Determining Oil, Water and Gas Saturations Simultaneously Through Casing by Combining C/O & Sigma Measurements
Authors: Schnorr DeWayne R. 
Published: December 2007

Abstract
Combining C/O ratio (inelastic mode) and Sigma (capture mode) measurements into one logging tool has developed into a new interpretation method for determining oil, water, and gas saturations simultaneously through casing. This interpretation method solves the limitations of both the C/O ratio and Sigma measurements when they are used alone and not in combination. The C/O ratio measurement, by itself, can not determine water saturation when gas is present. When gas is present with oil and water, the water saturation is too pessimistic and the oil saturation is too optimistic. The sigma measurement, when used by itself, has similar problems like the C/O ratio measurement, but in a different way. The Sigma measurement cannot determine oil from fresh water. The absorption rate of thermal neutrons is similar for oil and fresh water; therefore, oil and water saturations are impossible to determine. If there is some gas present in the zone, the calculated saturations are wrong.
This new interpretation model uses both the C/O ratio and Sigma data to determine oil, water and gas saturations behind casing. The C/O ratio data is cross plotted against the Sigma data. Next, the end points for oil, water and gas are determined from this cross plotted data, or calculated from the C/O ratio and Sigma equations. From the model that was determined from the data, the oil, water and gas saturations are then determined. The same interpretation method with a different model can used in shaly sands to determine oil, free water and bound water saturations.
Three case histories will be used to demonstrate the results of this new interpretation technique. These examples are:oil displacing gas due to pressure depletion,oil replaced by water, andthe effects of dispersed coal, (carbon) in the matrix on the C/O ratio and Sigma measurements, and how the oil and water saturations are corrected to give the correct oil and water saturations.
These wells are just a few of the wells that this method has been used on, and were selected to bring attention to what can be done with this method.

Introduction
Combining the inelastic C/O ratio and capture Sigma measurements into one tool, the Reservoir Saturation Tool, RST*, is now being used to evaluate reservoirs through casing in terms of oil, water and gas saturations. Combining the two measurements eliminates the uncertainty that has been a problem when using only the C/O ratio or Sigma measurements by themselves. C/O ratio and Sigma can he used in reservoirs that contain shale or silt to determine how much of the total water is free water - "producible", and how much of the total water is bound water - "not producible". If the reservoir has a small amount of coal in it, the C/O ratio-Sigma combination will correct for this effect and determine the correct oil and water saturations. This same evaluation can be done when other minerals, such as Pyrite or Siderite, are present in the reservoir matrix. All of this is done simultaneously from the cross plot of C/O ratio and Sigma.

ClO - SIGMA MODEL
Combining C/O ratio and Sigma data gives a simultaneous solution for the oil, water, and gas saturations that are present in the reservoir.
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