As the industry turns its attention to new and more challenging types of reservoirs, well completions are getting more complex and higher-tech
Ever heard of a Christmas tree? Not the evergreen, arboreal kind. The wellhead kind – the assembly of spools and valves installed during well completion to control the flow of fluids out of an oil well. A wellhead is an essential part of a completion – the process of kitting out a drilled well to become a producing one.
Most completions have several other features in common too: production tubing, the conduit for the flow of oil and gas to the surface; packers to channel the hydrocarbons up the tubing; and a safety valve to cut off the flow in the event of an emergency. And they all do the same fundamental job, says Ezio Toffanin, sales and marketing manager in Europe and Africa, for Schlumberger Completions. “A completion is what connects you to the reservoir.”
But the requirements and complexity of completions vary enormously, according to factors such as well location – onshore or offshore, for instance – reservoir depth and temperature, the corrosiveness of produced fluids and the formation’s me- chanical stability. All those things need to be considered in a plan that puts safety and efficiency first, says Adam Vasper, domain manager for the Europe and Africa region for Schlumberger completions.
It’s about teamwork
Completion engineers need to work closely with other engineering disciplines to determine how the reservoir should be drilled, how the casing should be perforated, the types of fluids that should be used downhole and whether additional sand-control measures or other processes and treatments need to be built into the completion plan.
The first priority is working with well-construction teams to make sure the well is drilled correctly. If drilling fluids are allowed to escape into the formation, for example, they can damage the rock pores and impair production later on.
In some reservoirs, meanwhile, technologies to get the fluids moving may be part of the completion plan – if, for instance, the natural pressure inside the reservoir is too low to push the oil out of the ground. Electrical submersible pumps can be
In multilateral wells, various lateral legs split off from a single motherbore to tap different deposits. Having one well at surface reduces the surface impact of drilling and overall costs, so more can be spent on completing each leg
Cased or open-hole?
Deep down within the reservoir, there are two basic types of completion: cased completions – in which the section of well going through the producing zone is, like the rock layers above it, lined with a special casing; or open hole (or barefoot) completions, in which the part of the drilled hole inside the producing formation is left bare.
Open-hole completions are a good idea if the rock is sturdy enough to withstand the rigours of drilling and production without disintegrating. They’re cheaper than casedhole completions because there is no need to buy, install and cement a casing string in the production zone. Also, because the sides of the hole are exposed, there is a very large surface area through which fluids can flow. But most sandstone reservoirs are too mechanically weak for an open-hole completion so they tend to be cased or include sand-control equipment.
In cased and cemented completions, completion includes an extra step called perforation, in which holes are made in the casing and its cement sheath – usually with explosive charges – through which fluids can then flow.
Cased or open-hole?
Sand-control technologies have become increasingly integral to the success of well completions worldwide. That’s partly because of the continuing advance offshore: a large proportion of the deep-water depositions being targeted at present were formed, in geological terms, relatively recently. Grains within newer sedimentary layers have had less time to cement together, and so form a more sand-prone rock. Indeed, more than half of the wells Schlumberger completes today require some form of sand control, says Ezio Toffanin, sales and marketing manager in Europe and Africa, Schlumberger completions.
Controlling sand is important because if sand particles flow into a well along with oil and gas, they can damage hardware and slow down or even stop production – with catastrophic results for operational continuity. Sand control usually involves the placement of a screen across the formation. These filter out sand but allow fluids through.
Gravel packs can be used in combination with screens to further tighten the sand filter. Specially sized grains of proppant are mixed into a carrier fluid, forming a slurry that can be pumped into the annular space between the screen and the sides of the hole. The proppant grains accumulate around the screen, forming a highly effective filter for formation fines.
placed downhole to help the flow. Or a gas can be injected into the bottom of the well to reduce the density of the oil and water mixture produced from the reservoir and buoy it towards the surface.
The requirements and complexity of completions vary enormously, according to well location, reservoir depth and temperature, the corrosiveness of produced fluids and formation stability
In unconventional oil and gas production, meanwhile, the hydrocarbons need additional help to get to the surface. Producing from shales, for example, requires hydraulic fracturing – the process that forces open, through high- pressure pumping, micro-pathways in impermeable rock in order to allow fluids to flow freely through the formation to the wellbore. In multilateral wells, in which various lateral legs split off from a single motherbore to tap different deposits, multiple completions are needed.
Intelligent completions, meanwhile, give an extra dimension to downhole control and enable greater precision in the reservoir- management process. In intelligent completions, different producing zones are equipped with sensors and valves that are connected to the surface through electrical and/or hydraulic conductors. The sensors send information to the surface about the reservoir and signals can be sent in the opposite direction to open and close different zones, as required. Advanced communications technologies mean that intelligent completions can be controlled from pretty much anywhere – “potentially from your desktop”, says Vasper.
The high-tech dimension of completions – with more monitoring and greater control downhole – is likely to remain a growth area. Certainly, as the industry turns its attention to new and more challenging types of reservoirs, completions are unlikely to get any simpler.