Perforation Inflow Test Analysis (PITA)

Abstract

Due to economics or time constraints, well-testing is sometimes reduced to perforating a well under-balanced, 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. However, special analytical procedures are required for analyzing the data, because these perforation inflow tests are shorter than conventional well tests and the influx rates are not measured. In this study, the working equations for analyzing these short tests are presented, and the procedure required for calculating meaningful estimates of the reservoir parameters is presented. Analyses of early-time and late-time data are the two major components of this approach. The early-time analysis is used for estimating the skin, and the late-time analysis is used for estimating the initial pressure and permeability. A distinctive feature of the PITA is that it does not require calculation of the influx rates, which are generally not available during a perforation test. A special derivative, called the impulse derivative, can be used to determine if the data collected is sufficient to yield meaningful results from a PITA. It is particularly important that the reservoir-dominated flow regime be reached, if the estimates of initial reservoir pressure, permeability and skin are to be acceptable. Good estimates of these parameters from a PITA will minimize the uncertainty associated with non-uniqueness in inverse problems, when modeling the test data.