High frequency NMR characterization of the Eagle Ford shale formation

Abstract

Conventional Nuclear Magnetic Resonance (NMR) measurements are conducted utilizing NMR logs in downhole in situ stress state as well as using low field (2 MHz) NMR measurements on preserved core samples. These measurements can reveal some of these pore space characteristics and the relaxation mechanisms dominant in these pores for medium to high permeability samples. As shale cores have very low permeabilities and extremely small particle sizes and associated small pore spaces, their investigation at high field (400 MHz) can reveal much more details. The main objective of of using a high field high frequency (400 MHz, 9.4 Tesla) Nuclear Magnetic Resonance (NMR) instrument is to distinguish these much smaller pore spaces with higher resolution and establish the kerogen, bitumen content, organic matter (OM) hosted porosity, fluids held in OM porosity, hydrocarbons, water/brine in inter-particle (IP) or intra-granular porosity. In this study, core samples from the Eagle Ford shale basin are studied to characterize them on the basis of their organic matter (kerogen and bitumen) as well as their inter-particle and intra particle hosted pore space characteristics. The samples were obtained from the Eagle Ford shale in Gonzales county in Texas. These are preserved as-is cores used in this study. This study aims to study the shale pore size distributions and their fluids properties in order for their characterization with better detailed understanding and exploitation in the future. The shale core samples were imbibed under pressure of 5000 psig in 8% Sodium Chloride (KCl) brine, 17.8% KCl brine, and n-dodecane for a period of 72 hours. The imbibition of different fluids should distinguish between the organic matter hosted porosity and inter/intra particle porosity. This assumption was made on the fact that hydrocarbons such as n-dodecane relax differently from brine/water in the organic matter pores. This is due to the hydrocarbon-hydrocarbon interaction, and the Van der Waals force between the same molecules. Similarly, water/brine present in the inter/intra particle pores would relax differently than dodecane, as this relaxation mechanism is dominated by ionic attraction forces. Again, the relaxometry and the interaction between the non-organic matter pores and water is different from that of organic matter pores and dodecane, because both are hydrocarbons. Comparing the relaxation between the organic matter pores and n-dodecane and brine/water in inter/intra particle would give us a better contrast in the relaxation parameters for both the pore space and hence, a better comparison of their pore sizes and distribution in the shale core samples. The two-dimensional (2D) NMR T1-T2 maps, measured with high frequency NMR, should give different T2 cutoff values for the irreducible fluids and organic matter. This is due to the differences in the relaxation mechanisms of the hydrocarbons and water/brine in the organic matter pores and inter/intra particle pores. The 2D maps further help us in identifying the fluid typing and setting up the T2-cutoffs for the shales studied in this research and hence in distinguishing the shales on the basis of organic matter, oil and gas contained therein. These 2D NMR T1-T2 maps can be interpreted as a fingerprint of each shale play. Identifying and categorizing them are another step towards making the study of shale plays easier to classify and possibly easier to exploit by quantifying the moveable fluids from non-movable fluids.