Monitoring CO2 behaviour during injection into reservoir sandstone

Abstract

Storing CO2 in deep subsurface aquifers is considered to be a good solution for reducing the increasing atmospheric emissions of CO2. To mitigate the possibility of stored CO2 leaking out to the atmosphere, geophysical monitoring techniques are applied. These techniques must be able to detect small and big changes in CO2 saturation. In this thesis acoustic and electrical resistivity measurement will be used to detect and monitor the injection of CO2 into three brine saturated samples. Two sandstones from the Gres des Vosges formation with different orientation relative to bedding and one Berea sandstone were selected. Each sample is in turn placed in a nitrile sleeve with piezo-elements for acoustic and resistance measurements. A hydrostatic pressure vessel is used to simulate reservoir condition by using pumps to apply an effective pressure of 15MPa on the sample and 10MPa pore pressure when saturated. These samples are will undergo several experiment exposing them to different conditions. Each sample undergoes a set series of loading cycles (dry, fully CO2 and full brine saturated) before drainage and imbibition. In drainage CO2 is injected from the top and pushed downwards and in imbibition brine is injected from the bottom. Pressure and temperature are assumed constant during drainage and imbibition. For Gres des Vosges (drilled perpendicular to bedding) two additional drainage and imbibition experiments were done to asset flow rate influence on CO2 injection. Rock physical and pore fluid analysis were used to interpret acoustic velocity and electrical resistivity measurements. Analysis shows CO2 saturation and front movement are affected by injection flow rate, orientation of beddings, permeability and prior injection experiments. Acoustic velocities in axial direction decreases by 7.8%, 7.4% and 4.8% for respectively Gres des Voges drilled perpendicular to bedding, parallel to bedding and Berea. For saturation of CO2 passes 20%, the acoustic velocity has little to no significant changes. Front movement of CO2 can be seen on both acoustic and resistivity measurement. At high saturation of CO2 resistivity can be used to estimate saturation. Saturations found for Gres des Voges in axial direction are 60% for perpendicular to bedding and 49% for parallel to bedding. An increase in flow rate decreases the saturation of CO2 shown constant flow rate from start to finish of 2.5mL/min results in 57% while 0.5mL/min results in 58%. Results from Gres des Voges suggest a continuous usage after first drainage and imbibition alters permeability of the sample.