Combining the simplified analysis and the more rigorous Laminar Inertial Turbulent (LIT) analysis of isochronal and modified isochronal tests, it has been shown that these tests are valid, within engineering accuracy, for all values of the inverse sloe, n. A sequence of increasing rates produces less errors in a modified isochronal test than decreasing rates. The Absolute Open Flow (AOF) calculated by the simplified analysis and that calculated by the LIT analysis, can be significantly different. A table of correction factors has been generated, which permits conversion of the AOF obtained from the simplified analysis to the value that corresponds to the more complicated LIT analysis. When the ratio of the AOF to the test rate is less than 3 the difference between the simplified AOF and the LIT AOF is less than 10%.
The isochronal and modified isochronal tests are standard and routine tests in the natural gas industry. The inverse slope, n, of the plots is known to lie between a half and one, yet the theoretical validity of these tests has only been demonstrated(1) for the case n=1. It is taken for granted that n is the same for stabilized flow as it is for transient flow (the isochronal part of the test). It is the purpose of this paper to show that the change in n from transient to stabilized conditions is small and may be practically ignored for all values of n.
The isochronal and modified isochronal tests are described in great detail in the Gas Well Testing manual(2). They consist of four flow periods of short but equal durations (isochronal) followed by a single extended flow to stabilization. The isochronal data are plotted to give a straight line of inverse slope n, and a parallel line is drawn through the stabilized point resulting in the deliverability line for the well. A close look at the equations of flow reveals very readily that, theoretically, these two lines should not necessarily be parallel. In practice, they are always drawn parallel. For the case of n=1, it has been shown (1) that the two lines are indeed parallel; for n 1 there is no published verification.
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