Using a New Gradient-Based Ultra-Deep Resistivity Inversion Model to Map Oil-Water Contacts in the Gekko Fields

Abstract

This paper evaluates a new ultra-deep azimuthal resistivity (UDAR) inversion model in the context of transition zones above fluid contacts, these are common in gradational resistivity settings and typically cause difficulty with previous inversion models. The new inversion model is applied to oil-water contact mapping from within a thin oil column, and the results are verified against conventional methods for oil-water contact determination. A new gradient-based enhanced focus UDAR inversion model allows for model layers with continuously sloped resistivity profiles. Previously, approximation using multiple layers with discrete resistivity steps was required to model formations without sharply contrasting boundaries, as in transition zones above fluid contacts. We compare the performance of previous and new inversion models using real data from an extensively drilled sandstone reservoir with stepped or tilted gas/oil and oil/water contacts. The new UDAR inversion model is applied to map the oil-water contact across the entire structure, which is then compared with proven depths in exploration and geopilot wellbores. The new inversion model improves UDAR imaging of the Gekko reservoir of the Alvheim area in Norway, allowing for geosteering of nine horizontal wellbores within a thin oil column to maximize oil and gas recovery. The reservoir is comprised of stacked turbiditic sandstones ranging in configuration from thick and homogenous to complexly layered sands between shale sequences. The application of an enhanced focus inversion is shown to better define both subsurface geology and oil-water contacts than the previous inversion model. This allows more confident differentiation of the oil-water contact from shale beds. The oil/water contact derived from UDAR imaging is found to closely match results from formation pressure gradient analysis and contact penetrations in nearby wellbores. Confident mapping of the oil-water contact reveals it has an uneven tilt across the structure. This paper demonstrates how a new inversion model improves UDAR imaging in a gradational resistivity setting. It presents data that practicing engineers can use to evaluate its performance and limitations and demonstrates the capability to confidently map unevenly tilted oil-water contacts in homogenous oil-bearing reservoirs from within the transition zone.