A Short Cut to Stabilized Gas Well Productivity

Abstract

Here are simplified correlations that quickly provide correction factors for obtaining stabilized productivity from short-term flow tests. The only data needed to use the correlations are reservoir pressure, duration of test, and estimated permeability. Introduction For most studies involving gas wells and gas reservoirs, a very important consideration is the stabilized productivity or the ability to deliver gas for a productivity or the ability to deliver gas for a considerable time against a given backpressure. This knowledge is usually most needed in the early stages of development of an area, before extensive well test data or production information can be collected. Even in fields with considerable history, an engineer may frequently need a long-range performance prediction without having access to any data except the short-term test data normally required by state regulatory bodies. Short-term test data typified by a four-point back pressure with only 1 or 2 hours' flow on a point, or pressure with only 1 or 2 hours' flow on a point, or even a one-point 24-hour test may be grossly in error in low permeability reservoirs. Much of our nation's gas reserves is contained in the San Juan, Anadarko and Arkoma basins, which have the common characteristic of low permeability. In these areas it is not at all unusual for a well to require 30 days' flowing time to reach stabilization. Use of short-term test data for such wells would obviously lead to poor-quality estimates of either producing rates or reserves. The long-term flow test, to actually achieve stable flow, may be impractical for any of several reasons. The next best alternative would probably be two-rate flow tests. These require collection of detailed and closely controlled test data and can be costly even though much less time-consuming than the long-term flow to stable conditions. Another possible approach would be to determine interwell permeability and a skin factor from a pressure buildup test. Substitution of these factors with estimates of other reservoir parameters in the Darcy steady-state flow equation parameters in the Darcy steady-state flow equation would provide a reasonable estimate of stable flow capacity. In the absence of adequate field test data to define stabilized producing capacity, a stabilization factor to apply to short-term flow data would be useful; and that is the subject of this paper. For a stabilization factor to be most useful, it should be determinable with a minimum of field test data and calculation and still retain a reasonable degree of accuracy. Correlations have been developed that fulfill the requirement of minimum data and calculation. A check of the correlations with test data from several fields indicates a fair degree of accuracy. Basis for Correlations The well known Darcy steady-state flow equation neglecting turbulence may be written for gas wells as follows: (1) This equation and all others here are in practical field units and the terms are in the nomenclature. In the equation s is the skin factor defined by van Everdingen. JPT P. 537