Reservoirs with a fluid bank, or a buming front, reservoirs with a reduced or an increased permeability region around the wellbore, and geothermal reservoirs are often modeled as composite reservoirs. Reservoirs with a fluid bank include reservoirs undergoing waterflood, chemical flood, polymer flood, C02 flood, and steam injection. Eight well tests reported in the literature exhibiting composite reservoir behavior have been analyzed using the deviation time method. The dimensionless deviation times obtained from pressure and pressure derivative responses for a well in a composite reservoir have been used for analyzing the well tests. Analysis shows the estimate of discontinuity radius to be sensitive to both the real and the dimensionless deviation times used. The estimated discontinuity radius from the deviation time method may represent a lower bound for discontinuity radius, if the swept region is not cylindrical Also, obtaining an accurate deviation time for small mobility contrasts may be difficult.
Limitations on the deviation time method due to wellbore storage effects have been quantified indicating a need to minimize wellbore storage in composite reservoir well tests. The effects of a transition region on the dimensionless deviation time have been studied using an analytical solution for a three-region reservoir.