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→Single-point tests
* ''s''′ = apparent skin factor
* ''s''′ = apparent skin factor
To solve for permeability, an arbitrary value of permeability is assumed (0.1 md is often a good first estimate), and Equation 2 is solved for ''r''<sub>d</sub>,. Then, this value for ''r''<sub>d</sub> is used in Equation 3 to solve for permeability. For each iteration after the first, use the permeability calculated from Equation 3 in solving for ''r''<sub>d</sub> from Equation 2; this procedure usually converges in three to four iterations.
To solve for permeability, an arbitrary value of permeability is assumed (0.1 md is often a good first estimate), and Equation 2 is solved for ''r''<sub>d</sub>. Then, this value for ''r''<sub>d</sub> is used in Equation 3 to solve for permeability. For each iteration after the first, use the permeability calculated from Equation 3 in solving for ''r''<sub>d</sub> from Equation 2; this procedure usually converges in three to four iterations.
The need to estimate an apparent skin factor, which is usually not known, is the biggest limitation of this method. Pressure buildup tests run in other wells in the same reservoir often provide a good estimate of typical skin factors. Low permeability wells are generally broken down and balled out after completion and prior to testing; in these wells, a skin factor of –1 to –2 is often a reasonable assumption. If a well has been damaged by [[drilling fluid]]s and the perforations have not been broken down, a skin factor of +2 to +5 (or more) is appropriate (see [[Fundamentals of fluid flow]]).
The need to estimate an apparent skin factor, which is usually not known, is the biggest limitation of this method. Pressure buildup tests run in other wells in the same reservoir often provide a good estimate of typical skin factors. Low permeability wells are generally broken down and balled out after completion and prior to testing; in these wells, a skin factor of –1 to –2 is often a reasonable assumption. If a well has been damaged by [[drilling fluid]]s and the perforations have not been broken down, a skin factor of +2 to +5 (or more) is appropriate (see [[Fundamentals of fluid flow]]).