Time-lapse borehole gravity imaging of CO2 injection and withdrawal in a closed carbonate reef

Three borehole gravity (BHG) surveys were performed in 2013, 2016, and 2018 to monitor the changes in gravity/density as a result of the injection and withdrawal of carbon dioxide (⁠$CO2$⁠) into and out of the Dover 33 carbonate reservoir reef in Northern Michigan. The observed gravity changes and inferred density changes have been modeled to determine the flow and storage zones of the injected $CO2$ in the reef. The high quality and low level of uncertainty of the data collected make them useful for delineating the $CO2$ plume position over time and for identifying the oil sweeping extent and mechanisms in the Dover 33 reef. The time-lapse gravity results indicate the effects of the changing $CO2$ mass within the reservoir, consistent with increasing mass from 2013 to 2016 (following $CO2$ injection) and a decreasing mass from 2016 to 2018 (after $CO2$ withdrawal). Three-dimensional imaging of fluid migrations in the reef has been obtained by coupling the time-lapse BHG results to a 3D porosity and permeability model. This coupled approach allows the evaluation of the volume of the reef affected by the injection of $CO2$ between 2013 and 2016, the efficiency of the oil sweeping between 2016 and 2018, and the location of the residual $CO2$ plume in the reef after 2018.