Abstract

Downhole annular pressure is becoming a standard measurement in all drilling environments. When monitored in the context of other drilling parameters, and with a fundamental understanding of hydraulics principles it is possible to identify undesirable drilling conditions, suggest remedial procedures, and help prevent serious problems from developing. For wells with only a small difference between fracture gradient and pore pressure, drilling without realtime downhole annular pressure information may not be possible. Indeed, this is particularly true for extended reach wells, for high temperature / high pressure (HTHP) wells, and in deep water environments where large flowing friction pressure losses or very narrow pressure margins can exist.

Previous publications have discussed how with realtime downhole annular pressure measurements a driller can more effectively maintain the equivalent circulating density (ECD) and equivalent static density (ESD- when not circulating) within a desired range in order to prevent lost circulation and maintain borehole integrity (including managing swab, surge and gel breakdown effects). Annular pressure data are used to evaluate the effects of flow and pipe rotation on the ECD, and to evaluate formation integrity tests. With accurate downhole data the driller can apply conventional drilling practices more effectively to reduce both rig time and potentially also the number of required casing strings.

In order to interpret annular pressure measurements it is important to first understand the physical hydrodynamic principles. It is also important to analyze pressure data in the context of all downhole and surface drilling parameters.

Recent experiences with downhole pressure data have demonstrated how ECD trends can be used to anticipate drilling problems before they develop into serious events. Even with the ECD within its desired range, drilling problems not apparent from conventional procedures can be anticipated.

We present three case histories. The first is from an extended reach drilling project where the annular pressure data gave advanced warning of high cuttings loading, and helped avoid a potential lost circulation or stuck pipe event. This example also illustrates the significant flowing pressure losses that can occur along an extended reach wellbore. A second case history demonstrates the importance of using annular pressure measurements in deep water environments both to establish and maintain safe pressure margins. Downhole pressure data can also be used to evaluate kill procedures. The final example was from an extended reach well where the drilling fluid had sagged during a bit trip, and shows how by evaluating downhole annular pressure measurements, safer and more efficient tripping practices can be developed.

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