Thermal effects on single-well chemical-tracer tests for measuring residual oil saturation

The single-well chemical-tracer (SWCT) test for measuring residual oil saturation, S_or, often involves injecting cool fluid containing a reactive tracer into a warm formation. The S_or estimation with this method depends on the separation between reactant and product tracers. Because the reaction rate is temperature-dependent, accounting for the thermal effects may be necessary to obtain reliable results. Two simulator models are normally used to interpret SWCT tests. The ideal model is used for relatively homogeneous sandstone formations. The pore-diffusion model is used for heterogeneous carbonate formations. Both models have now been solved with appropriate heat-balance equations. These nonisothermal models have been used to reinterpret several previously reported field tests. For the worst case, the estimated S_or value from the nonisothermal model is 5% PV higher than that from the isothermal model. Inequality conditions have been developed that divide the parameter space of SWCT tests into two regions, depending on the location of the temperature front relative to the tracer bank during the reaction period. In the 'safe' region, the estimated S_or values from isothermal and nonisothermal models are essentially equal. The inequality conditions have been extended to include the effects of over-and underburden layers and intervening shales in layered systems.