- FLOW PERIOD (t) = I HOUR 

 _ SHUT-IN PERIOD (9) 



en 



OJ 



lu 



Z> 

 (/) 

 (D 

 UJ 



q: 



HQUfiS- 



'■'I' L 



i i l I I 



J L 



100 70 50 30 20 



10 8 6 4 3 2 1.5 



Figure 37-3. The effect of shut-in time on the accuracy of extrapolated pressure. 



build-up curves. The use of an average production rate determined from the total 

 recovery divided by the flowing time is generally sufficient for use in the formula 



kh _ Qa (2) 



162.6 % 



Unless the flowing curve is approximately straight, indicating constant pro- 

 duction rate, Equation 2 will not be strictly correct (see Appendix A). For- 

 tunately, accuracy requirements on permeability are not strict, and the appoxi- 

 mate value obtained from a DST is useful. Since the permeability so determined 

 represents the average effective value for an entire drainage area, it may in fact 

 be a better value than the permeability reconstructed from isolated core plugs 

 from the section. In vugular and fractured porous zones, the effective perme- 

 ability of the drainage area is all important and cannot be measured except by 

 testing. 



Field Method For Calculating Effective Permeability 



A practical field method for estimating effective permeability is illustrated 

 in Figure 37-4. It is necessary to have a successful duel shut-in pressure test, in 

 which the initial shut-in curve is nearly leveled out. The final shut-in pressure 

 need only be developed to about three-fourths of the way between the final 

 flowing pressure and the initial shut-in pressure. The procedure is as follows: 



747 



