DST Analysis - Two Boundary Effects

[Yusri Ibrahim]

Hi guys,

I have a quick question regarding DST analysis of boundary effect in Pressure Derivative Plots.

From the late time region of my pressure derivative plot, two boundary effect was apparent which was closed boundary (upward curve) and a constant pressure boundary (downward curve). The constant pressure boundary appears just after the closed boundary so my pressure derivative plot curves upwards then downwards in the late time region.

Here's the thing, both this effect is supported by the geological feature (seal and aquifer support) and the dipping of the reservoir actually accounts for the timing of these two features.

So my question is weather Pressure derivative plot can possibly show two very different boundary effects one after another or is this just another case of ambiguity?

Cheers,
Yusri

[brhm]

Dear Yusri
Is that a build up test?
if so could you please let me know what is the production period you have used before extracting the derivative plot?
what is the production period before the build up during the test and what is the acual production period of the well since first production?

[vibhore]

Dear Yusri,

From the late time region of my pressure derivative plot, two boundary effect was apparent which was closed boundary (upward curve) and a constant pressure boundary (downward curve). The constant pressure boundary appears just after the closed boundary so my pressure derivative plot curves upwards then downwards in the late time region.

About your querry of boundary effect on pressure derivative, if the derivative goes up during late times it shows the presence of a boundary/fault. If the test is for sufficient time for the derivative to stabilize then the derivative stablizes at twice the value that of the radial homogeneous, in case of single fault.
Now for closed boundary case, the derivative and the del P curve both goes up with a slope of +1 and that too only in the drawdown case. This is pseudo steady state case. the reason for the derivative to go up is that since the volume of the closed reservoir is fixed and the pressure transient has seen all the boundaries, so the pressure continues to decline, thats why dP and its derivative increases in drawdown.
And for constant pressure case, the change in pressure, after the transient reaches the constant pressure is negligible so the derivative falls. This is visible in both drawdown and build up.