The Effect of Scale on Seismic Monitoring of Fracture Alteration by Reactive Flow

The interpretation of fracture properties obtained from seismic measurements changes with the scale of seismic observation because of intrinsic length scales within the fracture. The length scales include those associated with the geometry of the fracture and the flow path, the wavelength of the seismic signal, and the size of the region probed by the seismic beam. In this study, we focus on how the scale of observation influences measured changes in fracture specific stiffness caused by reactive flow in a fracture. We used an acoustic lens system to produce pseudo-collimated acoustic beams with diameters of 15 mm, 30 mm and 60 mm from water-coupled spherically-focused 1 MHz piezoelectric transducers. The lens system was used to examine how the scale of observation alters the interpretation of seismic measurements made on a fractured carbonate rock (150 mm in diameter and 76 mm in height) and on a standard intact acrylic sample (approximately the same dimensions). Seismic measurements (as a function of beam diameter) and volumetric flow rates were made on the fractured sample prior to and after reactive flow with an HCl solution. In each case, the different beam diameters were used to probe the same region of the fracture. Thus, the number of signals obtained at the 15mm, 30 mm and 60 mm scales were 16, 4 and 1, respectively. Statistical analysis was used to compare measurements made at the 60 mm scale with average values from the 15 mm and 30 mm scale, and to compare measurements made before and after reactive flow. In addition, high resolution two-dimensional maps over an 80 mm x 80 mm region of the fracture were obtained through diffraction-limited acoustic mapping without the use of the lenses. Prior to reactive flow the mean transmission coefficient for the 30 mm scale were consistent with the 60 mm scale but the 15 mm exhibited a slight deviation from the mean (about 9%). This slight drop in the transmission at the 15 mm scale is caused by diffraction losses not captured within the numerical aperture of the lens. After reactive flow, the 30 mm and 60 mm scales again showed no significant change in seismic transmission relative to each other and relative to pre-reactive flow. This was despite the fact that the volumetric flow rate through the fracture decreased by 32%. A possible conclusion from these larger scales alone would have been that seismic measurements were not detecting the change in fracture geometry caused by the reactive flow. However, when the transmission coefficient for the 15 mm scale is added to the picture, it showed a 23% decrease in mean value compared to the 30 mm and 60 mm scales. Furthermore, a two-point correlation analysis of the high resolution seismic scans showed that the spatial correlation length of the fracture was roughly isotropic prior to reactive flow (horizontal: 12 mm & vertical: 11 mm) but became anisotropic (horizontal: 14 mm & vertical: 25 mm) after reactive flow. The 15 mm scale was the only probe scale that was below the correlation length, and was also the only probe scale to detect the alteration of the fracture. These results suggest that the ability for seismic measurements to detect changes in fracture geometry depends on the size of the region probed, with greater sensitivity attained at scales smaller than fracture-related correlation lengths. Acknowledgments: LJPN wishes to acknowledge the Geosciences Research Program, Office of Basic Energy Sciences US Department of Energy and the University Faculty Scholar program at Purdue University.