Seismic Cross-hole Surveying with Conventional Seismic and Distributed Acoustic Sensing (DAS) at the Svelvik Test-site

Abstract

A seismic cross-hole field experiment to monitor a CO2 injection using conventional seismic sources and receivers, Distributed Acoustic Sensing (DAS) and Distributed Temperature Sensing (DTS) using fiber-optic cables was carried out in September 2021 at the SINTEF CO2 field lab in Svelvik, Norway. At the site an injection well and four observation wells down to a depth of approximately 100 meters are installed. The wells are equipped with linear and helically-wound (HWC) DAS cables. The main aims of the experiment were to test seismic imaging capabilities with a source triplet (P-, SH- and SV-wave) using a newly developed SV-source, test the capabilities of DAS for cross-hole surveys using different types of cable, and investigate the feasibility of using full waveform inversion (FWI) for CO2 injection monitoring. The seismic experiment monitored the propagation of CO2 (gas) during a six day injection period using high-resolution P-wave tomography with hydrophone strings and the DAS system. Additionally, S-wave tomography using SH- and SV- sources was carried out using a multi-station borehole acquisition system consisting of eight triaxial geophones at two meter separation and the DAS system. A baseline survey was carried out before the experiment started and P-wave measurements were repeated on a daily basis whereas the more time-consuming S-wave surveys were carried out only at injection level depth and within the upper aquifer at later injection stages. First results from conventional P-wave tomography indicate a horizontally layered sedimentation with alternating high and low velocity zones, i.e. lower or higher permeable sediments. A comparison of two P-wave tomographic images between the baseline data and data acquired at injection day 4 shows the migration of CO2 along higher permeable zones within the upper aquifer. We also apply seismic FWI techniques to the hydrophone data. The DTS measurements appear to indicate CO2 migration that is consistent with that shown by the seismic data. A comparative analysis of conventional seismic data and DAS data was carried out to evaluate the potential of using DAS systems for P- and S-wave seismic tomography surveying. For the first time, we compute a P-wave tomogram from the DAS data showing similar results to the conventional instruments, with data from the HWC providing the best data. This confirms the utility of a HWC configuration in cross-hole P-wave surveys where wave incidence angles are close to broadside to the cable. Initial results indicate that a joint application of conventional seismic and DAS significantly increases the data interpretability in such a CO2 imaging experiment and for short-range cross-hole surveys in general because DAS provides repeatable measurements with dense spatial sampling over the full wellbore. Further work is required to optimize the cable type and deployment and improve the processing of DAS data for this application. DTS can also provide complimentary insights into the CO2 migration.