CO₂ near-miscible flooding for tight oil exploitation

Abstract

Tight oil resources have become increasingly important as massive hydraulic fracturing techniques breakthrough. Water flooding is generally applied to tight oil reservoirs; however, the oil recovery achieved by water flooding is quite low. A CO₂ miscible flooding process is regarded as a primary enhanced oil recovery (EOR) technique for conventional oil reservoirs as CO2 can extract oil even at a high water cut. Furthermore, many CO₂ field trials in low permeability reservoirs have been recorded as successful. As CO₂ utilization efficiency drops when formation permeability goes down, CO₂ injection in a miscible condition for tight oil exploitation may not be as profitable as that in conventional oil reservoirs. In tight formations, there exist small pore throats, even at nanoscale. As the confined space in nanopores may shift a phase envelop and lower CO₂ minimum miscible pressure (MMP), operating a well in a near-miscible region where pressure is slightly less than MMP as measured in the lab may result in a good chance of miscibility for some parts of a tight oil reservoir. In this paper, equations of state (EOS) calculations are conducted in order to see the effects of confinement on a CO₂ injection process in tight oil reservoirs. On the basis of Cardium reservoir properties, numerical reservoir simulations are run to investigate the effects of confinement caused by a small pore throat size in 50 nm and 10nm on the CO₂ injection process. Comparisons of CO₂ near-miscible and miscible processes are made with various pore throat sizes. Results show that confinement effects in tight formations help to lower the bubble point pressure and boost an oil rate during CO2 injection. However, CO₂ EOR efficiency goes down as formation pressure approaches MMP as mearsured in the lab. It is not necessary for CO₂ injection to operate in an above MMP condition in tight formations, where a nanopore size is present. In this way, the volume of CO₂ injected can be reduced. For tight oil reservoirs with a small pore throat size, a CO₂ near-miscible process is more suitable than miscible flooding.