AAPG Bulletin; February 2008; v. 92; no. 2;
p. 181-200; DOI: 10.1306/09210706140
© 2008 American Association of Petroleum Geologists (AAPG)
New insights into the volume and pressure changes during the thermal cracking of oil to gas in reservoirs: Implications for the in-situ accumulation of gas cracked from oils
Hui Tian1,
Xianming Xiao2,
Ronald W. T. Wilkins3 and
Yongchun Tang4
1 State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, People's Republic of China; tianhui{at}gig.ac.cn
2 State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, People's Republic of China; xmxiao{at}gig.ac.cn
3 Commonwealth Scientific and Industrial Research Organisation Petroleum, P. O. Box 136, North Ryde, New South Wales, Australia; ronwtwilkins{at}hotmail.com
4 Power, Environmental, and Energy Research Center, California Institute of Technology, 738 Arrow Grand Circle, Covina, California 91722; tang{at}peer.caltech.edu
Tian is an assistant researcher in the State Key Laboratory of Organic Geochemistry, where he received his Ph.D. in geochemistry in 2006. He earned his B.S. and M.S. degrees in petroleum geology from the University of Petroleum, China, in 2000 and 2003, respectively. His recent and current research focuses on the formation and preservation of gases in highly matured marine basins.
Xiao is a senior researcher at the State Key Laboratory of Organic Geochemistry, Chinese Academy of Sciences. He obtained his Ph.D. in geology from the Chinese University of Mining and Technology in 1989. His current research focuses on the kinetics of natural gas generation and accumulation.
Wilkins was formerly a chief research scientist with Commonwealth Scientific and Industrial Research Organisation Petroleum, where he worked on fluid inclusions, laser instrumentation, and novel methods for maturity determination based on fluorescence of organic matter.
Tang is currently the director for the Power Environmental Energy Research Center at the California Institute of Technology. Tang has published more than 80 articles in the field of geochemistry, chemistry, and petroleum engineering. His major research interests are applying molecular modeling and experimental simulation techniques to energy-related problems.
Previous pressure-volume calculations during oil cracking to gas, based on the conventional model that presupposes oil cracking to be completed by approximately 150°C, underestimate the potential for gas accumulation in petroleum reservoirs. In this article, a compositional kinetic model of gas generation from oil cracking is suggested based on pyrolysis data using sealed gold tubes, and the pressure-volume changes are recalculated based on the new kinetic model under various geological conditions. The kinetic modeling of oil cracking confirms that crude oil begins cracking at about 160°C for a heating rate of 2°C/m.y., and that the oil-cracking process has two distinct stages with significant differences in gas composition. The first stage is characterized by dominant C2–5 wet gases, whereas the second is characterized by the recracking of C2–5 wet gases to methane and pyrobitumen, leading to a progressive increasing dryness of the gas. The pressure-volume-temperature simulations of oil cracking to gas show that initial oil saturation, temperature-pressure gradients, and openness of reservoirs are important geological factors that control gas accumulation in original petroleum reservoirs. For a reservoir that is geologically open and saturated with 100% oil, gas spills out of the trap at 196°C. The gas loss at 240°C is almost 50% of the total gas, far lower than the 75% based on the conventional model of oil destruction. With lower oil saturation, the gas loss decreases because the gas-water contact can shift downward, and the gas loss occurs mainly by solution. For effectively isolated reservoirs, oil cracking readily exceeds lithostatic pressure, leading to reservoir fracturing, which becomes more obvious when oil saturation decreases. The calculated fracturing temperatures for 100 and 50 vol.% oil saturations correspond to oil destructions of 95% and 86.4%, greatly exceeding the value of 1% as suggested by previous studies. A conceptual model of gas accumulation and loss in isolated and open geological conditions for a reservoir with 50% oil saturation is suggested. On the basis of this model, the Triassic carbonate gas pool in northeastern Sichuan Basin was discussed as a typical example for in-situ accumulation of gas cracked from reservoired oils. The present model infers that the reservoired oils were completely cracked into gas at 87.6 Ma, and that 75–85% of the gas has been preserved in the original reservoir rocks to form the in-situ gas pools with a huge amount of gas resources. We believe that gas accumulation from oil cracking in original petroleum reservoirs is much more prospective than previously thought, and that gas cracked from oil has great potential in other areas of the Sichuan Basin and the eastern Tarim Basin.
Copyright © 2008 by American Association of Petroleum Geologists (AAPG)
