Use of the NMR Diffusivity Log To Identify and Quantify Oil and Water in Carbonate Formations

Abstract

Abstract We report a successful example using the 2D NMR technique for determining oil-water contact (OWC) in difficult carbonate environments. We applied an NMR diffusion-based interpretation method to identify oil, water, and transition zones and to quantify the oil saturation in a limestone reservoir. Hydrocarbon typing and saturation determination from NMR logging usually require high contrasts of intrinsic or apparent relaxation times and/or diffusivity. Many carbonate reservoirs in the Middle East contain large-sized pores, which coupled with low relaxivity of carbonates results in long T1 and T2 times for water. Further, because light oil has a long relaxation time, there is little T2 contrast and small polarization contrast between water and oil. When these reservoirs contain very light or high-GOR oils, the diffusivity contrast between oil and water is also less pronounced. Hence, it is difficult to distinguish between the oil and water signal with most NMR hydrocarbon typing techniques. The example shows that a diffusion log reconstructed from 2D NMR interpretation works well even for marginal diffusion contrast cases. Furthermore, we present a modification to the Coates permeability model that is applicable to carbonate formations having partially connected vugs.