ESTIMATION OF WATER/GAS SATURATION AND EFFECTIVE PRESSURE FROM SEISMIC VELOCITIES AND QUALITY FACTORS AS FUNCTIONS OF FREQUENCY: PREDICTIONS OF AN EMPIRICAL MODEL

摘要

The two main reservoir parameters that change during hydrocarbon production are water saturation (S_w) and effective pressure (P_c), The predictions of an empirical model to estimate S_w and P_e. from seismic quality factors and velocities in partially saturated water-gas systems are investigated. The predictions include the following. Compressional velocity (V_p) is insensitive to frequency below～100 Hz. Shear velocity (V_S) is insensitive to frequency but sensitive to S_w suggesting that high frequency lab V_S measurements can be used for modeling at seismic frequencies. The P-wave quality factor (Q_P) shows diagnostic behavior as a function of S_w for frequencies greater than 500 Hz and S_w > 50%. At low frequencies (< 1 KHz) and S_w < 85%, Q_S~(-1) decreases with S_w; at frequencies > 1 KHz, Q_S~(-1) increases with S_w. The Q_S/Q_P ratio is predicted to be independent of frequency for water saturation greater than 90%. The dependence of predicted V_P and V_S on P_e is non-linear at all frequencies (30 Hz - 5 KHz). Q_S~(-1) is maximum at full saturation whereas Q_P~(-1) is maximum at partial saturation (～80%). V_P, V_S, Q_P and Q_S are relatively insensitive to P_e at P_e > 10 Mpa. Q_S/Q_P is independent of P_e but shows nearly linear dependence on S_W at S_W < 70%. The predicted Q_S/Q_P values are shifted downward and the V_P/V_S vs. S_w curves are flatter for tight sandstones, compared to porous sandstones. V_P/V_S and Q_S/Q_P ratios together are potentially diagnostic of P_e and S_w at frequencies from 30 Hz to 5 KHz. The results of this paper are empirical predictions that are testable.