Angel Acosta-Colon

Wave propagation across partially infill fracture

Natural fractures are often filled with debris. This debris originates from many different mechanisms: organic and/or inorganic chemical reactions (such as mineralization), sediment transport, formation of the fracture, mechanical weathering or combinations of these processes. In many cases, the presence of debris forms a sub-porosity within the fracture void space. In this study, we investigate how the existence of a subporosity affects seismic wave propagation and consequently our ability to probe changes in the fractures properties caused by the formation or alteration of a sub-porosity. Laboratory experiments were performed to examine acoustic wave scattering from packing of spherical beads used to create a sub-porosity within a synthetic fracture. The sub-porosity in the fracture was created by using spherical beads with a range of diameters from 500 μm to 7.79 mm. Compressional waves were transmitted across the fracture using contact piezoelectric transducer. The analysis was performed for a fixed aperture with different sub-porosity created by single and multiple layers. Interpretation of the acoustic response depends critically on understanding the length scales associated with the layers (bead diameter, layer thickness), the aperture of the fracture and seismic length scales (wavelengths, field of view). The detection of multiple layers of debris within a fracture is possible by understanding wave interference, dispersion and the reflections in the waveform.

Reactive flow in a fracture: scale effects in the interpretation of seismic measurements.

Summary In this study, a fracture in a carbonate rock is chemically eroded to determine if changes in hydraulic properties can be determined from seismic measurements. Hydraulic and seismic measurements were made prior to and after the invasion of a hydrochloric acid in an initially watersaturated fracture in limestone. Seismic measurements were collected before, during and after the reactive fluid flow. Two methods for monitoring the seismic response of the sample were used: 1) a detailed compressional-wave transmission map of the sample; and 2) seismic measurements as a function of scale. In the second method, the region probed by the seismic beam varied from 15 mm to 30 mm to 60 mm to determine the effect of scale in interpreting the changes in fracture properties. To vary the size of the region probed, a set of acoustic lens was developed to collimate the seismic wavefront over different widths. The measured signals for both experiments were analyzed for wave attenuation and frequency content as a function of scale. The results show that the scale of measurement affects the interpretation of the effect of acid etching on the seismic properties of fractures. Using seismic techniques to monitor geochemical changes in fractures requires the appropriate choice of the seismic probe in terms of size and area of coverage.