Coke Formation and Coupled Effects on Pore Structure and

Significant open ISC questions include the effect of coke formation on the pore structure and permeability. In the study, an experimental apparatus wa...
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Coke Formation and Coupled Effects on Pore Structure and Permeability Change during Crude Oil in Situ Combustion Qianghui Xu,† Hang Jiang,‡ Cheng Zan,§ Wenbin Tang,† Ran Xu,† Jia Huang,‡ Yang Li,‡ Desheng Ma,‡ and Lin Shi*,† †

Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Thermal Engineering, Tsinghua University, Beijing 100084, China ‡ State Key Laboratory of Enhanced Oil Recovery, Research Institute of Petroleum Exploration & Development, China National Petroleum Corporation, Beijing 100083, China § Power Environmental Energy Research Institute, Covina, California 91722, United States ABSTRACT: In situ combustion (ISC) is an important thermal recovery technique. Significant open ISC questions include the effect of coke formation on the pore structure and permeability. In the study, an experimental apparatus was constructed to not only physically simulate coke formation similar to the crude oil ISC process but also to in situ measure postdeposition permeability. Effects on coke deposition with the Xinjiang crude oil were studied, including reaction atmosphere, temperature, and time. The results indicate that the critical coking temperature differs significantly by at least 200 °C between low-temperature oxidation (LTO) runs with air flow and coking runs with nitrogen flow for the Xinjiang crude oil. The coke generation promoted by LTO and the coke consumption via high-temperature oxidation (HTO) result in a maximum coke production with temperature in the LTO runs. In addition, the study found that many resins and the small amount of asphaltenes in the Xinjiang crude oil prolonged the induction coking period in the coking runs. This understanding of the coke deposition process led to the production of core samples with different amounts of coke deposition for selected reaction conditions. The pore structure of the coked grain clusters was viewed with a scanning electron microscopy (SEM) and mercury porosimeters. The results showed the complicated pore structure and increasing number of micropores with increasing coke deposition, which not only reduced the permeability rapidly so that it deviated from the Kozeny−Carman relationship at the Darcy scale but also further promoted the Klinkenberg effect. In addition, the global permeability damage would be further underestimated regardless of the coke concentration heterogeneity in the core samples. The permeability change was then correlated with coke deposition for numerical simulations of ISC or ToeHeel Air Injection (THAI) processes in sandstone reservoirs. (HTO).8,9 The amount of fuel deposited per unit volume of the reservoir is a key parameter which affects the energy generation and is used to estimate the air requirement as the combustion front propagation.10 Hascakir and Kovscek ran six combustion tube experiments with different crude oils. The results showed that the combustion front was quenched in one of the experiments since the crude oil could not form enough coke, resulting in insufficient energy.11 The sustainability of the combustion front was concluded to depend primarily on adequate fuel formation and the injected air, which determines the success of ISC in the reservoir. Therefore, parametric studies of the fuel formation during the ISC process have been conducted for specific reservoirs before project design and economic assessment.9,12 The fuel available was correlated with the physical and chemical properties of the crude oil by Alexander et al., including the API gravity, viscosity, and H/C ratio.9 The fuel availability varied greatly with the oil properties. Banerjee et al.13 conducted kinetic studies of coke formation from hydrocarbons in crude oil. The results found that coke formed more rapidly from asphaltenes than from resins and small aromatics.13 The large amount of resins and the small

1. INTRODUCTION In situ combustion (ISC) is a thermal recovery technique, involving parallel and series reactions with a small portion of the oil in the reservoir and oxygen in the injected pressured air. An enormous amount of thermal energy is released with production of carbon oxides, hot water, and steam, which help displace the oil into production wells by various flooding mechanisms.1 The ISC method has many economic and environmental advantages. Hundreds of ISC projects have been implemented in various crude oil reservoirs since the first ignition operation in 1933.2−4 Meanwhile, the integration of the horizontal well and in situ combustion process has been proposed such as ToeHeel Air Injection (THAI), as the mature horizontal well technology. The THAI process may provide more efficient short-term oil displacement with the oil drawn-down by gravity into the horizontal production wells at the formation bottom with injection wells placed at the formation top.5 ISC and THAI involve complex multiphysics mechanisms and reactive transport processes.6 A limited understanding of the processes results in only a small fraction of the ISC projects worldwide being successful with about 80% being economic failures.7 The reactions during ISC are usually categorized as lowtemperature oxidation (LTO), middle-temperature reactions/ fuel formation reactions, and high-temperature oxidation © 2016 American Chemical Society

