Changes

===Single-rate flow tests===

===Single-rate flow tests===

A single-rate flow test is run using the following procedure. Start with the reservoir at a uniform, stabilized pressure. Then place the pressure measuring device as near the perforations as possible. Flow the well at a strictly constant rate (control with a variable choke or other control device). An alternative method for gas wells is to measure surface pressures and calculate bottomhole pressures from surface measurements. The rate at which pressure changes with time reflects the formation properties. An example of this type of test is a single-point deliverability test in a gas well (see “[[Production testing]]”).

A single-rate flow test is run using the following procedure. Start with the reservoir at a uniform, stabilized pressure. Then place the pressure measuring device as near the perforations as possible. Flow the well at a strictly constant rate (control with a variable choke or other control device). An alternative method for gas wells is to measure surface pressures and calculate bottomhole pressures from surface measurements. The rate at which pressure changes with time reflects the formation properties. An example of this type of test is a single-point deliverability test in a gas well (see [[Production testing]]).

===Pressure buildup tests===

===Pressure buildup tests===

In a pressure buildup test, one should stabilize the rate in the tested well for several days, that is, maintain the rate approximately constant. Then place a pressure measuring device as near the perforations as possible several hours before shut-in. Shut the well in and let the pressure build up. The rate at which pressure builds up with time reflects the formation properties. (For more details of pressure buildup and flow tests, see <ref name=pt09r16>Matthews, C. S., Russell, D. G., 1967, Pressure buildup and flow tests in wells: Dallas, TX, Society of Petroleum Engineers Monograph Series No. 1, 172 p.</ref>.)

In a pressure buildup test, one should stabilize the rate in the tested well for several days, that is, maintain the rate approximately constant. Then place a pressure measuring device as near the perforations as possible several hours before shut-in. Shut the well in and let the pressure build up. The rate at which pressure builds up with time reflects the formation properties. (For more details of pressure buildup and flow tests, see Matthews and Russell.<ref name=pt09r16>Matthews, C. S., and D. G. Russell, 1967, Pressure buildup and flow tests in wells: Dallas, TX, Society of Petroleum Engineers Monograph Series No. 1, 172 p.</ref>)

==Multi-rate flow tests==

==Multi-rate flow tests==

===How the tests are analyzed===

===How the tests are analyzed===

Wellbore effects dominate early test data. The end of wellbore effects is found using log-log plots of test data, which are compared to preplotted type curves, as illustrated in Figure 1. The shapes of test data plots are also used to identify the reservoir type, such as homogeneous acting, naturally fractured, layered, or hydraulically fractured. Derivative type curves (basically the slope of a plot of pressure versus the logarithm of time) are particularly helpful for identifying reservoir type and wellbore effects, as shown in Figures 2(a) and (b).

Wellbore effects dominate early test data. The end of wellbore effects is found using log-log plots of test data, which are compared to preplotted type curves, as illustrated in [[:file:pressure-transient-testing_fig1.png|Figure 1]]. The shapes of test data plots are also used to identify the reservoir type, such as homogeneous acting, naturally fractured, layered, or hydraulically fractured. Derivative type curves (basically the slope of a plot of pressure versus the logarithm of time) are particularly helpful for identifying reservoir type and wellbore effects, as shown in [[:file:pressure-transient-testing_fig2.png|Figures 2(a) and (b)]].

Traditional analysis is focused on semi-logarithmic plots of test data, with slopes of straight lines on these plots used to determine [[permeability]]. [[:file:pressure-transient-testing_fig3.png|Figure 3]] is a typical semi-log plot of flow test data, and [[:file:pressure-transient-testing_fig4.png|Figure 4]] is a typical semi-log plot of buildup test data. The “correct” semi-log straight line is indicated on these figures; the line is identified with the help of type curves (see [[Production histories]]). On the buildup test plot, shut-in bottomhole pressure is plotted versus the logarithm of the ratio of producing time, ''t''_p, plus shut-in time, Δ''t'', to shut-in time. This plot is called a ''Horner plot'', named for the person who proposed it in the [[petroleum]] literature.

 

 

Traditional analysis is focused on semi-logarithmic plots of test data, with slopes of straight lines on these plots used to determine [[permeability]]. Figure 3 is a typical semi-log plot of flow test data, and Figure 4 is a typical semi-log plot of buildup test data. The “correct” semi-log straight line is indicated on these figures; the line is identified with the help of type curves (see “[[Production histories]]”). On the buildup test plot, shut-in bottomhole pressure is plotted versus the logarithm of the ratio of producing time, ''t''_p, plus shut-in time, Δ''t'', to shut-in time. This plot is called a ''Horner plot'', named for the person who proposed it in the petroleum literature.

