Magnetic Resonance Permeability in Low-Porosity Carbonate Wash

Kneedy, Jason ¹; Smith, Charles H.²; Ramakrishna, Sandeep ³
(1)Chesapeake Operating, Oklahoma City, OK. (2) Halliburton, Oklahoma City, OK. (3) Halliburton, Houston, TX.

The Wichita Mountain uplift that extends through western Oklahoma and the Texas panhandle is the source of the formation matrix material for many of the productive formations in the Anadarko Basin in Oklahoma. The rock ranges in age from Precambrian to Middle Pennsylvanian. Portions of this so-called wash segment are characterized by a large component of carbonate material. The energy of deposition determines the composition of the formation, and there is great vertical variation in texture and generally poor particle sorting through the entire section.

This variation contributed to confusion over the ability of the rock to produce hydrocarbon and the volume of reserves available. Standard triple combo logs would generally characterize the formation as a low-porosity and high-resistivity formation. The high resistivity was not clearly identified with the presence of hydrocarbon or with the incidence of low porosity. Historically, attempts were made to complete the reservoir when density/neutron cross-plot porosity was in excess of 8%. The reservoir itself was remarkably inconsistent. Wells that had the same apparent matrix and same log porosity would produce at very different rates. Conventional logs were unable to provide any reservoir quality indication for the formation. Sonic logs and image logs also failed to provide any definition or resolution to the problem. Drilling and completing this horizon became a discouraging and costly business.

The addition of NMR measurements cast this formation in a new light. The logs were run in a mode to allow valid responses in the relaxation, or T2 spectrum. The Bray-Smith permeability equation was applied to this result. This evaluation identified the existence of high permeability in areas with very low porosity. Some of these high permeability parts of the reservoir developed in zones with porosity as low as 3-5%. Completions focused on these NMR indicated productive zones with excellent results.

This paper compares the NMR-observed responses to traditional logs. The designation of productive intervals is based on that information. There is a comparison of the productivity calculated from this permeability versus actual production for a group of these carbonate wash wells. There is also a comparison of the Bray-Smith result for these wells versus the Coates and Timur permeability values.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.