Performance demonstration of gas-assisted gravity drainage in a heterogeneous reservoir using a 3D scaled model.

Gas-assisted gravity drainage (GAGD) is an effective method for oil recovery. Gravity increases the stability of the Gas-Oil Contact (GOC), thus delaying gas breakthrough and promoting crude oil production. Studying the effects of fluid and reservoir parameters on the stability of GOC could help understand the mechanism of GAGD. In this study, a series of high-pressure GAGD tests were conducted on a 3D heterogeneous scaled model established according to the heterogeneity of the oil reservoir. During the tests, GOC was monitored with electrical resistivity tomography (ERT) to study the effects of gas injection rate, gas type, and gas injection direction on GOC and oil recovery factor (RF). The results showed that N2-GAGD achieved the most stable GOC, the largest sweep volume but a poor RF. CO2-GAGD achieved the best RF of 63.33% at the injection rate of 0.15 m d-1 under 15 MPa. CO2 and CH4 could interact with crude oil and reduce the advancing rate and transverse swept area of GOC. CO2 and CH4 could lead to a higher RF as they reduce the viscosity of crude oil, cause swelling when dissolved, and have low tension. Therefore, the effects of gas dissolution, swelling, and viscosity reduction must be considered in addition to those of gravity, viscous force, and the capillary force so that RF could be increased while ensuring the stability of the displacement front. Accordingly, a new non-dimensional number N new was proposed with comprehensive considerations of gravity, viscous force, capillary force, gas-oil viscosity ratio, the viscosity reduction by gas, and reservoir properties. Finally, a prediction model was proposed, which could accurately predict the RF of heterogeneous reservoirs applying GAGD.

The Vapour-Vapour Interface Observation and Appraisement of a Gas-Condensate/Supercritical CO2 System.

Injecting supercritical CO2 into gas reservoir is a novel trial to improve condensate gas recovery and decrease the hydrocarbon liquid dropout. A good understanding of the effect of supercritical CO2 on the phase behavior properties of these hydrocarbons is essential for accurately forecasting the displacing performance and storing process of the reservoirs with numerical simulators. This paper presents novel phase behavior experimental procedures and phase equilibrium evaluation methodology for gas-condensate phase system mixed with supercritical CO2 over a wide range of temperatures and pressures. A unique phase behavior phenomena was also reported. The mass transfer between two vapour phases was also measured. In order to interpret and identify the interface property between condensate gas and supercritical CO2, a multiphase thermodynamic VLV equilibrium model was established. Finally, taken YKL condensate gas in Northwest China as an example, the region where the conditions in terms of pressure, temperature and CO2 concentration can yield VLV equilibrium was found. The calculation results of multiphase thermodynamic model for condensate-CO2 system in this paper are close to the experimental data and can truthfully reflect the phase behavior of interface between CO2 and condensate gas. The research results indicate that it is the existence of the interface between CO2 and condensate gas that makes CO2 possible be an attractive option to successfully displace condensate gas and decrease CO2 emissions.