# Gas Cap Ratio

on . Posted in Reservoir Engineering

Gas cap ratio,  a dimensionless number, in a reservoir is the ratio of the volume of free gas in the gas cap to the volume of oil in the oil zone within a petroleum reservoir.  It is a parameter in reservoir engineering as it helps to determine the overall recovery strategy and production plan.  The gas cap acts as a natural pressure maintenance mechanism for the reservoir.  As oil is produced and pressure drops, the gas in the gas cap expands and helps to maintain reservoir pressure, which can enhance oil recovery.

### Gas Cap Ratio formula

STB (stock tank barrel) is the same volume defined at some surface standard conditions in a stock tank, which are usually 60°F and 14.7 psi.

$$n_i \;=\; G_c \; G_f \;/\; OIP \; B_o$$     (Gas Cap Ratio)

$$G_c \;=\; n_i \; OIP \; B_o \;/\; G_f$$

$$G_f \;=\; n_i \; OIP \; B_o \;/\; G_c$$

$$OIP \;=\; G_c \; G_f \;/\; n_i \; B_o$$

$$B_o \;=\; G_c \; G_f \;/\; n_i \; OIP$$

Symbol English Metric
$$n_i$$ = Gas Cap Ratio $$dimensionless$$ -
$$G_c$$  = Initial Gas Cap Volume $$ft^3 \;/\; bbl$$ -
$$G_f$$ = Initial Gas Formation Volume Factor $$ft^3 \;/\; bbl$$ -
$$OIP$$ = Initial Oil in Place $$bbl$$ -
$$B_o$$ = Initial Oil Formation Volume Factor $$bbl \;/\; STB$$ -

Tags: Reservoir