Case Studies Underbalanced Drilling
Underbalanced Drilling using Coiled Tubing
Drilling Scenario
An operator in the Michigan Basin of the United States desired to deepen a well producing from a Niagaran Reef carbonate reservoir. Conventional drilling techniques were not previously successful, because of the overpressure conditions present in this interval. The feasibility study performed by the service company determined that underbalanced drilling technology could provide a potential solution for drilling this well in order to exploit the deeper reserves.
Technical Challenges
The average pore pressure in this reservoir was 3,600 psi at around 4,000 feet, leading to overpressured reservoir conditions. However, since the currently produced reservoir interval would be exposed to drilling activity, there were concerns about damaging the existing reservoir pay zone. Underbalanced drilling would provide an excellent option to maximize production rate in the reservoir interval to be drilled by eliminating the risk formation damage. It would also minimize the chance of fluid loss, thus minimizing the well clean-up time.
In order to optimize the benefits of drilling underbalanced, a plan was devised to complete and test the well underbalanced without tripping. Because of the presence of H2S in the produced gas, a closed circulation system had to be installed. Underbalanced drilling with coiled tubing was chosen to address the drilling and safety needs of this operation.
Project Results
The use of underbalanced technology to deepen this well resulted in flow test rates of approximately 60 MMSCF/d. H2S levels of up to 12% were measured during the underbalanced operation. Through careful engineering and project planning, this well was drilled and completed, without any incidents. Underbalanced drilling technology allowed the operator to successfully drill an interval previously identified as un-drillable using conventional means.
A major economic benefit was achieved by underbalanced drilling technology to tap additional reserves without damage to previously established production.
