What differentiates deepwater drilling? Greater water depths, riser manipulation, temperature gradients and added costs.

It is fashionable these days to use the deepwater label to distinguish a particular type of drilling. But is this so different from shelf or onshore practices?

The common denominator of all drilling activities is the management of people, technology and locations. Customs, environmental and legal issues exist, also. And so does that detail of prospect selection. That's fine. But this logistical labyrinth is essentially the same whether you're sitting in a company man's office offshore Angola or onshore Azerbaijan.

Technology isn't exclusive to deepwater, either. Smart completions using fiber optics and satellite communications are enabling the production of onshore production zones to be commingled and controlled. Acidification through water injection lines permits live well intervention without skidding land rigs. New gravel packing and filtering techniques can be used to control sand production in shelf fields. In fact, it seems an equally compelling case can be made for technology to be used in onshore or shelf locations to improve marginal economics as can be made for deepwater operations.

What really differentiates and impacts deepwater activities are the challenges associated with incredible sea depths. Of course, block size in deepwater frontier areas such as Brazil can reach huge proportions: 9,653 sq miles (that's 1,000 GOM blocks). This makes picking and drilling prospects tough, irrespective of operator resources or experience. But it is greater water depth that leads to higher pressures and overburden, and that's where the problems arise. The drilling engineer has to consider and overcome bottomhole pressures that can exceed 22,000 psi and mudline temperatures that can fall as low as 35°F.
So where is the deepwater line drawn? According to Petrobras, waters between 3,281 ft and 6,562 ft (1,000 m and 2,000 m) are classified as "deep." Beyond this are the ultradeep waters (this line goes to 11,484 ft [3,500 m] for the present). Deepwater definitions aside, deeper seas mean deeper pockets.
Deep waters are characterized by strong currents that create a need for high specification rigs that are capable of maintaining station and in some instances of suppressing vortex induced vibration (VIV). Such rigs are expensive. Contracting one in the GOM can cost a cool US $250,000 per day. Deep waters are also characterized by young depositional formations that differ from shelf and onshore scenarios. Exemplifying this is the typically narrow window between pore pressures and fracture gradient. Low fracture gradients can necessitate lighter drilling fluids and lighter cement slurry, while rising pore pressures can often upset the delicate fracture gradient, destabilizing the wellbore and jeopardizing the section, if not the entire well.

A consequence of narrow pressure windows is the need for close tolerance and contingency casing schemes, accompanied by a need for concentric underreaming. (It's easier to set casing with gauge hole). In short, deepwater operators must have an excellent knowledge of wellbore stability to avoid a formation influx (kick) or fracture at the shoe, which would result in losses. New well control procedures are being developed for just such an eventuality. These include the "dual gradient system." Petrobras is presently studying gas or hollow sphere injection systems. For this technology to work, risers must be resistant to collapse forces as soon as gas is injected into their bases.

Further engineering challenges are added by temperature gradients. A negative gradient runs from surface to seafloor, but this turns positive below the mud line. Equations become more complicated as cooler surface mud alters the temperature profile as it is pumped downhole. While gas hydrate formation is a common problem, it is difficult to resolve. Hydrates trap natural gas inside water molecules and bond with metal. This can result in tubing blockages and affect valve and BOP operation. Unfortunately, deepwater environments present the ideal combination of low temperatures, high seabed pressures, gas and water that cause hydrate formation. Extensive modeling is required to minimize hydrate formation.
Low temperatures alter the properties of cement, which means new designs of cement slurry composition are required. Existing API norms do not cover low deepwater temperatures, and stringent test procedures are now determining the properties of cement slurries in deepwater operating conditions.

Riser manipulation is another challenge found in ultradeep waters - 6,562 ft (2,000 m) - and beyond. Research is being carried out on innovative lightweight risers. By reducing the weight of the risers and their joints, it should be possible to use lower-cost, fit-for-purpose rigs in ultradeep water. A parallel technology being developed is the "slender well" concept to permit the use of smaller diameter well bores and lighter risers. Petrobras has drilled more than 150 slender wells.

Undoubtedly, deeper waters add greater cost and complexity to operations. But expenses can be cut in three ways. Firstly, by simplifying well design. Well trajectories should not only be compared in terms of how effectively targets are reached, but also on their overall cost effectiveness. Secondly, by reducing casing strings, casing can be set deeper, based on real-time pore pressure and fracture gradient detection. Accurate prediction will reduce contingency casing. Offset data can help to refine pore pressure models, and enhanced pore pressure detection will make the best of the casing program while drilling. Modeling steady and dynamic state fluid behavior will reduce surprises. Last but not least, costs can be cut by contracting "fit-for-purpose" technology, especially rigs.

Editor's Note: I would like to thank Joao Carlos Placido of Petrobras CENPES for his help with this article.

Comments