M. Verliac, A. Berzanskis
Keywords: CO2 storage, geophysical monitoring, optical sensors, borehole
Summary:Carbon Capture and Storage (CCS) in underground reservoirs has been identified as a potential solution to partially reduce the CO2 concentration in the atmosphere, and consequently the global warming on Earth. However, injection of tens of millions of tons in depleted hydrocarbon reservoirs or in saline aquifers is not without risks, either for reservoir integrity or for the surrounding geological formations, including fresh aquifers contaminations or induced seismicity. Geophysical monitoring will be mandatory for such projects. Such a monitoring can be done either from surface (surface seismic, soil or surface fluid sampling, ground deformation) or from boreholes (seismic imaging and induced microseismic listening). Deep boreholes, down to the reservoir depth, have the advantage of being closer to the injection point and consequently allowing an early detection of possible anomalies (via acoustic images or microseismicity tracking). Existing borehole geophysical systems are based on geophones (sensors using moving coils generating an electrical signal when being hit by an acoustic wave). They integrate in situ electronical chips and use electrical current. They have been a proven technology for surface acquisition for decades but do not represent a reliable technology for permanent borehole systems mainly due to their weakness when exposed for long periods to high temperatures. Optical technologies, like Distributed Acoustic Sensing (DAS) or Optical Point Sensors (OPS), have been introduced in the recent years to the Oil and Gas Industry for acoustic geophysical applications. DAS systems are slim and well designed for borehole imaging applications but are sensitive to only one of the three directions in space (along the fiber axis). This makes them unsuitable for microseismic challenges when using only one fiber (well). OPS sensors have been used for surface seismic imaging from seafloor but were not designed for high temperature and high pressure of deep boreholes and were, also, too big to fit in such a limited environment. MagiQ has developed an innovative solution, with the support of TotalEnergies, for a combined system, called GeoLite, including slim OPS and embedded DAS optical fibers. The result is a multilevel, multicomponent, high sensitivity, large temperature range (low and high) and large frequency range tool surpassing geophones performances. It has also the advantage of being cheaper and having a longer life expectancy (tens of years) than geophones (years) permanently deployed in deep boreholes, due to the fact that they do not embark downhole electronics. A first laboratory prototype was tested in a Research Well in Houston (TX) in 2020. A fully field proven hybrid tool (GeoLite/DAS) will be tested in real deep borehole conditions during 2023 for both seismic imaging and microseismicity detection. It will be benchmarked with a standard commercial geophone tool deployed in the same environment. If successful, this technology is expected to have a large impact in the near future for numerous CCS projects in the USA and abroad.