The paper reports on an investigation of the applicability of imaging microfractures with reflection tomography methods. The reconstruction and imaging algorithms are based on Kirchoff migration and are modifications of Center for Wave Phenomena/Seismic UNIX (CWP/SU) code, which is a free software package. Sensitivity modeling and preliminary laboratory experiments have yielded results indicating that microfractures and the surrounding process zone in rock can be successfully imaged. Reflection techniques, as opposed to transmission, require access to only one side of a body in question and has wide-spread applications in non-destructive evaluation.
This investigation tests an active acoustic method for imaging flaws and structure inside rock and other solids. The immediate purpose is to provide near-real-time imaging of crack growth in cooperation with a large-scale laboratory study to optimize hydraulic fracture design at the Shell Bellaire Technology Center. This large scale study is supported by the Gas Research Institute. The method requires access to only one surface of the test specimen, and this presents advantages over transmission methods. The imaging is achieved using seismic evaluation code based on CWP/SU (Cohen et al., 1994) developed at the Center for Wave Phenomena at the Colorado School of Mines. Originally designed for imaging of kilometer-scale geologic structures, adaptations now enable it to resolve images on a millimeter scale. Current efforts focus on determining the suitability of the technique and optimization of imaging methods and of future experiments through parametric simulations. Analysis of simulation results shows that it is possible to distinguish crack thickness of less than 1 mm and crack lengths to within 5 mm. In addition, Shell has provided test data that is used to image the growth of hydraulically induced fractures in a Tony Buff sandstone
A parametric modeling study was conducted to determine whether reflection seismology methods and Kirchoff migration are sensitive enough to image hydraulically-induced microfractures, and possibly the surrounding process zone. A sensitivity analysis was undertaken to refine imaging parameters and to determine the feasibility of imaging velocity perturbations. Synthetic seismograms were generated for a matrix of different imaging parameters including fracture and process zone geometry and transducer spacing, and source pulse center frequency.
Artificial source reflection seismology has been used extensively in recent decades to image the earth's subsurface for the purposes of exploring and developing hydrocarbon reservoirs (Anstey, 1977). Reflection seismology relies on the transmission of sound waves through solids and liquids according to well-understood elastic principles. As an acoustic wave travels through a body, it is relatively uninterrupted until it encounters a change in acoustic impedance (the product of density and velocity), such as would occur in a change of rock type, stress regime, or saturation. When such a discontinuity is encountered, a portion of the acoustic energy is transmitted through the boundary and a portion is reflected back toward the source. The relative amounts of signal transmitted and reflected are determined by the degree of the impedance contrast (e.g. Anstey, 1977).
