Azimuthal Investigation of a Fractured Carbonate Reservoir

Abstract

Oil production and enhanced oil recovery in carbonate reservoirs in Abu Dhabi, UAE, are largely affected by fracture systems that control the fluid path and the permeability of reservoirs. Most fracture properties, such as fracture orientations and density, are obtained by interpreting petrophysical data acquired at the wellbores, whereas fracture properties between wells are typically derived from nonzero offset seismic data. However, deriving fracture properties from seismic data is challenging, as it requires a robust methodology and a careful seismic processing procedure. In the current case study, we used the azimuthal amplitude vs. offset (AVAz) method on 3D seismic data acquired in onshore Abu Dhabi, to generate maps of fracture orientation and density in a carbonate reservoir. A sophisticated processing series was carefully performed to increase signal-to-noise ratio (SNR) and preserve seismic amplitudes. The main parameters controlling the AVAz method were investigated and optimized before being applied to the 3D seismic data. The reservoir has a high fracture density in the lower regions, but a low fracture density in the upper parts, indicating a weaker anisotropy. The resulting dominant fracture directions span from north-northwest/south-southwest to north-northeast/south-southwest, as well as from northwest/southeast to east/west, which is consistent with the primary fracture orientations determined from the interpretation of fullbore formation microimager (FMI) data acquired at well locations. These fracture systems are the result of the Late Cretaceous obduction of the Semail ophiolite, which was oriented east/west and northeast/southwest, followed by the south/north to southwest/northeast trending Late Oligocene-Miocene continent-continent collision of the Arabian and Central Iran plates along the Zagros orogenic front.