After the high-temperature front passed through the zones where the oil-filled core plugs were placed (5–6 and 8–9), the next temperature zone appeared to have a dome-like curved shape compared to the sharper peaks observed for the other zones. This phenomenon can probably be explained by the additional oil flow filtering through the core plugs and joining the mainstream.
As the combustion propagates toward the end of the tube, the accumulated light hydrocarbon gases react with the O2, causing the temperature rise to be resumed after a short interruption, as shown in Zones 8, 10 and 11.
Oil recovery during an ISC recovery process is controlled by the oxidation and displacement history at each location downstream of the leading edge of the combustion zone. The steam bank and the fire front propagation are two of the most important driving forces of oil mobilization.
The temperatures of 150 and 350 °C require special attention, as they are, respectively, the temperatures at which oxygen addition (LTO) reactions generate temperature rises that are visually identifiable and generate well-defined heat waves, in addition to carbon oxides and water, and bond scission reactions occur in the heavy oils. The temperature of 350 °C is characterized as the transition to the high-temperature range (HTR), in which the highest levels of energy generation and O2 uptake occur. The reaction zone temperatures in the HTR correspond to effective oil mobilization.
3.1. Product Gas Composition Analysis
Gas composition identification is a routine type of analysis in CT tests. In this work, a couple of issues noted during the experiment execution introduced challenges that needed further assessment. First, due to the operator error, the high-pressure He gas cylinder, which was used prior to the ignition start-up and during the helium purge period, should have contained pure He. Instead, it was inadvertently N2-contaminated. Secondly, after approximately 8 h from the ignition time, a small He leak originating from the pressure jacket annulus and combining with the effluent gas stream was detected. Finally, a higher-than-expected level of CO2 production was detected. This was determined to be the result of the decomposition of the dolomite crushed core and core plugs under high-temperature and -pressure conditions. The N2 impurities in the cylinders and the detected He leaks did not affect the quality of the CT tests, because the gases entering the system were non-reactive. It was necessary to develop a methodology so as to correct the gas composition data. The separation of the N2 associated with the injected air from the N2 contained in the contaminated He cylinder was vital, as the N2 was used as a tie component to complete the material balance of the consumed gases.
Three high-pressure cylinders containing He were utilized during the CT experiment. Cylinder 1 (He) and Cylinder 2 (He) were N
2-contaminated with an insignificant amount of O
2. The compositions of the used He cylinders are presented in
Table 5.
Cylinder 3 (He) was used to purge the remaining N
2 from the tube. As the O
2 content in the He cylinders was minute, it was excluded from the mass balance calculation. The behavior of the produced gases and the switching times from one injection cylinder to another are shown in
Figure 9.
As expected, for an ISC test using air injection, the combined N2 and He (Cylinder 1) content in the effluent gas was detected throughout the experiment. Between 14 and 17 h, the N2 and He concentrations of Cylinder 1 (He) were 42 and 57%. At 15.8 h, when the gas cylinder was switched to Cylinder 2 (He) and after a 1.7 h delay, the gas composition changed to 72 and 27% for He and N2, respectively. After 21.6 h, the gas bottle was changed to Cylinder 3, which comprised pure He, allowing the N2 to be completely purged from the system. An unexpected increase in the He concentration in the product gas was observed during the 6.5 to 13 h period. First, a sharp increase from 0 to 3%, followed by a decline to 1.5%, was observed. At approximately 8.5 to 9 h, the He concentration increased to more than 30%, indicating a leak from the He-filled annulus.
Figure 10 focuses on the produced gas components with a concentration of less than 2 mole % in the produced gas (O
2, light hydrocarbons, H
2S and H
2). The O
2 present in this plot was associated with the injected air and the contaminated He from Cylinders 1 and 2. The O
2 concentration disappeared after switching the injection gas to Cylinder 3.
The rate of the produced gas (see
Figure 11) showed an increased product gas flow associated with a leak from the pressure jacket. It was found near the core pack outlet or production line. Vertical black colored lines indicate the switch from helium to air and back to helium injection.
