DST process earlier (Dolan and Hill, 1955). All other things being equal, if 

 shut-in curves from tests having a production rate of q u q n and q a constant 

 through the flow period are compared with a curve having a production rate 

 changing with time, dq/dt = a, from q ± to q n , the varying production-rate curve 

 will approach the average production-rate curve with negligible error as the 

 shut-in time increases. 



APPENDIX B The damage ratio defined in this chapter is an 



empirical factor intended for practical im- 

 mediate evaluation when most of the necessary data for precise determinations 

 are unavailable. The relation of conventional treatments in the literature to the 

 damage ratio here denned is explained as follows: The equation for total pressure 

 drop across the wellbore is 



oft- = " -&-Kf--^-)+qS, (id 



4-n-kh 



where Ei ( - X) = - I — c -» du, which may be 



x 

 approximated as Ei{ — X) = In X + .5772 if X < .01. 



Damage ratio, D.R., is defined as the dimensionless ratio of kh/fi to instan- 

 taneous P. I., where the P.I. is taken to mean the measured ratio of production 

 rate to differential pressure, P.I. = q/(P s ~ Pf)- 



D.R. WP ' - Pf) (12) 



\xq 

 This D.R. is measurable from the flowing and shut-in curves on a DST chart. 



Replacing kh/ ' \x.q with — — from the shut-in curve, the following equation is 



obtained. 



D.R. = (.183) P * ~ n P] (13) 



Using Equation 12, one may define a D.R. from Equation 11 for two cases. 

 Case I: 5 = 0, the normal D.R., without skin damage. 



(D.R.K = - ±- Ei (- ^-) 



4tt \ 4kt J 



(14) 



758 



