The Unified Analysis of Well Tests

Abstract

American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc. Abstract A technique has been developed for the unified analysis of pressure data, using information from all drawdown and buildup periods of virtually any test schedule. Best periods of virtually any test schedule. Best statistical estimates are obtained for values of initial reservoir pressure, permeability, skin factor and, if applicable, the nondarcy flow constant. The fundamental reliability of the method is demonstrated by the precision with which it analyzes artificial test data. Analysis of field data from both oil and gas wells provides further strong support for the dependability of the technique. The method is easily programmed on a computer for a regression analysis on many observations to give fast, reliable and inexpensive analysis of well-test data. Introduction The design of a well test is generally bound by constraints such as the need for constant flow rates, isochronal flow periods or stabilized pressures. These are required so that the test data can be reliably analyzed by an appropriate method. The execution of a well test is often influenced by factors beyond the control of the testing crew. For example, it may be impossible to maintain a constant flow rate because of a varying backpressure on the well; a test may be disrupted by equipment failure; flowing pressures might not stabilize for weeks or pressures might not stabilize for weeks or months. Thus, test data may fail to meet precisely the specifications required by the precisely the specifications required by the particular analytical method. The choice is to particular analytical method. The choice is to retest the well in the hope of obtaining more suitable data, or to use the existing data recognizing that the results may not be entirely reliable. Many analytical techniques ignore a significant amount of the data collected during the test. Horner's buildup analysis, for example, ignores all but the final flowing pressure of the drawdown period before shut-in. pressure of the drawdown period before shut-in. An isochronal test ignores the pressure data preceding the endpoints of each flow and shut-in preceding the endpoints of each flow and shut-in period. The multiple flow-rate analysis period. The multiple flow-rate analysis proposed by Odeh and Jones ignores all shut-in proposed by Odeh and Jones ignores all shut-in periods. periods.