the low-capacity (kh) formations, even the initial shut-in will fail to develop 

 in reasonable time. Mud leakage from the annulus also produces abnormal 

 pressures. Again it is the low-capacity (kh) formation which is susceptible 

 to mud-leakage effect. The measurement of pressure in the low-capacity (kh) 

 formation is a continuing problem that merits considerable attention. 



Closely related to low-capacity (kh) is the question of proper shut-in time. 

 Other things being equal, the error in extrapolation is proportional to the 

 amount of log (t + 6) /9 remaining at the end of the shut-in period. Figure 

 37-3 illustrates the relationship between extrapolation error and shut-in times. 

 One of the greatest causes of non-usable DST pressure charts is insufficient 

 shut-in time relative to the flowing time and capacity (kh) of the formation. 

 The lower the capacity of the formation, the longer the shut-in time must be to 

 obtain an accurate extrapolated pressure. 



Effective Permeability 



The effective formation permeability may also be determined within limits 

 from the DST chart by using the well-known methods (Horner, 1951; Miller, 

 Dyes, and Hutchinson, 1950; van Everdingen, 1953; Hurst, 1953) for pressure 



100 70 50 

 I I I I I 



30 20 



10 7 

 I I I I 



1.0 



© DST, 44 DAYS BEFORE COMPLETION 

 © PRESSURE SURVEY, 24 DAYS AFTER COMPLETION 

 ® PRESSURE SURVEY, 99 DAYS AFTER COMPLETION 

 PRESSURES CORRECTED TO A DATUM PLANE 



2800 



90 



80 



70 



60 



2750 



40 



30 



20 



10 



2700 



Figure 37-2. DST pressure build-up curve compared with conventional pressure build-up 

 curves. 



746 



