Improved Fracture Characterization by Utilizing Seismic-Derived Attributes including Anisotropy and Diffraction Imaging in a Giant Offshore Carbonate Field, UAE

Abstract

We present a case study of fracture characterization by integrating borehole data with a variety of seismic attributes in a carbonate reservoir from a giant offshore field, United Arab Emirates. The objectives are to determine to what extent seismic data may be confidently used for mapping spatial distributions of subtle faults and fracture corridors in the reservoirs and to better understand the distribution of overburden anomalies (karsts, high impedance channels) for field development planning. Borehole data used in our study include information from core descriptions (fracture density and orientations), image logs, cross-dipole shear-wave anisotropy analysis, and dynamic data (well testing, PLT, tracer, and mud-loss). The seismic attributes include standard and advanced post-stack geometrical attributes; pre-stack seismic azimuthal AVO attributes, and recently developed pre-stack diffraction imaging. We conclude that (1) there are common features that can be identified in different attributes, and the differences may indicate different scales of fractures; (2) There is a qualitative correlation in the area of history match challenges and strong anisotropy, where seismic anisotropy can identify relatively high fracture intensity regions/zones instead of pinpointing individual fractures and complements other attributes as differences do exist between seismically identified fracture zones and well data due to overburden anisotropy, resolution and sampling issues; and (3) diffraction attributes have revealed more detailed geological features in overburden (e.g. karsts) and reservoirs (e.g. lineaments) than in reflection data and a comparison with mud loss data in the shallow zones looks promising with good correlation between mud loss and collapsed features. This work has provided an improved understanding of the applicability and limitations of the using multi-seismic attributes for fracture characterizations in carbonate reservoirs.

Compressional and shear wave attenuations from walkway VSP and sonic data in an offshore Abu Dhabi oilfield

“…The average reservoir porosity ranges from 10 to 25% and the permeability from 10 to 100 md. Fractures have been reported in TH1 and TH3 [Edwards et al, 2006, Skeith et al, 2015. These fractures generally occur at the boundaries of dense zones and reservoir units [Shekhar et al, 2017].…”

Investigation of fractured carbonate reservoirs by applying shear-wave splitting concept

“…Interpretation of fullbore microimager (FMI) and core data are commonly used to obtain geometrical properties of fractures at well location. Curvature, dip, azimuth, coherency, similarity, semblance, variance, spectral decomposition (e.g., Marfurt et al, 1998;Chopra and Marfurt, 2007;Hale, 2013;Gao and Di, 2015;Jaglan et al, 2015;Skeith et al, 2015;Mandal and Srivastava, 2018) are the seismic attributes mostly used to derive fractures properties between wells. However, these attributes may only be able to identify the geometrical properties of fractures rather than physical ones.…”

“…However, these attributes may only be able to identify the geometrical properties of fractures rather than physical ones. Full waveform inversion is a suitable tool for an accurate mapping of media with high fracture density, but with low resolution (Takougang et al, 2020). Seismic waves attenuation also, has a great potential for investigating physical properties of fractures as well (Bouchaala et al, 2019) because of its closure to petrophysical properties of reservoirs, such as fluid type and saturation (e.g., Bouchaala and Guennou, 2012;Matsushima et al, 2017).…”