effect of damage is in reducing recovery. The shut-in rate is also faster where 

 a damage effect exists. 



Barrier Detection 



In principle, the detection of changes in transmissibility, kh//x, in the 

 vicinity of the wellbore can be determined by study of the pressure build-up 

 curves (Horner, 1951). When formation conditions are favorable, DST charts 

 may be analyzed to detect nearby barriers. For example, in the case of a linear 

 barrier fault, the expected result is a break in the linearity of the plot on semi- 

 logarithmic paper. The early part of the curve will have a slope, AP, in 

 psi/cycle exactly one-half the AP of the latter part of the curve. In practice, 

 an exact one-half ratio is not measurable. Figure 37-7 shows an actual DST curve 

 indicating the possibility of barrier interference. The ISIP appears reliable. The 

 permeability measured on the latter part of the shut-in curve, is in agreement 

 with core data on the interval tested. The test recovered 1600 ft of free oil 

 while the off-set well was tested dry in the same zone. Had an initial shut-in 

 pressure not been taken and had the final shut-in time been insufficient, the 

 change in the slope in the latter part of the pressure build-up curve would not 

 have been observed and the detection of the barrier would not have been possible. 



Extending the pressure build-up analysis to DST charts for barrier detection 

 presents the following difficulties. 



100 70 50 

 I I I 1 



I — T 



30 



20 



10 8 6 

 I I I I I 



4 3 



1.5 



i.O 



i r 



INITIAL SHUT-IN CURVE 



P s = l680±psig 



FINAL SHUT-IN CURVE 



t+e 



e 



1800 



1700 



1300 



Figure 37-7. DST pressure build-up curves indicating possible barrier. 



752 



