Changes

==Multi-rate flow tests==

==Multi-rate flow tests==

These tests are run much like single-rate tests, except that the rate is changed by discrete amounts one or more times while the test continues. An example of this type of test is a flow after flow deliverability test on a gas well, which is also called a four-point or back-pressure test.

These tests are run much like single-rate tests, except that the rate is changed by discrete amounts one or more times while the test continues. An example of this type of test is a flow after flow deliverability test on a gas well, which is also called a four-point or back-pressure test.

===When the tests are applicable===

===When the tests are applicable===

Flow tests can be useful when the reservoir is at uniform pressure, such as when a new well is completed or when a well has been shut in for a lengthy period. Flow tests are appropriate when a well must continue to produce revenue even though a test is needed. Analysis of flow tests is simplest when the rate is held strictly constant and in all cases requires known rates at all times during the test.

Flow tests can be useful when the reservoir is at uniform pressure, such as when a new well is completed or when a well has been shut in for a lengthy period. Flow tests are appropriate when a well must continue to produce revenue even though a test is needed. Analysis of flow tests is simplest when the rate is held strictly constant and in all cases requires known rates at all times during the test.

===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).

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

[[file:pressure-transient-testing_fig1.png|thumb|{{figure number|1}}Type curve that identifies the end of wellbore effects.]]

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)]].

[[file:pressure-transient-testing_fig2.png|thumb|{{figure number|2}}(a) Derivative type curve used to match derivatives of test data. (b) Shapes of derivatives of test data for various reservoir conditions.]]

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

 

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).]]

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.

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:

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.

[[file:pressure-transient-testing_fig5.png|left|thumb|{{figure number|5}}(a) Typical buildup curve shape with flow barrier, (b) Doubling of slope on Horner plot for well near 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. 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.

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.

[[file:pressure-transient-testing_fig5.png|thumb|{{figure number|5}}(a) Typical buildup curve shape with flow barrier, (b) Doubling of slope on Horner plot for well near 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==