Time‐lapse PS‐wave time‐shifts and quantitative pressure‐saturation discrimination applications

Abstract

We show that time-lapse (4D) time-shifts from PS-wave data are valuable as they are very sensitive to pressure changes, whereas 4D time-shifts from PP-wave data are sensitive to both pressure and saturation changes. This gives another opportunity from using 4D amplitudes alone to understand and discriminate areas of pressure changes from areas of saturation changes in field data. 4D PS time-shifts sense transmission related changes taking into consideration propagation through the altered medium, whereas amplitude changes are reflection or backscattered signals from the boundary. Therefore, these two attributes will be complimentary in 4D interpretation. Rock-physics-based time-shift cross-plots are generated to analyse how pressure and saturation changes in a reservoir impact PP-and PS-wave time-shifts. The developments of 4D time-shifts are shown fixing the reservoir properties and varying reservoir layer thickness. Synthetic seismic data from a wedge is modelled to understand differences between 4D amplitude and time-shift changes and the value of PS time-shifts. Cases with pressure changes only and saturation changes only are also analysed with the wedge model. The method is next applied to two 4D field data sets. One is in the North Sea and the other offshore Brazil. In these field data sets, with different levels of noise in the 4D data, we see interpretable results from PS-wave time-shifts that can help in 4D signal understanding and improve reservoir characterization and monitoring practices. We believe that with modern acquisition and processing, this technology is realizable and provides quantitative and accurate 4D interpretations. It is shown that the use of PS-wave data can lead to a larger chance of success in placing development wells.

Quantitative pressure and saturation engineering values from 4D PP and PS seismic data

“…Estimation of pressure and saturation changes from 4D Pand S-impedances has been previously demonstrated using pre-stack PP seismic data (Landrø, 1999;Tura & Lumley, 1998, 1999 and using PP and PS seismic data (Landrø et al, 2003;Tura et al, 2022). In addition, complimentary to this work, Tura et al (2023) covered the use of 4D time-shifts for pressure and saturation estimation. Here, we introduce a new method to transform time-lapse (4D) P-and S-impedance changes to seismic pressure and saturation change axes.…”

“…The acquisition and processing of the 4D seismic data for PP and PS waves was conducted by the field operator (see Held, 2023). 4D joint PP/PS inversion was applied to Baseline and Monitor 1 data and published by Daneshvar (2020), Dhelie et al (2021) and Tura et al (2022).…”

“…After extensive testing, we also allowed equal weighting of PP and PS seismic data in the joint inversion results. Details of the inversion steps and test results of the parameters discussed above can be found in Held (2023).…”

“…Figure 11 shows a cross-section and fieldwide map view of the negative values (average amplitude between green horizons). Once again, taking only negative values allows the removal of side-lobes (positive values), making the calculation and displays more accurate (see, e.g., Held, 2023). Moreover, for this case, map view attributes are calculated in the shallow section window.…”