THEORY It has been shown (Muskat, 1937; Horner, 



1951) that the following equation may be 

 used for analysis of pressure build-up curves: 



P -P w = 162.6-^lo, r 



kh 



(^) 



When this equation is applied to the curves obtained in drill-stem testing, 

 the assumptions and boundary conditions are more nearly realized than in 

 conventional flow and build-up tests on producing wells. Zak and Griff en (1957) 

 have recently discussed in detail the use of this equation in analyzing DST charts. 



One of the problems with DST curves is the lack of reservoir data for 

 precise analysis. Therefore, it is necessary to develop empirical rules and field 

 methods for analyzing DST charts in quantity. For this reason, the empirical 

 methods presented in this paper have been developed, and their derivation is 

 found in Appendices A and B. 



METHOD USED FOR In order to apply the pressure build-up theory 



ACCURATE READING to DST charts, it is necessary to obtain a 



OF DRILL-STEM digitized expression for the pressure and time 



TEST CHARTS data recorded by the pressure gauge. These 



data may be either provided by the service 

 company or interpolated by projecting a photographic reproduction of the 

 chart against a cartesian wall screen and converting the scale readings to 

 pressure and time values based on the reported readings of key points. 



The authors have used an optically linear opaque projector and a standard 

 cross-section-millimeter paper screen to tabulate intermediate pressure points 

 between the key points normally reported on the charts. Use of this technique 

 depends upon correct calibration of the gauges. 



Comparing the measured mud pressure with mud weight and the measured 

 flowing pressure against recovery weight are independent methods for checking 

 gauge accuracy. The variation between measured mud pressures and estimated 

 mud pressures, which are calculated from mud weight and depth, usually check 

 within 2 percent, as shown in Figure 37-1. 



RESULTS Experience in plotting a large number of 



DST charts on semi-logarithmic paper has 



Pressure Extrapolation shown that a straight line is usually obtained 



when the indicated kh/p is greater than 10 



md ft/cp. In the ranges of M/m less than 10 md ft/cp, curved plots are usual. 



Curved developments also occur when non-radial flow is present. Figure 37-2, 



744 