Received: November 4, 2015 Revised: December 18, 2015 Published: January 14, 2016 933

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allow measurement of the permeability after coke deposition. The system was used to study the amount of coke production for the Xinjiang crude oil with air or nitrogen flows. The results were compared with published studies to show the difference in the coke formation from the Xinjiang crude oil. The effect of reaction time on the amount of coke production with air flow was also investigated. The coke deposition for various reaction conditions was then analyzed to select several specific reaction conditions to form a series of core samples with different amounts of coke deposition. The permeability change by coke clogging was measured using gas. Grain clusters filled with coke were scanned with SEM, while the pore distributions postdeposition was analyzed by mercury porosimeters. The results were correlated to relate the permeability changes to the amount of coke deposition, which was then compared with the Kozeny−Carman relationship.

amount of asphaltenes in Chinese crude oil are significantly different from the rich asphaltenes in American and Canadian crude oil, particularly the Xinjiang crude oil. ISC of the Xinjiang crude oil may result in different coke formation processes thereby influencing the amount of coke deposition with air or nitrogen flow. The coke or other solid/semisolid residuals deposited in the porous medium reduce the porosity and permeability. Osman et al.14 investigated an in situ sand consolidation technique by LTO of light crude oil (18 to 20 oAPI) at low temperatures between 100 and 150 °C at 50 PSI and measured the permeability loss after consolidation, which ranged from 4.53 to 21.59%. The residue deposited in the core included the oxidation residue and residual water, which were clearly not coke or coke-like residues. A core holder was used to prepare the consolidated sample and measure the permeability. However, the core holder could not be exposed to the high temperatures of up to 600 °C in the ISC process. Therefore, the LTO reaction conditions in the study were significantly different from the actual conditions in the ISC process. Recently, Fau et al.15 studied coke formation by n-dodecane pyrolysis in a stainless steel porous medium at temperatures up to 547 °C and 3.5 MPa and the effect on the permeability, but this is also not directly related to the ISC process. The deposited coke filled about 30% of the pores inside the porous medium, and the permeability dropped by 2−3 orders of magnitude from the original permeability of 196 mD, demonstrating significant coke blockage. Furthermore, Gaillard et al.16 and Chen et al.17 combined X-ray microtomography and Lattice Boltzmann simulations to estimate the permeability changes due to ZrO2 colloidal particles deposited in a granular porous medium. The significant reduction in the permeability by 2 orders of magnitude after the great colloid deposition was not captured by either the Kozen−Carman relationship or the power-law ϕ−k relationship.17 Similarly, the existing empirical or semiempirical relationships are still questionable to model the permeability change due to coke deposition. Coke deposition during the ISC process increases the permeability heterogeneities which will affect the oxygen transport in the formation. The effect of coke deposition on the oxygen transport further promotes the oxygen bypassing, especially at the combustion front. The bypassing oxygen then impacts the air participating in the LTO in the evaporation and vis-breaking zones18 and then the combustion front spreading characteristics. In the THAI process, the coke deposition inside the production well creates a gas seal that prevents air breakthrough into the producer and stabilizes the THAI process.19,20 The STARS simulator by Computer Modelling Group Ltd. provides a function to specify the variable permeability affected by coke deposition. However, the parameters still need to be supported by reliable experimental data guided by the tutorial.21 Unfortunately, little experimental study is available about the effects of permeability changes on the amounts of coke deposition in ISC or THAI processes. Lack of the experimental results hinders modeling of the variable permeability caused by the coke deposition, which aggravates the simulation uncertainties for the combustion front expansion during ISC and THAI processes. Therefore, the permeability change during coke deposition for ISC or THAI processes needs to be characterized more accurately. The present study needs an experimental apparatus that could not only physically simulate coke formation for conditions similar to actual crude oil ISC process but also

2. EXPERIMENTAL SECTION 2.1. Materials. A crude oil from the Xinjiang Reservoir was selected for this study. The crude oil properties are listed in Table 1.

