P‐ and S‐wave anisotropy of a synthetic sandstone with controlled crack geometryl¹

Abstract

A method is reported for the production of synthetic porous sandstones containing cracks of known dimensions and geometry with respect to the matrix. A synthetic sandstone was manufactured from Sand cemented with an epoxy glue. The cracks of known geometry were introduced into the material in the manufacturing stage, by emplacing thin metallic discs in the Sand-epoxy matrix. These discs were chemically leached out of the consolidated porous sandstone. Acoustic anisotropy. and shear-wave splitting were observed in the synthetic sandstones. For the dry sample the observed angular dependence of the P-and S-wave velocities (at 100 kHz) compares well, qualitatively, with the theoretical models of Hudson and of Thomsen. Quantitatively, however, the experimental data fits Hudson's model better. For the case of a saturated sample the experimental results are in excellent agreement with Thomsen's model. Hudson's model, on the other hand, predicts a different angular dependence for P-waves. This demonstrates that the concept of fluid transfer between cracks and the ambient porosity can be a significant process. The results reported here are from the first successful experiment in which the theoretical models were tested on a porous material containing a known crack geometry .

Experimental observation of water saturation effects on shear wave splitting in synthetic rock with fractures aligned at oblique angles

“…Theoretical models are needed to interpret seismic data in terms of fracture properties and fluid properties. The combined effect of fluids (100% gas or liquid saturation) and aligned fractures have been studied theoretically and experimentally (e.g., Thomsen, 1995, Hudson, 1981, Hudson et al, 2001, Chapman, 2003, Ass'ad et al, 1992, Rathore et al, 1995. The fluid independent relationship between fracture density and shear wave splitting for wave propagation at 90 o to the fracture normal has been observed in laboratory experiments (e.g., , Rathore et al, 1995 in line with theoretical predictions (e.g., Hudson, 1981).…”

“…The fractures are just visible on the surface of the sample in Figure 1a but Figure 1b shows the idealised distribution throughout the sample volume. The synthetic rock samples were made from a mixture of sand, kaolinite and aqueous sodium silicate gel using a similar approach to Rathore et al (1995). A predetermined number of 2 mm diameter aluminium discs of 0.2 mm thickness were arranged on successive 4 mm layers of sand mixture.…”

“…The combined effect of fluids (100% gas or liquid saturation) and aligned fractures have been studied theoretically and experimentally (e.g., Thomsen, 1995, Hudson, 1981, Hudson et al, 2001, Chapman, 2003, Ass'ad et al, 1992, Rathore et al, 1995. The fluid independent relationship between fracture density and shear wave splitting for wave propagation at 90 o to the fracture normal has been observed in laboratory experiments (e.g., , Rathore et al, 1995 in line with theoretical predictions (e.g., Hudson, 1981). Theoretical predictions of fluid sensitivity of the slow shear-wave propagating at oblique angles to the fracture normal was observed in laboratory experiments by Tillotson et al (2011) using a synthetic fractured sample cored at 45 o to the fracture normal and saturated with single fluid phases of different viscosities.…”

Effective field method for seismic properties of cracked rocks

“…[62] Unfortunately, Rathore et al [1994] give no information about permeability coefficients of the samples; hence we cannot estimate Biot's frequency. However, one can assume that the frequencies for which the experiments were carried out are much higher than the frequencies where the Brown-Korringa theory works well.…”

“…[57] Rathore et al [1994] presented the measurement results of elastic waves velocities in sandstone containing artificial cracks of given geometry and orientation. The measurements were carried out for saturated by water and dry samples.…”