Drilling Sideways - A Review of Horizontal Well Technology and Its Domestic Application
April 1, 1993
Highlights
The use of horizontal drilling technology in oil exploration, development, and production operations has grown rapidly over the past 5 years. This report reviews the technology, its history, and its current domestic application. It also considers related technologies that will increasingly affect horizontal drilling's future.
Horizontal drilling technology achieved commercial viability during the late 1980's. Its successful employment, particularly in the Bakken Shale of North Dakota and the Austin Chalk of Texas, has encouraged testing of it in many domestic geographic regions and geologic situations. Of the three major categories of horizontal drilling, short-, medium-, and long-radius, the medium-radius well has been most widely used and productive. Achievable horizontal bore hole length grew rapidly as familiarity with the technique increased; horizontal displacements have now been extended to over 8,000 feet. Some wells have featured multiple horizontal bores. Completion and production techniques have been modified for the horizontal environment, with more change required as the well radius decreases; the specific geologic environment and production history of the reservoir also determine the completion methods employed. Most horizontal wells have targeted crude oil reservoirs. The commercial viability of horizontal wells for production of natural gas has not been well demonstrated yet, although some horizontal wells have been used to produce coal seam gas. The Department of Energy has provided funding for several experimental horizontal gas wells.
The technical objective of horizontal drilling is to expose significantly more reservoir rock to the well bore surface than can be achieved via drilling of a conventional vertical well. The desire to achieve this objective stems from the intended achievement of other, more important technical objectives that relate to specific physical characteristics of the target reservoir, and that provide economic benefits. Examples of these technical objectives are the need to intersect multiple fracture systems within a reservoir and the need to avoid unnecessarily premature water or gas intrusion that would interfere with oil production. In both examples, an economic benefit of horizontal drilling success is increased productivity of the reservoir. In the latter example, prolongation of the reservoir's commercial life is also an economic benefit.
Domestic applications of horizontal drilling technology have included the drilling of fractured conventional reservoirs, fractured source rocks, stratigraphic traps, heterogeneous reservoirs, coalbeds (to produce their methane content), older fields (to boost their recovery factors), and fluid and heat injection wells intended to boost both production rates and recovery factors. Significant successes include many horizontal wells drilled into the fractured Austin Chalk of Texas' Giddings Field, which have produced at 2.5 to 7 times the rate of vertical wells, wells drilled into North Dakota's Bakken Shale, from which horizontal oil production increased from nothing in 1986 to account for 10 percent of the State's 1991 production, and wells drilled into Alaska's North Slope fields.
An offset to the benefits provided by successful horizontal drilling is its higher cost. But the average cost is going down. By 1990, the cost premium associated with horizontal wells had shrunk from the 300- percent level experienced with some early experimental wells to an annual average of 17 percent. Learning curves are apparent, as indicated by incurred costs, as new companies try horizontal drilling and as companies move to new target reservoirs. It is probable that the cost premium associated with horizontal drilling will continue to decline, leading to its increased use. Two allied technologies are currently being adapted to horizontal drilling in the effort to reduce costs. They are the use of coiled tubing rather than conventional drill pipe for both drilling and completion operations and the use of smaller than conventional diameter (slim) holes.