**Most wells drilled for water, oil, natural gas, information or other subsurface objectives are vertical wells** – drilled straight down into the earth. ** However, drilling at an angle other than vertical can obtain information, hit targets and stimulate reservoirs in ways that can not be achieved with a vertical well.** In these cases, an ability to accurately steer the well in directions and angles that depart from the vertical is a valuable ability.

**Many low-permeability gas reservoirs are historically considered to be non-commercial due to low production rates**. Most vertical wells drilled in tight gas reservoirs are stimulated using hydraulic fracturing and/or acidizing treatments to attain economical flow rates. In addition, to deplete a tight gas reservoir, **vertical wells must be drilled at close spacing to efficiently drain the res**ervoir. This would require a large number of vertical **wells. In such reservoirs, horizontal wells provide an attractive alternative to effectively deplete tight gas reservoirs and attain high flow rates.** Joshi (1991) pointed out that horizontal wells are applicable in both low permeability reservoirs as well as in high-permeability reservoirs. The excellent reference textbook by Joshi (1991) gives a comprehensive treatment of horizontal well performance in oil and gas reservoirs.

In calculating the gas flow rate from a horizontal well, **Joshi (1991) introduced the concept of the effective wellbore radius rw into the gas flow equation. The effective wellbore is given by this:**

**For a pseudosteady-state flow**, Joshi (1991) expressed Darcy’s equation of a laminar flow in the following two familiar forms:

**For turbulent flow, Darcy’s equation must be modified to account for the additional pressure caused by the non-Darcy flow by including the rate-dependent skin factor DQg** in practice, the back-pressure equation and the LIT approach are used to calculate theflowrate and construct the IPR curve for the horizontal well. Multirate tests, i.e., deliverability tests, must be performed on the horizontal well to determine the coefficients of the selected flow equation.

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