Extend the initial shut-in pressure curve until it intersects the pressure 

 ordinate where (t + 6) / 6 = 1. Connect this point with the final shut-in pres- 

 sure point which has been plotted according to the value of (t + 6) /6 from the 

 open time (t) and shut-in time (6). Extend this line until it intersects the 

 pressure ordinate where (t + 6)/8 = 10. Using this AP across one logarith- 

 mic cycle, calculate the effective permeability (kh/fx) according to Equation 2. 



As a specific example and referring to Figure 37-4; DST: open 45 minutes. 

 Shut in 15 minutes. Recovery: 540 ft water in 4i/g in. drill pipe. Sand thickness: 

 20 ft. Estimated fluid viscosity: 1 cp. ISIP = 1800 psig. FSIP = 1620 psig. 

 Average production rate: 



q = 54 ° X "Off X 144 ° = 245 B/D, 



Connecting the ISIP with FSIP and extending this line until it intersects 

 the pressure ordinate where (t + 0)/6 = 10, 



P 10 = 1500 psig 

 Ps — P 10 — 300 psig/cycle 



kh 162.6(245) md ft 



OAA -LOO 



m 300 cp 



k 

 — = 6.65 md/cp 



k =7 md ± 



Therefore, from the reported data we are able to calculate the effective 

 permeability. This calculation can be performed at the well immediately after 

 removing the chart from the testing tool. 



Productivity Index and Wellbore Damage 



Productivity index and damage ratio can also be determined from DST 

 data. Two values of productivity index are obtainable. The first comes from 

 the flow curve and is determined by the amount of fluid recovered, the length 

 of flowing time, and the pressure differential between the flowing pressure and 

 the true formation pressure. The second value of productivity index comes from 

 an analysis of the final shut-in curve. The first value of productivity index is 

 affected by any kind of wellbore damage, because during the flow period the 

 fluid recovered had to pass through the damaged zone. The second value of 

 productivity index is nearly independent of damage because essentially no flow 

 takes place during the final shut-in time. 



748 



