8. Using the simplified methods outlined in this chapter, calculate the 

 effective permeability and wellbore damage as soon as the DST chart has been 

 recovered. These calculations may greatly alter the evaluation of the zone 

 tested, which may be based on recovery alone, and may warrant retesting the 

 zone, a change in the drilling schedule, or a change in the completion interval. 



CONCLUSIONS 1. Aside from limited possible application to 



low-capacity (kh) formations, a true original 

 formation pressure can be determined by drill-stem testing long before com- 

 pletion and before any drawdown due to production takes place. 



2. Effective permeability and wellbore damage can be calculated at the 

 well site by using the simplified methods presented in this chapter. 



3. Barriers such as faults, pinchouts, changes in formation permeability, 

 etc., can sometimes be detected by proper analysis of DST charts, provided 

 formation conditions are favorable. 



Acknowledgment 



The authors wish to express their appreciation to Petroleum Research Corp. 

 for permission to publish this chapter. 



APPENDIX A Aside from the difficulties in reducing reservoir 



conditions to the usual simplified constant- 

 parameter radial flow picture, there is the question of the effect of non-constant 

 production rate, which obviously exists during many drill-stem tests. It is the 

 practice to approximate the varying production rate problem by breaking the 

 flow period, t , into a finite series of sub-periods, At, each having a different 

 but constant production rate, q u q 2 , q 3 . . . q n - Then, if the superposition 

 theorem is used, the basic equation becomes: 



> A t + 



* W 



_ r ln (»* 



h L v (re - 1 



4-rrkh I \(n - 1) At + 6 



+ ln { (n-2)A t + e ) + • • • + ^ [—0 ) 



(4) 



Assuming 



J-% jfJLQ 



—ry-r < -01 , q, = constant. 



755 



