The location of a downhole gauge may not match the formation top.
In order to translate it to the Datum one needs:
- Estimate gauge readings at formation top
p_{\rm top} = p(z_{\rm top}) using the wellbore fluid density
\rho_f gradient:
\displaystyle
p_{\rm top}(t) = p_{z_{\rm gauge}}(t) + g \cdot \int_{z_{\rm gauge}}^{z_{\rm top}} \rho_f(z) dz
- Recalculate the pressure at formation top to the Datum using regional hydrostatic pressure gradient GP: \displaystyle p_{\rm dat}(t) = p_{\rm top}(t) + GP \cdot (z_{\rm dat} - z _{\rm top})
where
z_{\rm gauge} | TVDss of downhoile gauge |
z_{\rm top} | TVDss of formation top |
p_{\rm gauge}(t) | gauge pressure readings |
p_{\rm top}(t) | estimated wellbore pressure at formation top |
p_{\rm dat}(t) | estimated Datum Pressure |
GP | regional hydrostatic pressure gradient |
\rho_f(z) | wellbore fluid density as function of TVDSS |
g | Standard gravity constant |
When wellbore fluid density is fairly condant between the gauyge location and formation top then ine can simplify the datum pressure calculation to:
(1) | p_{\rm dat}(t) = p_{z_{\rm gauge}}(t) + g \cdot \rho_f \cdot (z_{\rm top} - z_{\rm gauge}) + GP \cdot (z_{\rm dat} - z _{\rm top}) |
See Also
Petroleum Industry / Upstream / Subsurface E&P Disciplines / Pressure Testing & Production Analysis