a printing resolution of 0.025 mm. The micro cracks were elliptical in cross-sections (2 mm long by 1 mm wide), through the 50 mm thickness of sample, and spaced 3 mm apart (center-to-center for adjacent cracks). Broadband transducers (0.2-1.5 MHz) were used to transmit and receive P and polarized S wave signals that were propagated at normal incidence to the linear array of micro cracks.

Examination of transmitted P-P and S-S waves, and P-S converted mode reveals that the P-S converted mode exhibited the most sensitivity to fluid viscosity. We observed changes in the spectral content of the P-S mode when a viscous fluid filled the voids. The central frequency of the P-S converted mode was observed to increase (100-200 kHz) while P-P and S-S transmitted signals through silicon oil-filled cracks exhibited a smaller shift (50-100 kHz) compared to air-filled micro cracks. The P-P and S-S wave modes showed slight increases and decreases, respectively, in amplitude (less than 10%) when the fluid viscosity increased. However, the amplitude of the P-S mode exhibited a significant decrease (about 30% between 1 - 100k cSt). This suggests that P-S converted mode provides a potential method to remotely probe changes in fluid viscosity in fractures.

Acknowledgment: This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Geosciences Research Program under Award Number (DE-FG02-09ER16022).