 

[[file:pressure-transient-testing_fig3.png|thumb|{{figure number|3}}Typical flow test data graph.]]

 

[[file:pressure-transient-testing_fig4.png|thumb|{{figure number|4}}Typical buildup test graph (Horner plot).]]

Simple equations allow us to estimate permeability and skin factor once the correct semilog straight line is identified and its slope, ''m'', is estimated. These equations apply to both drawdown and buildup tests. The following equations are used for oil wells:

Simple equations allow us to estimate permeability and skin factor once the correct semilog straight line is identified and its slope, ''m'', is estimated. These equations apply to both drawdown and buildup tests. The following equations are used for oil wells:

* Δ''p''_lhr = pressure change in first hour of test (psi)

* Δ''p''_lhr = pressure change in first hour of test (psi)

* ''ϕ'' = [[porosity]] (fraction)

* ''ϕ'' = [[porosity]] (fraction)

* ''μ'' = viscosity (cp)

* ''μ'' = [[viscosity]] (cp)

* ''c''_t = total compressibility of formation and its fluids (psi^–1)

* ''c''_t = total compressibility of formation and its fluids (psi^–1)

* ''r''_w = wellbore radius or hole size (ft)

* ''r''_w = wellbore radius or hole size (ft)

Similar equations are used for gas well test analysis.

Similar equations are used for gas well test analysis.

Extrapolation of pressure in a buildup test to Horner time ratio of unity provides an estimate of original reservoir pressure (new well) or “false” pressure, which serves as the basis for determining current drainage area pressure, <math>\bar{p}</math>, for a well with some pressure depletion in its drainage area caused by extended production of fluids. Figure 4 illustrates extrapolation of pressure to time ratio of unity.

Extrapolation of pressure in a buildup test to Horner time ratio of unity provides an estimate of original reservoir pressure (new well) or “false” pressure, which serves as the basis for determining current drainage area pressure, <math>\bar{p}</math>, for a well with some pressure depletion in its drainage area caused by extended production of fluids. [[:file:pressure-transient-testing_fig4.png|Figure 4]] illustrates extrapolation of pressure to time ratio of unity.

Also, distance to boundaries of flow barriers is found from semilog plots by deviation from a previously established semilog straight line. In the simplest case, which is uncommon except for wells very close to boundaries, the late time slope doubles. Figure 5(a) shows the usual case, where the slope increases at late times but does not double. Figure 5(b) shows the less common case where the slope actually doubles at late times. The time at which the deviation occurs and the amount of deviation can be used to estimate the distance from the tested well to the flow barrier.

Also, distance to boundaries of flow barriers is found from semilog plots by deviation from a previously established semilog straight line. In the simplest case, which is uncommon except for wells very close to boundaries, the late time slope doubles. [[:file:pressure-transient-testing_fig5.png|Figure 5(a)]] shows the usual case, where the slope increases at late times but does not double. [[:file:pressure-transient-testing_fig5.png|Figure 5(b)]] shows the less common case where the slope actually doubles at late times. The time at which the deviation occurs and the amount of deviation can be used to estimate the distance from the tested well to the flow barrier.

 

==Long-term production tests==

==Long-term production tests==

The same information is available from long-term production tests as from short-term flow tests, including permeability, skin factor, and distance to boundaries. Hydrocarbons in place in the tested well's drainage area can frequently be estimated from these test data. Once boundaries have affected the test data, long-term production data can be extrapolated to provide a forecast of future production to the economic limit and can thus provide a reserve estimate for the well.

The same information is available from long-term production tests as from short-term flow tests, including permeability, skin factor, and distance to boundaries. Hydrocarbons in place in the tested well's drainage area can frequently be estimated from these test data. Once boundaries have affected the test data, long-term production data can be extrapolated to provide a forecast of future production to the economic limit and can thus provide a reserve estimate for the well.

===How the tests are analyzed===

===How the tests are analyzed===

The simplest of these tests—those with production data from wells produced at constant BHP or with smoothly changing rates and BHP—can be analyzed with simple type curves, such as Fetkovich's, illustrated in Figure 6. Formation properties are obtained from matches of actual field data to the type curve. Forecasts are made by projecting future performance along the type curve that matches post-production performance.

The simplest of these tests—those with production data from wells produced at constant BHP or with smoothly changing rates and BHP—can be analyzed with simple type curves, such as Fetkovich's, illustrated in [[:file:pressure-transient-testing_fig6.png|Figure 6]]. Formation properties are obtained from matches of actual field data to the type curve. Forecasts are made by projecting future performance along the type curve that matches post-production performance.

