Extreme Deepwater Wells Push Drillers to Begin Using Managed-Pressure Methods
Authors: Rassenfoss Stephen 
Published: February 2015

Dual-Gradient Drilling

Dual-gradient drilling has long been described as the drilling method of the future for challenging offshore wells. Now there are indications that it could start being used with some regularity, with multiple-well campaigns possible as early as next year.

This method of managed-pressure drilling has been used a handful of times on a floating drilling rig, mostly to precisely manage the pressure in wells while drilling in weak, low-pressure formations prone to major fluid losses. Currently, Chevron and Statoil are seeking to use the system for deep, difficult, high-pressure formations in the US Gulf of Mexico.

A drillship leased by Chevron, Pacific Santa Ana, is testing dual-gradient equipment on the ocean bottom while drilling a conventional well, and the company has leased a second drillship from Pacific Drilling, Pacific Sharav, that was built to accommodate dual-gradient drilling equipment.

Statoil is seeking a permit to drill what could be the first dual-gradient well in the deep waters of the Gulf of Mexico, using another version of the managed-pressure method. “When we came here to the Gulf of Mexico, it was a perfect match with the challenges we had here,” said Uno Holm Rognli, vice president of drilling and wells in the offshore US for Statoil. “We started a program locally to develop this system” for conditions in the Gulf.

It is the next step for the method used by Statoil off Norway, and by three companies in water 7,500 ft deep off Cuba. In both places it was used to successfully drill wells where pressure was so low, and the rock so fragile, that wells drilled using conventional methods were plagued by major fluid losses.

While a few pioneers are working on dual-gradient drilling, there is a broader, growing base of operators using managed-pressure systems offshore. These are closed-loop systems with metering to allow precise measurement of change in fluid flows and chokes that allow them to apply backpressure when needed.

Petronas and Petrobras are embracing managed-pressure drilling to deal with offshore wells where total fluid loss makes some wells impossible to drill using conventional methods. Managed-pressure equipment allows them to drill through sections of wells with cavernous underground hazards that can cause major fluid loss by using managed-pressure methods such as mud-cap drilling.

’New Ways To Manage Pressure Raise Well Control Questions’

When discussing dual-gradient drilling, there is an important distinction between controlling wells and well control.

Controlling wells requires maintaining the balance between preventing influxes that could lead to trouble and exceeding the limit that could push drilling fluids out of the well and into the formation. Well control refers to the hardware, procedures, and training associated with dealing with situations in which an influx has occurred and needs to be quickly stopped before it grows into a bigger problem, and then circulated out of the well.

For companies seeking to win approval from the US Bureau of Safety and Environmental Enforcement (BSEE) to drill the first deepwater well in the US Gulf of Mexico using dual-gradient drilling, questions about well control are critical. Offshore regulators want to be sure that all the new equipment has been thoroughly tested because this is a new way to drill, and the BSEE wants to be sure that it is understood that when it comes to well control, the requirements are not changing.

“With Dual-Gradient Drilling, the Name Requires Explanation”

Dual-gradient drilling covers a variety of methods for managing pressure within an offshore well while drilling. Dual refers to systems built to reduce or eliminate the pressure added by drilling fluid in the riser. The goal is to create distinct pressure gradients above and below the mudline.

It is a departure from the simpler traditional approach using a single fluid combining water or oil plus drilling mud to make it viscous enough to remove cuttings while drilling, and just heavy enough to control pressures in the open hole without causing damage.

As drilling moved into deeper waters the gap has narrowed between the effective mud weight-per-gallon to offset the formation pressure, and the level that can fracture it. Drillers refer to this slim margin as working within tight drilling windows.