Well concerns prompt wide-ranging review of drilling practices

Sept. 1, 1999
A research program aimed at tackling the problems of ultra-deepwater drilling hydraulics and well control is getting underway at Rogaland Research (RF).

A research program aimed at tackling the problems of ultra-deepwater drilling hydraulics and well control is getting underway at Rogaland Research (RF). Many assumptions made for drilling in normal water depths cannot be extrapolated to operations in deepwater and ultra-deepwater depths, says Project Manager Rolv Rommetveit. In a deepwater well, more than 50% of the wellbore may be in the sea. The severe cold typical of these depths creates a completely different situation for handling pressures inside the wellbore, says Rommetveit. More knowledge is also needed about the temperature and pressure effects on drilling mud properties such as viscosity and density and its thermophysical properties.

"The program has been designed to address the kind of problems which have actually been experienced in recent deepwater wells, including underground blowouts and failure to reach objectives," he says. "Practical solutions need to be found if operations are to proceed both safely and efficiently. There are significant cost savings to be achieved by improving the success rate."

The two-year program consists of seven projects with an overall budget of NKr 21.8 million. Some of the research is to take place at Heriot-Watt University in Scotland and Institut Français du Petrole, while service companies will also provide input related to drilling fluids products. In addition to desktop studies and laboratory experiments, some of the work will involve RF's Ullrigg test rig and field experiments at a deepwater rig sites. Negotiations are in progress with Petrobras in Brazil and with operators of upcoming wells in the North Sea and Gulf of Mexico for the field experiments.

Two of the projects are already underway, at RF and Heriot-Watt, while funding is still being sought to get the others started. Oil companies and contractors have the flexibility to support one, several, or all projects, and European Union funds are also being sought.

  • The first project, on ultra-deepwater riser hydraulics and temperature, focuses on downhole pressure effects during fluid circulation, drillstring rotation, and static fluid gel development at cold riser temperatures. Objectives are to develop a useful understanding of fluid hydraulics and temperatures inside an ultra deepwater riser, to improve hydraulic and thermal models for ultra deepwater operations, verify riser hydraulic and thermal models through full-scale field tests at a deepwater rig site, and recommend improved drilling practices for fluid hydraulics optimization. The field data will be used to refine RF's Presmod hydraulics model for ultra deepwater applications.
  • Critical pressure effects for ultra deep-water drilling is the subject of the second project, which focuses on pressure surges resulting from the break-up of gels during tripping and circulation startup. The aim is to develop guidelines for minimizing downhole pressure peaks expected to occur while breaking gel at the start of circulation or due to causes such as surge/swab effects. PC programs for computing such pressure surges at the rig-site are also being developed.
  • The starting point for the third project is the fact that the chances of gas passing through the BOP and into the riser prior to a kick being detected is much greater during ultra deepwater drilling. In order to prepare for the gas coming to the surface, it is important to know the gas migration velocity in the drilling mud.

The gas expands as it travels up the riser, perhaps sufficiently to expel some of the mud in the riser. The project aims to establish the rates at which mud is expelled from the riser, develop and verify rules and recommendations on gas migration rates within the riser, and also recommend innovative methods of removing gas from the riser once it has passed the BOP. A PC program to enable gas migration rates to be computed at the rig-site for given riser configurations will also be developed.

Hydrate issues

The remaining four projects focus on the problems of hydrate formation and the development of tools to avoid them.

The first concerns the development of a hydrate probability model during well control operations. It will take the form of a module to be used with RF's Kick simulator, which allows the user to investigate possible problems with hydrate formation in steady-state situations and in transient situations such as taking a gas kick. The module will also allow the user to follow changes in hydrate formation probability as hydrate inhibitors are added to the drilling mud.

A second project focuses on the development of environmentally friendly and improved hydrate inhibitors for deepwater spudding and drilling. Existing water-based fluids are unable to prevent hydrate formation in deepwater drilling, where the pressure at the sea-bed is several hundred bar at temperatures ranging from -2 to +4°C depending on the location. Low density drilling fluids are required, which means the extensive use of salts has to be avoided. The project seeks to develop and verify 'green' chemicals that can be added to drilling fluids to improve hydrate control properties while maintaining the existing mud properties. As new products are developed, they will be given to service companies to investigate the other drilling fluid properties.

RF will investigate kinetic inhibitors, which are known to provide additional hydrate control to at least 10°C subcooling and in which it has already developed substantial expertise. It will also investigate hydrate flow modifier additives, another area in which it has worked for many years, to identify a chemical of this type which will allow hydrate to flow in glycol/salt/water blends rather than oil, as this is more suitable for green water-based drilling fluids.

A third project will investigate circulation procedures to avoid hydrate formation, focusing on the most critical phase after shut-in or during circulation out of a kick. In these phases pressure is high, gas is mixed into the drilling fluid and temperatures may be low, creating the worst-case conditions for hydrate formation. By means of experimental work with Petreco Wheels and a test program at IFP's loop facilities at Solaize, the project seeks to identify procedures to circulate out a kick in such a way as to minimize the likelihood of hydrate formation. RF intends in a future full-scale project to develop the resulting report into a manual for drilling engineers on procedures for circulating out kicks.

The final hydrate project, on which work has started at Heriot-Watt, focuses on the thermodynamic properties of drilling fluid components with a view to avoiding hydrate formation in water and oil-based drilling fluids.