NMR-Supported Near-Wellbore Data Analysis to Improve the Petrophysical Evaluation of a Sandstone Reservoir Drilled With Barite-Enriched Water-Based Mud

Abstract

The effect of mud filtrate and eventually fine migrations in high-quality rocks considerably influence the reading of NMR and conventional logs. It poses a challenge in extracting useful information that can contribute to the petrophysical evaluation of the virgin zone, e.g., the rock quality index. This is critical for determining the permeability index and the saturation exponent n required for accurate determination of the water saturation.

The rock quality index is understood as the ratio between free-fluid volume (FFV) and bound-fluid volume (BFV). This is equivalent to the pore-throat radius, which dominates fluid flow, that is negatively affected by the fine migration. This is reflected in a reduction of the porosity filled by free fluid. The free-fluid porosity undercall is corrected with the help of deeper-reading density data. After this correction, the NMR-derived permeability in the flushed zone, probably damaged by migrated fines, is used to calculate the permeability in the virgin zone.

The determination of the oil saturation in the flushed zone from NMR logs was done in two ways—first, by applying statistically based machine-learning tools, and secondly, by applying the widely used 2D-NMR method, diffusion vs. T₂ maps. Comparative analysis of these two methods shows that both lead to consistent results. The water saturation in the flushed zone is estimated to be eight times higher than in the virgin zone because of the high degree of invasion.