Drilling world’s longest wells at Liberty

Extending the horizontal reach of oil wells reduces the surface impact

BY ALAN BAILEY FOR GREENING OF OIL 

After years of planning and preparation, sometime this year a drill bit will augur its way into the ground towards BP's Liberty oil field offshore
Alaska's North Slope. The drill bit, attached to a massive drilling rig, will test the limits of how far it is possible to drill from a surface well site to penetrate a distant oil reservoir in the deep subsurface.

Success will mark a new chapter in reducing the surface footprint of oil and gas development.

The 100-million-barrel Liberty field lies under the shallow waters of the Beaufort Sea, about five miles from land, in an area covered by thick but mobile sea ice during the frigid Arctic winter, and populated by marine mammals such as seals and polar bears. During the summer open water season, when the sea ice typically recedes to the north, bowhead whales migrate through the region, while running the gauntlet of Inupiat subsistence hunters along the North Slope.

Traditional technique is a gravel island

The traditional way to develop an offshore oil field such as Liberty in this shallow, ice-prone water would be to build an offshore gravel island over the field, which would be fortified from the ice by layers of sand bags around its perimeter and connected to shore by a subsea oil pipeline. The island would house the field’s drilling rig, as well as the production facilities that separate crude oil, water and natural gas from the fluids flowing from the field’s oil wells.

In 2000, BP used this approach when developing its Northstar field in shallow Beaufort Sea waters directly north of the giant Prudhoe Bay field in Alaska’s central North Slope.

But in 2002, following major cost and schedule overruns in the environmentally sensitive Northstar development, BP abandoned an initial plan to use a similar development strategy for Liberty, electing instead to investigate ways of drilling into Liberty from shore locations, using oil wells that would deviate into near horizontal trajectories, running under the several miles of ocean that lie between the field and the land.

Eventually the company determined that the optimum drilling site for this venture would in fact be an existing gravel island, a part of the Endicott field infrastructure east of Prudhoe Bay. Endicott is a decades-old oil field situated a short distance offshore in the Beaufort Sea and operated from two artificial islands, both connected to the mainland by a manmade causeway.

Five miles from shore, but 8 miles from Endicott

In 2009 BP moved into the construction phase of its Liberty project.

Although the use of an existing gravel island has minimized Liberty’s environmental impact, accommodating the Liberty drilling rig has involved enlarging the island somewhat.

However, BP determined that it would be able to process the Liberty production through the existing Endicott facilities for export through Endicott’s oil pipeline, thus eliminating the need to construct and install a new oil production infrastructure specifically for Liberty.

But the technical challenges of drilling into the Liberty field from Endicott are huge.

Although just five miles offshore, some of Liberty’s wells will be located about eight miles from Endicott, thus requiring Liberty to extend over world-record horizontal distances of 34,000 to 44,000 feet to reach the Liberty field reservoir, some 11,000 feet beneath the seafloor.

Using a technique called ultra-extended reach drilling, a well will drive vertically downwards below the drilling rig before bending into a near horizontal path towards the oil field. On approaching the field, the well will bend back in a vertical direction before penetrating down into the field reservoir.

BP reckons that it will need up to six of these wells to bring Liberty into full production, with four of the wells producing oil and the two other wells injecting water into the field, to flush oil from the field’s reservoir rock.

BP has already used ultra-extended reach drilling to develop its Wytch Farm field on the south coast of England, producing oil onshore from a field reservoir underneath the picturesque and environmentally sensitive Poole Bay. But the Wytch Farm wells extend horizontally 35,000 feet (6.6 miles) to access a reservoir 5,500 feet below the surface, distances and depths that are both less than those at Liberty.

The evolution of drilling techniques

In fact the Wytch Farm and Liberty developments represent new stages in the evolution of oil-well drilling techniques that have progressively reduced the surface land area impacted by drilling activity while increasing the area of the subsurface that can be accessed from a single drilling site.

This evolutionary process goes back many years to the invention of a technique known as rotary drilling in which, rather than simply pounding a well hole into the ground using a metal spike, drillers would use a drill bit at the end of a rotating length of drill pipe to augur a hole vertically downwards, rather like drilling a hole with a carpenter’s wood drill.

