NMR Fluid Substitution – Pursuing the Fundamental Controlling Parameters of a Low-Mobility Reservoir
Authors:   AkerBP, Christensen Soren A. , Thern Holger ,   Baker Hughes, Petersen Jon Torkel ,   Baker Hughes, Eiane Tor ,   Baker Hughes
Published: November 2024

Chalk is a highly uniform low-mobility endmember of the carbonate rock group bordering unconventional reservoirs with matrix oil permeability often below 1 md. The fundamental control of the distribution of saturation and flow is most often pore-throat radius (PTR) and capillary entry pressure (Pce). For clastic sand reservoirs, the interpreter doing saturation-height modeling (SHM) may not appreciate that the proxies for these controls applied in modeling, typically porosity and permeability, are, in fact, nothing more than proxies. When modeling a low-mobility reservoir, however, it is commonly observed that modeled saturation fails to match calculated saturation from logs. The reason is often that the assumed, and typically unconsciously applied, link between the proxy and PTR/Pce is broken. It is, therefore, important to establish a workflow that enables the characterization of PTR and a refined permeability (k) to facilitate understanding when modeled Swt from SHM fails to match calculated saturation. Downhole petrophysical measurements are typically governed by a combination of rock and fluid properties. The concept of fluid substitution (FS) is to create a measurement response at a different saturation than at which data was acquired downhole. For nuclear magnetic resonance (NMR), FS is used to eliminate the hydrocarbon contribution from T1 and T2 distributions and, thus, establish the pure response of a water-filled rock. In 2015, Christensen et al. presented the first study conducting successful FS for a chalk reservoir. Based on theoretic considerations and laboratory measurements, the T2 geometric mean (T2gm) of the water-saturated rock was estimated and converted into PTR and k. By linking T2gm and total porosity to PTR and thus Pce, fundamental parameters controlling the distribution of saturation and flow properties of a low-mobility chalk reservoir were derived. In this study, the NMR FS methodology is applied to a more complex North Sea chalk reservoir – the Norwegian twin fields Valhall and Hod. Data for calibration of the model were acquired from 10 core samples, and the established models were calibrated from core to log scale for application to downhole NMR logs. The very basis of the FS model was, this time, challenged by the core data. The core T2 and PTR distribution data acquired for calibrating the estimation of PTR and k deviated from earlier observations. Closer inspection of the rock samples revealed the presence of micro-stylolites and foraminifera as possible explanations. However, a modified workflow was developed, enabling a successful calibration and application of the FS workflow. The NMR FS methodology appears to be more versatile than originally concluded. On the one hand, the established model is capable of identifying important flow units that otherwise would not have been appreciated. On the other hand, it identifies less prolific intervals from a flow perspective that otherwise would have entered 3D reservoir modeling as “normal” rock. This study adds to the limited work published in the field of NMR FS, and the results point to an underexplored and untapped potential of NMR. Further work in different formations and environments can pave the way for broader applications