Shock-induced Stoneley waves in carbonate rock samples

Abstract

To assess the productivity of oil- and gas-bearing carbonate reservoirs, acoustic waves are used to extract information on the reservoir properties. We performed shock-induced broadband linear acoustic wave experiments on fractured carbonate samples with different fracture apertures. Samples were prepared from a full-diameter carbonate rock core in which the aperture of a single fracture could be varied by means of spacers. Microseismograms were constructed from multiple wave experiments. We measured the wave speeds of different wave modes in the annulus along the cylindrical core sample and the transmission coefficient of the Stoneley wave over the fracture. We found that the transmission coefficient of the annulus Stoneley wave can directly be related to the fracture aperture and the fracture length. Comparison with a straightforward quasi-1D theory yielded best-fit values for fracture aperture and length that were in agreement with the actual values, for the fracture aperture determined by the size of the spacers. The values of the fracture length were not accurately predicted probably due to the underestimation of the fracture compliance in the experiment. The inversion method can be optimized for carbonate reservoir conditions to extract fracture data from Stoneley wave transmission and reflection coefficients.