Real-Time 3D Anisotropy Analysis Enables Lithology Identification at Distance

Abstract

Ultra-deep ElectroMagnetic (EM) inversion is used to resolve multiple layers far from the wellbore. Since it primarily responds to resistivity variations, shale and water bearing sand layers of similar resistivity values can’t be resolved using a traditional resistivity inversion alone. It can be critical to identify the presence of water at distance, so that a standoff from the oil/water contact can be maintained to optimize production. Analysis of the resistivity alone means that a well path may be optimized to maintain a standoff from any low resistivity zone, to avoid drilling close to a potential water contact. Identifying the lithology at distance would remove the uncertainty, allowing a well to drill closer to a shale zone or maintaining distance from a water zone to optimize both well’s position and future production. Inversion for resistivity and anisotropy in three dimensions provides a solution that can improve lithological and fluid analysis in real time, allowing for more sophisticated well placement decisions to be made, by identifying any critical water bearing zones at distance. Real-time transmission of nine ultra-deep resistivity measurement components allows 3D inversion for both resistivity and anisotropy far away from the wellbore. The interpretation of these inversions is examined in clastic reservoirs where it is critical to differentiate between shale and water bearing sand layers at a distance from the well trajectory. Although shale zones could have very low resistivity values similar to that seen in water bearing sand, anisotropy values are generally much higher, due to the laminated nature of the shales. Therefore, resolving both resistivity and anisotropy at a distance allows the shale zone to be distinguished from water filled sand in real time, enabling successful geo-steering and keeping a desired offset above the water level. An example is presented where ultra-deep 3D inversion successfully mapped both resistivity and anisotropy. The lithology has a significant impact on the anisotropy; hence, it was possible to differentiate low resistive shale from the low resistive water bearing sand at a distance from the well trajectory. This information was used to optimize geosteering, geomapping and geostopping processes, and has the potential to be applied to future wells, where applicable. Interpretation results also provide valuable inputs for optimizing completions designs and reservoir management strategies. This paper presents the global first real-time well placement application of 3D resistivity and anisotropy inversions to avoid drilling into a water zone. This novel approach using ultra-deep azimuthal resistivity technology addressed the existing challenge of identifying shale and water filled sand zones of similar resistivities.