Accurate knowledge of oil saturation and its distribution is essential for any reservoir being considered for redevelopment or for secondary and tertiary operations. Determination of oil saturation forms the only rational basis for prediction of recoverable oil reserves and it is critical in the selection, design and economic evaluation of an oil-recovery process.
As technology advances, more of this oil will come within our reach. However, it is essential that we understand the reservoir involved so that proper, economically viable technology can be applied. Thus, the importance of residual oil saturation (SOR) which in turn is the basis for the estimation of remaining oil-in-place.
Five techniques to determine remaining oil saturation are currently used. They are coring, logging, chemical tracers, reservoir engineering studies and pressure transient tests. Normally, two or more of these methods are used together and the data evaluated as a whole since each technique has its own advantages and limitations. When evaluated together, a clear understanding of the total reservoir can be obtained.
Conventional logging tools and procedures have been used extensively for many years for determination of reservoir and fluid properties. Although SOR determination normally require a higher degree of accuracy than is commonly achieved by conventional logging methods, the incentive to use logs for SOR measurements remains high. They are often cheaper and easier to use than is the case for other methods which are currently available for measuring SOR.
Two logging methods used to estimate residual oil saturation after waterflood (Sorw) and after gas flood (Sorg) in QGPC's reservoirs will be discussed to illustrate the techniques. The log-inject-log technique for Sorw and the Borehole Gravity Meter (BHGM) for Sorg will be evaluated along with their uncertainties.
It is possible to get reliable water saturation answers from clean formations if gas saturations are not present using the Pulsed Neutron Capture (PNC). This technique is quite useful in the evaluation of Sorw. The procedure in this method uses log-inject-log techniques. The major advantage of this technique lies in the fact that the capture cross section of the injected water is known. When sigma log (Slog) reading has a greater spread between water-filled and hydrocarbon-filled zones, a more accurate evaluation of water saturation is possible. This means having accurate values for Smatrix, Shydro and Slog to start with.
The problem becomes more apparent when gas is introduced into the system. This volume of gas makes the error much greater. It introduces a third term of Sg into the equation.
With such a large error in the case of oil being displaced by gas in the Sorg determination from the PNC a more accurate tool needs to be used. The only other tool, currently available that is believed to provide better accuracy for Sorg determination is the Borehole Gravity Meter (BHGM). The BHGM is used to examine the bulk density large distances away from the borehole. The bulk density of a rock complex is a function of the matrix density of the rock, the porosity of the rock, and the interstitial fluids in the porous void space. Fortunately, these various factors are related by simple superposition and by taking the derivative, one obtains a relationship which governs change in bulk density as a function of the changes in fluid density1.
The theory of log-inject-log for Sorw determination is well known.2,3