Ratio of cumulative oil production $\begin{array}{l}Q^{\uparrow}_O\end{array}$ to STOIIP $\begin{array}{l}V_{\rm STOIIP}\end{array}$ :

(1)	$\begin{array}{l}\displaystyle {\rm RFO} = \frac{Q^{\uparrow}_O}{V_{\rm STOIIP}}\end{array}$

Proxy Model NDR

For waterless period of production:

(2)	$\begin{array}{l}\displaystyle {\rm RFO} = \frac{c_t \cdot (p_i - p_e)}{1-s_{wi}}\end{array}$

where

$\begin{array}{l}E_S\end{array}$	sweep efficiency
$\begin{array}{l}E_{Dow}\end{array}$	water-oil displacement efficiency
$\begin{array}{l}p_i\end{array}$	initial formation pressure
$\begin{array}{l}p_e\end{array}$	current formation pressure
$\begin{array}{l}s_{wi}\end{array}$	initial water saturation
$\begin{array}{l}c_t = c_r + s_{wi} c_w + (1-s_{wi}) c_o\end{array}$	total compressibility
$\begin{array}{l}c_r\end{array}$	pore compressibility
$\begin{array}{l}c_w\end{array}$	water compressibility
$\begin{array}{l}c_o\end{array}$	oil compressibility

Proxy Model WF

For isotropic homogenous water propagation and pressure maintenance at initial formation pressure:

(3)	$\begin{array}{l}\displaystyle {\rm RFO} = E_S \cdot E_D\end{array}$

where

$\begin{array}{l}E_S\end{array}$	sweep efficiency
$\begin{array}{l}\displaystyle E_{Dow} = \frac{1-s_w}{1-s_{wi}-s_{orw}}\end{array}$	water-oil displacement efficiency
$\begin{array}{l}s_w\end{array}$	current formation water saturation
$\begin{array}{l}s_{wi}\end{array}$	initial water saturation
$\begin{array}{l}s_{orw}\end{array}$	residual oil saturation to water sweep