The ∆P Curve in the log-log plot
If we assume the following multiple-rate production profile:
From t= tP onwards, the well is shut-in to perform a PBU test of duration Δt
Rate-normalized pressure and derivative data from the PBU test are plotted versus the shut-in duration in a log-log scale.
For most softwares, the ΔP curve represents the pressure signal
versus the shut-in duration Δt in hours. (q=qn-1 the latest rate before PBU test)
Use of the ΔP plot
The ΔP plot is used to:
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Calculate the Skin value:
Although in theory, this plot could be used to recognize some flow regimes, it is mainly recommended so as to derive the total skin value. Indeed the vertical separation between the derivative stabilization level and the ΔP curve is indicative of the skin value. The higher the ΔP curve is located, the higher the skin value is.
By overlaying ΔP curve from different PBU tests, the skin value can be monitored over time.
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Detect any error on the start of a PBU test
The ΔP curve depends on the shut-in pressure value Pws(Δt=0) and start time Δt=0. Any error on the start of the PBU test will mainly distort the ΔP curve, and in that sense could be recognized and mitigated. The start of the PBU shut-in needs to be inline with the start of the rate shut-in.
The figure below shows the impact of an incorrect start of the PBU test. In general, the derivative plot remains unchanged while the ΔP curve is distorted at early times.
This error is very characteristic and as a result it can be easily recognized and mitigated.
However, if the start of the shut-in period is taken during the PBU test itself, the curve will not look distorted, but shifted down, giving the impression of a lower skin.
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Verification tool:
The ΔP plot is also a verification tool. It is used to compare the actual data versus the simulated ΔP data from the interpretation model.
This plot should be matched. If not, it is probably explained by an erroneous skin value in the interpretation model, as shown in the figure below.
For more information or for a discussion on this topic, please don’t hesitate to contact us.
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