Vegard Elverhaug and Anders Torland
Texas Tech University, Petroleum Engineering
This study is directed toward investigating possible effects of the anisotropy ratio on the analysis and result of buildup tests using Horner method. Even though most reservoirs are more or less homogeneous but not necessarily isotropic, there is no direct method for estimating the anisotropy ratio at reservoir scale despite the fact that this parameter affects to a great extent the reservoir behavior.
The approach used in this study is to solve numerically the one phase 3-D equations that governs the fluid flow in a reservoir. After checking the solution by simulating a test well under buildup, a parametric study to investigate a possible sensitivity of the results to the anisotropy ratio has been conducted.
A MATLAB computer program using a finite difference approach has been written for this purpose. Buildup tests have been simulated successfully and an estimation of the permeability using Horner analysis showed that the results are consistent with the data used for the simulated reservoir. Even though these preliminary results show that Horner method cannot discern between isotropic and non-isotropic reservoirs for fully penetrating wells, the behavior of partially penetrating wells shows clearly that the anisotropy ratio has a significant effect on the calculated permeability. More specifically, this study shows that the effect of the anisotropy ratio is proportional to the penetration ratio of the partially perforated wells.
These results have been generated by simulating very thick and pretty coarse reservoirs. It is expected that more simulation runs will shed more light on the effect of anisotropy ratio on well behavior during routine well testing procedure.