One of the highest percentages of the produced gas stream was associated with the CO2 concentration. It was apparent from the high CO2 concentration and N2 concentration, which was lower than 79%, that more CO2 than the combustion reactions could have produced was generated. This required careful analysis to establish whether the origin was related to the oil or dolomite core. Normally, when a CT test is performed within a sandstone reservoir, the CO2 production range is approximately 12–14%. In this study, the CO2 production was greater than 30%. The decomposition of the dolomite core samples was confirmed by the loss-on-ignition test.
To introduce corrections to the gas composition, some assumptions were made based on the extensive experience and observations of the ISCRG:
1. The mixture of helium and nitrogen that was added before the air injection was treated as pure He, and the N2 component was “removed” using high He dilution ratios. The term “helium dilution” refers to the proportion of gas that is He. Using this factor, the gas composition was expressed as if there was no He present.
2. The He dilution factor was also used to correct for the He leak.
3. The CO2 was divided into two parts: (a) oxidation/combustion reactions and (b) dolomite core decomposition.
From the CT experiments, two ratios based on the produced combustion gas mole fraction compositions were calculated and compared to the typically reported values, viz.,
It is recognized that, in some cases, these values may vary; however, the above is a reasonable approximation that can be used for this study. Using these ratios, the excess CO2 was assigned to dolomite decomposition as the difference between the total CO2 detected by the GCs and the calculated CO2 associated with the oxidation/combustion processes. The decrease in CO, recorded after 13.58 h, was considered during the computation of CO2 using the (CO2 + CO)/CO molar ratio. CO2 gas is usually accompanied by CO, which, to some extent, is represented by a constant molar ratio if a stable combustion period is observed. After the air injection termination, the CO production declined. After 15 h, the measured COx resulting from the combustion reaction was assumed to be zero, and all the subsequent CO2 was assigned to dolomite core decomposition.
4. A combination of the He dilution factor and prior knowledge based on past experiments conducted under similar pressure conditions were used to correct for the N2 contamination effect during the purge. The CO2 gas generated as a result of dolomite decomposition was not included in this correction practice, as it was treated as a separate stream.
The modified gas composition of the produced components is presented in
Figure 12 and
Figure 13, demonstrating stable burning in the bond scission mode that takes place in the HTR. The declining He dilution was tracked from the time when the air injection began to 8.5 h, when a distinctive peak was detected, indicating a leak from the annulus tube. Overall, the unexpected He stream made up approximately 2% or 160 L(ST) of the total effluent gas during the air injection period. As with previous figures, the black-colored vertical lines indicate the switch times between the various gas bottles, labelled on each figure.
The light hydrocarbon gases, as presented in
Figure 13, were most likely products of the thermal cracking reactions and displayed a stable production during the experiment. The H
2 and CH
4 curves showed an increased or continuous stream after the air was shut down. The H
2 disappeared after 13 h once the He dilution factor approached zero.
3.2. Combustion Parameters Based on Product Gas Analysis
When designing an ISC-based project for a field scale, the air and fuel requirements are the most important parameters. They are often estimated from laboratory experimental studies mimicking reservoir conditions [
2]. Generally, when the ISC is operated using a dry or normal wet regime, the common air and fuel requirement values for heavy oils are 240 m
3(ST)/m
3 and 22 kg/m
3, respectively.
In the traditional literature, the calculation of the fuel requirements is based on the C and H
2 balance, i.e., on the produced volumes of CO
2 and CO, assuming that all the H
2 is consumed to form water. The ISCRG practice aims to obtain the fuel based on the injected air requirements and estimate the fuel based on the air/fuel ratio (AFR). This avoids uncertainties related to water formation and fuel oxygenation. Using combustion stoichiometry, the burnt carbon was computed considering the amount of carbon oxides produced, while the apparent H
2 was estimated assuming that all the reacted O
2 was consumed to form carbon oxides or water. It is known that a portion of the reacted O
2 is consumed by LTO reactions, but an accurate water balance is required to separate the O
2 consumed to form water from the O
2 consumed by LTO reactions. This is why the H/C ratio is normally referred to as the apparent H/C ratio. All the equations used to calculate the apparent H/C ratio, AFR, fuel and air requirements and other parameters mentioned here can be found in Barzin et al. [
36].
