6000 



5000- ,.r 



4000 



3000- 



2000- E 



1000 



1000 2000 3000 4000 5000 60( 



Figure 37-1. Measured mud pressures compared with estimated mud pressure. 



which is an example of the result that can be expected, illustrates the comparison 

 of formation pressures obtained by DST 44 days before completion and by 

 extended pressure surveys. Fortunately in this case, two pressure surveys 

 were available for comparison. 



There are several causes of error in extrapolation to original pressure. 

 Aside from a multiplicity of tool, packer, and gauge troubles, which can 

 usually be identified, there is the problem of low-capacity (kh) formations. 

 The production of even a small quantity of fluid is frequently enough to draw 

 the formation pressure down, so that a prohibitively long shut-in time is nec- 

 essary to obtain a usable build-up curve. The initial shut-in pressure technique 

 is used to minimize the effects of excessive fluid production. Entrapped mud 

 pressure is bled off, presumably just enough to equalize the formation pressure, 

 by opening the formation into a limited air chamber sealed off from the 

 main drill pipe. This technique is very useful in medium-to-good perme- 

 ability formations, since a level formation pressure is quickly obtained. In 



745 