Table 1. Properties of the Selected Xinjiang Crude Oil parameter −3

density (g·cm ) viscosity (at 50 °C, mPa·s) saturate (%) aromatics (%) resins (%) asphaltenes (%)

value 0.94 2339 46.17 26.47 25.96 1.34

The SARA fractions of the crude oil were measured according to the China Petroleum and Chemical Standard, SH/T 0509-92. The mass fraction of asphaltenes in the Xinjiang crude oil was 1.34% while that of resins was 25.96%. The permeability and porosity of the core samples packed with the poorly sorted sand vary significantly, which makes it difficult to quantify the effect of coke deposition on permeability change. Therefore, 160−180 mesh glass beads were used for the porous medium with the main composition being silica. The glass bead porosity was 0.35−0.37, while the bulk density was 1.45− 1.49 g·cm−3. 2.2. Experimental Apparatus. The experimental apparatus is shown in Figure 1. The apparatus can subject the core samples to a programmed experimental environment similar to rock in reservoirs undergoing in situ combustion. Brooks SLA5850 HP massflow meters were used to control the air or nitrogen injection rates. A backpressure regulator was used to adjust the reactor pressure. The liquid trap separated the oil and gas produced from the reactor. The tubular reactor, which is also referred to as a kinetics cell, was mounted inside the furnace either horizontally or vertically. The furnace provided various heating modes, such as ramped temperature heating and constant temperature heating up to 800 °C. The kinetics cell consisted of a Hasyelloy tube 700 mm long and 15.22 mm in inner diameter. Stationary Grayloc hubs were welded to both tube ends. Seal rings were clamped between the stationary and active hub faces to seal. Two Swagelok tube fittings were welded on the sides of the tube near the top and bottom ends for fluid injection and outflow. A thermowell was inserted into the center of the kinetics cell through a thermowell-tube fitting welded on the active Grayloc hub. A hole was drilled inside the thermowell-tube fitting to embed O-rings and Viton-glaskets, which are tightened by a gland to prevent gas leakage from the thermowell. A bundle of three K-type thermocouples was soldered together at intervals of 10 cm to measure the temperatures along the axis. A 30 cm long annular section inside the kinetics cell was packed with glass beads and oil sand, segmented as top, middle, and bottom zones each with a length of 10 cm. The electric heating PID controls and temperature measurements in the three zones were independent. The 934

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Figure 1. Schematic of the experimental apparatus. three independent PID controls help uniform temperature distribution along the kinetics cell. A data acquisition system automatically recorded the temperatures inside the kinetics cell, various temperatures on the tube wall and in the furnace, the inlet and outlet pressures, and the inlet to outlet differential pressures as well as gas injection rates. The differential pressures were simultaneously measured by two differential pressure transmitters with small (8 PSI) and large (50 PSI) ranges to allow for precise measurements over a large range. The measurement uncertainty of the pressure was 1 PSI and that of the differential pressure was 0.02 PSI. The differential pressure and gas injection rates are important parameters used to calculate the sample absolute permeability according to Darcy’s law. 2.3. Experimental Procedures. The experiment to study the amount of coke deposition started with reactant preparation, following by sample packing. The oil sand reactant was a uniform mixture of 160−180 mesh glass beads and oil at a ratio of 4.5:1 to approach oil saturation. The top and bottom zones of the kinetics cell were only filled with glass beads, with the oil sand packed in the middle zone. The packing pattern provided a more uniform temperature distribution along the reactant, as well as preheated and steady gas flow through the reactant, which helped produce approximate coke properties in each core sample. The packed oil sand was weighted on a Mettler-Toledo AL204 balance with 0.1 mg precision. Each time small portions of the oil sand were introduced into the middle zone and tamped tightly using a plunger. In addition to the glass beads and oil sand, some metal meshes and temperature-resistant cotton were positioned at the top and bottom ends of the kinetics cell adjacent to the glass beads to hold the glass beads in place. The temperature resistant cotton is composed of artificial glassy silicate fiber for continuous high-temperature (1050 °C) applications. Short metal tubes machining inside each active Grayloc hub were inserted into the kinetics cell to tightly pack the samples. The kinetics cell was installed horizontally inside the furnace to suppress gravity drainage effects during the reactions. After hooking up all the connections and sensors, the cell was checked for leaks by pressuring to approximately 1.5 MPa. The back pressure was adjusted to the desired level, and gas injection was initiated at 5 SL/h. When the pressure in the kinetics cell reached the setting point, the furnace started to heat with the constant temperature heating mode. After the required time, the heating was

