Unconventional-Reservoir Characterization With Azimuthal Seismic Diffraction Imaging

Abstract

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper URTeC 2695232, “Unconventional-Reservoir Characterization With Azimuthal Seismic Diffraction Imaging,” by Dmitrii Merzlikin, Sergey Fomel, Xinming Wu, and Mason Phillips, The University of Texas at Austin, prepared for the 2017 Unconventional Resources Technology Conference, Austin, Texas, USA, 24–26 July. The paper has not been peer reviewed. The complete paper proposes an azimuthal plane-wave-destruction (AzPWD) seismic-diffraction-imaging work flow to efficiently emphasize small-scale features associated with subsurface discontinuities such as faults, channel edges, and fracture swarms and to determine their orientation by properly accounting for edge-diffraction phenomena. The work flow is applied to characterize an unconventional tight-gas-sand reservoir in the Cooper Basin in Western Australia. Extracted orientations of edges provide valuable additional information, which can be used by the interpreter to locate finer-scale features and distinguish them from noise. Introduction Unconventional reservoirs may exhibit high structural variability, which is difficult to characterize with a discrete wells network. 3D reflection seismology allows the extraction of additional information about the subsurface with significantly denser spatial sampling intervals. However, conventional images of the subsurface have low spatial resolution and are dominated by continuous and smooth reflections, which carry the information associated with only large-scale heterogeneities. Diffraction images are more capable than conventional reflection images in emphasizing small-scale features associated with subsurface discontinuities. Many studies employ diffraction images as a source of additional information for interpretation. Past work has proposed an AzPWD work flow, which extends a plane-wave destruction diffraction imaging framework to account for edge-diffraction orientation and allows efficient extraction of these orientations on the basis of scanning of different azimuths. Two modifications to the previously proposed AzPWD work flow are considered in this paper. Edge-diffraction-orientation determination is performed through a structure-tensor estimation based on PWD. A PWD-based structure tensor allows determination of edge-diffraction orientation from two volumes corresponding with PWDs applied in inline and crossline directions. Thus, no scanning over the PWD azimuth is required. Because reflection/diffraction separation is performed on the basis of the PWD filter in the data domain, the approach pertains to a diffraction imaging framework. The structure-tensor estimation approach has been applied previously in the image space on the basis of image gradients, the highest contribution to which is connected with strong and coherent reflection events masking diffractions. The second improvement presented by the AzPWD work flow is based on additional smoothing along the edge orientation, which makes the diffraction/ reflection-separation operator equivalent to the structure-oriented Sobel filter.