Fracture Characterization from Tubewaves in Boreholes

Abstract

The ability to characterize the flow properties of fractures in the vicinity of a borehole is of considerable importance for reservoir engineering; prediction of reservoir performance and for making decisions about drilling required to improve reservoir performance. While there are several methods for characterizing fractures that intersect a borehole, the ability to reliably predict their flow properties is lacking. We propose an improved model for using the amplitude of the tube wave that is generated during Vertical Seismic Profiling (VSP) surveys at the location where a permeable fracture intersects a wellbore for estimating the mechanical compliance of the fracture, which may be related to the flow characteristics of the fracture. Since the tubewave is generated by flow into the borehole from the fracture in response to the incident compressional wave from the VSP source, the tubewave provides direct evidence about which fractures have flow paths connected to the borehole. We use the amplitude of the tubewave to estimate the compliance of the fracture. We describe a new model to understand tubewave generation at a fracture intersecting a borehole. By numerically solving the dispersion relation in the fracture, the amplitude ratio of generated tubewave to incident P-wave was studied over all frequency ranges. Based on the results of the model, we propose that measuring amplitude ratios near a frequency where the pressure regime in the fracture transitions from wavelike to diffusion like in response to the incident seismic wave can help constrain fracture compliance and a parameter related to fracture aperture. For typical in situ conditions, this transition frequency is at the lower range of that used for VSP surveys. However, measurements in a high frequency limit can place a lower bound on fracture compliance. Applying the model to previously published VSP data, we argue that compliance values of the order 10-10 to 10-9 m/Pa are possible in the field. These values are large enough to allow surface seismic data to be influenced by scattering from fractures so that they may be mapped in the subsurface from surface measurements made under appropriate conditions. In addition, by estimating compliance of fractures that intersect a borehole we may obtain relative information about their flow properties.