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• Each plot uses a different time function depending on the length and variation of the injection rate preceding the falloff. These plots can give different results for the same test, so it is important that the appropriate plot with the correct time function is used for the analysis. Determination of the appropriate time function is discussed below. • The slope of the semilog straight line is then used to calculate the reservoir transmissibility-kh/µ, the completion condition of the well via the skin factor- s, and also the radius of investigation- r; of the test. Determination of the Appropriate Time Function for the Semilog Plot The following four different semilog plots are used in pressure transient analysis: 1. Miller Dyes Hutchinson(MDH)Plot 2. Horner Plot 3. Agarwal Equivalent Time Plot 4. Superposition Time Plot These plots can give different results for the same test. Use of the appropriate plot with the correct time function is critical for the analysis. • The MDH plot is a semilog plot of pressure versus At, where At is the elapsed shut-in time of the falloff. 1. The MDH plot only applies to wells that reach psuedo-steady state during injection. Psuedo-steady state means the pressure response from the well has encountered all the boundaries around the well. 2. The MDH plot is only applicable to injection wells with a very long injection period at a constant rate. This plot is not recommended for use by EPA Region 6. • The Horner plot is a semilog plot of pressure versus (t +At)/At. The Horner plot is only used for a falloff preceded by a single constant rate injection period. I. The injection time,t =VP/q in hours, where VP injection volume since the last pressure equalization and q is the injection rate prior to shut-in for the falloff test. The injection volume is often taken as the cumulative injection since completion. 2. The Horner plot can result in significant analysis error if the injection rate varies prior to the falloff. • The Agarwal equivalent time plot is a semilog plot of the pressure versus Agarwal equivalent time, At.. 1. The Agarwal equivalent time function is similar to the Horner plot, but scales the. falloff to make it look like an injectivity test. 2. It is used when the injection period is a short, constant rate compared to the length of the falloff period. 3. The Agarwal equivalent time is defined as: Ate log(tp At)/(tp+At), where t is calculated the same as with the Horner plot. A-9