An in situ combustion process for recovering heavy oil using scaled physical model
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In order to study the effects of formation thickness and structural dip on in situ combustion and the combustion performance in the 1/4 of inverted nine-spot injection pattern, the scaled experimental system was developed based upon the ISC scaling law. The laboratory results show that within 1/4 of inverted nine-spot injection pattern the main combustion zone was swept completely with the oil recovery of 75%, leaving 5–15% oil saturation in the cracking/vaporization zone. Gas override and early breakthrough became more and more serious when the formation thickness increased, and oil recovery in the reservoir with structural dip was much lower than that in the 0° structural dip reservoir under same operating conditions. Conclusions have been drawn that employing nine inverted nine-spot injection pattern ISC can achieve a good oil recovery in G3-6-18 reservoir of Liaohe Oilfield in China. However, the formation thickness and the structural dip shall be taken into account when a project is designed as they play a major role on the sweeping efficiency. The sweeping efficiency can be enhanced by optimizing the operational parameters.
KeywordsHeavy oil In situ combustion Thermal recovery Scaled model Laboratory
In situ combustion (ISC) is a thermal recovery process with heat being generated in the reservoir by burning some of the original oil in place and then propagating the fire front under continuous air injection. The heat generated in situ increases the mobility of the unburned oil by decreasing the oil viscosity in the vicinity. ISC mechanisms are largely a function of oil composition and rock mineralogy (Sarathi 1999). The extent and nature of the chemical reactions between crude oil and injected air, as well as the heat generated in situ, depend on the oil matrix system. Laboratory studies, using crude and matrix from a prospective ISC project, are required prior to the design of any field operations (Ghafoor and Arvin 2010).
The one-dimensional combustion tube experiments are mandatory to determine the basic parameters needed to design and implement field projects, such as the apparent atomic H/C ratio, the volume of injected air and the volume of consumed fuel, etc. (Cheng 2012). These data are significant for mitigating the investment risks and making predictions of field test performance. However, it is beyond its ability to evaluate the effects of geological parameters, which have been proven to play a major role in the combustion process. The effects of geological parameters can only be studied using a scaled physical model (Greaves and Al-honi 2000).
Studies of ISC using scaled geometries are very limited, because of the extra complexity and cost. Nevertheless, the effort in this direction is worthwhile due to the valuable understanding to be gained from a physical scaled model of the process. Moreover, a scaled model can also be utilized to study and optimize well positioning and provide a more detailed picture of what is happening in the complex process of ISC (Garon et al. 1982).
In the past few decades, most of the scaled experiments mainly focused on injected gas override, the steady-state analysis of the combustion front, flame velocity and reaction rate, oil recovery, etc. Very limited scaled experimental studies of ISC were reported with regard to the effects of geological and operational parameters on the combustion (Coats 1980; Oklany 1992; Anis et al. 1983).
This paper mainly focuses on the influences of the well pattern, the structural dip and the formation thickness on the combustion front propagation and the sweeping efficiency using a scaled model, which had only been discussed theoretically in the previous studies, but not experimentally.
Scaled model setup
Scaling laws shall be followed for scaled physical studies. The earliest scaling laws of a simplified ISC model was given in 1967, and a complicated scaling law taking into account geometry, gravity and dynamics was then developed for ISC (Caron et al. 1984). Liu et al. developed a more sophisticated scaling law taking the combustion reaction kinetics into consideration (Liu et al. 2013). This experimental system was developed based on Liu’s scaling law.
The scaling factors between the model and prototype
Air injection rate
9.62 × 10−7
4.49 × 10−7
Prior to activating the ignitor, air was injected at low rate into the model to ensure the connectivity. The power of ignitor was incrementally increased to make the various compositions of heavy oil crack or evaporate at different temperatures, leaving sufficient fuels for the oxidation. The accurate temperatures were obtained at 139 TMPs, and temperature distribution during combustion process was analyzed through interpolation method.
Experimental parameters with different reservoir thickness
Reservoir thickness, m
Oil saturation, %
Cell dimension, cm
Dehydrated crude oil and quartz sands
50 × 9.5 × 4
50 × 19 × 4
50 × 28.6 × 4
Results and discussion
The temperature data were collected from TMPs, and the temperature profiles were plotted using interpolation and inversion method. Considering the potential impacts caused by excessive sensors on porous medium, we did not place sufficient pressure sensors in the model to determine the pressure contours. The production pressure drop was maintained between 1.1 and 1.3 MPa, and the production in all cases was stable under such circumstances. The CO/CO2/O2 contents in the effluent gases were analyzed to assist in determining the ISC performance and fire breakthrough. If required the optimization would be applied by adjusting the producers. The CO/CO2/O2 contents under stabilized conditions in different cases were 4.0 ± 0.5%, 10.1 ± 0.4% and 3.2 ± 0.4%, respectively. The details of different cases are discussed as follows.
