Scattering by hydraulic fractures: Finite-difference modeling and laboratory data

Reservoir production can be stimulated by creating hydraulic fractures that effectively facilitate the inflow of hydrocarbons into a well. Considering the effectiveness and safety of the operation, it is desirable to monitor the size and location of the fracture. In this paper we investigate the pos sibilities of using seismic waves generated by active sources to characteri ze the fractures. First, we must understand the scattering of seismic waves by hydraulic frac tures. For that purpose we use a finite-difference modeling scheme. We argu e that a mechanically open hydraulic fracture can be represented by a thin, fluid-filled layer. The width or aperture of the fracture is often small c ompared to the seismic wavelength, which forces us to use a very fine grid spacing to define the fracture. Based on equidistant grids, this results in a large number of grid points and hence computationally expensive problems . We show that this problem can be overcome by allowing for a variation in grid spacing in the finite-difference scheme to accommodate the large-scale variation in such a model. Second, we show ultrasonic data of small-scale hydraulic fracture experimen ts in the laboratory. At first sight it is difficult to unravel the interpr etation of the various events measured. We use the results of the finite-di fference modeling to postulate various possible events that might be presen t in the data. By comparing the calculated arrival times of these events wi th the laboratory and finite-difference data, we are able to propose a plau sible explanation of the set of scattering events. Based on the laboratory data, we conclude that active seismic sources can potentially be used to de termine fracture size and location in the field. The modeling example of fr acture scattering illustrates the benefit of the finite-difference techniqu e with a variation in grid spacing for comparing numerical and physical exp eriments.