In recent years, more horizontal well targets in unconventional reservoirs are being designed to stay within narrow stratigraphic-target windows. These narrow windows can be as thin as 10 feet and require active geosteering to keep the wellbores within the target. It is a difficult challenge to meet the target objectives when steering with a standard omnidirectional gamma-ray (GR) tool alone. The lack of azimuthal sensitivity in these tools makes it difficult to determine whether the wellbore is approaching the top or bottom of the target window while using real-time data during drilling, or using memory log data even after drilling the lateral section.
To solve this problem, the industry is beginning to turn to azimuthal GR tools. GR tools assist in determining the relative stratigraphic position of the drilling assembly. This is done as it cuts up-section or down-section, when approaching or crossing bed boundaries. In real-time, only up- and down-looking azimuthal GR curves are available from some service providers. However, others have real-time multisector data and may even provide real-time GR images. The quality and amount of the real-time data is impacted by telemetry limitations, drilling noise and vibrations, and the rate of penetration.
There is a wide range of commercial azimuthal GR tools available that vary in design and quality. Azimuthal GR tools can be contained within almost any section of the bottomhole assembly (BHA). This includes the drilling system (at- or near-bit), or a component of a sonde-based measurement-while-drilling (MWD) system, or built into the body of a logging-while-drilling (LWD) system. The different tool designs that dictate the placement of the GR detector within the tool cross section can lead to different azimuthal sensitivities and tool performance.
We compare the expected performance of three different azimuthal GR tools based upon Monte Carlo nuclear modeling to the actual performance using real data from three wells. The modeling results demonstrate different vertical resolution among the different tools and different quadrants of the same tool. The geometrical relationship between the well path and the formation layer is shown in measured depth and true-vertical-thickness (TVT) reference spaces. This is the basis for processing and interpreting GR and other borehole images. We also demonstrate a method to reverse-engineer the depth of image (DI) for each of the three tools. We use forward-modeling geosteering correlation techniques to closely match to the actual field data. Then the up- and down- GR sector data acquired in horizontal well sections are verticalized in TVT reference space, noting the total off set and deriving a DI. The theoretical analysis-based modeling results are shown to compare favorably to the field results.