This paper presents an integrated approach on HPHT well control in narrow pressure margin wells. The approach includes theoretical evaluations, computer simulations with advanced modeling tools, laboratory studies, well control training as well as implementation of learnings in procedures and operations. Examples from the North Sea on the application of advanced transient well control modeling for the planning of HPHT drilling as well as training are described in detail.
A special focus is directed towards gas diffusion during drilling operations; with its impact on a number of important parameters like rheology and barite sag. Both theoretical and experimental work is presented.
The drilling of HPHT wells pose special challenges compared to standard wells:
High pressures and temperatures impact mud properties in a dynamic way, and can have effects on well control
Small margins between pore and fracture pressures will prevail in sections of the well
The conditions are above the critical point for the gas/oil/condensate influx; which means that the hydrocarbon influx is infinite soluble in the base oil of the mud.
Hydrocarbon influx will totally mix with the base oil in oil based mud (OBM), and infinite amounts of gas can dissolve in the mud
Drilling of inclined and horizontal wells will make the consequences of barite sag serious
Significant quantities of gas can diffuse into a horizontal section of a well if OBM is used even if the well is overbalanced
An HPHT well can be categorized as an integrated physical system. The well is defined as the flow conduit from the mud pump down the drill string, drill bit and up the annulus to the mud pits; with the drilling mud, etc. filling up this flow conduit. Maintaining the control of the well at all times is a question of understanding the physics of the well during changing conditions and using this knowledge to optimize the design of the well, to develop sound drilling procedures and to handle unexpected situations during drilling in an optimal way.
Traditional and well proven drilling practices and rules of thumb have been developed by "trial & error". In many cases, these represent optimal solutions. However, when the drilling situation differs significantly from the traditional, old rules may not apply, and one will need to analyze the problems scientifically to revise the practices. One can use transient computer models with the correct physics built into them to develop new procedures and practices for these wells.
Below the main physical parameters and interactions are discussed.