10 8 6 4 3 2 



i — i i i i — i — i — i r 



P s , INITIAL SHUT-IN PRESSURE 



/N 



< 



^» 



* FINAL SHUT-IN 

 PRESSURE 

 (FROM DST CHART) 



P ^ 

 10 



TRANSMISSIBILITY = 



kh 



= 162.6 



AP 



<r 



t±e 







CO 

 Q. 



LU 



cr 



Z) 

 CD 



C7) 

 LU 

 DC 

 CL 



Figure 37-4. Technique for field interpretation of effective permeability. 



The ratio between these two values of productivity index is therefore 

 indicative of wellbore damage. This damage is commonly caused by a mud 

 filtrate water block on the formation face or pressure loss across perforations 

 in the anchor or across the testing tool. 



Field Method For Calculating Damage Ratio 



Although more precise methods are described in the literature, (van 

 Everdingen, 1953; Hurst, 1953; Arps, 1955) the damage ratio may be calculated 

 at the well site immediately after the DST chart is recovered by using the 

 following empirical equation (see Appendix B) : 



D.R. = .183 



P.-Pt 

 AP 



(3) 

 749 



