Formation Rate Analysis in an Anisotropic Formation and It's Practical Applications

Abstract

Proposal In probe-type formation tests, due to the geometry of the wellbore and the sealing effect of mudcake, the flow pattern is not perfectly spherical. To account for the deviation from spherical flow, several geometric correction factors were proposed for different analysis techniques. A geometric factor is used in Formation Rate Analysis (FRA), a technique used in analyzing a probe test to estimate formation pressure and permeability. Like other geometric correction factors, the geometric factor is a strong function of permeability anisotropy that is generally unknown before a test. When analyzing the test, we would logically assume an isotropic formation and use the corresponding geometric factor. Consequently, the FRA estimated permeability does not represent the true spherical permeability. In contrast, the spherical permeability can be estimated from buildup analysis without prior knowledge of permeability anisotropy. Therefore, the discrepancy between the permeability estimates from the two analysis methods cannot be resolved. In addition, if separately considered, neither FRA nor buildup analysis can decompose the estimated permeability into its horizontal and vertical components. This paper presents the derivation of numerical values of several geometric correction factors. Using the concept of geometric factor, we resolve the discrepancy between the permeability estimates from FRA and buildup analysis. This paper also presents procedures to estimate horizontal and vertical permeabilities by combining FRA and buildup analysis or by combining buildup analysis and pressure history matching. These procedures are verified with a simulated probe test. An example of an actual test analysis is also presented.