Resolution Analysis of Azimuthal Directional Resistivity Images
Authors: Fang Sheng 
Published: October 2011

Abstract
Azimuthal Propagation Resistivity (APR) images have been widely used in geosteering applications to identify the direction of a nearby boundary to maintain the drilling in a target zone. However, it is possible to miss some thin layers if only an APR image is used, especially when the azimuthal resistivity tool approaches a boundary at a medium-to-high incident angle.
The APR image indicates the azimuth direction of a more conductive layer that the tool is approaching by looking at the angle position (vertical axis) of a darker color strip. For example, a darker color strip is shown in the middle of the image plot (180 degree) if the tool is approaching a more conductive bed that is exactly below the resistive formation, whereas the strip is shown in the upper (0 degree) and lower (360 degree) parts if the bed is sitting above. The APR image shows a sharp transition of the darker color strip between two bed boundaries due to the existence of a zero-crossing of APR signals. This indicates the tool is sensing a different bed. More attention should be paid to the use of these features when a thin layer exists, especially for the case with a medium incident angle.
The resolution of the APR image and raw measurements is analyzed using a model study of different three-layer models with varying thicknesses of a central layer for both resistive and conductive formations, at different incident angles, and using multiple thin-layer models. For a thin-layer model, multiple sharp transition features may exist so that a multiple-layer model could be misinterpreted. On the other hand, for a multiple thin-layer model, the APR image may only show a simple sharp transition feature so that a single boundary could be misinterpreted. A better understanding of the formation resistivity distribution can be achieved from carefully evaluating the APR image and all azimuthal resistivity measurements.

Introduction
Proactive drilling becomes possible after the introducing of an azimuth propagation resistivity (APR) tool (Li et. al., 2005; Bell et. al., 2006; and Bittar et. al., 2007) into a logging-while-drilling (LWD) measurement system. An APR tool provides measurements that can enable detection of the azimuthal direction of a nearby bed-boundary. This azimuthal detection capability is particularly useful for all geosteering applications including, but not limited to, well landing and well placement.
An APR image (Bell et. al., 2006) is developed, as a virtual representation of APR measurements, to display the relative location of a bed-boundary of a resistivity contrast. Geologists and drilling engineers can use such a contrast to make a precise well landing and to maintain drilling strictly within a target layer (Bell et. al., 2006). A mistake could happen if a bed boundary is mis-identified due to the resolution limitation of APR image.
Understanding of APR image at a near-horizontal well (85 to 95 degrees) for different themes and motifs can be found in the paper by Kennedy et al (2009). The themes are categorized for easier interpretation, and each theme contains one or several image motifs of different characteristic patterns. The motifs for themes of penetrating a thin-bed at 85 and 95 degrees or penetrating multiple thin-beds at a closer horizontal well (closer to 90 degrees) are documented to help in geosteering a well. However, the APR image features for thin-layer models at a different borehole inclination that is further away from 90 degrees can be very different. Synthetic model studies in the paper indicate a multi-layer APR image feature occurs for a single thin-layer model, whereas a single thick-layer APR image feature appears for a multi-thin-layer model. A careful interpretation of the APR image at a medium-to-high inclination together with all APR responses is needed to correctly identify possible boundaries so that accurate proactive geosteering becomes possible.
In the subsequent sections, the items that are measured and processed from an APR tool are introduced and the basics of an APR image are explained. Resolutions of APR images for a single thin-layer or multi-thin-layers (resistive or conductive) models at different inclinations are analyzed and concluded.