In the 1960s and 1970s drillers perfected a technique in which they could deviate a rotary-drilled well up to about 30 degrees from the vertical, thus enabling wells to splay outwards to access relatively large sections of an oil field from a central location such as an offshore oil platform. But the technique required the time-consuming pulling of the drill pipe from the well, to adjust the drilling configuration, so that over the years people developed new drilling methods, culminating in the 1990s with what is referred to as “rotary steerable technology.” Rotary steerable technology enables the drillers to steer the direction in which the drill bit is grinding its way through the subsurface rocks while allowing the drill pipe to continue to turn, without having to pull the drill pipe from the well.

New techniques involving the transmission of signals up and down the well while drilling, using a system of pressure pulses sent through the drilling fluids that the drillers pump through the well, have enabled the continuous control of the equipment at the bottom of the well and the transmission of drilling data to the drillers at the surface.

At the same time, a machine called a top drive has taken over from the traditional technique of using a gripping device on the floor of the drilling rig the task of rotating the drill pipe in the well. Powered by a massive electric motor and slung from the derrick structure of the drilling rig, a top drive can handle multiple lengths of pipe, and can enable much closer control over the drilling operation than had previously been possible.

These various evolving technologies enabled drillers to deviate wells further and further from the vertical, steering the drill bits with great precision through the subsurface, with the ratio between the horizontal extent of the well to the well depth progressively increasing from around one to one in the 1960s and 1970s to around five to one in modern directional drilling.

A key technology for Liberty

By eliminating the need to pull the drill pipe from the well when changing the drilling direction, rotary steerable technology is a key to the extended reach drilling that BP plans for the Liberty field: BP has estimated that it might take as long as a week to pull the drill pipe out of a Liberty well, thus rendering the frequent pulling of the pipe impractical and prohibitively expensive.

But BP requires a massive drilling rig for Liberty, to handle the weight of perhaps eight miles of drill pipe and to overcome the frictional resistance to rotating that length of pipe in the well bore.

The company commissioned Parker Drilling Co. to build the world’s largest drilling rig for the project. The $215-million rig, which BP has barged to the North Slope and which Parker has been assembling ready for use, has eight 2,640-horsepower engines, stands 240 feet tall and has a top drive rated at 105,000 foot-pounds of torque—a typical North Slope rig has a top drive rated at around 40,000 foot-pounds.

To minimize the load on the rig, BP has come up with a new form of steel alloy for the drill pipe, an alloy that combines light weight with the necessary strength to withstand the torque applied by the top drive.

But to lower the frictional forces on the drill pipe while drilling is in progress, BP plans to place friction reducing devices on the outside of the pipe, where the pipe passes through steel well casing, the casing that drillers use to seal and stabilize sections of a well that have already been drilled. And the drillers will need to float some sections of the well casing into position, using a combination of compressed air and drilling fluid.

Seismic data critical to well planning

Another state-of-the-art technology, vital for drilling at Liberty, as in any modern precision drilling, is 3-D seismic surveying, in which sound signals transmitted from the surface and echoed from subsurface rock formations generate detailed three dimensional images of the subsurface.

BP has conducted a 3-D seismic survey on the Liberty field itself, to determined precise targets for the Liberty wells. But the company is also using 3-D seismic images of the subsurface between the field and the Endicott drilling site, to enable the well planners to work out optimum routes for the wells.

And accurate planning and design of those wells is critical to success. For example, as the drill bit moves almost horizontally along the well path, out towards the field, an unexpected encounter with an especially hard rock layer could deflect the bit into some direction that results in a “dog leg” bend in the well. Unplanned bends like this would increase the drag on the drill pipe, thus increasing the force that needs to be applied to the pipe, potentially compromising the possibility of continuing the drilling.

Other challenges at Liberty include the need to drill nearly parallel to some known coal seams, with the possibility of the well bore becoming too weak if it continues along a seam for an extended distance.

But despite the many challenges facing the Liberty development, BP remains confident that the more than $1 billion that it anticipates eventually plowing into the project will pay off. And North Slope communities support a project that offers a way of developing offshore oil without the environmental risks of an offshore oil infrastructure.

But nerves must be tightening in the Liberty team as the day approaches when that first drill bit hits the ground on the Endicott island.

“We’ll be doing a number of things that have never been done before,” said Doug Suttles, the then president of BP Exploration (Alaska), when in July 2009 he announced the BP board’s final approval of the Liberty development. “We’ll be drilling the longest wells ever drilled.”

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Contact Alan Bailey at abailey@petroleumnews.com