Gullfaks Underbalanced Drilling Project

SINTEF Petroleum Research has participated in the Statoil UBD project. Persons now in SINTEF Petroleum Research were involved from the very start of the project in dynamic and steady state modeling of the operation. In well C-05A particular focus has been on

• Dynamic flow modeling
• Flow test modeling
• Design and real time modeling of cementing under pressure
• Displacement modeling

Dynamic Modeling of Flow Test

Flow Model. Models available for simulating the dynamics during the underbalanced drilling operation were not appropriate for modeling the flow test with the expected oil influx, and would have given unrealistic results. Hence a more realistic model was set up for this case with the following characteristics:

• Slip between oil and mud is modeled
• Gas boils out when the bubble point pressure of the oil is reached
• Gas rises faster than mud
• A transient coupling between well and reservoir

Sensitivity analysis prior to operation. Prior to the operation sensitivity evaluations of the expected influx alternatives were performed with the flow models. For oil influx the new flow model  was used, and for gas influx a transient UBD design model was appropriate.

Post-analysis. After the operation the flow test model has been used with the real test data, giving estimates of reservoir properties in the high pressure zone.

Dynamic Modeling the Displacement of kill fluid
After the flow test the well had to be killed. Heavy kill fluid was bullheaded down the annulus. This mud had to be displaced from the well in order to resume the underbalanced operation and drill the Shetland formation. This displacement was performed in two steps:

• First the kill fluid was removed from 2000m MD with sea water
• Then the string was stripped down to 2900 m MD and the rest of the kill fluid was pumped out

The displacement operation was modeled both prior to the operation, and also after it was finished using the real mud properties measured as input to the simulations. During the displacement the standpipe pressure and thus the bottomhole pressure was kept constant by regulating the return choke.

Dynamic UB Cementing Model. A transient multi-fluid flow model has been developed. The model transports a sequence of fluids down through the inside of a pipe and up through an annulus outside the pipe. The fluids may be transported through a surface choke with the purpose of controlling the bottomhole pressure during the cementing operation. The fluids may have different density, rheology, volume, and may be pumped with different pump rates.

Cementing design. The model was used to design the operation and also developing detailed procedures for controlling the choke.

A detailed analysis of the cementing operation with comparison of calculations and measured data has been presented in separate papers.

More operations; well C-09A
The project is ongoing. Next well will be drilled near balance with a full UBO package as a back-up. SINTEF Petroleum Research will in particular work on the cementing operation desing and analysis, and will also do real time modeling at the rig site in order to optimize procedures.