Monitoring in Situ Upgrading of Heavy Oil and Tar Sands under Simulated Thermal Recovery Conditions
In situ upgrading of oil sands is a process that requires several parameters to be assessed remotely, including temperature, degree of conversion and sources and mobility of hydrogen, among others. Monitoring the conditions achieved in the underground reactor and the different sources of hydrogen is key to optimize upgrading strategies.
In this study, we assess molecular and isotopic proxies to monitor upgrading reactions and sources of hydrogen during processes of in situ upgrading of oil sands. One of the main aims is to determine whether hydrogen from water, hydrogen gas, or hydrogen from other pools in the reservoir is being transferred to target molecules in the bitumen during in situ upgrading procedures. With this purpose, heavy oils and oil sands samples were pyrolyzed under hydrous conditions at up to 350°C for time periods of up to three days using waters of known δD, enriched with respect to the bitumen, in order to trace hydrogen movement. Pyrolysates were analyzed for molecular composition and hydrogen isotopes ratios (2H/1H).
Main results demonstrate that oil sands bitumens that are characterized by a lack of n-alkane and isoprenoid hydrocarbons, generate a relatively uniform pattern of n-alkanes up to C30 under simulated thermal recovery conditions. More degraded tar sand bitumens produce higher concentrations of n-alkanes, suggesting that the asphaltene fraction is the main source of the newly formed compounds. Bulk composition (SARA) changes with increasing pyrolysis duration support this interpretation. Biomarkers such as terpanes are resistant to the experimental conditions, while monoaromatic and triaromatic steroids are destroyed. Molecular maturity parameters based on terpane distributions, as well as methyl phenanthrenes and methyl anthracenes formed during pyrolysis, appear to be good proxies of the thermal conversion achieved.
δD of bulk bitumen increases with increasing pyrolysis duration for periods up to 24 hours when it seems to reach constant values, suggesting being a consequence of pure exchange. Based on these results, the addition of major amounts of water hydrogen to the n-alkanes does not seem to play an important role in n-alkane generation at 350°C in the presence of pure water. Isotopic analyses to different fractions of the oil are being conducted to determine the possible preferential exchange or transfer of hydrogen from water to specific oil fractions with more acidic hydrogen positions.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009