Rock structural changes monitored by fibre Bragg Grating sensors and Nuclear magnetic Resonance during static and dynamic carbonated brine core flooding experiments

Abstract

One proposed solution to reduce greenhouse gas emissions is the capture and storage of carbon dioxide (CCS) in geological formations such as depleted oil and gas reservoirs. Injected carbon dioxide (CO2) forms carbonic acid once dissolved in the formation water, which can lead to dissolution of certain types of rock minerals. This may weaken rock geomechanical properties that can jeopardize the safety of long-term storage. In this work, the use of Fibre Bragg Grating (FBG) sensors associated with Nuclear Magnetic Resonance (NMR) was investigated to measure the change in rock strain during core flooding experiments. Optical fibres were glued onto two synthetically calcite cemented sandstone rock samples (called CIPS). The samples were saturated with dead brine followed by live brine.The FBG sensors accurately monitored a change of approximately −30 µstrain during dead brine migration at a fast rate within 10–20 min, and then the FBG strain became constant when the sample reached 100% saturation. Exposing the CIPS to live brine induced up to −1000 µstrain in a dynamic injection exercise and – 250 µstrain in the static condition, these changes occurred after 5 h exposure, which was interpreted as the result of fluid-rock interactions occurred within the sample. Those changes continued to increase over the next 35 h, albeit at lower rate. The NMR T2, spatial T2 and FBG data confirmed structural changes within the rock samples. X-ray Computer Tomography (CT) imaging also supported the structural change at certain locations in the sample while featuring good agreement with FBG results.Overall, FBG sensing technology associated with NMR can accurately measure the changes in rock strain during core flooding experiments, while allowing it to monitor the reservoir’s geo-mechanical strength which is a key factor to ensure the long-term safety of CO2 storage.