Source: U.S. Department of Energy / Office of Fossil Energy

Oil and Gas wells traditionally have been drilled vertically, at depths ranging from a few thousand feet to as deep as 5 miles. Depending on subsurface geology, technology advances now allow wells to deviate from the strictly vertical orientation by anywhere from a few degrees to completely horizontal, or even inverted toward the surface. About 90 percent of all horizontal wells have been drilled into carbonate formations, which account for about 30 percent of all U.S. reserves.

Directional and horizontal drilling enable producers to reach reservoirs that are not located directly beneath the drilling rig, a capability that is particularly useful in avoiding sensitive surface and subsurface environmental features. New methods and technology allow industry to produce resources far beneath sensitive environments and scenic vistas in Louisiana wetlands, California wildlife habitats and beaches, Rocky Mountain pine forests, and recreational areas on the Texas Gulf Coast.

Horizontal drilling may also allow a producer to contact more of the reservoir, so that more resources can be recovered from a single well. Advances in directional drilling also facilitate multilateral drilling and completion, enabling multiple offshoots from a single wellbore to radiate in different directions or contact resources at different depths. Recent and very rapid development of such radial drilling technology has spurred a boom in horizontal drilling. Since the mid-1980's, the drilling of horizontal wells has grown from a few to more than 2,700 wells per year worldwide. In the United States, horizontal drilling now accounts for 5 to 8 percent of the land well count at any given time.