Enhancements to Landro's method for separating time‐lapse pressure and saturation changes

Abstract

Landro’s (1999) method is an inversion technique that solves for time-lapse pressure and saturation changes from seismic amplitude-versus-offset (AVO) data. The method relies on the fact that pressure changes affect the slope and intercept of the AVO curve differently than do saturation changes. Specific assumptions are made about the range of validity of various parameters and the behavior of the Pand S-velocity and density as functions of pressure and saturation. We present two modifications of Landro’s method that are valid under different, but less restrictive, conditions, and we test these modifications using synthetic seismic data from a North Sea reservoir model. In the first modification, P- and S-wave impedances are used instead of the slope and intercept, while in the second modification, the P-velocity is assumed to vary quadratically, instead of linearly, with saturation. The first modified method yields results similar to Landro’s original method when the slope is calculated exactly. Inversion results from the second modification yield more accurate saturation values than those estimated by Landro’s method. In all cases, however, pressure changes “leak” onto the saturation map, and pressure decreases are not accurately determined, a result that can be explained by the poor sensitivity of P- and Swave velocities to decreases in pore pressure.