Pressure Transient Analysis for Wells With Variable Sandface Flow Rate

Abstract

Abstract In the last two decades, with the availability of new state-of-the- art production logging devices that are capable of providing accurate downhole rate and pressure measurements, variable rate testing has been given special concern. This is mainly because it is sometimes impractical and often impossible to hold a constant flow rate long enough to perform a drawdown test. Furthermore, the use of sand face flow rate and pressure data in well test analysis has a few major advantages over conventional methods. Variable-rate tests minimize the wellbore storage effects, reduce the test duration, and provide better description of the area in the vicinity of the wellbore. This paper presents a new technique that does not require a constant flow rate for the entire duration of the test. The technique consists of two flowing periods: a short constant flow rate period followed by a variable flow rate period. In deriving the final working equation, the variable flow rate period is approximated by a polynomial. The new method can be also used to analyze pressure drawdown data dominated by wellbore storage and pressure buildup data pertaining to early time afterflow. Two simulated cases and two field examples are presented to illustrate the applicability of the new technique. Introduction The pressure drawdown test has long been recognized in the oil industry to be an excellent technique to determine reservoir characteristics. However, one of the major drawbacks in analyzing pressure draw downs is the theoretical requirement of these techniques that requires the flow rate to be held strictly constant during the entire test duration. Moreover, in most wells, the wellbore storage effects dominate the early test data and force the operator to run the test for a long period of time. This is especially true in low-productivity pumping wells and low-permeability tight reservoirs. Furthermore, in the case of small reservoirs, the boundary effects may disturb the test data even before the appearance of the semi log straight line on the test plot. Sometimes, even when the plot displays a straight line, the data analysis may not reflect the near wellbore conditions. In all of these cases, the well test fails to achieve its goals. In addition, for both oil and gas wells, the long constant-rate production condition is hard to fulfill in the field, especially during the early life of the reservoir. For all of these reasons, well test researchers tried to come up with new testing techniques that do not require a constant rate, minimize the wellbore storage effects, reflect the actual conditions in the vicinity of the wellbore, and reduce the test duration. Variable-rate tests are designed to solve most of these problems. The idea of using afterflow data to analyze well tests is not new. As early as 1955, Gladfelter et al.(1) presented the first attempt to use approximated afterflow rate and pressure data to estimate the reservoir parameters.