The isochronal test[1] is a series of single-point tests developed to estimate stabilized deliverability characteristics without actually flowing the well for the time required to achieve stabilized conditions at each different rate. Both the Rawlins and Schellhardt and Houpeurt analysis techniques are presented in terms of pseudopressures. The isochronal test is conducted by alternately producing the well then shutting it in and allowing it to build to the average reservoir pressure before the beginning of the next production period. Pressures are measured at several time increments during each flow period. The times at which the pressures are measured should be the same relative to the beginning of each flow period.

This article discusses the implementation and analysis of the modified isochroncal testing for gas well deliverability tests. Both the Rawlins and Schellhardt and Houpeurt analysis techniques are presented in terms of pseudopressures. The time to build up to the average reservoir pressure before flowing for a certain period of time still may be impractical, even after short flow periods. Consequently, a modification of the isochronal test was developed[1] to shorten test times further. The objective of the modified isochronal test is to obtain the same data as in an isochronal test without using the sometimes lengthy shut-in periods required to reach the average reservoir pressure in the drainage area of the well.

Abstract

This paper presents a review of the practical backpressure test analyses available for estimation of the stabilized absolute open flow (AOF) potential of natural gas wells. Linear regression analysis techniques have been used to correlate the field-recorded deliverability data and statistical influence tests have been used to identify possible out-liers in the test data.

The types of backpressure tests considered in this study are the conventional flow-after-flow (four point), single point, regular and modified isochronal backpressure tests, and the multiple modified isochronal test. The deliverability analyses considered in this paper are the Rawlins-Schellhardt pressure-squared, and the Houpeurt (quadratic) real gas pseudopressure and pressure-squared analyses. Modified versions of these analyses are used in the analysis of multiple modified isochronal tests.

The analysis techniques developed for multiple modified isochronal tests were reviewed and found to permit a rapid and adequate means of estimating the stabilized AOF potentials of slow-in-stabilizing wells in homogeneous reservoirs, using only the semilog transient isochronal deliverability data. Theoretical considerations are also introduced which may provide a means of estimating stabilized AOF potentials of gas wells completed in naturally fractured reservoirs. A discussion is also included on the estimation of stabilized AOF potentials of wells completed in homogeneous reservoirs, which have been vertically fractured to increase their productivity.

Introduction

Deliverability testing of natural gas wells for the estimation of stabilized absolute open flow (AOF) potentials is generally performed using backpressure tests. A backpressure test is a drawdown flow test in which a well is produced at a series of flow rates and associated sandface pressures in order to establish the deliverability behavior of the well.

Varying definitions of stabilized AOF potential of a gas well can be found in the literature. While the lack of consistency in the definition of AOF potential generally does not significantly affect the values of stabilized AOF potential obtained, it does add confusion to a discussion about stabilized AOF potential determination. Since a natural gas well will not exhibit a flowing sandface pressure of less than atmospheric pressure for normal production operations, we shall use the definition of stabilized AOF potential as the theoretical stabilized rate at which the well would produce at a stabilized flowing sandface backpressure of atmospheric pressure. While this definition of stabilized AOF potential has the limitation of variable atmospheric pressure values, the limitation is negligible since in most areas, the standard atmospheric pressure is regarded to be about 14.7 psia.

Estimates of stabilized AOF potentials of gas wells have been used by the natural gas industry and regulatory agencies for several purposes, such as setting allowable production rates, pipeline and gathering system design, planning field development, and for the negotiation of sales contracts. The various types of backpressure tests and analyses available were reviewed to determine their applicability to the various types of reservoirs commonly found today.

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Basic concepts, which include flow equations for unsteady-state, pseudosteady-state, and steady-state flow of fluids, are discussed first. Various flow geometries are treated, including radial, linear, and spherical flow. The pseudosteady-state equations provide the basis for a brief discussion of oil well productivity, and the unsteady-state equations provide the basis for a lengthy discussion of pressure-transient test analysis. For pressure-transient test analysis, semilog techniques, type curves, damage and stimulation, modifications for gases and multiphase flow, the diagnostic plot, bounded reservoirs, average pressure in the drainage area, hydraulically fractured wells, and naturally fractured reservoirs are included. The chapter also discusses transient and stabilized flow in horizontal wells and gas-well deliverability tests. It concludes with considerations of coning in vertical and horizontal wells. Many important applications of fluid flow in permeable media involve 1D, ...

The new approach will provide a single and useful tool for estimating gas/condensate well productivity. The productivity of gas and oil well plays an important role in gas and oil field development, particularly gas Introduction condensate well that is characterised by severe loss of well deliverability due to liquid drop out below the dew Evaluation of well productivity is necessary for the point, and for establishing field proration schedules planning and development of gas/condensate reservoirs, and setting maximum allowable production rates for and for establishing field proration schedules and individual wells by regulatory agencies. Houpeurt setting maximum allowable production rates for wells equation and Rawlins-Schellhardt equation are the under specific reservoir conditions. In the natural gas most widely used gas deliverability equations by industry, engineers mostly used the Rawlins-engineers. Although the Houpeurt equation has a Schellhardt equation and Houpeurt to estimate the theoretical basis and is rigorously correct, the equation production capabilities of gas wells. The standard dry assumes that Darcy's law is still valid under high gas flow equations based on flow-after-flow, isochronal velocity of gas flow and that an additional term must be and modified isochronal testing have also been used to added to account for the increased pressure drop. The analyze the gas condensate well productivity when the Rawlins-Schellhardt equation still continues to be used retrograde condensate is viewed immobile at reservoir by the natural gas industry, however it was empirically conditions. Rawlings and Schellhardt (1936) put formulated and the deliverability plot is traditionally forward the empirical equation (1) after they observed reversed in order to estimate the deliverability that a log-log plot of the difference between the squares exponent.