Changes

==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., and A. P. Roberts, 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>

where

where

* <math>\bar{p}</math> = initial or current average reservoir pressure (psia)

* <math>\bar{p}</math> = initial or current average reservoir pressure (psia)

* ''p''_wf = flowing bottomhole pressure (psia)

* ''p''_wf = flowing bottomhole pressure (psia)

* ''μ'' = viscosity (cp)

* ''μ'' = [[viscosity]] (cp)

* ''B'' = formation volume factor (rcf/MSCF)

* ''B'' = formation volume factor (rcf/MSCF)

The productivity index can be a useful indicator of well productivity and wellbore condition during the life of a well. PI will generally decrease over time due to declining reservoir pressure, changes in producing conditions, and/or [[production problems]].

The productivity index can be a useful indicator of well productivity and wellbore condition during the life of a well. PI will generally decrease over time due to declining reservoir pressure, changes in producing conditions, and/or [[production problems]].

Single-point tests can also be used to estimate formation permeability<ref name=pt09r15>Lee, W. J., Kuo, T. B., Holditch, S. A., McVay, D. A., 1984, Estimating formation permeability from single-point flow data: Proceedings of the 1984 SPE/DOE/GRI Unconventional Gas Recovery Symposium, Pittsburgh, PA, p. 175–186.</ref> with an iterative solution of the transient radius of drainage equation (Equation 2) and the pseudosteady-state flow equation (Equation 3), as follows:

Single-point tests can also be used to estimate formation permeability<ref name=pt09r15>Lee, W. J., T. B. Kuo, S. A. Holditch, and D. A. McVay, 1984, Estimating formation permeability from single-point flow data: Proceedings of the 1984 SPE/DOE/GRI Unconventional Gas Recovery Symposium, Pittsburgh, PA, p. 175–186.</ref> with an iterative solution of the transient radius of drainage equation (Equation 2) and the pseudosteady-state flow equation (Equation 3), as follows:

:<math>r_{\rm d} = \left(\frac{kt}{376\phi \mu c_{\rm t}}\right)^{1/2}</math>

:<math>r_{\rm d} = \left(\frac{kt}{376\phi \mu c_{\rm t}}\right)^{1/2}</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''_d. Then, this value for ''r''_d 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''_d 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''_d. Then, this value for ''r''_d 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''_d 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 [[Fluid flow fundamentals]]).

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==

[[file:production-testing_fig2.png|thumb|500 px|{{figure number|2}}Multi-point test used to estimate absolute open flow potential.]]

[[file:production-testing_fig2.png|thumb|300 px|{{figure number|2}}Multi-point test used to estimate absolute open flow potential.]]

Multi-point tests are typically used to establish gas well deliverability and absolute open flow potential; these tests may also be referred to as gas well deliverability tests, backpressure tests, or flow-after-flow tests.<ref name=pt09r1 /> Multi-point tests typically require the measurement of gas flow rates and surface pressures at four stabilized flow conditions; surface shut-in pressure is also measured. Generally, an increasing flow rate sequence is preferred to a decreasing rate sequence.

Multi-point tests are typically used to establish gas well deliverability and absolute open flow potential; these tests may also be referred to as gas well deliverability tests, backpressure tests, or flow-after-flow tests.<ref name=pt09r1 /> Multi-point tests typically require the measurement of gas flow rates and surface pressures at four stabilized flow conditions; surface shut-in pressure is also measured. Generally, an increasing flow rate sequence is preferred to a decreasing rate sequence.

The surface shut-in and flowing pressure measurements are converted to bottomhole conditions and a log-log plot of <math>\bar{p}^{2} - p_{\rm wf}^{2}</math> versus flow rate, ''q'', is generated ([[:file:production-testing_fig2.png|Figure 2]]). The four points define a straight line with a slope that is generally between 0.5 and 1.0. This straight line is extrapolated to determine gas flow rate at a point where the flowing bottomhole pressure is zero; this rate is referred to as the absolute open flow (AOF) potential of the well.

The surface shut-in and flowing pressure measurements are converted to bottomhole conditions and a log-log plot of <math>\bar{p}^{2} - p_{\rm wf}^{2}</math> versus flow rate, ''q'', is generated ([[:file:production-testing_fig2.png|Figure 2]]). The four points define a straight line with a slope that is generally between 0.5 and 1.0. This straight line is extrapolated to determine gas flow rate at a point where the flowing bottomhole pressure is zero; this rate is referred to as the absolute open flow (AOF) potential of the well.

Multi-point test data can also be used to estimate permeability using a variable rate flow test analysis.<ref name=pt09r20>Odeh, A. S., Jones, L. G., 1965, Pressure drawdown analysis, variable-rate case, in Pressure Analysis Methods: Dallas, TX, American Institute of Mining, Metallurgical and Petroleum Engineers, Society of Petroleum Engineers Reprint Series No. 9, 256 p.</ref> For gas wells, the data are plotted as

Multi-point test data can also be used to estimate permeability using a variable rate flow test analysis.<ref name=pt09r20>Odeh, A. S., and L. G. Jones, 1965, Pressure drawdown analysis, variable-rate case, in Pressure Analysis Methods: Dallas, TX, American Institute of Mining, Metallurgical and Petroleum Engineers, Society of Petroleum Engineers Reprint Series No. 9, 256 p.</ref> For gas wells, the data are plotted as

:<math>\frac{\bar{p}^{2} - p_{\rm wfn}^{2}}{q_{n}} \mbox{ versus } \frac{1}{q_{n}} \sum\limits_{j=0}^{n-1} \Delta q_{j} \log (t_{n} - t_{j})</math>

:<math>\frac{\bar{p}^{2} - p_{\rm wfn}^{2}}{q_{n}} \mbox{ versus } \frac{1}{q_{n}} \sum\limits_{j=0}^{n-1} \Delta q_{j} \log (t_{n} - t_{j})</math>

[[Category:Production engineering methods]]

[[Category:Production engineering methods]]