Case Studies Underbalanced Drilling
Gasified Liquid Drilling for Field Development
Drilling Scenario
This case history describes the use of underbalanced drilling through a partially pressure depleted reservoir for field development in Southern Venezuela. While drilling in this reservoir interval, differential sticking was repeatedly experienced, to the point that the operator had previously sidetracked three-times before becoming differentially stuck again at a depth of 9,150 ft.
Technical Challenges
Seeking to prevent the loss of the bottom hole assembly and drill pipe valued at $2 million, the operator was determined to identify a solution, which would prevent such a loss. They faced the situation of having to sidetrack a fourth time, or possibly abandon the drilling project altogether.
To overcome the extreme differential sticking problems caused by the low bottom hole gradients, it was decided to put the borehole into an underbalanced state. This would be achieved by the annular injection of nitrogen to gasify the drilling fluid column. Nitrogen required for gasification was generated on-site, using membrane filter units.
Project Results
The drill string had been differentially stuck for 21 days when the operator decided to aerate the drilling fluid to free the stuck pipe. Equipment was mobilized over a distance of 2,300 kilometers to the drilling location. The drill string was free after four hours of nitrogen injection, thus avoiding the fishing, sidetrack, or well abandonment situations the operator was previously facing as their only option.
After this experience, the operator was convinced of the contribution that underbalanced drilling could make to the field development program. Gasified drilling fluid was evaluated during the next development-drilling program in this field. It is planned to drill up to 30 additional development wells in the field utilizing underbalanced technology.
