Onshore Time-Lapse Borehole Seismic Project for CO2 Injection Monitoring

Abstract

An innovative time-lapse borehole seismic project for onshore CO2 injection monitoring was conducted in 2010 for ZeroGen in Australia. The goal of the seismic project was to investigate borehole seismic methods to monitor the effects of supercritical CO2 injection in tight, saline reservoir rocks. Results from processing data acquired before and after injection show that a time-lapse, pseudo-4D VSP approach detected likely changes in subsurface acoustic behaviour. A comprehensive borehole seismic program, consisting of two rig source VSPs, an offset VSP, and four Walkaway VSP surveys, was acquired pre-CO2 Injection in the vertical well and then repeated post-injection. Data were acquired using an eight-shuttle VSI seismic tool down-hole and a vibroseis mini-buggy as source. Data processing focused on achieving a repeatable workflow to ensure that time-lapse effects can be effectively analysed. A near surface basalt layer significantly affected both measured time and recorded signatures. This required special handling of surface statics and of source signature variations. The pre- and post injection survey results were analysed for acoustic changes related to the injection program. A time-lapse change in acoustic response in the injection interval was observed on all available borehole seismic datasets. The processed offset and Walkaway VSP images also provided estimates of lateral extent and orientation of these anomalies. Detection of time lapse changes in such a small injection test requires an investigation into the underlying causes such as fluid and pressure effects and the possibility that such techniques might be used to monitor pressure evolution in similar injection sites.