stopped, and the kinetics cell was immediately air-cooled to room temperature, following by depressurization. All samples produced in the kinetics cell were collected and filtered with toluene using a Soxhlet extractor until the filtrate was clear. Toluene dissolves the SARA fractions with coke retained in the filter residue. After the residue was dried overnight in a vacuum oven, the residue was graphed and then burned out in a muffle furnace at 500 °C, which is a box-type oven. The residue mass loss was the amount of coke deposited at the specified reaction conditions. The experimental procedure for measuring the permeability of the coked sample inside the kinetics cell had a coke formation period and an in situ permeability measurement period. Based on the understanding of the amount of coke influenced by reaction conditions, core samples with different amounts of coke deposition could be produced by controlling the reaction conditions. The coke formation procedure in the present experiments had some differences from that used in the effects study of the coke deposition. This study focused on the effect of coke deposition on the core permeability which was related to the amount of coke as the only variable. Other parameters that may affect the permeability measurements should be eliminated during the serial experiments. Therefore. the masses of different samples added into the kinetics cell were fixed and listed in Table 2. The length of the packed

Table 2. Masses of Different Samples in the Kinetics Cell samples

weight (g)

cotton adjacent to the top zone glass bead in the top zone oil sand in the middle zone glass bead in the bottom zone cotton adjacent to the bottom zone

3.2 20.5 53.5 18.5 4.8

oil sand was extended to 20 cm to highlight the permeability change by the coke deposition by reducing the amount of glass beads and cotton in the rest of the section. The kinetics cell was pressured and depressurized with very low gas flow rate to avoid damaging the core structure. Moreover, since the coke properties differ at different conditions, especially the coke density, the same coke weight in the core would lead to different coke volumes and different effects on the 935

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Energy & Fuels permeability change. Therefore, the experiments were extended for 2 h with N2 flow at 425 °C following the coke formation period. The extra time helped keep the coke density approximately constant during the additional experiments. Furthermore, the light components in oil, which had been distilled and stored at the end of the kinetics cell, significantly influenced the core permeability. Therefore, petroleum ether flowed through the core at a slow rate of 1 mL/min to dissolve and sweep away the light oil until no black color was visible in the wash so that only the coke and coke-like residue were retained. The kinetics cell was then reheated to 120 °C with nitrogen injection at 100 mL/min for 1 h to vaporize the remaining petroleum ether. After the kinetics cell was cooled to room temperature, the experimental system was again checked for leaks, and the pressure transmitters and differential pressure transmitters were calibrated. Since the total gas resistance was due to the gas flow through the samples in the kinetics cell and all the tubing with various sectional areas and lengths, the total transmissibility, Tg, was used to easily represent the total gas resistance instead of the total permeability. The total transmissibility was measured at four different pressures to evaluate the Klinkenberg effect. The pressures, inlet to outlet differential pressures, and the temperatures along the core axis were recorded at each transmissibility measurement. The pressures and temperatures were used to calculate the air viscosity. Note that the total core transmissibility was contributed by the gas flow resistance in the zone with the coke and coke-like residue deposition and the other zones filled with the glass beads, cotton, and metal meshes as well as the tubing. However, the permeability of the zone with coke and coke-like residue was the objective in the study. Therefore, the gas flow resistance from the other zones was measured and then excluded in the calculations. Thus, all the samples were tamped in the kinetics cell except for the oil sand in the middle zone. The transmissibility of the samples was measured with the same method, which was called the blank experiment. Furthermore, the porosities and pore distributions of the grain clusters with coke and coke-like residues were analyzed using mercury injection with a Micromeritics’ AutoPore IV 9500. A FEI Quanta 200 SEM system was also used to view the grain clusters microstructure with magnifications up to 1,000,000X to observe coke texture and distribution. The mass fractions of coke in the small grain clusters were measured by thermogravimetry analysis (TGA). About 15 mg of coked sand was taken from each small grain cluster which usually weighted less than 100 mg and then burned out with 100 mL/min air passing through the sample platform at 500 °C. The automatically recorded mass loss represented the coke concentration in the grain cluster.

Figure 2. Browner color coke deposited at different temperatures in the LTO runs.