The calculated values of the apparent incremental H/C and air/fuel ratios presented in
Figure 14 exhibited stable burning after approximately 3 and 6 h, respectively, from the beginning of the experiment. The AFR parameter range varies between 10.8 and 15.0 for heavy oils and bitumen [
37,
38] and 10.8 to 12.5 m
3 (ST)/kg for light oils [
39].
Tracking the apparent H/C ratio curve, the values at the start of the experiment were much greater than 3, indicating that the combustion propagation was operating in the O
2 addition mode [
2]. At the same time, an increase in the CO
2 concentration following stabilization was observed (see
Figure 14). The stabilized apparent H/C ratio displayed an average value of 2.1.
A summary of the air (O
2)–fuel parameters, O
2 utilization and the volume of gases produced can be found in
Table 6. These values were obtained based on the burned volume of the composite core of 11.3 × 10
−3 m
3 or 78% of the CT volume.
3.4. Compositional Analysis
The oil samples were analyzed for changes in viscosity, density, the asphaltene content, elemental analysis of CHNSO (obtained using LECO CHN-2000 and Antek 9000 analyzers, with the oxygen calculated as the difference between the measured CHNS components, assuming CHNS+O made up 100 mass %), cut-point analysis (obtained using ASTM D7500) and composition of saturates, aromatics, resins and asphaltenes (SARA).
Figure 18 reflects the viscosity change obtained from the produced oil samples. During the air injection process, the viscosity of the produced oil significantly decreased.
In
Figure 19, a continuous upgrade of the API gravity and viscosity (measured at 25 °C), along with the asphaltene content reduction, can be observed. The results of the simulated distillation percent, showing the split between distillate (400 °C- fraction) and residue (400 °C+ fraction), confirm the upgrading of the produced oil samples.
The extraction of the core samples started from the production end. Core 1 and Core 2 correspond to the sample located at the production end, representing frac sand with the meshes of 16 and 20/30, respectively. As expected, the hydrocarbons remaining in the core exhibited a higher asphaltene content because the original oil-heavy components precipitated onto the surface of the mineral skeleton. The highest accumulation of asphaltenes was observed in areas beyond the point where the air injection stopped (corresponding to the collected sample Core 8), where a coke-like wall was evident.
The average oil upgrading values for the samples produced during the run were as follows: API°—from 14.2 to 17.4, asphaltenes—from 11.13 to 9.7 wt.% (12.8% decrease), distillate @ 400 °C—from 29 to 44 wt.%, and residue @ 400 °C+—from 60 to 45 wt.%. Note that the viscosity values deviated significantly and started declining after 4.8 h of the ISC operation. Thus, the viscosity varied from 4091 to 59 cP (measured at 25 °C), with the trend decreasing as the produced oil samples corresponding to the combustion front approached the production outlet.
Due to the insufficient amount of extracted oil, viscosity and API gravity measurements of some of the extracted samples were not possible. The produced oil samples and the extracted residual oil samples demonstrated a slight change in the values of H and S, while the rest of the elements (except for O) presented a stable mass fraction throughout the run (
Figure 20). The oxygen mass noticeably reduced between 7.0 and 8.8 h, slowly increasing towards the end of production.
Using the simulated distillation results, cut-point analyses were performed from the initial boiling point to 700 °C+ on both types of produced liquid oil samples: the oil produced as liquid during the test and the toluene-extracted oil from the excavated core. The data indicate that most of the oil upgrading was associated with an increase in the light-intermediate components (C
4 to C
20) and a reduction in the asphaltenes and heavier ends (C
44 to C
100). Some discrepancies were observed in the oil property results obtained from the core extractions. While an increase in C
44 to C
100 was observed, the concentration of the lighter components, C
4–C
20, were underestimated in the oil samples, as they were exposed to toluene extraction (
Figure 21). Thus, the average percentage of the increase in the concentration of C
4 to C
44 in the produced oil samples was approximately 30%, 54% of which was associated with the increase in the C
4–C
20 fractions.
While
in situ oil upgrading obtained through the other thermal methods is mainly a function of the temperature rise and asphaltene precipitation, in the case of ISC, the upgrading occurs due to the bond scission reaction, and the products may have a significantly lower carbon number content [
40].