Determination of Stabilization Factor And Skin Factor From Isochronal And Modified Isochronal Tests
Authors: Lin Chung , Mattar Louis 
Published: March 2010

A large number of isochronal and modified isochronal tests which do not meet the criterion of stabilized flow are performed. The deliverability prediction from these tests is optimistic and results in oversized pipelines and gas plants. In order to correct for the effect of a non-stabilized flow period, a graphical procedure is described which results in the calculation of a stabilization factor. The procedure uses the backpressure equation q = C(∆p2)n to calculate two values of C at two different times, from which the stabilized value of C at time to stabilization is obtained either graphically or from tables. The only other piece of information that is necessary is the order of magnitude of the permeability. In addition to the stabilization factor, an excellent estimate of the skin factor can be obtained from these tests without having to do a traditional plot of p2 versus log time. The results are also applicable to the LIT approach.

Introduction

One of the most common tests performed on a gas well is a modified isochronal test. This usually consists of four flow and shut-in periods of short duration 1 to 12 hours and an extended flow period until stabilization is achieved. The results are plotted as shown by lines I and III in Figure 1. The four short-term flow data points are run to obtain the inverse slope, "n", of the back pressure curve. A line drawn parallel to that, but going through the stabilized flow data point, gives the stabilized deliverability line, often called "the back pressure curve". This curve is used to forecast the deliverability potential of a gas well at all stages of reservoir depletion against any pipeline pressure. The Absolute Open Flow (AOF) potential of the well is also obtained by extrapolating the line to reflect a flowing pressure equal to zero.

The testing and analysis procedures are very simple, but with the coming on stream of low-permeability wells, the time required for stabilization of these wells is often prohibitively long. The extended rate is still carried out for approximately 72 to 120 hours, but the time to stabilization may be in the order of 3 months to 1 year. In such a situation, the lines obtained on Figure 1 are I and II instead of I and III. If line II is assumed to be the true stabilized deliverability then the production forecast will be unduly optimistic, resulting in oversized pipelines and gas plants.

There are several ways of obtaining a stabilized line from theoretical calculations. These involve analyzing drawdown or buildup data, using semilog techniques, and calculating permeability and skin factor. This aspect is covered in great detail in numerous publications on pressure transient analysis, both in the petroleum literature and in the authoritative ERCB (1979) publication(l) on gas well testing. These and other techniques such as the SIT method(2) require an understanding of transient pressure analysis to be properly applied.

This paper uses the theoretical basis of isochronal and modified isochronal tests, to develop a simplified graphical solution to the problem of determining the stabilization factor.