Numerical Interpretation of Single Well Chemical Tracer (SWCT) Tests to Determine Residual Oil Saturation in Snorre Reservoir
Authors: Khaledialidusti R. , Kleppe J. , Skrettingland K. 
Published: May 2015

Determination of residual oil saturation (Sor) is heavily required for evaluating the effect of EOR methods. While there are some methods to determine Sor such as coring, logging, and tracer methods, it is generally accepted that tracer methods are more efficient mainly due to the measurement of large reservoir volume and non-dependency on porosity. Among all proposed tracer methods to determine Sor of EOR methods, only Single Well Chemical Tracer (SWCT) method is widely implemented in the industry.

Several SWCT tests have been carried out in Snorre reservoir to evaluate the effect of injection of high, drill, and lowsal waterflooding. Since characteristics of each SWCT test is unique and often is different even on the same well in various tests, simulation procedure for finding multiple history matches to the measured tracer production from the field is a challenging process. In this paper, the numerical interpretation of first SWCT test after high salinity waterflooding is done. The necessary parameters to model the field test and the value of numerical interpretation of SWCT tests is also introduced. UTCHEM is used as the numerical simulation tool to get an acceptable history match to the measured field data.

The procedure of interpretation is started with a single-layer (ideal) model. The unknown parameters that affect tracer production are grid size, hydrolysis reaction rate, and average Sor are obtained. The Sor is determined 0.24 in the present test in the original reservoir temperature of 194 °F. While ideal model shows a worthy fit, there is still some deviation between measured and modelled profiles at the tail of the curves because of non-ideality effects. Flow irreversibility in layered test zones is considered for departing ideal model from the real test in this sandstone reservoir. The three new unknown parameters of non-ideal model are the number of layers in the test zone, the fraction of total fluid injected and produced from each layer, and the Sor for each layer. In the interpretation of the present test, two other layers are added to determine the effect of flow irreversibility in test zone. The main layer, the early layer, and the late layer accepted and produced 52%, 17%, and 31% of the total injected tracer, respectively. The average Sor of 0.22, 0.24, and 0.26 is resulted to the main layer, the early layer, and the late layer, respectively. The best history match without any deviation between modelled and measured profiles is obtained using non-ideal model.

According to the reservoir cooling and heating from shale layers above and below the flooded sand interval, the temperature of the fluids near-wellbore will change. Since the tracer bank is injected with lower temperature than reservoir temperature, the effect of reservoir cooling on the ester partitioning coefficient is investigated in order to determine Sor more precisely. Using this numerical SWCT interpretation, it is achievable to examine other EOR methods to reach more oil recovery in this sandstone reservoir via obtained unknown parameters.