Evaluation of Carbonate Reservoir — Secondary Porosity — By Nuclear Magnetic Resonance Logs

Donahue, Joel ¹; Smith, Charles H.²; Menendez, Eli ²
(1)Sanguine Gas Exploration, Oklahoma City, OK. (2) Halliburton, Oklahoma City, OK.

The lower member of the Atoka formation of Oklahoma, proximal to the Wichita mountain front, is a classic example of an altered carbonate-wash reservoir. The matrix of the formation is very dense and demonstrates very little primary porosity. The alteration of the rock by successive leaching processes creates a large amount of secondary porosity development in the form of vugs. There are also areas of the formation that include natural fractures. Fluid flow through the fractures allows enhanced adjacent alteration in the form of increased vugular development.

These features create an admirable reservoir for oil and gas. When these vugs are linked to each other through inter-vugular networks, or along the flow path of the fracture, they can coalesce to become a very prolific reservoir. The ability to properly evaluate the reservoir for production is a desirable objective. Many evaluation techniques have been applied through the years with varying degrees of success.

Basic density/neutron logging failed in nearly every case to define porosity because they do not respond well to the secondary development. Sonic logs showed some promise, as the sonic travel paths can indicate secondary porosity. Image logs also had some value in that they could determine the presence of fractures and at least define the presence, but not the interconnectivity, of the vugs.

The addition of NMR logs brought a great deal of additional information. These logs only investigate pores, so every response from the logs can be directly attributed to some component of porosity. Evaluation of the relaxation, or T2 time, provides a precise understanding of the pore structure, thus creating a situation where the amount of secondary porosity can be directly measured. The quality of that secondary porosity and its value to the character of the reservoir can be understood.

The NMR response observed in the subject wells is shown along with an analysis of the quality of the secondary porosity. Productive capacity or permeability can be estimated by various means. This paper compares the results of this permeability calculation using the Bray-Smith equation and compares that result to the Coates equation. Both of these permeability calculations are used to estimate expected flow rates, with the results compared to actual production.

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