Unraveling Anticorrelation Interpretation in High Resistivity Geosteering During Coiled Tubing Drilling Operations
Authors: Zhong X. , Chang Y. , Santoso G. , Gremillion J. , Conghui S. , Beltran V. , Murphy J. , Cabas L. 
Published: February 2025

Abstract
Unusual logging-while-drilling (LWD) propagation resistivity responses were observed during a recent coiled tubing drilling (CTD) job in a high-angle well. Throughout field testing, four accurate resistivity measurements, along with gamma ray data, were acquired and integrated to geosteer the lateral section of the reservoir effectively. Periodic instances of anticorrelation between the apparent attenuation resistivity (Rad) and phase shift resistivity (Rps) were noted. Such anticorrelation behavior had not been previously encountered in resistivity logging data obtained from either wireline or LWD tools.
To understand the underlying causes, comprehensive numerical modeling was undertaken. This modeling relied on tool response simulations, which take as input the formation properties, well trajectory, and tool orientation to generate logging data mirroring actual tool measurements during logging operations. Numerous scenarios were modeled to encompass the diverse influencing factors inherent in wellbores and surrounding formation properties. The results of the numerical study unequivocally demonstrate that the interplay of factors such as high-angle or horizontal well configurations, wellbore tortuosity, trajectory proximity to bed boundaries, and high resistive formations with shoulder bed contrasts can lead to the observed anticorrelation between Rad and Rps.
The inverse relationship of Rad and Rps witnessed during field test may be attributed to the synergistic impact of high-angle well trajectories combined with tortuosity within high-resistivity formations. Resistivity sensitivity maps were generated for various distance-to-boundary, associated with the different magnitudes of wellbore tortuosity. These sensitivity maps enable visualizing the combined effects of distance-to-boundary, formation resistivity, and shoulder bed resistivity on the likelihood of observing anticorrelation of Rad and Rps. A workflow was developed to integrate the distance-to-boundary and the wellbore tortuosity for tool response prediction and logging data matching. This innovative approach promotes the real-time geosteering boundary interpretation upon the recognition of anticorrelation, enhancing the drilling accuracy. When drilling in resistive segments of a high angle well, leveraging the anticorrelation recognition offers a robust real-time interpretation tool. Consistent anticorrelation between Rad and Rps curves indicates the tool is approaching a bed boundary. The magnitude of this anticorrelation aligns to the variance of wellbore tortuosity. Moreover, this indicator could also serve as a valuable quality check for surface inversions, facilitating the quantification of multilayer resistivities and distances-to-boundaries, enhancing the overall interpretation process.
The novel anticorrelation recognition and interpretation based on simple resistivity measurements is not only limited to slim hole CTD resistivity tool in underbalanced conditions, but it could also be applied to any other LWD resistivity petrophysics interpretation. This methodology of utilizing anticorrelation recognition in LWD propagation resistivity logs can be a robust indicator for boundary approaching interpretation in geosteering. It plays a pivotal role in decision-making processes, ultimately contributing to the success and efficiency of drilling operations.