In order to translate pressure from downhole gauge to the Datum one needs to:

1

Estimate gauge readings at formation top $\begin{array}{l}p_{\rm top}\end{array}$ using the wellbore fluid density $\begin{array}{l}\rho_f\end{array}$ gradient

(1)	$\begin{array}{l}\displaystyle p_{\rm top}(t) = p_{\rm gauge}(t) + g \cdot \int_{z_{\rm gauge}}^{z_{\rm top}} \rho_f(z) dz\end{array}$

2

Recalculate the pressure at formation top to the Datum using regional hydrostatic pressure gradient $\begin{array}{l}GP\end{array}$

(2)	$\begin{array}{l}\displaystyle p_{\rm dat}(t) = p_{\rm top}(t) + GP \cdot (z_{\rm dat} - z _{\rm top})\end{array}$

where

$\begin{array}{l}z_{\rm gauge}\end{array}$	TVDss of downhoile gauge
$\begin{array}{l}z_{\rm top}\end{array}$	TVDss of formation top
$\begin{array}{l}p_{\rm gauge}(t)\end{array}$	gauge pressure readings
$\begin{array}{l}p_{\rm top}(t)\end{array}$	estimated wellbore pressure at formation top
$\begin{array}{l}p_{\rm dat}(t)\end{array}$	estimated Datum Pressure
$\begin{array}{l}GP\end{array}$	regional hydrostatic pressure gradient
$\begin{array}{l}\rho_f(z)\end{array}$	wellbore fluid density as function of TVDSS
$\begin{array}{l}g\end{array}$	Standard gravity constant

When wellbore fluid density is fairly constant between the gauge location and formation top then one can simplify the Datum Pressure calculation to:

(3)	$\begin{array}{l}\displaystyle p_{\rm dat}(t) = p_{\rm gauge}(t) + g \cdot \rho_f \cdot (z_{\rm top} - z_{\rm gauge}) + GP \cdot (z_{\rm dat} - z _{\rm top})\end{array}$

In flowing conditions the equations (1) and (3) are both inaccurate as they do not account for pressure losses due to friction.

The equation (1) should be replaced with a proper solver for wellbore pressure profile.

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Estimating Datum Pressure from downhole pressure gauge

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