Investigations on Unconventional Tight Carbonate Rock Cores by Low-Field Nuclear Magnetic Resonance Relaxometry

Abstract

Low-field nuclear magnetic resonance (LF-NMR) is a user-friendly tool for analyzing rock core samples. NMR relaxation measurements on these samples provide a possibility to quantify the presence of different fluids (water vs. crude oil). In this study, we determined the permeability and porosity-type properties of rock cores from Kuwait by analyzing the longitudinal (T₁) and transverse (T₂) NMR relaxation data. The intricate structure of carbonate is reflected in the NMR analysis as fluctuating T₁ and T₂ values depending on the depth of rock cores. T₁ results showed a lower percentage of clay-bound water, while T₂ results suggested a higher fraction. This discrepancy between T₁ and T₂ values is attributed to the differences in a surface relaxivity. Additionally, the T₁/T₂ ratio, a crucial parameter for the mineral affinity of the fluid, is lower for the signal associated with clay-bound water and higher for those associated with hydrocarbons. The oil/water contact depth was measured at 17776 feet. The m (cementation factor) and n (saturation exponent) values were determined for each rock core individually, enabling the validation of permeability. This study’s most important and novel finding is the detection of the highest permeability (as indicated by the analysis of the T₁ and T₂ relaxation curves) alongside the highest m and n values at approximately the same depth as the water/oil contact.