International Journal of Advanced Engineering Research and Science (IJAERS) [Vol-7, Issue-8, Aug- 2020] https://dx.doi.org/10.22161/ijaers.78.7 ISSN: 2349-6495(P) | 2456-1908(O) www.ijaers.com Page | 51 Analysis of the effectiveness of the alternating water and gas injection method (WAG) Matheus Andrade de Almeida¹, Alessandra Terezinha Silva Souza², Vitória Felicio Dornelas 3 , Ana Paula Meneguelo 4 ¹Graduate in PetroleumEngineering, Federal Universityof Espírito Santo (UFES), São Mateus - ES, Brazil. ²Energy Master’s Student, Federal Universityof Espírito Santo (UFES), São Mateus - ES, Brazil. ³Energy Master’sStudent, Federal Universityof Espírito Santo (UFES), São Mateus - ES, Brazil. 4 Doctoral degree in Chemical Engineering, Federal University of Santa Catarina (UFSC), Professor of Reservoir Engineering at the Federal University of Espírito Santo (UFES), São Mateus - ES, Brazil. Abstract—Being the main source of primary energy in the world, oil is an increasingly used resource, and at the same time more difficult to be explored and produced. Special recovery methods have been extensively studied to increase the oil recovery. The WAG method consists of the alternating injection of water and gas for the recovery of the residual oil in the reservoirs. The objective of the present work is to simulate the alternating injection process and to evaluate its recovery efficiency when compared to the water injection and CO2 injection method using the MRST software (Matlab Reservoir Toolbox Simulator). In general, an increase of about 4% in oil production was found, when the WAG methods and CO2 methods are compared.The results show that, for three scenarios analyzed, up to 12 years of production the water flooding has the same volumes of recovered oil as the WAG or CO2 mechanisms. After that period, the WAG mechanism resulted in greater volumes of oil produced. Keywords—Carbon dioxide, EOR, Petroleum, Reservoir engineering, WAG, MRST. I. INTRODUCTION The constant evolution of technology, which provides a better quality of life for society, is closely related to an increase in global energy demand. This increase in turn leads to an accelerated pace of high investments in the search for new sources of energy and/or improvement of existing ones. However, according to the IEA (International Energy Agency), fossil fuels account for about 81% of the world's primary energy. Of this total, 54% are still oil and natural gas exploring and production [1] Both the burning of these fuels and the petroleum and gas exploration and production leads to a large amount of CO2 emitted to the atmosphere, which has created a worldwide discussion on what to do with this gas and how to reduce its emission rate. CO2 emissions become a challenge for the exploration and production of pre-salt reservoirs. Petroleum in the pre-salt reservoirs in Brazil has very high gas-oil ratio (GOR) with high content of carbon dioxide[2] According to [3] there are three ways to reduce the accumulation of these gases in the air: increase the efficiency of energy production, i.e. produce a smaller amount of CO2 per unit of energy; use of renewable energies; CO2 capture for use in enhanced oil recovered (CO2-EOR) and geological storage (part of the CCS technology). In the CCS technology the produced gas with high content of CO2 is treated at the production platform or in a subsea CO2 separation process [4]. In the CO2-EOR process the remainder gas with CO2 content of 70% to 80% is reinjected into the reservoirs in the initial age of the reservoirs production. Where it mixes with the oil to swell it and reduce the oil viscosity, making it lighter and detaching it from the rock surfaces. However not all the injected CO2 is produced, as a significant fraction of the CO2 is retained in the reservoir. Therefore, the CO2-EOR process is essentially a closed loop for CO2[5]. Then, to maintain a specified injection ratio of CO2, the recycled CO2 is supplemented with the purchased CO2, contributing to the reduction of CO2 emissions from the atmosphere. However, CO2 has a lower viscosity than