Thermal Visbreaking of Heavy Oil During Steam Recovery Processes
- W.R. Shu (Mobil R and D Corp.) | K.J. Hartman (Mobil R and D Corp.)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Engineering
- Publication Date
- September 1986
- Document Type
- Journal Paper
- 474 - 482
- 1986. Society of Petroleum Engineers
- 5.5.8 History Matching, 5.1 Reservoir Characterisation, 5.8.5 Oil Sand, Oil Shale, Bitumen, 5.3.2 Multiphase Flow, 5.2.1 Phase Behavior and PVT Measurements, 4.3.3 Aspaltenes, 2.4.3 Sand/Solids Control, 4.1.1 Process Simulation, 4.1.5 Processing Equipment, 4.1.2 Separation and Treating, 5.4.6 Thermal Methods
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Summary. This paper describes a simulation study on the effect of visbreaking on heavy oil recovery during steam injection processes. The kinetic rate constant for in-situ visbreaking was derived from in-house kinetic data and then used in conjunction with a thermal compositional simulator to assess the effect of visbreaking on recovery through a series of numerical experiments. Various steam injection strategies were tested and the effect of visbreaking studied. In some cases, physical heating, not thermal visbreaking, was the dominant recovery mechanism. In other cases, visbreaking had a large effect on recovery. The difference was primarily a result of the placement of the visbroken oil with respect to primarily a result of the placement of the visbroken oil with respect to the direction of flow. In cases where the visbroken oil zone was perpendicular to the flow, it formed a mobility transition zone that perpendicular to the flow, it formed a mobility transition zone that improved sweep, thus enhancing oil recovery.
A major constraint in the production of heavy oil and tar sand bitumens is the high viscosity. Thermal recovery techniques, such as steam flooding, are effective in temporarily lowering the oil viscosity and enhancing its recovery. Field evidence, however, indicates that a permanent reduction in oil viscosity, or visbreaking, often accompanies these processes owing to the cracking of the oil. In one incident, oil as light as 21 deg. API [0.93 g/cm ] was produced from steam stimulation in a reservoir produced from steam stimulation in a reservoir containing 11 deg. API [0. 99-g/cm 3 ] heavy oil. Steam distillation could not account for this phenomenon because the oil had a very small fraction that boiled below the steam temperature. In the laboratory, numerous experiments confirmed that heavy oils experience significant visbreaking at temperatures in the range of 500 to 700 deg.F [260 to 371 deg. C). Thermal visbreaking, a well-known refinery process for upgrading heavy crudes, has been subjected to much study. Few studies, however, were conducted under the relatively mild conditions commonly encountered in insitu steam recoveries. In-situ visbreaking is characterized by mild decomposition, minimum coke formation, and the retention of the products in the liquid phase. Heavy components, such as asphaltenes, are cracked into lighter components that act as an internal solvent, thereby lowering oil viscosity. Little is known of the importance of in-situ visbreaking on heavy oil recovery. To the best of our knowledge, no studies on the subject have been reported in the literature. Shu and Venkatesan reported a kinetic study of in-situ visbreaking. Laboratory visbreaking data for a Cold Lake oil were collected at 500 to 617 deg.F [260 to 325 deg. C] with residence times of up to I month. An algorithm was derived that allowed the calculation of a kinetic rate constant directly from viscosity data. The rate constant was used in the present study as input to the simulator. A compositional thermal process simulator with capability to handle reaction kinetics was used. We investigated the effects of visbreaking in three modes of steam recovery: cyclic steam stimulation, continuous steamflooding, and a steam slug process.
it is instructive to first consider some kinetic aspects of visbreaking and to delineate the rate-influencing parameters. The visbreaking reaction may be represented by the parameters. The visbreaking reaction may be represented by the following first-order reaction.
The extent of visbreaking can be represented by the conversion of the heavy oil.
where CA is the concentration of Component A, and k is represented by the usual Arrhenius expression:
For the Cold Lake oil under study, w= 2.952 x 10 day -1 and E= 31,800 cal/g mol, as determined in the earlier study.
if we substitute Eq. 4 into Eq. 3, we obtain
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