HEAVY OIL Kinetics of thermal visbreaking of a Cold Lake bitumen W.R. SHU and V.N. VENKATESAN Mobil R&D Corp. Dallas, Texas, U.S.A. ABSTRACT This paper presents a study on the kinetics of visbreaking under conditions commonly encountered in thermal recovery processes. Batch visbreaking of a Cold Lake bitumen was con-ducted at 260 -325'C with residence times of up to one month. A simple phenomenological model was developedfor the visbreaking kinetics, wherein the oil was first cracked to a less viscous species and the latter then imposed a solvent effect on the former. The Cold Lake data were used to validate the model. A comparison with some literature data on an Atha-basca bitumen was also made. The first order kinetic constant of visbreakingfor Cold Lake bitumen has an activation energy of 31 Kcallg-mote. The kinetic data may be used to assess the importance of visbreaking in a thermal recovery process. It also supplements visbreaking data at more severe conditions requiredfor surface upgrading of heavy crudes and bitumens. Introduction High viscosity is a major constraint in the production and pro-cessing of heavy oils and oil sand bitumens. Thermal recovery processes such as steamflooding or in-situ combustion are effective in temporarily lowering the oil viscosity and enhanc-ing its production. Field evidence, however, indicates that a permanent viscosity reduction, or visbreaking, also accom-panies these processes which can result in partial upgrading of the heavy crude. In-situ visbreaking is characterized by mild decomposition, minimum coke formation, and the retention of products in the liquid phase.In-situ visbreaking of heavy oils has been the subject of several earlier studies. For example, McNab et al.(1) studied the visbreaking of Athabasca bitumen and associated the observed chemical changes to its origin and evolution. Erdman and Dickie(2) made a similar study on asphaltic crudes. Henderson and Weber(3) studied the visbreaking of several heavy oils and obtained time-temperature relationship for the physical up-grading of each oil. Hayashitani et al. (4) reported an extensive study on the thermal cracking of Athabasca bitumen. All of their data except two points, however, were taken in the temperature range of 344 -452'C, beyond the mild condition studied here for in-situ visbreaking.The present work reports the results of our systematic in-vestigation on the visbreaking of a Cold Lake bitumen in the temperature range of 260 -325'C for up to 720 hours. The ob-Keywords: Heavy oil, Visbreaking, Cold Lake, Athabasca bitumen, Kinetics, Thermal recovery, Viscosity. Paper reviewed and accepted for publication by the Editorial _ 60 jective was to provide kinetic data for visbreaking as input to reservoir simulators. A simple phenomenological model for visbreaking was developed to derive kinetic rate constants from the Cold Lake data. The model was also applied to Henderson and Weber data(3) on Athabasca bitumen to fur-ther validate the model. Experimental The Cold Lake bitumen sample was obtained from the...
SPE Members Abstract A field test of wet in-situ oxygen combustion has been carried out in the Esperson Dome field in southeast Texas. The objectives of the pilot test were to evaluate the potential of oxygen combustion technology, to assess its advantages over air fireflooding, and to gain experience in the safe handling and downhole injection of high purity oxygen in an oil field environment. The Miocene sandstone chosen for the pilot is relatively deep (2700 feet), thin (20 feet), and had been substantially watered-out by a strong natural water drive. At the beginning of the combustion pilot an estimated 850 bbl/acre-ft (36% saturation) of 90 cp (21 degrees pilot an estimated 850 bbl/acre-ft (36% saturation) of 90 cp (21 degrees API) oil remained in place. The natural water drive dominated the combustion process and assisted in displacing the oil. At project termination, 200 MMscf of oxygen and 55 MMscf of nitrogen had been injected, 90,000 barrels of oil recovered, four existing older wells failed, four new wells drilled, and three wells burned out. Determination of incremental oil is uncertain due to these operational changes. The objectives of the project were met, and improvements in the project design and operation are expected to lead to an economic project design and operation are expected to lead to an economic process for further applications. process for further applications Introduction In-situ combustion has been used since the early 1940's to thermally stimulate oil reservoirs. Traditionally, compressed air has been injected into the formation at a flux great enough to drive a combustion front across the reservoir. Gates and Ramey reported results of the first successful field test of air fireflooding in South Belridge in the mid 1950's. A review, of all reported field tests is given by Chu. The oxygen-driven in-situ combustion process is considered to be more efficient than air, due to the absence of nitrogen. Advantages include:a lower gas flux mitigates sanding, erosion, and pumping problems at the production well and allows a higher effective pumping problems at the production well and allows a higher effective permeability to oil in the reservoir;a higher CO2 concentration permeability to oil in the reservoir;a higher CO2 concentration in the flue gas allows more CO2 dissolution in the oil