A Simplified Method for Determining Gas-Well Deliverability

Abstract

Summary. This paper presents a simplified method for predicting the performance of a gas well. A method for determining the deliverability of performance of a gas well. A method for determining the deliverability of an unfractured gas well by use of a single-point flow test and a dimensionless Vogel-type inflow performance curve was proposed by Mishra and Caudle. Their procedure necessitates the calculation of real-gas pseudopressures for shut-in and flowing bottomhole pressures (BHP) obtained pseudopressures for shut-in and flowing bottomhole pressures (BHP) obtained from pressure-buildup and stabilized-flow tests, respectively. This paper offers a simplification of this technique in which a range of pressure values is defined over which pressure-squared terms can be substituted for pseudopressures. A comparison is made between results obtained from analysis of well-test data on several gas wells made with conventional multipoint test methods, with the Mishra-Caudle technique, and with the simplified method presented in this paper. The simplified method offers the engineer who might not have access to a pseudopressure computer program or pseudopressure tables a method for pseudopressure computer program or pseudopressure tables a method for estimating gas-well deliverabilities. The method of Mishra and Caudle and the simplified method were both observed to yield slightly conservative estimates of gas-well deliverabilities compared with the deliverabilities calculated from multipoint flow-test analyses. The simplified technique was found to be useful for predicting the performance of fractured gas wells as well as unfractured wells. Introduction Predicting the performance of gas wells is a process that has relied Predicting the performance of gas wells is a process that has relied almost exclusively on some form of multipoint well-testing procedure. The conventional backpressure or flow-afterflow, the procedure. The conventional backpressure or flow-afterflow, the isochronal, and the modified isochronal tests have been used to predict the short- and long-term stabilized deliverability of gas wells. predict the short- and long-term stabilized deliverability of gas wells. In a typical multipoint deliverability test, a well is produced at a minimum of four different flow rates with shut-in periods of various lengths separating flow periods. Pressure is monitored during both the flow and shut-in periods throughout the test. Analysis of the BHP vs. flow rate yields results that, when plotted on log-log paper as shown in Fig. 1, produce a straight line that reflects the paper as shown in Fig. 1, produce a straight line that reflects the stabilized deliverability behavior of a gas well. The empirically derived relationship given by Eq. 1 represents the equation of a stabilized deliverability curve such as the one shown in Fig. 1.(1) The constant C reflects the position of the stabilized deliverability curve on the log-log plot. The value of the exponent, n, is equal to the reciprocal of the slope of the stabilized deliverability curve and normally has a value between 0.5 and 1.0. The stabilized deliverability curve or its equation may be used to predict the ability of a well to produce against a given sandface backpressure. The absolute open flow (AOF) of the well is also frequently calculated. The AOF is the theoretical maximum flow rate a well can maintain against a zero surface backpressure. The AOF is used mainly in comparing wells and by regulatory billies in establishing production allowables. Multipoint backpressure tests yield very reliable deliverability projections when correctly conducted in the field. Frequently, projections when correctly conducted in the field. Frequently, however, these tests require a commitment of manpower, equipment, and time that may render the tests cost-prohibitive. This is particularly true in the case of low-permeability reservoirs, where testing particularly true in the case of low-permeability reservoirs, where testing times may be very long. The problem is further compounded in terms of lost revenues if gas must be flared throughout the test. Alternative methods for forecasting as-well deliverability have been proposed by several authors. A replot of the stabilized deliverability curve shown in Fig. 1 on Cartesian coordinate graph paper (Fig. 2) produces an inflow performance, or IPR, curve paper (Fig. 2) produces an inflow performance, or IPR, curve similar to those observed in the testing of oil and gas producing wells. Russell et al. showed that IPR curves constructed with Eq. 1 gave predicted gas-production rates lower than those observed in the field. predicted gas-production rates lower than those observed in the field. Russell et al. proposed an equation that depicted gas-inflow performance more accurately performance more accurately (2) Greene documented that Neely rewrote Eq. 2 by collecting the parameters that were constant for a given well in a constant, C, parameters that were constant for a given well in a constant, C, yielding the gas-well inflow performance equation: (3) The constant C, in Eq. 3 may be determined from a single flow test if the shut-in BHP is known. The constant C, will not vary with flow rate; however, it may change over the life of the well because of changes in the producing condition of the wellbore or formation. Greene noted that a valid IPR curve could be constructed for a well from a single C1-factor determination and a known shut-in BHP. This could be done by assuming values of pwf, calculating corresponding ug and z values, and substituting in Eq. 3 to find corresponding values of q. BHP could then be plotted vs. flow rate to obtain the IPR curve. Vogel extensively studied the inflow performance of solution-gas-drive reservoirs. He suggested that the dimensionless IPR curve shown in Fig. 3 could be used to generate actual IPR curves for wells in which oil and gas were flowing. With Vogel's method, only a value for shut-in BHP and a single flow-test point are necessary to generate an IPR curve for a well completed in a solution-gas-drive reservoir. Browns reported that field experience has shown that Vogel's dimensionless IPR curve also yields good approximations of flow behavior when the method is applied to wells producing oil, gas, and water. Vogel suggested that dimensionless producing oil, gas, and water. Vogel suggested that dimensionless IPR curves could be constructed for wells producing only liquids or only gas, as shown in Fig. 3. However, he did not propose an actual dimensionless IPR curve that could be used to predict gaswell performance. Mishra and Caudle presented a method for predicting the deliverability of a gas well in an unfractured reservoir. SPERE P. 1090