Abstract Drilling on top of the Mesa in the Piceance Basin presents a significant loss circulation and stuck pipe challenge to operators wanting to exploit the huge gas reserves in the area. Operators have experienced losses that exceed 4000 barrels of mud when the intermediate section is drilled using conventional techniques. This is due to a combination of natural fractures and weak rock. Various strategies have been deployed to tackle the problems, including under-balance drilling operations and Direction Casing While Drilling (DcWD). A new technique described in this paper is now the best practice for ConocoPhillips in the area. It involves acquiring realtime circulating density (ECD) measurements and control of mud weight in the annulus, using direct air injection through a parasite aerating string (PAS).
During the development stages of this new process an annular pressure sub (APWD) was run to gather diagnostic data. Analysis of the data shows conventional drilling practices often yield up to 3 ppg variation in circulating density exposed to the formation. Analysis of the data also suggests there is a fracture reopening gradient of approximately 8.3 ppg and there are huge circulating density variations during connections. The new strategy shows these wells can be drilled with an ECD in the range of 5–7 ppg using conventional water based mud systems. This strategy allows wells with a very narrow mud weight window to be drilled safely. This simple approach avoids the use of complex multiphase models, giving the flexibility to quickly deploy the technique to the well site without the need for expert personnel. The information enables the driller to control and keep the ECD within recommended limits, delivering a safe and productive well.
The alternative approach of using conventional well design techniques would result in multiple casing strings and cost overruns, while more advanced techniques such as DcWD and under-balance drilling 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 excellent results. This paper presents a simple risk management technique using today's conventional technologies to successfully manage loss circulation risk in the Piceance basin.
Introduction In conventional drilling, bottomhole circulating pressure (BHCP) is managed by controlling the density of the drilling fluid to maintain ECD profile above the pore pressure throughout the wellbore. In underbalanced drilling, the wellbore pressure profile is intentionally kept below the pore pressure of the exposed formation in the wellbore. Under Balanced Drilling (UBD) has allowed development of oil and gas reservoirs that otherwise would not be exploited due to technical or economic limitations. The International Association of Drilling Contractors (IADC) defines UBD as "drilling under conditions where pressure being exerted inside the wellbore from the drilling fluids is less than the pressure of the water, oil or gas in the formation being drilled". In between the extremes of conventional drilling and under balanced drilling, the industry has seen various techniques used to control the annular pressure profile and overcome the constraints imposed as a result of the ECD of the mud system.