All of these new technniques have enabled oil and gas producers to develop new oil reserves on the North Slope for less than $2.50 a barrel. By lowering the finding costs of new oil, North Slope producers have been able to compensate for other costs, such as transportation, that rise because overall production from the North Slope is falling.

Alaska's oil drillers have pioneered yet another new technology. The ARCO-BP Shared Services Drilling group that manages drilling in several North Slope oil fields have completed four successful through-tubing rotary drilled or TTRD, wells.

Through-tube rotary drilling involves a new well to be drilled through the production tubing of an older well. Because the old tubing doesn't have to be pulled out of the ground by a drilling rig, the technique saves time and money. The potential is to save up to $1 million per well.

Drill crews can pull and replace tubing in the ground at a rate of 59 feet per minute, but if the trip doesn't have to be done, if the new well can be drilled through the production tubing of an older well, it saves money.

With TTRD, drillers also can rotate pipe inside the completion tubing, reducing the friction from sliding.

TTRD follows several other technological breakthroughs in drilling done on the North Slope. Some, like horizontal drilling of production wells, are revolutionizing the way oil fields are developed worldwide.

Horizontal Production Wells
The first of the North Slope's drilling innovations was the drilling and completion of horizontal production wells, which allow producers to run long sections of horizontal tubing through a thin layer of oil down to 6 feet draining the deposit through several perforations, or openings, along the section of pipe.
Horizontal technology has developed rapidly since the first well was drilled in 1990. Now 90 percent of the wells drilled in Prudhoe Bay are horizontal.

The amount of oil-bearing sands penetrated horizontally also has increased as the technique has improved. Early horizontal wells in the late 1980s and 1990s penetrated 500 to 800 feet of reservoir laterally. Earlier this year, through a multilateral completion, about 8,000 feet of reservoir was penetrated horizontally.

Coiled Tubing Units
The second innovation was the drilling and completing of wells with coiled-tubing units instead of drilling rigs. For certain types of wells, mainly "sidetracks" drilled off the bore of an older well, the coiled tubing units represented an alternative.
Coiled tubing units have been used for years on the North Slope, and elsewhere, for doing maintenance work on wells. Using them to drill certain types of wells became possible when new types of down-hole directional equipment and drilling motors became available. It was done for the first time on the North Slope.

Multilateral Wells
The third new technology was the multilateral well completion. Like a sidetrack, this involved a new well drilled off from another. The difference was that both wells are producing where with a sidetrack the older well is cemented and closed off.
Multilaterals involve new wells drilled off older wells as well as two new wells drilled with both sharing the vertical section that reaches to the surface. The savings is in the sharing of the part of the well that reaches to surface, as well as the facilities at the surface.

The first multilateral was drilled on the North Slope a year and a half ago. There are now 10 multilateral wells, eight drilled with drilling rigs and two with coiled-tubing units.

The multilaterals drilled to date on the North Slope have involved two wells, but a "trilateral," three wells flowing into one hole to the surface, is possible and may be drilled sometime.

Designer Wells
Another innovation is in designer wells. These are drilled with a high degree of precision to reach small oil targets, several small oil pockets or to reach through or around faults to isolated traps.
It is made possible by use of three-dimensional seismic, which allows reservoir engineers to plot the locations of faults and small oil traps to within 100 feet of accuracy.

It also is made possible by new technologies allowing tight turns in drilling. Drillers can now turn wells 55 degrees in 100 feet and 100 degrees, about a quarter circle, in 200 feet.

Tighter turns, as tight as 100 degrees in 100 feet, have been made elsewhere in the world, but it was in rock that was solid enough to support "open hole" drilling, without a liner. Tight turns with a liner are a first for the North Slope drillers.

One designer well drilled earlier this year turned 270 degrees, almost a full circle. Another turned 180 degrees to tap four separate oil pockets, with a horizontal length of 5,800 feet.

A full-circle well, to be drilled 360 degrees, is planned for the near future.

Reduced Cost of New Oil
All of these new technniques have enabled oil and gas producers to develop new oil reserves on the North Slope for less than $2.50 a barrel.
By lowering the finding costs of new oil, North Slope producers have been able to compensate for other costs, such as transportation, that rise because overall production from the North Slope is falling.

It also has allowed more drilling to be done on the Slope since drilling and well completion is less expensive.

The Prudhoe Bay field alone will see about 100 new reservoir penetrations this year, about twice the number in 1985. Most are sidetracks and multilateral wells.