Oil Material Balance

Brief

The general form of the Oil Material Balance equation was first published by Schilthuis in 1941.

Math & Physics

Equating all underground withdrawals to the sum of the volume changes:

$N_p * B_o + N_p * (R_p - R_s) * B_g$

Np * Bo + Np * (Rp - Rs) * Bg = N * (Bo - Boi) + N * (Rsi - Rs) * Bg + m * N * Boi * (Bg / Bgi - 1) + (Pi - Pres)* N * (1 + m) * Boi * (cw * Swc + cf) / (1 - Swc) - (Wp * Bw - Wi * Bw - Ggi * Bgi - We * Bw)

Pres = Pi - (Np * Bo + Np * (Rp - Rs) * Bg + (Wp * Bw - Wi * Bw - Ggi * Bgi - We * Bw) - (N * (Bo - Boi) + N * (Rsi - Rs) * Bg + m * N * Boi * (Bg / Bgi - 1))) * (1 - Swc) / (N * (1 + m) * Boi * (cw * Swc + cf))

$GIIP \times B_{gi}= (GIIP - G_p) B_g$

See also

P/Z plot

Nomenclature

$B_g$ = gas formation volume factor, ft³/scf

$B_{gi}$ = initial gas formation volume factor, ft³/scf

$GIIP$ = gas initially in place, scf

$G_p$ = cumulative gas produced, scf