The application of horizontal wells is one of the most rapidly growing areas of petroleum engineering technology today. Typically, horizontal wells are applied In problem producing areas such as fractured reservoirs, heavy oils, tight rocks, thin formations, or coning/cusping situations.
The Midale field is part of a Mississippian carbonate trend in southeastern Saskatchewan. The Midale reservoir conditions many natural vertical fractures, which are oriented predominantly in a NE-SW direction. Fracture spacing varies from several Inches to a few feel. The oriented fracture system creates a permeability anisotropy which averages about 25:1 fieldwide.
Based on available data, the Midale field appears to be an excellent candidate for horizontal well drilling. Horizontal wells can be positioned to intersect a large number of vertical fractures, which should result in a significant Increase in oil production. Three horizontal wells will be drilled in the Midale Unit in 1989, This paper was written concurrent With the drilling of the first well, and describes the thought processes which went into the planning of the first Midale horizontal wells.
Many excellent overviews of the application of horizontal well technology are available in the literature (References 1–3). A very brief review of some general applications of horizontal wells is presented here for completeness.
The basic philosophy behind horizontal wells is simple. Petroleum reservoirs are often relatively flat objects: they are thin but a really large (dimensions like a coin). Normal vertical wells are typically drilled thousands of feet into the earth to produce hydrocarbons from just a few feet of pay at the bottom of the hole. If the producing section of the well at the bottom was turned and drilled horizontally through the pay rather than vertically, the producing section could be much longer and more productive. The increased contact area between the reservoir and the horizontal well should result in increased offlake rates and/or reduced pressure drawdown. A typical horizontal well may deliver 2 to 10 times the performance of a conventional well (Ref. 1).
Obviously, engineers cannot control the thickness (h) of the reservoir, as this was dictated by geologic events millions of years ago. However, the length (L) of the horizontal well can be controlled, and as Uh gets larger, potential benefits from the horizontal well also get larger. Thus, the simplest situation where horizontal wells can be put to goad use is in thin formations.
A second application where horizontal wells can be of benefit is where coning or cusping is a problem. Gas or water coning in vertical wells is caused by the very large pressure drop which occurs near the wellbore due to converging (radial) flow. Horizontal wells can significantly reduce the near-wellbore pressure drop, and thus allow water- or gas-free production at higher offlake rates.
Other applications also exist for horizontal wells. In tight formations, a long horizontal section can often economically produce reserves which could not be obtained with normal vertical wells. Exciting possibilities exist in heavy-oil fields using horizontal wells and steam-assisted gravity drainage.