1. The distance of penetration can be shown to be proportional to the 

 flowing time. An empirical relationship, b 2 = kt (where b is the distance to the 

 barrier in feet, and k is the permeability in md) may be used to estimate the 

 range of penetration detectable on a DST chart (see Appendix C). For most 

 drill-stem tests, the capacity (kh) of the formation will be unfavorable for large 

 radial penetration within reasonable flowing time. 



2. Production rate is not constant. Effects similar to the break in the plot 

 can also be caused by a decreasing production rate (see Appendix A) . 



3. Reservoir characteristics do not agree with the simplified assumptions. 

 Since the analysis is based on a simplified radial-flow picture, any departure 

 from the assumptions can obviously cause curvature, which could be mistaken 

 for breaks. 



RECOMMENDATIONS 1. For accurate pressure readings, use proper 



clock speeds and pressure elements to get the 

 maximum size pressure curve on the DST chart. 



2. Always take a double shut-in test. This will permit a much more accurate 

 extrapolated formation pressure and may permit the detection of changes in 

 formation transmissibility or barriers in the event the tool is not shut in a 

 sufficient length of time. 



3. Where practical, set the flowing time on the basis of the observed blow. 

 If the blow indicates that substantial recovery has been obtained, there is no 

 need for a longer flow period; therefore, the tool may be shut in at that time. 

 The weaker the blow, the longer the tool must be left open to sample the 

 formation effectively. 



4. The final shut-in time should be at least equal to the flowing time if an 

 accurate extrapolated formation pressure is to be obtained and if nearby 

 changes in transmissibility are to be detected. The final shut-in time should be 

 as long as possible but should also be consistent with rig-time cost and safe 

 hole conditions. 



5. The lower the permeability of the zone to be tested, the longer should 

 be the shut-in time. If the capacity (kh) is expected to be below 10 md-ft, a 

 long initial shut-in time ranging up to two hours is recommended. For capacities 

 above 10 md-ft, a shut-in time of 30 minutes is usually sufficient. 



6. Measure the mud weight several times during the circulation period 

 immediately prior to running the DST in order to get an accurate and repre- 

 sentative mud weight over the entire length of the well. 



7. Record accurately inside diameters of drill collars and drill pipe used 

 in the testing string; the location, relative to the bottom of the hole, of all 

 pressure gauges used; and the number and location of packers used. 



753 