[[file:pressure-transient-testing_fig6.png|thumb|{{figure number|6}}Fetkovich's type curve for analyzing long-term production data. (After <ref name=pt09r8>Fetkovich, M. J., 1980, Decline curve analysis using type curves: Journal of Petroleum Technology, June, p. 1065–1077.</ref>.)]]

An essential requirement in this method of analysis is that there is enough production history for boundary effects to have influenced production data. This same requirement also applies to conventional decline curves and decline curve analysis—if boundary effects have not been felt, the decline curve projection is totally meaningless and certainly incorrect. [[:file:pressure-transient-testing_fig7.png|Figure 7]] shows a type curve match of past performance and indicates how production data can be extrapolated into the future.

An essential requirement in this method of analysis is that there is enough production history for boundary effects to have influenced production data. This same requirement also applies to conventional decline curves and decline curve analysis—if boundary effects have not been felt, the decline curve projection is totally meaningless and certainly incorrect. Figure 7 shows a type curve match of past performance and indicates how production data can be extrapolated into the future.

More complex tests, with abrupt changes in rate and BHP, are more readily analyzed with computer reservoir simulators. These simulators are used to history-match production data to obtain a reservoir description, which is then used to obtain a long-term production forecast and thus to estimate reserves. [[:file:pressure-transient-testing_fig8.png|Figure 8]] shows an example of a history match of production data and a forecast of future performance of the well using the reservoir description obtained from the history match.

[[file:pressure-transient-testing_fig9.png|thumb|300px|{{figure number|9}}Active and observation wells in an interference pulse test.<ref name=pt09r7>Earlougher, R. C., Jr., 1977, Advances in Well Test Analysis: Dallas, TX, American Institute of Mining, Metallurgical and Petroleum Engineers, Society of Petroleum Engineer's Monograph 5, 264 p.</ref>]]

 

[[file:pressure-transient-testing_fig8.png|thumb|{{figure number|8}}History match of production data and forecast of future performance.]]

 

Interference and pulse tests are run to obtain the following information:

Interference and pulse tests are run to obtain the following information:

===How the tests are run===

===How the tests are run===

 

file:pressure-transient-testing_fig10.png|{{figure number|10}}Schematic illustration of rate history and pressure response for an interference test. (After <ref name=pt09r7 />.)

[[file:pressure-transient-testing_fig9.png|thumb|{{figure number|9}}Active and observation wells in an interference pulse test. (After <ref name=pt09r7>Earlougher, R. C., Jr., 1977, Advances in Well Test Analysis: Dallas, TX, American Institute of Mining, Metallurgical and Petroleum Engineers, Society of Petroleum Engineer's Monograph 5, 264 p.</ref>.)]]

file:pressure-transient-testing_fig11.png|{{figure number|11}}Schematic Illustration of rate (pulse) history and pressure response for a pulse test.<ref name=pt09r7 />

 

[[file:pressure-transient-testing_fig10.png|thumb|{{figure number|10}}Schematic illustration of rate history and pressure response for an interference test. (After <ref name=pt09r7 />.)]]

Pulse tests are performed by first producing (or injecting into) the active well for a few hours. The active well is then shut-in, then returned to production, shut-in again, and so on in a regular, repeating pattern. The response in the offset wells is then measured while continuing to produce all wells in the field except those directly involved in the test. This is possible because the “noise” caused by continued production of wells not directly involved in the test can be filtered out using the response caused by the repeated on-off pattern in the active well. Figure 11 shows a typical response in a pulse test observation well.

Interference tests are run by first shutting in the portion of the reservoir in the area affected by the test. Then one produces (or injects into) one well (called the active well) and measures the pressure response in the [[offset]] wells. [[:file:pressure-transient-testing_fig9.png|Figure 9]] shows a typical interference test pattern, and [[:file:pressure-transient-testing_fig10.png|Figure 10]] is a plot of a typical response in an observation well.

[[file:pressure-transient-testing_fig11.png|thumb|{{figure number|11}}Schematic Illustration of rate (pulse) history and pressure response for a pulse test. (After <ref name=pt09r7 />.)]]

Pulse tests are performed by first producing (or injecting into) the active well for a few hours. The active well is then shut-in, then returned to production, shut-in again, and so on in a regular, repeating pattern. The response in the offset wells is then measured while continuing to produce all wells in the field except those directly involved in the test. This is possible because the “noise” caused by continued production of wells not directly involved in the test can be filtered out using the response caused by the repeated on-off pattern in the active well. [[:file:pressure-transient-testing_fig11.png|Figure 11]] shows a typical response in a pulse test observation well.

===Comparison of pulse and interference tests===

===Comparison of pulse and interference tests===

[[Category:Production engineering methods]]

[[Category:Production engineering methods]]