The Basis And Use of the API RP43 Flow Test For Shaped-charge Oil Well Perforators (1997)

Abstract The recently revised API (American Petroleum Institute) Recommended Procedures for Evaluating Shaped-charge Perforators (RP43, 5th ed.) includes test procedures for measuring perforation flow performance under simulated in situ conditions. In this paper, we review these procedures and develop the basic equations used to interpret the results. Using numerical analysis, we provide curves for relating core flow efficiency (CPE) values from older 4th edition tests with those from the new test. We also provide curves for interpreting the older style tests in arbitrary sample diameters. CPE values are related to thickness and permeability of the damaged zone and to skin values. We find that CPE for the older tests tend to lie between 0.8 and 1.0 for a wide range of damaged zone characteristics, while the newer test using radial boundary conditions, is much more sensitive to damaged zone characteristics. We investigate the effects of non-Darcy flow arid anisotropy on ideal flow. We confirmthat non-Darcy effects are generally negligible for steady-state liquid flows. As previously noted, average-permeability formula provided by API do underestimate ideal flow, but to a much lesser extent than previously stated. We provide revised formulae for averagepermeabilities to be used in analytic calculation of ideal flow rates. Introduction Flow efficiency of perforations plays a major role in determining well productivity in naturally-completed production or injection wells. Even in stimulated wells, good perforation flow is needed to allow fracturing or acidizing fluids to enter the formation. Well tests and their interpretation are also affected by perforation depth and flow efficiency. Flow efficiency for a given set of perforations is affected by charge size and design, formation rock and fluid properties and by under- or over-balanced conditions during perforating. To provide a standard method for measuring flow efficiency under simulated in situ conditions, the American Petroleum Institute provides a flow test procedure as part of its Recommended Procedures for Evaluation of Oil Well Perforators (RP 43) ((1, 2) and has recently updated it to provide more realistic simulation of down-hole conditions. This new procedure is beingused in several projects studying underbalance and sand production and has been used in several formation-specific testprograms. This paper is intended to review the basis of the test procedures and to provide some guidance in its performance and interpretation. In particular we will derive the equations used, show how the core flow efficiency (CPE) is related to that from earlier versions of the test and how it is related to skin and damaged-zone permeability. We also investigate the effect of anisotropy and core orientation. The paper is divided into the following sections:Review of basic test proceduresUseful modifications to the proceduresMeasurement of pretest permeability dataCalculation of core flow efficiency CCPE)Relations between CFE values, skins, characteristics of the reduced permeability zone. To assist in interpretation of the tests we have developed an unsteady-state, three-dimensional, finite difference model to simulate the fluid flow inside test targets with or without perforations and using arbitrary flow boundary conditions.