Estimation of changes in saturation and pressure from 4D seismic AVO and time-shift analysis

Abstract

A reliable estimate of reservoir pressure and fluid saturation changes from time-lapse seismic data is difficult to obtain. Existing methods generally suffer from leakage between the estimated parameters. We propose a new method using different combinations of time-lapse seismic attributes based on four equations: two expressing changes in prestack AVO attributes (zero-offset and gradient reflectivities), and two expressing poststack time-shifts of compressional and shear waves as functions of production-induced changes in fluid properties. The effect of using different approximations of these equations was tested on a realistic, synthetic reservoir, where seismic data have been simulated during the 30-year lifetime of a water-flooded oil reservoir. Results found the importance of the porosity in the inversion with a clear attenuation of the porosity imprint on the final estimates in case the porosity field or the vertically averaged porosity field is known a priori. The use of a first-order approximation of the gradient reflectivity equation leads to severely biased estimates of changes in saturation and leakage between the two different parameters. Both the bias and the leakage can be reduced, if not eliminated, by including higher-order terms in the description of the gradient, or by replacing the gradient equation with P-and/or S-wave time-shift data. The final estimates are relatively robust to random noise, as they present fairly high accuracy in the presence of white noise with a standard deviation of 15%. The introduction of systematic noise decreases the inversion accuracy more severely.

Volumetric bounds on subsurface fluid substitution using 4D seismic time shifts with an application at Sleipner, North Sea

“…For instance, Landrø (2001) introduced a method for discrimination of pressure and saturation changes from AVO data. Subsequently, Trani et al (2011) refined this method by also including time-shifts in the analysis. For compacting reservoirs, time-shifts are used in order to monitor production progress and related geomechanical issues (e.g.…”

Inversion of Dynamic Properties from 4D Seismic, Ensuring Coherency between Geology, Engineering and Rock Physics.

“…It can be argued that the integration of 4D data is made simpler if interpreted (or inverted) dynamic changes (ΔSw, ΔP, ΔSg) rather than elastic changes are provided to the geologist or engineer. Several inversion schemes exist for the inversion of ΔP and ΔSw (Landro 2001, Meadows et al 2001 with some theoretical examples of the integration of results (Trani et al 2011). Here we use a scheme similar to that of Meadows et al (2001), empirically approximating the PEM as a function of the dynamic parameters only, whilst ensuring that the empirical relationship is a valid approximation (Validation 1 -Figure 1).…”

Time-lapse difference static correction using prestack crosscorrelations: 4D seismic image enhancement case from Ketzin

“…Productionrelated or injection-related processes can cause considerable velocity changes (Hatchell and Bourne, 2005;Fuck et al, 2011;Haugvaldstad et al, 2011), which leave time-shift imprints on time-lapse seismic data that can be very similar to those of near-surface velocity variations. In this context, accurate static correction is an essential prerequisite for quantitative time-lapse interpretation methods that are based on time shifts (Landrø and Stammeijer, 2004;Rickett et al, 2007;Trani et al, 2011;Chadwick et al, 2012;White, 2013).…”