Natural Oil Depletion
The EUR during the natural oil depletion can be assessed with the following formula:
(1) | {\rm EUR}_{\rm NDR} = \frac{Q_o}{V_o} = \frac{ (p_i - p_{wf}) \, c_t}{(1-s_{wi})\, B_o} = \frac{ (p_i - p_{wf}) }{(1-s_{wi})\, B_o} \, \big( c_r + s_{wi} c_w + (1-s_{wi})c_o \big) |
where p_{wf} is flowing bottom-hole pressure, p_i – initial formation pressure, B_o – formation volume factor for oil, Q_o – cumulative oil production, V_o – STOIIP, s_{wi} – initial water saturation in oil pay.
For the naturally flowing wells the production bottom hole pressure can be assessed as:
(9) | p_{wf} = p_s + \rho_g \, g\, h + \bigg( 1- \frac{\rho_g}{\rho_o} \bigg) \, p_b |
where p_s – tubing-head pressure defined by the production gathering system, h – is the true vertical depth at formation top, \{ \rho_o, \, \rho_g \} – oil and gas densities, p_b – bubble-point pressure.
Natural Gas Depletion
The Expected Ultimate Recovery during the natural gas depletion can be assessed with the following formula:
(10) | EUR_{GD} = \frac{Q_g}{V_g} = 1- \frac{p_{wf}}{p_i} |
See also
Petroleum Industry / Upstream / Production / Field Development Plan / Recovery Methods