A refraction method to detect reservoir velocity and anisotropy

Abstract

Since its inception in the early 1980s, detection of fractures and stress using P-wave reflection amplitudes and traveltimes has proved to be challenging. A significant amount of theory has been developed, but convincing and calibrated applications of the theory to field data have been lacking. This is mainly because of the physical limitation that P-wave reflection amplitudes sample only a small areal region (Fresnel zone) of the reservoir. Similarly, P-wave reflection traveltimes sample only a limited thickness (twice near-vertical reservoir thickness) compared with total traveltime from the source to the receiver. As a result, estimation of reservoir anisotropy from P-wave reflection data is inherently limited. A new method is used to detect fracturing and stress in addition to reservoir velocity. When available, the use of refracted P-wave traveltimes from a target of interest can provide robust information on reservoir velocity and anisotropy caused by fractures and horizontal stress variations. This is because refracted waves travel horizontally inside the medium under investigation and sample a large section of the target, integrating the anisotropic variation along different azimuths. Azimuthal variation of refraction traveltimes from the investigated medium can be used to invert for velocity and anisotropy. This traveltime method is applied to the highly fractured Joanne reservoir in the U. K. sector of the Central North Sea.