Advanced Resistivity Modeling to Enhance Vertical and Horizontal Well Formation Evaluation

Abstract Accurate formation evaluation relies critically, among other inputs, on the correct true formation resistivity (Rt). The common practice in the industry is to use the deep resistivity log as Rt. A single resistivity curve from a deep resistivity measurement often does not represent Rt due to the shoulder bed effect, polarization at boundaries, and many other effects. This is especially true in high angle and horizontal wells. The main objective of the paper is to demonstrate that advanced resistivity modeling workflows for both wireline and logging while drilling (LWD) help in determining Rt by removing or minimizing the impact of all different effects and improving the formation evaluation in vertical and horizontal wells. Extensive work and computational advances have provided the oil and gas industry with codes to model resistivity tools in a wide variety of formations/conditions. The workflow used is a model-compare-update approach and provides an interface where a layered earth model (layer geometry and property) can be constructed. If available, image logs are used to provide dip information for each layer. After several iterations, and if an agreement between the forward model and measured logs has been achieved (called the "reconstruction check"), this will be a confirmation of the structural model and assigned layer properties. Reconstruction of actual data to modeled data is the confidence indicator. The model changes or iterations can be done manually or automatically. The practical process is usually a combination of both. As a result, the resistivity logs free of shoulder bed, polarization and other effects are extracted as squared logs for each layer and are used to improve the interpretation methodology and minimize the associated uncertainties to reservoir evaluation. The workflow and benefits of advanced resistivity modeling for improving formation evaluation in vertical and especially high angle and horizontal wells will be discussed. Several field log examples will be used to demonstrate the capabilities of the proposed workflow to enhance formation evaluation in vertical and horizontal wells. Conclusions fromthe work with suggestions/recommendations for the way forward will be presented. Advanced resistivity modeling when used by the petrophysicists can have many advantages, especially in complex situations. We propose workflows and case studies which demonstrate the value of such advanced modeling in enhancing vertical and horizontal well formation evaluation.