1962 Paper G

A COMPUTER PROGRAM FOR OBTAINING APPROXIMATE POROSITY AND WATER SATURATION FROM RESISTIVITY AND VELOCITY LOGS IN CONSOLIDATED SAND OR SANDSTONE RESERVOIRS

J. C. Wells and J. E. Walstrom

Standard Oil Company of California San Francisco, California

ABSTRACT

The RESON computer program is a statistical best fit method for obtaining approximate porosities and water saturations in selected sand intervals in sections of several hundred to a thousand feet or more where formation water resistivity and mineralogical composition of the sands is relatively uniform. A maximum of 40 levels can be processed in one operation by the IBM 7090 computer.

Input consists essentially of a set of Rt values obtained from a deep investigation resistivity or induction log, the corresponding Dt values from a continuous velocity log, and an assumed average fluid velocity. An optimum matrix velocity and the A factor in the relationship F=A/ز are worked out by the program from the fully wet sands in the section analyzed. This subset routine requires a minimum of five wet sand intervals with different porosities in order to obtain good results from the statistical best fit approach. An apparent Rw is obtained concurrently from the wet sand intervals. Porosity,, formation factor, and water saturation are then calculated for all levels from the formulary Dt=Ø /VF+(1-Ø)/VM, F=A/ز, and Sw=ⁿ√F Rw/Rt. Three values of water saturation are obtained assuming n of 1.5, 2.0, and 2.5. The log analyst can then select the most realistic value or range for Sw depending upon his knowledge of clay or cementing material in the section analyzed.

The principal advantages of this program are minimum of input data and assumptions, maximum utilization of the memory capacity of a large computer through the statistical approach, and adaptability to automatic processing of digitized logs. Additional refinements and a more sophisticated treatment for selected intervals may be added to the program as warranted.