On the Characterization of Carbonate Reservoirs Using Low Field NMR Tools

Abstract

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractAlberta contains significant deposits of oil and gas in carbonate formations. Carbonates tend to have fairly tight matrix structures, resulting in low primary porosity and permeability. As a result, laboratory characterization of carbonate properties is a slow and tedious process. Low field NMR is an emerging technology shows great promise in rock characterization. In a single NMR experiment, rock properties like porosity, permeability and S wi can in theory be measured.Experiments were performed on approximately 80 core plugs from six carbonate formations.Porosity measurements were performed through gas expansion and brine saturation (Archimedes' principle). Air permeabilities were also measured. Cores were also saturated with brine and spun to irreducible water saturation. NMR measurements were taken at both saturation stages (S w = 1.0 and S w = S wi ). NMR data were interpreted using the conventional core analysis results as guides. Observations were made regarding NMR trends and corresponding rock properties.Preliminary analysis of the data shows that NMR can successfully predict the content and distribution of the fluids in the porous media. Also, the NMR spectra of carbonate samples seem to suggest that NMR can be used as a tool for classifying cores into different pore systems.Using T 2cutoff values as a tool, the cores can be divided into groups. Group 1 has T 2cutoff < 80 ms, while group 2 has T 2cutoff in the range of 80 to 200 ms and group 3 has T 2cutoff values > 200 ms. Group 1 cores generally have higher porosity values than groups 2 and 3. Group 1 also has low values of T 2gm , compared to the other two groups. Air permeability was compared to the geometric means and T 2gm_cutoff values of the three groups and some general trends were observed, but more analysis is required before these trends can be quantified.

Evaluation of Grain Size Effects on Porosity, Permeability, and Pore Size Distribution of Carbonate Rocks Using Nuclear Magnetic Resonance Technology

“…To analyze the porosity characteristics and to determine the T 2 cutoff values (T 2C ) of the limestone samples, all samples at 100% water saturation (S w ) were set for NMR measurement. Four core samples (C1, C2, C3, C4) were then centrifuged using a CSC-10 Super Core Centrifuge to obtain an irreducible water condition with a 6000 RPM corresponding to centrifuge pressure of 143 psi [22], after which NMR measurements were performed again.…”