Real-Time Image Data Aids Horizontal Drilling in Carbonate Reservoirs Onshore Abu Dhabi
Authors: Denney Dennis 
Published: February 2015

This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 101916, "Real-Time Image Data for Successful Horizontal Drilling in Carbonate Reservoirs Onshore Abu Dhabi, UAE," by Maria Teresa Ribeiro, Mohamed Al Neaimi, Irfan Hendrawan, Sultan Karrani, Mohamed Ben Saad, Mohammed R. Ayoub, Kamal Zubair, Khalid Al Marzouqui, and Suaad Khoori, Adco, prepared for the 2006 Abu Dhabi International Petroleum Exhibition and Conference, Abu Dhabi, 5–8 November.
Real-time image interpretation was used during an intensive horizontal-well drilling campaign in two major carbonate fields onshore Abu Dhabi. Major and smaller faults were identified by use of sophisticated methods for interpreting seismic data. These faults were confirmed with image logs while drilling the horizontal field-development wells. Real-time image-data interpretation enabled geosteering decisions while drilling into reservoirs with complex structure or unknown geology or in areas with sparse geological or structural information.

Introduction
The success of drilling horizontal wells depends on accurate placement of the wellbore in the reservoir. Generally, placement is based on offset-well information, seismic data, and geological models. Although seismic data provide continuous coverage, the resolution is poor, and velocity errors may lead to wrong depths at planned well locations. As more wells are drilled, additional detailed measurements are acquired, including formation dip and acoustic velocity, and the reservoir geometry is refined. Dip data can be used to recognize structures such as folds and faults and to aid the interpretation of the depositional environment. Depth uncertainty related to survey measurements could lead to structural uncertain-ty, especially for highly deviated wells. The presence of unexpected subseismic faults can cause problems with correct well placement while drilling.
Logging-while-drilling (LWD) real-time azimuthal resistivity and density images can be used for accurate estimation of dip and strike of the intersected formations. Structural heterogeneities, such as faults, can be detected and immediate actions taken while geosteering the well. This action is particularly important for horizontal wells in thin reservoirs, in structurally complex geology, or in an area where structure is not calibrated because of sparse control points. In such situations, the use of image data while drilling the well can help drilling of a good well in terms of well placement and accessibility.

Real-Time Imaging Technology
LWD technology uses the rotation of bottomhole-assembly sensors to scan around the interior surface of a borehole. Technologies to acquire images of formation gamma ray, density, photoelectric factor, and resistivity are available.
The amplitude of an image sinusoid gives quantitative information on the relative dip and azimuth between the borehole and the formation layer. Most LWD images are oriented relative to the top of the borehole (generally, these images are acquired in high-angle wells). If the borehole is perpendicular to the layering, the amplitude of the sinusoid is low. With increasing relative angle, the amplitude of the sinusoid increases until the borehole is parallel to the formation and the layering appears as lines running down the image. Generally, the sinusoid magnitude on the image gives sufficient information while drilling to aid well geosteering. Measurement-while-drilling data are used to derive true formation dip and azimuth, helping update the reservoir-structure information.