Azimuthal Seismic Pilot for Fault and Fracture Detection - Abu Dhabi Case Study

Abstract

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 137338, ’Azimuthal Seismic Pilot for Fault and Fracture Detection - An Abu Dhabi, U.A.E., Case Study,’ by Klaus W. Mueller, Mohamad Samir Al Nahhas, William L. Soroka, SPE, and Mariam al Baloushi, ADCO, and Rick Sinno, Ruben D. Martinez, Waleed Hussein, and Paul LeCocq, PGS, prepared for the 2010 Abu Dhabi International Petroleum Exhibition & Conference, Abu Dhabi, UAE, 1-4 November. The paper has not been peer reviewed. An azimuth-dependant-processing pilot study was carried out in a large Middle East field to determine if this technology has potential to identify fracture-permeability pathways. The field is heavily faulted and fractured, with good well control. Anisotropy can be linked in a full evaluation to fault/fracture density and orientation. The anisotropy can be measured by differences in seismic-travel times or amplitudes/seismic attributes measured in different azimuth seismic cubes. Interpretation and validation of the results suggest that value is maximized by integrating multiple attributes, including horizon mapping for time differences, amplitude extractions for reflectivity differences, and result validation with available well calibration. Introduction The field under study is of Cretaceous age and at a depth of approximately 9,000 ft. The main reservoir has an average thickness of 180 ft, but other important layers vary in thickness from a few feet to tens of feet. Core and well data show complex carbonate geology with laterally varying reservoir properties and stratigraphy. The field is heavily faulted and fractured, and several different main fault directions are identifiable. Fault throws vary from several hundreds of feet to less than 5 ft. Reservoir porosity ranges from 20 to 30%. Use of the faults in a geologic model affects recovery, so faults must be accounted for properly in the reservoir model and production plan. A high-resolution, 640-fold, wide-azimuth (WA) 3D land-seismic survey was acquired to collect information to evaluate azimuthal differences and help identify permeability pathways. The acquisition was limited to a 2000-m offset in the cross-line direction, and the in-line direction had offsets up to 4000 m that could be used in the analysis. A 50-sq-km area was chosen for detailed processing. Two processing methods, or flows, were applied. One method used a more-conventional azimuth-sector approach. The other meth-od was a common-Cartesian-offset-bin (CCOB) approach, which includes Fourier-regularization processing. The main objective of this WA-processing and interpretation pilot was to demonstrate potential value of a WA image to better characterize fractures and faults that may not have significant throw associated with them. Additional objectives were to determine velocity changes including anisotropy at different geologic levels or amplitude changes to determine reservoir characteristics. Another objective was to evaluate the minimum-offset distance needed to produce reliable WA results, which could affect future survey designs and cost.