6.6 Tests Utilizing Middle-Time and Late-Time Data

In Chapter 5 it has been shown that early-time data may be used to determine the time of start of transient flow data. A similar analysis applies to the earlytime portion of a buildup. Data should be obtained in the transient flow regime since reservoir parameters calculated by an analysis of middle-time data are much more reliable than those calculated from early-time data.

Data obtained from a properly conducted buildup test that follows either a single-rate or a two-rate drawdown test and also a variable-rate drawdown test may be analyzed to yield values of kh, s ?, and p _R. The pressure buildup behavior during the middle-time period is analogous to that during the transient flow period in a drawdown test. In other words, the reservoir is infinite acting and boundaries do not affect the pressure time data. The analysis of middle-time data yields a semilog straight line, which should not be confused with the semilog straight line for a drawdown test. This straight line, when extrapolated, yields values of a false reservoir pseudopressure, ?*, corresponding to p*, which is used to calculate the average reservoir pseudopressure, ? _R, corresponding to the average reservoir pressure, p _R.