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Unlocking new resources at Kuparuk Coiled tubing drilling has enabled access into isolated pockets of oil in the complex reservoir of the Kuparuk River field Alan Bailey Petroleum News
With more than 6 billion barrels of original oil in place, the Kuparuk River field on Alaska’s North Slope is a giant among North American oil fields. But with the oil lying in two major reservoir zones — the A and the C zones — and with a multiplicity of geologic faults fracturing the reservoir into multiple compartments, teasing as much oil as possible from the reservoir sands has proved a significant challenge.
Although field operator ConocoPhillips has used water to flush oil into production wells, the compartmented nature of the reservoir and the complexities of production from the two reservoir zones have limited the effectiveness of this conventional “waterflood” technique, Bryn Clark of ConocoPhillips Alaska told the Pacific Section, American Association of Petroleum Geologists, in Anchorage on May 10. And so, over the years coiled tubing drilling, targeting specific reservoir compartments has proved to be a key technique in maximizing oil recovery, Clark said.
Continuous pipe Coiled tubing drilling, a technique pioneered in Alaska, makes use of a continuous coil of flexible three-inch diameter steel drill pipe rather than the 30-foot or 90-foot stands of rigid drill pipe used in a conventional drilling operation. A motor, powered by drilling mud, turns a drill bit at the bottom end of the tubing, while other tools in the bottom hole assembly control the direction in which the hole is drilled. While the drill bit gouges its way through the rock formations, the operators in the drilling rig slowly unwind the coiled tubing into the well hole.
In a typical coiled-tubing operation drillers thread the tubing down an existing well bore and then direct the drill bit out from the original well to form a lateral “sidetrack” well, accessing new areas of the field reservoir. The technique first came into use in 1998 with the drilling of simple, single lateral wells, Clark said. By the mid-2000s, multiple lateral wells were being drilled out from older well bores, with the first quadrilateral well being drilled in 2005, he said.
And it is now possible to drill lateral wells up to 3,500 feet long, with up to five laterals extending from a single parent well, Clark said. Techniques such as the use of an agitator to shake the well pipe, and the planning of a well trajectory to cause the well to slope somewhat downhill towards its end, help drillers to maximize the length of a well, he said.
Although the coiled tubing drilling initially targeted the relatively thick and straightforward sands of the C zone, ConocoPhillips is now drilling multilateral coiled tubing wells in the more challenging A sands, where waterflood techniques have proved especially difficult to apply, Clark said.
3-D seismic The shooting of a high-resolution 3-D seismic survey in 2005 has enabled ConocoPhillips to map the pressures within the field reservoir, determine locations where faults are blocking the flow of oil and then target specific fault blocks using precisely steered coiled tubing multilaterals, he said.
A new drill-bit steering technology implemented in 2009 has enhanced the accuracy with which a well can intercept a specific sand body. Drillers have also developed techniques for steering a drill bit through difficult underground geology, perhaps, for example, causing the bit to penetrate an unstable shale layer at a steep angle to prevent the shale from sending the bit off course.
However, it is still only possible to run two types of well logs — gamma ray and resistivity logs — through a coiled tubing well. Future enhancements will likely include the ability to use additional downhole tools.
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