Application of Statistical Analysis on Vertically Variable Azimuthal Anisotropy: Case Study from a Carbonate Field Offshore Abu Dhabi, UAE
Authors: Nakayama Shotaro , Belaid Kamal 
Published: May 2016

Abstract
In the mature oil and gas fields offshore Abu Dhabi, reservoir properties required for static and dynamic models are relatively well-defined by a number of wells. On the other hand, seismic data are considered as a fundamental and primary source to determine and optimize well placements. Azimuthal velocity analysis using wide-azimuth 3D-2C OBC seismic data is performed on different formations where several drilling issues arose mainly due to two dominant tectonic forces in the region. The results reveal different magnitude and orientation of azimuthal anisotropy from overburden to reservoir level. Available FMI and DSI logs confirm consistency between seismic and borehole-driven azimuthal anisotropy. The analysis results are also in agreement with the regional geology and tectonic history.
Azimuthal anisotropy analysis generally provides two types of information such as the orientation of anisotropy and the amount of anisotropy. The amount of anisotropy can be simply quantified while the information obtained from the azimuth data has some complexity as it is a periodic function. In this respect, a statistical model of the bipolar von Mises distribution is proposed to determine the preferred orientation of azimuthal anisotropy. The model also provides the concentration parameter that can quantify the degree of preferred dimensional orientation of azimuth data. Additionally, we show utilization of the azimuthal anisotropy analysis particularly on a non-fracture layer and its benefit to field development by the analysis of spatially varying mud weight prediction.

Introduction
Different magnitude and orientation of azimuthal anisotropy exists from overburden to reservoir levels offshore Abu Dhabi. This is due to the inherent heterogeneity of carbonate sediments along with two dominant tectonic forces in the region:the ophiolite obduction in Oman yielded E-W compressive stress during the Mesozoic to Late Cretaceous forming NW-SE fault system; andthe subduction of Arabian plate beneath Zagros mountains have yielded NNE-SSW compressive stress throughout the Cenozoic to present forming N-S fault system (Marzouk and Sattar, 1993).
A lack of understanding of the local influence of these tecntonic events results in vertically varying degrees of drilling risk and hinders drilling operations particularly at highly deviated well paths.
Azimuthal variation in seismic signatures derived from amplitude, velocities, travel time etc. often provides insights into reservoir properties such as fracture orientation and intensity. The selection of seismic signatures sometimes determines feasibility of azimuthal anisotropy analysis. Ikawa and Mercado (2010) showed a comparison of azimuth anisotropy analysis based on both amplitude and seismic velocity using wide-azimuth OBC seismic data from offshore Abu Dhabi. The AVOAz analysis faced difficulties in obtaining consistency between borehole and seismic data due mainly to overburden effect and remaining noises including acquisition footprint. On the other hand, seismic velocity field showed reasonable alignments with available static data.