BRINE COMPATIBILITY FOR IMPROVED WATER ALTERNATING GAS (CO2 -WAG) FLOODING IN LOW PERMEABLE OIL RESERVOIRS

High capillary forces present in tight formation resulting in trapped oil in very narrow pore channels causes low performance in waterflooding. Continuous gas injection (CGI) and Water Alternating Gas-CO₂(WAG-CO₂) are technologies which propose to utilize the CO₂ as a working fluid, not only to increase the oil recovery ratio but also to sequester greenhouse gas. CGI improves microscopic displacement efficiency but factors such as gravity segregation and viscous fingering of CO₂ in high permeable zones significantly bypass trapped oil and keeps performance of CO₂-geological storage and volumetric sweep efficiency low. WAG-CO₂ increases oil recovery to about 5% more than CGI efficiently. Alternating gas with water increases macroscopic sweep efficiency through mobility control and also builds up reservoir pressure to increase miscibility of injected gas and reservoir oil. The success of WAG however, depends on how well waterflooding can do in such a formation.

Despite the use of waterflooding, there has been a number of failures of which the case study (Farnsworth Unit, FWU-Texas) of this research is an example. FWU has two sections (East and West). The west performed on waterflooding (hence its success with CO₂-WAG) whiles the East section failed. We suspect that injectivity issues, brine compatibility and engineering issues (injector – producer pattern arrangement) could be the results of failure.

In this work, geological and petrophysical data is analyzed. We also performed experimental studies considering testing various salinities for selecting the most compatible injection fluid. We carried out core-flooding experiments in elevated pressure and temperature to mimic reservoir conditions. Zeta potential approach and Methylene Blue Test (MBT) are conducted to verify swelling potentials of the reservoir rock. Production and injection field data analysis indicates injectivity issues. Geological and mineralogical composition of the field present significant contents of clay minerals. Clays in high contents are kaolinite and chlorite which are evident in preliminary analysis of experimental results as susceptible to swelling and migration. Establishing the causes of failure and engineering an injection water composition greatly improves the success of CO₂-WAG operations.