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Nuclear Magnetic Resonance Permeability Response in Carbonate Reservoirs with Secondary Porosity

Borell, Jarret¹; Donohue, Joel²; Smith, Charles H.³
¹SandRidge Energy, Oklahoma City, OK.
²Sanguine Gas Exploration, Oklahoma City, OK.
³Halliburton, Oklahoma City, OK.

Permeability calculations from nuclear magnetic resonance (NMR) devices have been proposed in various ways. The Coates permeability equation has been used almost universally since the design of the tools. This equation requires some laboratory work to characterize the formation. This characterization is limited to isotropic reservoirs.

Mandrel based NMR devices, because the immense amount of data captured, are capable of measuring the entire spectrum of T2 or relaxation measurements within a reservoir. This complete characterization enables an accurate description of the textural changes of the porosity within the reservoir. The Bray-Smith equation for direct calculation of permeability is an equation that characterizes these textural changes as permeability variations as the T2 porosity distribution is altered. This has been applied with great success in siliceous reservoirs—even those with fractures present.

Carbonate reservoirs can be more complex. Secondary porosity from fractures is almost identical to that observed in siliceous reservoirs. Solution enhancement or detraction from secondary events may also be present. These take the form of open or filled vugs and fractures.

This paper investigates the application of the Bray-Smith permeability equation to determine the productive capacity of these formations. It includes examples of NMR T2 permeability in pure unaltered carbonates, intervals with secondary porosity from fractures, secondary porosity from solution enhancement, and vugular porosity development. It also compares the mandrel based NMR porosity texture description to image logs over these intervals. A comparison and contrast of predicted production rates vs. actual production rates is exhibited for each type of reservoir.

 

AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012