Analyzing the Performance of Gas Wells

Abstract

Summary Methods for analyzing gas well performance are presented. Inflow, outflow, and tubing performance presented. Inflow, outflow, and tubing performance curves are defined, and examples of each are given. These curves are the primary tools used to visualize and graphically predict the effects of declining reservoir pressure, increasing water production, changing tubing pressure, increasing water production, changing tubing sizes, or installing gas compressors. A graphical method for determining an accurate reservoir abandonment pressure is also presented. The concept of "flowpoint" pressure is also presented. The concept of "flowpoint" and it importance in understanding gas well behavior is explained. Introduction Understanding the flow characteristics of gas wells has become increasingly important to those in the petroleum industry as the economic importance of gas has increased. During 1970–80 the average annual wellhead price of all natural gas in the U.S. rose from $0.171 to price of all natural gas in the U.S. rose from $0.171 to $1.496/Mcf. Gas purchasers are currently paying in the $5.00/Mcf range and have paid more than $9.00/Mcf in some areas. It is now possible to understand and to predict the flowing characteristics of any gas well with a good degree of accuracy from a minimum of field test data. The entire range of a gas well's flowing performance may be determined from a single well test, a current reservoir pressure, and published mathematical correlations. Inflow Performance Curves One of the most widely used methods for mathematically describing the downhole performance of a gas well is the empirically derived relationship (1) where q = rate of flow, Mcf/D, C1 = a numerical coefficient, characteristic of theparticular well, p = shut-in reservoir pressure, psia, p = shut-in reservoir pressure, psia, pwf = flowing bottomhole pressure (BHP), psia, pwf = flowing bottomhole pressure (BHP), psia, and n = a numerical exponent, characteristic of theparticular well. This equation represents a straight line drawn through well-test data points plotted on log-log graph paper as shown by Curve A in Fig. 1. The numerical constant, C1, controls the placement of the performance curve to the tight or left along the horizontal axis of the graph, and the exponent, n, represents the reciprocal of the curve's slope. In Fig. 1, n is equal to the cotangent of the angle theta. This angle is usually about 45 deg., and n is about 1.0. Under normal conditions, a performance curve may be approximated by plotting a single well-test point and drawing a 45 deg. plotting a single well-test point and drawing a 45 deg. performance curve line through it. performance curve line through it. The procedure for collecting these well-test data is commonly called a "backpressure test" or "four-point test. "The primary purpose of this type test and graphic plot is to project a well's performance to what it would plot is to project a well's performance to what it would be with a flowing BHP, pwf, of zero. The gas rate established by this projection is obviously the maximum possible and is appropriately called the "absolute open possible and is appropriately called the "absolute open flow" (AOF) of the well. The calculated AOF of a well is a fundamental measure of its ability to produce. It is a good measure to use when comparing gas wells in different areas because the many and varying effects of well depth, tubing size, wellhead backpressure, etc., are eliminated by working with downhole other than surface measurements of rates and pressures. JPT P. 1378