Infinite Acting Radial Flow (IARF)

Time interval of pressure transition when pressure variation $\begin{array}{l}\Delta p\end{array}$ is evolving logarithmically in time:

(1)	$\begin{array}{l}\displaystyle \Delta p = p_{wf}(0) - p_{wf}(t) \sim p_{wf}(0) - p_i - \frac{q_t}{4 \pi \sigma} \, \bigg[ - 2S + \gamma - \ln \bigg( \frac{4 \chi t}{r_w^2} \bigg) \bigg]\end{array}$

and hence the pressure log-derivative stays constant:

(2)	$\begin{array}{l}\displaystyle \Delta p ' = \frac{d \, \Delta p}{d \, \ln t} = \frac{q_t}{4 \pi \sigma} = \rm const\end{array}$

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

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

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


Fig. 2. PTA Diagnostic Plot with IARF regime developed between 100 hrs and 1,000 hrs before PSS	Fig. 3. PTA Diagnostic Plot with IARF regime developed between 100 hrs and 1,000 hrs

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