Natural Depletion
Oil Depletion
The EUR during the natural oil depletion can be assessed with the following formula:
(1) | EUR_{ND} = \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.
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} |
Water flooding
Motivation = maintain formation pressure at sweep interface
The Expected Ultimate Recovery during the waterflood sweep can be assessed with the following formula:
(11) | EUR_{WF} = E_S \, E_D + (1-E_S) EUR_{ND} |
where E_D – displacement efficiency, E_S – sweep efficiency.
Sweep efficiency
Water-Oil displacement efficiency
Gas flooding
Motivation = maintain formation pressure at sweep interface with gas in case of high water mobility \frac{k_{rw}}{\mu_w} \gg \frac{k_{ro}}{\mu_o} which makes waterflood inefficient.
WAG flooding
Motivation = maintain formation pressure at sweep interface with alternating inejction of water and gas in case of high residual oil to water sweep is high s_{orw} and gas sweep is less than to water sweep s_{org} < s_{orw}.
(12) | E_D = \frac{1-s_{wi}-s_{org}}{1-s_{wi}} |
Chemical EOR
Motivation = maintain formation pressure at sweep interface with chemical injection and reduce residual oil to EOR sweep s_{or \, eor} < s_{orw}.
(13) | E_D = \frac{1-s_{wi}-s_{ori}}{1-s_{wi}} |
where s_{wi} – inititial water in oil pay, s_{ori} – residual oil to injection sweep.
CО2 injection
See also
Petroleum Industry / Upstream / Production / Field Development Plan