This paper demonstrates that the combination of two innovative technologies, Underbalanced Drilling (UBD) and rotary drilling with casing, proved a viable solution to successfully drill a well with varying stratified pressure regimes, including CO2, in a mature field. The primary objectives of this project were to:
Minimize drilling induced formation damage to maximize injection capabilities
Minimize induced losses thus differential sticking
Improve drilling efficiencies by reducing flat time caused, NPT, well conditioning, drillstring tripping and casing running operations.
This case study shows how this gas injector well overcame the challenges of conventional drilling approaches that were potentially costly and challenging from a well control perspective due to lost circulation and the resultant formation influxes. In addition, since the drilling took place with the casing string, the operator did not have to be concerned with any potential well control problems due to surges whilst tripping conventional drillstring, short trips to condition the well nor the casing running itself. To account for potential surface pressures due to the differential between the mud column and pore pressures, the operator used a closed-loop pressurized separation system at the surface to manage potential flow and pressures. Although the technical capabilities of the separation system exceeded the potential flow rates and pressures of the well, this virtually assured well control at all times.
As a result of applying the combination of these two technologies, Underbalanced Drilling (UBD) and rotary drilling with casing, the operator successfully drilled and cased the well to TD. Compared with conventional drilling practices employed in the area, the operator benefited by:
Not having to kill the well to run their casing string (Note: Well killed prior to cementing)
Significantly eliminated fluid lost into formation
Potential for improving injection capabilities
Reduced flat time
Observed excellent hole cleaning
Improved the cement job and bond log which is an issue in the field
Managed produced fluids and formation pressures using a pressurized separation system.
Operational benefits, productivity increases and savings (monetary and time) arising from drilling underbalanced with rotary casing are potentially crucial to the future viability of drilling these types of wells in mature fields. Equally important and not to be understated, this was an immense success from the Health, Safety and Environmental (HS&E) perspective. Due to drilling the well in a controlled environment, the project experienced no issues caused by CO2 freezing of pipes at surface.
Located north of Casper, Wyoming, the Salt Creek Field uses CO2 and water injection for flooding purposes.
Due to the expected over-pressured reservoir, maintaining equilibrium conditions to avoid losses posed a major challenge. These conditions in wells previously drilled required high mud weights to control CO2-pressured zones which resulted in frequent differential sticking across non pressured zones.
This paper discusses the well design and the associated challenges with underbalanced drilling conditions using casing as a rotary drilling string.
