Rock Type-Based Geosteering in a Multilayered Reservoir by Combining LWD NMR Measurements and Ultradeep Azimuthal Resistivity Images

Abstract

Deep-azimuthal resistivity images are widely used to place wells accurately inside of a target reservoir structure. However, the resistivity measurement does not provide much sensitivity to the reservoir quality, particularly in carbonates, where large pore-size variations are often encountered. For optimized well placement, combining the deep-resistivity images with logging-while-drilling (LWD) nuclear magnetic resonance (NMR) measurements establishes the link between reservoir structure and rock types while drilling. Factor analysis is the technique used to reduce the large NMR data set to a smaller number of underlying, porofluid components. These components are clustered into groups called porofluid facies, which reflect the most common combinations of pore volume and fluid NMR properties. Based on the NMR data, four petrophysical rock types are generated while drilling: RT-1 has good porosity and long T2 components, indicating large pores; RT-2 also has good porosity but medium T2 components, indicating smaller pores; RT-3 has medium porosity and long T2 components, while RT-4 has medium or low porosity and medium or short T2 components, indicating the poorest facies.20 The NMR-based rock typing technique and ultra-deep resistivity images were combined in three laterals drilled back-to-back in a thin-carbonate reservoir where pore size variations were expected. In the first lateral, targeting layer 10, fair (RT-2) rock type was present over the first half of the lateral. Only a small part of the drilled footage encountered the best rock type RT-1. The decision was then made to explore layer 11 under layer 10. The NMR showed that layer 11 was mostly of the poorest quality (RT-4). Based on the first lateral results, the second lateral targeted layer 10. Layer 10 was found to be of excellent quality in which most of the rock was RT-1 type, with some sections of RT-2; hence, no change of layer was required in this lateral. The third lateral explored layer 5. The first half of the section encountered RT-2 and RT-4 types. After having observed that a resistivity increase corresponded to a poorer rock type, the second part of the lateral was completed in one layer that intersected RT-1 and RT-2. Combining NMR rock typing with reservoir delineation by means of deep-azimuthal resistivity images while drilling is a new methodology that combines the strengths of both techniques to optimize well placement and reservoir understanding. One example is a well that has three laterals, which illustrates this technique; each lateral was placed not only in the best rock type, but also in the target structural location.