Development of Equations and Procedure for Perforation Inflow Test Analysis (PITA)

Abstract

Abstract Recently short emission tests have been getting attention due to economics, environmental issues or time constraints. Thus, well-testing is sometimes reduced to perforating the well and analyzing the inflow characteristics. The objective of a Perforation Inflow Test Analysis (PITA) is to estimate the initial reservoir pressure, permeability and skin, immediately after perforating the well. This information can be used for evaluating future development strategy. During the entire test period, the surface valve is kept closed, and the formation fluid enters the closed chamber (casing or tubing space) as initiated by a lower cushion pressure than the initial reservoir pressure. However, special analytical procedures are required for analyzing the data, captured through surface or subsurface monitoring, because these perforation inflow tests are shorter than conventional well tests. In this study, the working equations for analyzing these short tests are developed and presented for both gas and liquid (oil or water) wells, and the procedure required for calculating meaningful estimates of the reservoir parameters will be highlighted. The equations are based on some approximations of the slug-test solution in Laplace space. The radius of investigation during the perforation tests will be estimated, and it will be shown that in presence of measurement errors, radius of investigation will grow to a maximum value. Running the tests for anytime longer will detect just the noise. A special derivative, called the impulse derivative, will be used to determine if the data collected is sufficient to yield meaningful results from a PITA. It is particularly important that the wellbore flow has diminished to a low level and the reservoir-dominated flow has fully established, if the estimates of initial reservoir pressure, permeability and skin are to be acceptable.