In the process of the experiment, the significant observations were that the combustion front advanced to the production wells in relation to the air flow, and the flame could sweep any part in the model where the air flew through. The direction of the combustion front propagation was more likely toward nearby wells rather than the diagonal well due to the larger distance. The combustion front propagation could be controlled in order for the better sweep efficiency by optimizing the operational parameters.
In the experiment of 20 m scaled pay zone, the flame was formed at the bottom of the well at the very beginning and advanced to the production well. The tongue fingering phenomenon was observed while the combustion front moved forward to the production well, and gases broke through into the production well when the combustion front approached. The peak temperature was 634 °C, and the average was 500 °C in the combustion process. The oil recovery was 64.7%.
As is known, the large difference in density between air and the reservoir fluids gives the air a tendency to override the oil column and consequently bypass much of the oil if the reservoir exceeds a critical thickness. A thin oil sand, 20 m thickness in the experiment, tends to counter this override tendency and favor a more uniform displacement and vertical sweep. In comparison, rapid transfer of heat to the bottom of the sand in a thin reservoir will permit combustion front to advance at the bottom more rapidly than it would be possible in a thick reservoir. The gas override and early breakthrough exert a major impact in a thick reservoir on sweeping efficiency resulting in a reduction in oil recovery.
In order to minimize the gas override and early breakthrough, it is necessary to optimize the operational parameters. A few production wells were placed with various perforated intervals for selective zone production at the breakthroughs. The middle and the bottom of Fig. 9 shows the combustion front movement after the optimization. As the perforated intervals were adjusted, the combustion front movement could be controlled and eventually sweep the bottom section of the reservoir, which led to a higher sweep efficiency. It is applicable to control the combustion by optimizing the operational parameters to minimize the gas override and early breakthrough.
Theoretically the injected air and combustion front movement will be more rapid toward up dip wells. In dipping reservoir, it is advisable to locate the air injectors downdip and production wells up the structure to compensate for the expected flow of air up dip.
The scaled experimental system was developed based upon the ISC scaling law. Three experiments were conducted to study ISC performance in 1/4 of inverted nine-spot injection pattern (Case #1), the effects of formation thickness (Case #2: 20 m, 40 m, 60 m) and structural dip (Case #3: 0°,15°) on combustion front propagation and performance in Liaohe heavy oil reservoir.
The laboratory results of Case #1 show that the direction of the combustion front propagation was more likely toward nearby wells rather than the diagonal well. The main combustion zone was swept completely with the oil recovery of 75%, leaving 5–15% oil saturation in the cracking/vaporization zone.
The laboratory results of Case #2 show that gas override and early breakthrough became more and more serious when the formation thickness increased. Compared with 64.7% and 61.7% oil recovery in 20-m and 40-m reservoir, it could only achieve 21.4% in 60-m reservoir. But it could be elevated to 59.97% by optimizing the operational parameters.
The laboratory results of Case #3 show that the oil recovery of ISC in 40-m reservoir with 15° structural dip was 42.3%, much lower than the 71.8% in the 0° structural dip reservoir under same operating conditions.
It is concluded that when employing nine inverted nine-spot injection pattern ISC can achieve a good oil recovery in G3-6-18 reservoir of Liaohe Oilfield in China. However, the formation thickness and the structural dip shall be taken into account when a project is designed as they play a major role on the sweeping efficiency. The proper adjustment of operating parameters could control the fire breakthrough and effectively enhance the oil recovery.
The authors thank PetroChina Innovation Fund (Grant Number 2018D-5007-0212), National Science and Technology Major Project (Grant Number 2016ZX05002006) and for financial support to this research. The Northeast Petroleum University Scientific Research Start-up Support is also appreciated.
Compliance with ethical standards
Conflict of interest
The authors confirm that this article content has no conflict of interest.
- Caron AM, Kumar M, Lau KK, Sherman MD (1984) A laboratory investigation of sweep during oxygen and air fireflooding. SPE Reserv Eng 1:565–574Google Scholar
- Cheng HQ (2012) Physical simulation research on basic parameters of in situ combustion for super heavy oil reservoirs. Spec Oil Gas Reserv 19:107–110Google Scholar
- Ghafoor K, Arvin KS (2010) In situ combustion process, one of IOR methods livening the reservoirs. Pet Coal 52:139–147Google Scholar
- Greaves M, Al-honi M (2000) Three-dimensional studies of in situ combustion-horizontal wells process with reservoir heterogeneities. J Can Pet Technol 39:25–33Google Scholar
- Liu QC, Cheng HQ, Zhang Y, Zhao QH, Liu BL (2013) Research on similarity criteria of in situ combustion process. Spec Oil Gas Reserv 01:111–116Google Scholar
- Oklany SFAJ (1992) An in situ combustion simulator for enhanced oil recovery. Thesis, University of SalfordGoogle Scholar
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