2-D Finite Difference Modelling and Analysis of Shallow Gas Leakage Scenarios, Using Time-Lapse Refraction

Abstract

4D seismic is a relatively new technology used aggressively in the oil industry, over the years it has proved its potential. However, there is a need for development of new techniques with high precision that enable the record of subtle changes in seismic properties, where conventional monitoring methods have not succeeded. Experience shows that refraction time-lapse seismic might be a solution. By measuring timeshifts, between base and monitor surveys, on first arrival head-waves, the method aims the estimation of reservoir velocity changes at a much lower cost than conventional 4D seismic monitoring. Preliminary results show timeshifts in the order of $1ms$ to $10ms$, caused by variation in the reservoir P-wave velocity due hydrocarbon flow. Furthermore, the method offers an accurate estimation of lateral extension of the velocity anomalies, even when the extension is smaller than hundred meters. Limitations of this method are related with the existence of positive velocity contrast in the reservoir layer, existence of long offset data and repeatability of the seismic. Further investigation is needed, in order to identify crucial bottlenecks that are likely to meet for a real data case.