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p(t,r) = p_i + \frac{q_t}{4 \pi \sigma} {\rm Ei} \left(-\frac{r^2}{4 \chi t} \right)



LaTeX Math Inline
body{\rm Ei}(z)
exponential integral


Approximations

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Pressure Drop


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\delta p = p_i - p_{wf}(t) = \frac{q_t}{4 \pi \sigma} {\rm Ei} \left(-\frac{r^2}{4 \chi t} \right) 



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\delta p   \sim  \ln t + {\rm const}, \ t \gg \frac{r^2}{4\chi}


Log derivative


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p' = t \frac{d (\delta p)}{dt} = \frac{q_t}{2\pi\sigma} \exp \left( - \frac{r^2}{4\chi t} \right) 



LaTeX Math Block
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p'  \sim \rm const, \ t \gg \frac{r^2}{4\chi}


LaTeX Math Inline
body{\rm Ei}({\rm arg})

Fig. 1. PTA Diagnostic Plot for LSS pressure response for the 0.1 md reservoir in a close line source vicinity (0.1 m), which is about a typical wellbore size.

One can easily see that with wellbore effects neglected even for a very low permeability reservoir the IARF regime is getting formed very early at 0.01 hr (36 s).

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Radial Flow Pressure @model ] [ 1DR pressure diffusion of low-compressibility fluid ] [ Exponential Integral ]

LaTeX Math Inline
body{\rm Ei}(\xi)