--> NMR Relaxation Times and Related Pore Geometry in the Morrow Group (Pennsylvanian), Hemphill County, Texas and Texas County, Oklahoma, by R. Aivano and J. Howard; #90903 (2001)

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NMR Relaxation Times and Related Pore Geometry in the Morrow Group (Pennsylvanian), Hemphill County, Texas and Texas County, Oklahoma

R. Aivano and J. Howard
1Oklahoma State University
2Phillips Petroleum Company

The Lower Pennsylvanian “Morrow Formation” is a significant oil and gas producer in the Anadarko Basin, Oklahoma and Texas. Basin development is maturing, and continued discovery and exploitation of fields requires refined interpretations based partly on the use of new tools. Nuclear magnetic resonance (NMR) is a wireline tool that, with more accuracy and precision than conventional wireline tools, estimates porosity, permeability, producible fluids and determines the type of formation fluids. The addition of the NMR tool data to conventional wireline tool data aids in reservoir evaluation, furthering the exploitation effort of the Anadarko Basin.

Perhaps the most significant contribution of the NMR tool is its’ ability to estimate permeability. The field NMR tool uses a magnet 100 times stronger than the Earth’s magnetic field to align hydrogen protons with the NMR’s magnetic field. This static magnetic field is the Bo field. The tool’s antenna then uses a transmitter and receiver to apply in pulses a second magnetic field, the B1 field, perpendicularly to the Bo field. The NMR tool measures the hydrogen decay once the B1 pulses are turned off. Evaluation of this ‘decay-data’ allows geoscientists to evaluate complex lithology, identify fluid types and to better study low permeability/low porosity formations. The bench top NMR machine uses the same principles as the field NMR tool. The bench top NMR machine aligns hydrogen atoms with the static Bo magnetic field. By applying the B1 field in pulses, protons are excited from a low-energy state into a high-energy state. The machine measures the decay of hydrogen atoms from the Bo and B1 fields.

In this study, two permeability equations as they apply to NMR data are evaluated in the “Morrow Formation.” The first equation, K= (o¨ /a)2 * (FFI/BVI)2, uses the ratio of free fluid to bound fluid to estimate pore sizes (where o¨ = porosity, either from NMR readings, conventional log readings or core analysis (generally all are equivalent); a=a constant, usually 10 for sandstones; FFI=free fluid index and BVI= bound fluid index, FFI/BVI ratio calculated using a 33 msec cutoff). This equation is normally applied in the field. The second equation, K = a * o¨ 4 * T2 2, uses the T2 value to estimate pore size (where a=a constant, usually 4 for sandstones and the T2 value is taken from NMR lab measurements). Besides using the T2 value from lab measurements, a T2 value can also be weighed against changes in the NMR curve. This weighed T2 value generally correlates better to pore size than the lab measured T2 value.

Results of comparing data from the two permeability equations to measured core permeability indicate that, in the “Morrow Formation,” the FFI/BVI equation underestimates permeability and the T2 equation overestimates permeability. As a result of this disparity, the purpose of this study is to adjust the exponents and constants in the permeability equations such that calculated permeability more accurately and precisely correlates to observed permeability, better allowing geoscientists to evaluate the “Morrow Formation.”

AAPG Search and Discovery Article #90903©2001 AAPG Mid-Continent Meeting, Amarillo, Texas