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→Single-point tests
==Single-point tests==
==Single-point tests==
Single-point tests are usually simple productivity tests that typically involve a measurement (or estimate) of initial or average reservoir pressure and a measurement of flow rate and flowing bottomhole pressure (which can be estimated from flowing surface pressure) at stabilized producing conditions<ref name=pt09r1>Allen, T. O., Roberts, A. P., 1978, Production Operations, Volume 1 : Tulsa, OK, Oil and Gas Consultants International, 225 p.</ref>. From these data, the productivity index, PI, can be calculated as follows:
Single-point tests are usually simple productivity tests that typically involve a measurement (or estimate) of initial or average reservoir pressure and a measurement of flow rate and flowing bottomhole pressure (which can be estimated from flowing surface pressure) at stabilized producing conditions.<ref name=pt09r1>Allen, T. O., Roberts, A. P., 1978, Production Operations, Volume 1 : Tulsa, OK, Oil and Gas Consultants International, 225 p.</ref> From these data, the productivity index, PI, can be calculated as follows:
:<math>\mbox{PI} = \frac{q}{\bar{p} - p_{\rm wf}} (\mbox{for oil}) = \frac{q\mu B}{\bar{p}^{2} - p_{\rm wf}^{2}} (\mbox{for gas})</math>
:<math>\mbox{PI} = \frac{q}{\bar{p} - p_{\rm wf}} (\mbox{for oil}) = \frac{q\mu B}{\bar{p}^{2} - p_{\rm wf}^{2}} (\mbox{for gas})</math>
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]]).
The single-point test method for estimating permeability is valid for constant flow rate production, constant bottomhole pressure production, or smoothly changing bottomhole pressures and flow rates. The method is recommended for estimating permeability from prefracture flow test data only; it does not work well with postfracture flow data. The method is particularly useful in low permeability reservoirs where operators do not run buildup tests routinely because of the long test times required to overcome wellbore storage effects and reach radial flow (see “[[Pressure transient testing]]”).
The single-point test method for estimating permeability is valid for constant flow rate production, constant bottomhole pressure production, or smoothly changing bottomhole pressures and flow rates. The method is recommended for estimating permeability from prefracture flow test data only; it does not work well with postfracture flow data. The method is particularly useful in low permeability reservoirs where operators do not run buildup tests routinely because of the long test times required to overcome wellbore storage effects and reach radial flow (see [[Pressure transient testing]]).
==Multi-point tests==
==Multi-point tests==