Managed-Pressure Drilling Using a Parasite Aerating String
Authors: Okafor Zimuzor , Higgins-Borchardt Shannon , Akinniranye Goke , Bratton Tom , Avila Ricardo , Boone Danny 
Published: November 2010

Drilling on top of the mesa in the Piceance basin presents a significant lost-circulation and stuck-pipe challenge to operators wanting to exploit the gas reserves in the area. Operators have experienced losses that exceed 4,000 bbl of mud when the intermediate section is drilled using conventional techniques. This is because of a combination of natural fractures and weak rock. Various strategies have been deployed to tackle the problems, including underbalanced-drilling (UBD) operations and directional casing while drilling. A new technique implemented by an operator in the Piceance basin is described in this paper; it involves acquiring real-time equivalent-circulating-density (ECD) data and control of mud weight in the annulus by use of direct air injection through a parasite aerating string (PAS).

During the development stages of this new process, a real-time annular-pressure sensor was run in the bottomhole assembly (BHA) to gather diagnostic data. Analysis of the data shows conventional drilling practices often yield up to 3 lbm/gal variation in ECD exposed to the formation. The analysis also suggests that there is a fracture-reopening gradient of approximately 8.3 lbm/gal and that there are significant ECD variations during connections. The new strategy shows that these wells can be drilled with an ECD in the range of 5-7 lbm/gal using conventional water-based-mud systems. This strategy allows wells with a narrow mud-weight window to be drilled without significant mud loss to the formation. This approach avoids the use of complex multiphase models, and the downhole ECD can be displayed on the driller's console in real time from the data measured by the annular-pressure sensor and sent through the mud-pulse-telemetry measurement-while-drilling (MWD) tool. Alternatively, if an annular-pressure sensor is not included in the BHA, the downhole ECD can still be estimated accurately by use of a simple spreadsheet calculation (Scott 2009). This gives a measure of flexibility in deploying the technique to the wellsite. Expert personnel were used in the initial diagnostic stages to establish the procedures and validate the effectiveness of the technique. After that, they will not normally be required at the wellsite to manage the process in subsequent wells because the ECD data from the annular-pressure sensor can be understood by the driller, and the alternative spreadsheet solution, if used, can also be managed by the wellsite supervisor to calculate a reliable downhole ECD measurement. The ECD data enable the driller to control and keep the annular pressure within recommended limits, ensuring that lost circulation risk is reduced and wellbore stability is maintained.

The alternative approach of using conventional well-design techniques would result in multiple casing strings and in cost overruns, while more-advanced techniques such as directional casing while drilling and UBD would require specialized equipment and crews. This new technique uses existing and common drilling technologies along with new software tools for geomechanics analysis and drilling surveillance to achieve a fit-for-purpose solution. This paper presents a risk-management technique using today's conventional technologies to successfully mitigate lost circulation risk in the Piceance basin.