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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 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.
A single-point test for gas well deliverability is an attempt to overcome the limitation of long test times required for flow-after-flow tests. Both the Rawlins and Schellhardt and Houpeurt analysis techniques are presented in terms of pseudopressures. A single-point test is conducted by flowing the well at a single rate until the sandface pressure is stabilized. One limitation of this test is that it requires prior knowledge of the well's deliverability behavior, either from previous well tests or possibly from correlations with other wells producing in the same field under similar conditions. Ensure that the well has flowed long enough to be out of wellbore storage and in the boundary-dominated or stabilized flow regime.
Both the Rawlins and Schellhardt and Houpeurt analysis techniques are presented in terms of pseudopressures. Flow-after-flow tests, sometimes called gas backpressure or four-point tests, are conducted by producing the well at a series of different stabilized flow rates and measuring the stabilized BHFP at the sandface. Each different flow rate is established in succession either with or without a very short intermediate shut-in period. Conventional flow-after-flow tests often are conducted with a sequence of increasing flow rates; however, if stabilized flow rates are attained, the rate sequence does not affect the test. Fig 1 illustrates a flow-after-flow test.
Early estimates of gas well performance were conducted by opening the well to the atmosphere and then measuring the flow rate. Such "open flow" practices were wasteful of gas, sometimes dangerous to personnel and equipment, and possibly damaging to the reservoir. They also provided limited information to estimate productive capacity under varying flow conditions. The idea, however, did leave the industry with the concept of absolute open flow (AOF). AOF is a common indicator of well productivity and refers to the maximum rate at which a well could flow against a theoretical atmospheric backpressure at the reservoir. The productivity of a gas well is determined with deliverability testing.
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.
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.