For anyone wanting to see a case-book example of how environmental protection can make perfect business sense, Alaska’s North Slope oil industry makes a pretty good place to look. Technical innovations such as highly deviated directional drilling and high-resolution seismic data acquisition have not only improved business efficiency, they have also limited the environmental impact of a massive industrial operation in a sensitive Arctic setting.
Back in the 1970s, the huge Prudhoe Bay field was designed as a series of gravel pads, from which wells spread out like deep tree roots tapping the oil reservoir thousands of feet below the surface of the tundra. But the drill rigs in use at the time required relatively widely spaced surface wellheads in large well pads. Drilling waste was dumped into manmade ponds called reserve pits at the sides of the pads. And limitations on the amount of deviation from the vertical that a well could drill drove the need for a relatively large number of pads.
In the early days, when an Arctic oil industry hadn’t yet developed, people drilled just like they did in the Lower 48, Jerome Eggemeyer, engineering team lead for ConocoPhillips, told Petroleum News.
“They brought those rigs up here and dug big reserve pits and made holes in the ground the old way,” Eggemeyer said.
But a drive for efficiency and the need to access ever more elusive pieces of the oil reservoirs led to fundamental changes in the whole approach to drilling on the North Slope.
“When you look at the original development plan and what folks thought was going to happen, it changed pretty dramatically and that change was facilitated in a large way by drilling advances and the applications … that enabled us to become more efficient and increase the recoverability rates,” Gary Christman, director of Alaska drilling and wells for BP Exploration (Alaska), said.
Increased well deviationsThe invention of mud motors to drive drill bits around bends in well bores and the later development of rotary steerable technology enabled ever increasing amounts of well deviation — the bottom hole horizontal displacement from the well head maxed out at a distance of about the depth of the well in the 1960s, Eggemeyer said. But the ratio of the bottom hole displacement to the well depth then increased steadily to reach five to one or more today. And the introduction of horizontal drilling in the 1980s enabled wells to undulate their way through underground reservoir rock at horizontal distances of many thousands of feet from a surface well head.
At the same time more compact drill rig designs with cantilevered drilling platforms enabled well heads to be packed more closely together on a drilling pad, thus reducing the area of gravel pad needed to be able to drill a given number of wells.
“The evolution of the rig design helped us decrease the pad size … but it also increased the efficiency of the rig moves to where you can get more wells drilled in a year,” Randy Thomas, Greater Kuparuk Area drilling team lead, told Petroleum News.
Coiled tubing drilling techniques such as directionally drilled sidetracks were pioneered on the North Slope and involved the use of a mud-motor powered drill bit and continuous small diameter tubing, rather than a conventional rotating drill string assembled from multiple lengths of rigid drill pipe. The coiled tubing technique enables drillers to punch a sidetrack well into new segments of an oil reservoir from the bore of an existing well.
That dramatically reduces drilling costs. But it also reduces the need for new surface wellheads and the associated need for an additional area of well pad.
Reduced footprintAll of these drilling developments have together led to a progressive increase in the subsurface rock area that drillers can access from a single well pad, while at the same time reducing the required well pad area. According to BP a 20-acre well pad in 1970 could access about 502 acres of reservoir at a depth of 10,000 feet; nowadays a pad of just six acres can access about 32,000 acres of reservoir at the same depth.
The Alpine and Badami fields demonstrate this phenomenon, with wells accessing complete oil reservoirs from minimal-sized pads — just two pads in the case of Alpine and one at Badami.
Another North Slope-invented technique in which well cuttings are ground down and injected into a dedicated waste disposal well has eliminated the need for reserve pits, Christman said. That has further reduced the surface impact of drilling operations.
And drill rigs in the North Slope fields now use electrical power from the North Slope electrical grid, rather than using their own diesel power. That has significantly reduced diesel exhaust emissions on the slope.
“The North Slope is almost exclusively high line power,” Eggemeyer said.
Ultra extended reach drilling, with horizontal displacements in excess of perhaps 40,000 feet, is taking the whole concept of directional drilling to a new paradigm. BP has pioneered the technique at its Wytch Farm oil field on the south coast of England and now plans to use the technique to develop the Liberty oil field in the Beaufort Sea.
The use of ultra extended-reach drilling at Liberty will enable field development and production without the need to construct an offshore gravel island, Christman said. Instead, BP plans to make a relatively small extension to an existing gravel island at the Endicott field for the Liberty development.
Exploration drillingWhen it comes to exploration, the past 30 years have seen a progression to drilling techniques that involve virtually no environmental impact. Nowadays a lone wellhead in the midst of pristine tundra will likely be the only visible remnant of a suspended or abandoned exploration well. No one now uses gravel pads for drilling in untouched tundra — an ice pad that melts in the summer, leaving no trace, provides the platform for an exploration drilling rig. And an ice road that also completely disappears during the summer typically provides access to an exploration drilling site.
In addition, state-of-the-art 3-D seismic survey techniques have revolutionized the search for oil and gas prospects and reduced the risk of drilling a dry exploration well. That reduced risk translates to the need for fewer exploration wells, Michael Faust, offshore exploration manager for ConocoPhillips Alaska, told Petroleum News.
Improved safetyThe period since the 1970s has also seen a greatly increased emphasis on safety during drilling operations, Eggemeyer said. Drilling equipment has become safer and the rigs now have safer and more comfortable working spaces than in earlier days on the slope, he said.
“Safety and environmental is a priority at the rig site,” Terry Lucht, manager, drilling and wells, for ConocoPhillips Alaska, said. “We model our drilling operations around how to do it safely and be environmentally friendly, and still accomplish what we want to do. … As an industry we’ve made huge strides, I think.”