Feasibility Study of Gas-EOR Using CO2 and N2 Mixture in a Heavy Oil Reservoir: Experiments and Pilot Test

Introduction Since shallow-buried oil reservoirs of Nanpu Block, Jidong Oil Field, China belong to complex fault-block oil reservoirs, effective flooding networks are difficult to be established compared with other conventional oil reservoirs. Although horizontal wells are widely used in the field to expand drainage area, severe water invasions still happened after years of development [1,2]. Cyclic CO2 injection, also known as CO2 huff-n-puff, was then conducted to enhance oil recovery (EOR) since 2010, and great profits has been brought with 109.5×104 t of oil productions until the year of 2018. As a kind of solvent, CO2 can dissolve with the heavy oil, and then cause oil swelling and viscosity reduction [3, 4, 5, 6, 7]. However, production problems such as wellbore corrosions, gas channeling, etc. still cannot be avoided during the operation of CO2 injection, which then reduce the oil yielding and lower the CO2 utilization. N2 can also be used to enhance oil recovery according to the literatures [8, 9, 10], more importantly, it is not corrosive and usually cheaper than CO2 for EOR applications. Thus, CO2 and N2 mixture is proposed as an alternative gas-EOR technique for those shallow-buried heavy oil reservoirs. In order to figure out the interaction mechanisms between CO2/N2 mixture and heavy oil, a feasibility study is firstly evaluated using a PVT cell in the laboratory. Mixtures with different CO2/N2 molar ratios (1:0, 4:1, 7:3, 1:1, 0:1) are designed, and the high-pressure properties of gas/oil system including saturation pressure, volume factor and viscosity are investigated. Then, a series of cyclic gas injection experiments are conducted using outcrop cores to compare the oil recovery factors of CO2, N2, and CO2/N2 mixtures. A pilot test of CO2/N2 mixture injection is also introduced in this paper.