Summary

Unconventional reservoirs are becoming more and more conventional but successful drilling within these reservoirs has a unique set of problems. Most wells are drilled horizontally through the reservoir rock and fracking technology is applied to generate permeabilty and produce hydrocarbons. The pre-drill knowledge of natural fracture swarms and small offset faults is very important as these geological elements can interfere with the drilling and fracking process and influence the production rate. Seismic resolution from conventional reflection imaging is generally not sufficient to resolve these small scale rock properties.

Diffracted waves are events that are produced by the scattering of a wave after it meets a discontinuity such as fracture swarms, small amplitude faults and karsts that cause local sharp changes in the geometrical or lithological characteristics. A method for diffraction imaging that is based on coherent summation of diffracted waves was applied to a 3D data set over an unconventional oil reservoir. An integrated study that includes well information, diffraction energy and seismic attributes showcases the usefulness of diffraction events to predict fracture swarms within the Bazhenov formation, which is a black shale in West Siberia.

Introduction

Exploration and production of unconventional reservoir rocks is becoming more and more important. One of the main challenges is the detection of small scale geological objects such as faults and fracture corridors and swarms. We propose to utilize conventional seismic data to solve this problem as the wavefield generated by such subsurface elements is characterized by the presence of scattering or diffracted energy.

The amplitudes of diffracted waves are usually much weaker than those of specular reflections. Diffractions are essentially lost during the conventional processing/migration sequence, or they are masked in conventional seismic stacked sections. Local structural and lithological elements in the subsurface of a size comparable to the wavelength are usually ignored during processing and identified only during interpretation.

Efforts to image diffraction events were undertaken in Landa et al. (1987), Kanasewich and Phadke (1988), Landa and Keydar (1998), Fomel et al. (2007), Moser and Harpen (2006), Berkovitch et al. (2009). Separation of diffracted and reflected wavefields based on different kinematic properties was proposed in Khaidukov et al. (2004). Taner et al. (2006) and Klokov et al. (2011).

In this paper we present a generalization of the method proposed by Berkovich et al. (2009) to a 3D data set. The method is based on the MultiFocusing moveout time correction, which adequately describes not only reflection but also diffraction events. Optimal summation of the diffracted events and attenuation of the specular reflections allows creating an image that contains mostly diffraction energy. We briefly describe the theory of the MultiFocusing method and demonstrate the efficiency of the proposed diffraction imaging technique on a case study.

URTeC 1575917

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