thereby reducing its viscosity; anda higher net oxygen injection rate will result in an accelerated oil recovery and possibly permit a wider well spacing. The process can be applied to reservoirs where combustion would not be sustained with air. In the wet combustion mode, higher water injection rates are possible, thereby improving the sweep efficiency. With recent developments in cryogenic technology, oxygen prices are competitive with air compression costs and may be lower at injection pressures higher than 1000 psi. This emerging technology is being applied to recover heavy and medium gravity oils and shows promise. For example, the process has been applied to heavy oils in a secondary mode and to medium and light oils in a tertiary mode. To test the viability of this technology, Mobil designed and implemented a pilot test in the Esperson Dome field. This inverted seven-spot pilot was designed in 1983 based on laboratory and simulation results. The injector was a new well drilled low in the structure, with existing older wells as producers. The location of the injection well took advantage of the water influx to constitute a wet in-situ oxygen fireflood. This paper describes the design and performance of the pilot test. The laboratory efforts associated with the pilot design are briefly presented. presented. THE WET IN-SITU OXYGEN COMBUSTION PROCESS The oxygen combustion process with its associated zones and temperatures is shown in Figure 1. The process involves injecting oxygen or oxygen enriched air into an oil bearing formation through an injector well. After ignition occurs, continued oxidant injection causes the combustion front to move through the formation away from the injection well. The heat generated by combustion lowers the viscosity of the oil in place, thus enhances its mobility. Combustion or flue gases are absorbed by the reservoir oil or produced with the other fluids. Heat generated at the combustion front, where peak temperatures may range from 400 to 2000 degrees F, will vaporize formation water to create a steam zone. Most of the oil ahead of the combustion front is displaced by this advancing steam zone. The oil left behind, which is often the heavy hydrocarbons of the crude, is used as the fuel to sustain the combustion. P. 157
this article begins on the next page F F JCPT86-04-08 HEAVY OIL Alteration in heavy oil characteristics during thermal recovery V.N. VENKATESAN and W.R. SHU Mobil Research and Development Corp. Dallas, Texas ABSTRACT Significant visbreaking, ora permanent reduction in viscosity of heavy oils, can occur during in situ thermal recovery processes such as steamflooding and combustion. This paper reports a study on the mechanism of in situ visbreaking. Isothermal vis-breaking studies were conducted at 5 72 OF (300 OC) for up to 480 hours using heavy crudes from San Ardo, Celtic, and Cold Lake. The viscosity of the oil, as well as asphaltene content, dis-tillation, elemental analysis, etc., were monitored as a function of time. The experimental results showed a permanent reduction in oil viscosity by a factor as high as ten, without significant formation coke, after 120 hours of soaking. The observed initial rate of viscosity reduction increases with increasing hydrogen content of the oil. Heavy oil is viewed as a colloidal system con-taining asphaltenes dispersed in a solvent phase commonly known as maltenes. Under this framework, visbreaking is inter-preted as being caused by one of these three factors.-a decrease in maltene viscosity, a reduction in asphaltene content, or a decrease in a third parameter called "reduced specific viscosity", which characterizes the "solvent power" of the maltenes with respect to the asphaltenes. The decrease in maltenes viscosity and the increase in its solvent power dominate the visbreaking mechanism. The effect of reduction in asphaltene content on visbreaking is relatively minor. Introduction High viscosity is a major constraint in the production and pro-cessing of heavy oils and oil sand bitumens. Thermal recovery processes employing steam or combustion are effective in tem-porarily lowering the oil viscosity and enhancing its production. Field evidence indicates that a permanent reduction in viscosity or "visbreaking" can accompany these processes which can result in partial upgrading of the heavy oil(l). In situ visbreaking is characterized as a mild thermal cracking with the retention of products in the liquid phase. The extent of visbreaking depends upon the temperature, heating time, and the oil characteristics. Visbreaking as a recovery mechanism during steam recovery processes has been studied by Shu and Hartman(l) using numerical simulations. Their analysis showed that heavy oil visbreaking could have a significant effect on oil production depending upon the mode of steam process and the reservoir characteristics.The present work reports a study on the alteration in the oil characteristics during visbreaking for heavy crudes from S, Keywords: Viscosity, Visbreaking, Maltene viscosity, Asphaltene con-tent, Reduced specific viscosity. Ardo (California), Celtic (Saskatchewan), and Cold Lake (Alberta). Isothermal visbreaking experiments were conducted at 300-C for up to 480 hours. Such information will be useful for numerical simulation studies, for evaluation of field data, f...
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