FLUID FLOW IN A FLUVIAL FORMATION REVEALED BY CONTINUAL MONITORING OF FLUID CHEMISTRY DURING INJECTION OF CARBON DIOXIDE

At Cranfield, Mississippi, U.S.A., large-scale carbon dioxide (CO₂) injection through an injection well (~3,080 m deep) was closely monitored by a series of downhole tools in two observation wells. The injection to the Lower Tuscaloosa Formation injection zone, which consists of amalgamated fluvial point-bar and channel-fill deposits, presented a great opportunity to study the control of sedimentary architecture on fluid flow. U-tube samplers were installed in two observation wells for frequent fluid sampling and provided valuable insight for fluid flow in the CO₂ injection zone. Continual fluid sampling was carried out during the first month of CO₂ injection in December 2009. Two subsequent tracer tests (April and May 2010) using sulfur hexafluoride (SF₆) and krypton as vapor-phase tracers were conducted at different injection rates to measure injectate travel time and tracer dilution. The monitoring results show considerable heterogeneity of fluid flow between the wells (the observation wells are 68m and 112 m east to the injection well, respectively). It is found that the injector was connected to the observation wells through different flow pathways. Gas compositions after breakthrough suggest that multiple injectate fronts arrived at the observation wells through different preferential pathways at different times. The arrival of the flow fronts were marked by temporary increase of concentrations of methane and tracers. The change of gas flow velocity along different flow paths was not proportional to the change of injection rate. The flow pathways evolved with time. After five months of flooding, CO₂ travel time between wells slowed, suggesting increase of CO₂ saturation in and around flow paths or development of new flow paths. Reservoir flow simulation was performed to match the gas composition evolution. Modeling reproduces the fluctuations in CH₄ and CO₂ concentrations after the arrival of the injectate. It shows that CH₄ degasses from brine and is enriched along the contact of water-gas contact. Multiple flow paths bring methane-rich gas at the front of the flow into the observation wells at different times.