How Fast is Solvent Based Gravity Drainage? (2008)

Abstract A new correlation has been developed to predict bitumen/heavy oil production rates for solvent based gravity drainage. The correlated data set includes 60 individual rate measurements with both published and previously proprietary experimental data. The data set includes 11 crude oils, 4 solvents, permeabilities between 1.5 and 5,400 Darcies, viscosities between 90cP and 800,000 cP, condensing and non-condensing conditions. The oil production rates in the data set span a 30,000 fold range. The correlation only requires the raw bitumen viscosity and a grain (or pore) size parameter and provides a correlation coefficient R2 of 0.974 (R2=1 indicates a perfect correlation). Consequently, the correlation appears to be extremely robust and captures the full functionality of the solvent based gravity drainage extraction mechanism. The functionality of the correlation is consistent with both laboratory observations and mass transfer theory but contradicts and invalidates many of the assumptions of analytical and numerical models. The correlation suggests a common rate limiting step in both N-Solv and VAPEX and consequently, it is expected that this same rate limiting step may also apply to solvent- steam hybrid processes. The correlation resolves the long standing discrepancy between Hele-Shaw and packed bed experiments and provides a useful technique to assess the quality and reliability of experimental data and identify outliers. The correlation also provides an independent test to validate numerical models of solvent extraction. Most importantly, the correlation can help provide a basis to identify and rank extraction processes with target reservoirs. Introduction The Canadian tar sands are an enormous energy resource containing 1.6 to 2.5 trillion barrels of hydrocarbon liquids. More than 90% of the hydrocarbon resource lie in deposits that are too deep to mine economically. Steam Assisted Gravity Drainage (SAGD) is currently the most popular process for in situ extraction. SAGD uses twin horizontal wells in the pay zone, the upper wellbore is used to inject high pressure steam, while condensed water and "melted" bitumen drains via gravity and is collected in the lower wellbore. Gravity drainage is a very important technology advance as it can help avoid the bypassing (short circuiting) problem inherent in steam drive floods. Steam extraction requires heating about 8 kg of tar sand to a very high temperature (200–260 °C) to mobilize 1 kg of bitumen. Consequently, steam production requires combustion of enormous amounts of fuel (perhaps 30% of the heating value of the bitumen) and creates substantial carbon dioxide emissions. The subsequent upgrading of the raw bitumen to synthetic crude oil generates additional carbon dioxide emissions, along with volumetric shrinkage of about 15% due to coke rejection. The gross CO2 emissions just to produce a cubic meter of synthetic oil derived from SAGD bitumen can exceed 1000kg. A challenging issue for SAGD is that of chamber confinement. Virtually every SAGD project is operated at highly over pressured conditions relative to the original reservoir pressure. This enhances the heat transfer by driving steam into the bitumen and helps displace non-condensable gases from the chamber.