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Time interval of pressure transition when pressure variation 

LaTeX Math Inline
body\Delta p
 is evolving logarithmically in time:


LaTeX Math Block
anchorYUKTO
alignmentleft
\Delta p = p_{wf}(0) - p_{wf}(t) \sim p_{wf}(0) - p_i - \frac{q_t}{4 \pi \sigma} \, \bigg[ - 2S +  \gamma 

- \ln \bigg(  \frac{4 \chi t}{r_w^2} \bigg) \bigg]

and hence the

...

pressure log-derivative

...

 stays constant:

LaTeX Math Block
anchor9VWMA
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\Delta p ' = \frac{d \, \Delta p}{d \, \ln t}  =  \frac{q_t}{4 \pi \sigma} = \rm const


Since IARF is normally used to define a flow regime during a certain finite period of time, the term "infinite acting" is a misnomer.


This flow regime is native

...

...

 in vertical well, homogeneous reservoir and no boundary (see Fig. 1).


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Fig. 1. PTA Diagnostic Plot with IARF regime developing after 100 hrs











The other popular occurrence of IARF is before PSS (Fig.2) and SS (Fig. 3).

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Fig. 2. PTA Diagnostic Plot with IARF regime developing after 100 hrs until PSS at 1,000 hrsFig. 3. PTA Diagnostic Plot with IARF regime developing after 100 hrs until SS at 1,000 hrs