Evaluation of Nuclear Magnetic Resonance (NMR) Technology to Define Carbonate Pore Networks

Shinichi Sakurai, R. G. Loucks, and John Gardner

Within complex carbonate reservoirs, it is difficult to develop porosity-permeability relationships. Nuclear Magnetic Resonance (NMR) technology was evaluated to see if it could help define the petrophysics of carbonates and improve porosity-permeability transforms. Lower San Andres/Glorieta/Upper Clear Fork carbonates from the North Cowden field were used as a test case. They contain diverse pore networks of interparticle, intraparticle, intercrystalline, moldic, and microporosity developed in limestones and dolostones. NMR technology used to evaluate the carbonate rocks was based on laboratory-derived core analyzer measurements. Sample selection covered a range of porosity and permeability values, as well as different pore network types. NMR porosity was obtained from c rve fitting of the echo trains using an unconstrained bi-exponential model.

A comparison of "T2 distribution" (measure of pore size) measured by NMR and "pore-body-size distribution" from thin-sections (made from ends of plugs) show a strikingly similar pattern. NMR T2 distribution also agrees well with "pore-throat-size distribution" by mercury injection on the same plug used for the NMR tests. The general porosity-permeability relationship from core data for the North Cowden carbonates is very poor, because of their complex mineralogy, broad pore-size distribution, and range of pore-system configurations. However, an excellent relationship between macro porosity, derived from NMR core T2 measurements, and core permeability was obtained. The elimination of noneffective microporosity in these rocks is necessary to develop a useful porosity-permeability transf rm. NMR technology is useful in estimating pore-size distribution and permeability in carbonates.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California