Dynamic flow modeling has been an essential part of the planning for the Gullfaks C-5A well. This paper discusses the use of such modeling and presents the main results related to:
Design of operational window
Development of detailed procedures.
Flow test modeling
Design of under-balanced cementing
First, the operational window was defined by means of steady state modeling. Various combinations of influx rates and choke pressures were simulated to ensure that bottomhole pressure could be controlled. Also modeling was applied to ensure that proper hole cleaning was maintained during the operation. Modeling of the running of the 7" liner quantified the surge/swab pressures and laid the basis for the liner running procedure.
Next, a number of scenarios were developed by means of transient simulations. It was essential to evaluate the dynamics of the system if a fracture with high productivity was drilled through. This would assist the wellsite team detecting the situation and making the correct adjustments with the choke to maintain BHP. Also the effects of choke adjustments on the production rate as well as the transient conditions in the well were evaluated. Various procedures were simulated e.g. connection procedures in order to arrive at the optimal for this well.
Dynamic modeling was an important element in the well specific UBD training course which was held for all the personnel involved in the operation. This increased the understanding and preparedness of the crew prior to operation.
The displacement of heavy kill mud out of the well under pressure was modeled prior to the operation.
Modeling of the underbalanced cementing operation was performed for the purpose of developing guidelines for controlling the choke while maintaining constant bottomhole pressure.
When drilling new wells in the Gullfaks field, Statoil has experienced increased problems drilling in the Shetland formation. Underbalanced drilling has been chosen as one method to help overcome these problems. Underbalanced drilling is drilling with a hydrostatic pressure lower than the formation pressure and is typically applied for two reasons:
Increase productivity by reducing formation damage in the reservoir
Solve drilling problems like low ROP, kicks/losses due to pressure depletion etc.
After some modification the additional equipment required for underbalanced drilling has been integrated into the drilling and production facilities on the Gullfaks platform .
An essential part of the preparations for drilling the first underbalanced well; C-5A; has been to perform steady state and dynamic flow modeling in order to
Define operational window for the operation
Explore transient scenarios and develop and optimize operational procedures
Create material for training
A steady state and dynamic underbalanced drilling model has been used for this purpose [2,3].
Also, a flow test was performed in order to gather information on the reservoir properties in the cap rock above the main reservoir. A coupled reservoir-well flow model was established to assist in maintaining bottomhole pressure within the preferred window during the test . This model has been used for sensitivity evaluations prior to flow test.
A transient model for underbalanced cementing has been established  for the purpose of
Designing the cementing operation (providing a schedule for choke pressures)
Modeling during the operation with the most updated well and fluid information.