An Evaluation of the Application of Low-Field in the Characterization of Carbonate Reservoirs

ABSTRACT

Low field Nuclear Magnetic Resonance (NMR) as an analysis tool for reservoir studies is a relatively new and promising technology that is fast, nondestructive and able to yield a vast amount of information about the reservoir formation. In theory, a single NMR measurement can be used to determine porosity, permeability, and irreducible water saturation. Much of the earlier work with NMR was performed on sands or sandstones. When these models were applied to carbonates, the rock properties predicted were very different from those measured through core analysis, and were often incorrect. Thus the conventional method of interpreting NMR data needs to be changed to accommodate the difference between sandstones and carbonates. This paper details an investigation of the bound and free fluid components of carbonates through the use of NMR and Computed Tomography (CT) analysis. Such information is required for estimates of pore connectivity and recoverable reserves.

NMR T2cutoff values vary in carbonates. Correlations were observed between T2cutoff and the fully saturated NMR spectrum. These correlations could be used on a logging tool as a rough estimate of moveable fluid volume in different zones. T2cutoff was also observed to correlate well with the NMR spectrum at Swi, which represents the pores that are not drained. In this manner, NMR T2cutoff values are thought to be indicative of the connectivity of the pores. To test this hypothesis, CT data were obtained and visually compared to the NMR data in order to confirm the relationship between NMR T2cutoff and pore connectivity. This verifies that NMR T2cutoff analysis for estimates of moveable fluid volumes can be used to provide information about pore connectivity in carbonates.

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