The Challenge of Saturation Computation in High-Angle Wells: A Comparative Study for Evaluating Inverted Resistivity from Multi-Spacing Laterolog and Digitally Compensated Electromagnetic Wave Resistivity – A Case Study from Oman
Authors: Taher Ahmed , Al Sawafi Samya , Al Mahruqi Mohamed , Al Amri Hanan , Al Shukairi Saleem , Al Mamari Anwar 
Published: November 2024

Drilling high-angle and horizontal wells imposes a major challenge for saturation evaluation due to multiple factors that impact logging-while-drilling (LWD) resistivity measurements. LWD laterolog resistivity is shallow reading and is affected by mud invasion. Electromagnetic (EM) resistivity is subject to many effects in high-angle wells, including polarization at bed boundaries, which impact the different resistivity spacings and make evaluation difficult, especially in thin reservoirs. High formation resistivity also reduces the accuracy of EM resistivity.

Laterolog and EM resistivity tools are both commonly utilized to measure resistivity at various depths in a borehole. However, their responses in high-resistivity formations may differ significantly due to the characteristics of the formation and the limitations of the tool designs. This research investigates inversion processes applied to the data obtained from these tools, aiming to highlight the strengths and limitations of each technique. Laterolog resistivity was utilized for reservoir characterization in a high angle well. One of the key challenges was the high-resistivity formation, in which laterolog measurements provided a more accurate formation resistivity for saturation evaluation compared to conventional EM resistivity. Deep EM resistivity was utilized for reservoir mapping, utilizing long transmitter-receiver spacings and a multi-frequency design to provide high-definition data. The sensor also provided measurements of vertical and horizontal resistivity.

Multi-spacing laterolog resistivity was used primarily for revealing geological features and fractures, and secondarily for petrophysical evaluation where it was used to invert for true formation resistivity. The high-definition deep resistivity mapping revealed variations in reservoir boundary positions and dip changes, which were important for well-placement decisions, and resolved the thickness of the target layer in a laterally heterogeneous reservoir. The results of the analysis reveal insights into the sensitivity, resolution, and depth of investigation of each resistivity tool in high-resistivity formations. Factors such as tool response, formation anisotropy, and invasion effects are considered in the evaluation. This knowledge is valuable for optimizing well-logging strategies and improving the interpretation of subsurface resistivity data, aiding in more accurate reservoir characterization and efficient resource recovery in similar geological environments.

The use of inversion from lateral resistivity tools in high-resistivity formations is instrumental in overcoming challenges associated with conventional resistivity measurements. It provides a more detailed and accurate characterization of the formation, supporting better reservoir management and decision-making in the exploration and production of hydrocarbons. This research underscores the significance of integrating multiple measurement techniques to obtain a comprehensive understanding of subsurface formations in the quest for efficient and sustainable energy exploration and production.