Horizontal wells are high-angle wells (with an inclination of generally greater than 85°) drilled to enhance reservoir performance by placing a long wellbore section within the reservoir.
Horizontal well Vs extended-reach well
Horizontal Well contrasts with an extended-reach well, which is a high-angle directional well drilled to intersect a target point.
Growth of horizontal drilling
There was relatively little horizontal drilling activity before 1985. The Austin Chalk play is responsible for the boom in horizontal drilling activity in the U.S. Now, horizontal drilling is considered an effective reservoir-development tool.   
Advantages of horizontal wells
The advantages of horizontal wells include:
- Reduced water and gas coning because of reduced drawdown in the reservoir for a given production rate, thereby reducing the remedial work required in the future
- Increased production rate because of the greater wellbore length exposed to the pay zone
- Reduced pressure drop around the wellbore
- Lower fluid velocities around the wellbore
- A general reduction in sand production from a combination of Items 3 and 4
- Larger and more efficient drainage pattern leading to increased overall reserves recovery
Characteristics of horizontal wells
Horizontal wells are normally characterized by their buildup rates and are broadly classified into three groups that dictate the drilling and completion practices required, as shown in Table 1.
The “build rate” is the positive change in inclination over a normalized length (e.g., 3°/100 ft.) A negative change in inclination would be the “drop rate.” A long-radius horizontal well is characterized by build rates of 2 to 6°/100 ft, which result in a radius of 3,000 to 1,000 ft. This profile is drilled with conventional directional-drilling tools, and lateral sections of up to 8,000 ft have been drilled. This profile is well suited for applications in which a long, horizontal displacement is required to reach the target entry point. The use of rotary-steerable systems (RSSs) may be required to drill an extra-long lateral section because slide drilling may not be possible with the conventional steerable motors.
Medium radius horizontal wells
Medium-radius horizontal wells have build rates of 6 to 35°/100 ft, radii of 1,000 to 160 ft, and lateral sections of up to 8,000 ft. These wells are drilled with specialized downhole mud motors and conventional drillstring components. Double-bend assemblies are designed to build angles at rates up to 35°/100 ft. The lateral section is often drilled with conventional steerable motor assemblies. This profile is common for land-based applications and for re-entry horizontal drilling. In practical terms, a well is classified as medium radius if the bottomhole assembly (BHA) cannot be rotated through the build section at all times. At the upper end of the medium radius, drilling the maximum build rate is limited by the bending and torsional limits of American Petroleum Institute (API) tubulars. Smaller holes with more-flexible tubulars have a higher allowable maximum dogleg severity (DLS).
Short radius horizontal wells
Short-radius horizontal wells have build rates of 5 to 10°/3 ft (1.5 to 3°/ft), which equates to radii of 40 to 20 ft. The length of the lateral section varies between 200 and 900 ft. Short-radius wells are drilled with specialized drilling tools and techniques. This profile is most commonly drilled as a re-entry from any existing well.
- ↑ Burgess, T. et al. 1991. Horizontal Drilling Comes of Age. Oil Field Review 2 (3): 22-23.
- ↑ Fisher, E.K. and French, M.R. 1992. Drilling the First Horizontal Well in the Gulf of Mexico: A Case History of East Cameron Block 278 Well B-12. SPE Drill Eng 7 (2): 86-92. SPE-22545-PA. http://dx.doi.org/10.2118/22545-PA.
- ↑ Gust, D.A. and MacDonald, R.R. 1989. Rotation of a Long Liner in a Shallow Long-Reach Well. SPE Journal of Petroleum Technology 41 (4): 401-404. SPE-17183-PA. http://dx.doi.org/10.2118/17183-PA.
- ↑ Smith, R.C., Hayes, L.A., and Wilkin, J.F. 1994. The Lateral Tie-Back System: The Ability to Drill and Case Multiple Laterals. Presented at the SPE/IADC Drilling Conference, Dallas, Texas, 15–18 February. SPE-27436-MS. http://dx.doi.org/10.2118/27436-MS.