Figure 3. Blacker color coke deposited at different temperatures in the coking runs.

showed that the coked sand colors in the LTO runs are browner than those in the coking runs. The property differences such as the density and H/C contributed to the color difference.22 In order to quantitatively compare and then select the conditions for the permeability change study, the amount of coke deposition was expressed as coke mass per gram of the initially saturated oil. The coke deposition in the LTO runs and coking runs is plotted versus temperature in Figure 4. The average absolute difference in the amount of coke deposition at the same conditions in the repeated LTO runs was 0.020 g coke/g oil with an average variation of 7.1%. Thus, the experimental method was able to provide reproducible results with a single data point then used in the following study. The maximum coke deposition in the LTO runs averaged 0.363 g coke/g oil for temperature from 225 to 250 °C, which was 2.5 times more than that in the coking runs which had a maximum of 0.150 g coke/g oil at 425 °C. The different coking phenomenon was related to the different reaction mechanisms during the coke formation. In the coking runs, the coke formation was attributed to pyrolysis of the heavy oil fractions which destroyed the colloidal structure of the crude oil so that the asphaltene cores originally dissolved in the maltenes exceeded the solubility limit and precipitated, ending up as coke.23 However, the main LTO products were partially oxygenated hydrocarbons. The oxygenated hydrocarbons easily condensed and increased the heavy fractions in the crude oil such as secondary asphaltenes, which were precursors for coke formation. As a result, the

3. RESULTS AND DISCUSSIONS 3.1. Effects of Reaction Atmosphere and Temperature on the Coke Deposition. The effects of the reaction atmosphere and temperature on the coke deposition were investigated in a series of experiments with different maximum temperatures purged with air or nitrogen as the reaction gas at 5 SL/h injection rates. The experiments performed in the air environments were termed the LTO runs, while the experiments performed in the N2 environments were termed the coking runs. The constant temperature heating mode was used to control the temperature from room temperature to the maximum temperature at a rate of 10 °C/min, which was then maintained for 5 h. All the experiments were run at 5 MPa backpressure, which was close to the actual reservoir pressure. Filtered coked sand samples collected in the LTO runs and coking runs after removal of the toluene soluble fractions are depicted in Figures 2 and 3. As shown, little coke was observed at 200 °C in the LTO runs, while the critical coking temperature in the coking runs was 425 °C. Thus, the critical coking temperatures differed significantly with at least 200 °C difference between the LTO runs and coking runs, indicating that the coke formed more easily in the presence of air. Furthermore, the color analysis of Figure 2 and 3 quantitatively 936

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Figure 5. The differential heat flow reflects variations of the heat flow with the temperature. The high temperature oxidation

Figure 4. Amount of coke deposition in the LTO and coking runs. Figure 5. PDSC result with the filtered coked sand formed at 225 °C in the LTO runs.

instability of colloidal dispersion was advanced, and the coke deposition was promoted. Therefore, LTO plays a significant role in coke formation. Other researchers have experimentally studied coke formation in air or nitrogen flows.9,24,25 The experimental results of Cinar et al.24 for two different oils showed significant coke formation in the coking runs after 60 min for Sample F and 45 min for Sample Hamaca at 400 °C. The present study rarely showed coke formation after even 5 h at 400 °C in the coking runs with the Xinjiang oil due to the different oil properties. The Sample F density was similar to that of the Xinjiang oil. However, the asphaltene mass fractions in the Sample F and Hamaca crude oil were both reported to be 10%,24 much richer than in the Xinjiang oil. The Xinjiang oil consists of very few asphaltenes, as low as 1.34% by weight, which makes the crude oil colloidal dispersion more stable. The experimental result of Zhang et al.25 showed a critical coking temperature of the Liaohe crude oil of 400 °C in the coking runs, approximating that of the Xinjiang oil, but the critical coking temperature of the Liaohe crude oil increased to 280 °C in the LTO runs, about 80 °C higher than that of the Xinjiang oil. The different results are attributed to the different crude oil properties, core materials, and operating conditions. Another interesting result shown in Figure 4 is that the amount of coke produced increased from zero to an average maximum of 0.363 g coke/g oil at 225 °C−250 °C and then decreased to an average of 0.179 g coke/g oil at 300 °C in the LTO runs. The variations are similar to those reported by Alexander et al.9 in effluent gas analysis experiments. As the temperature increased from 200 to 250 °C, the LTO reactions promoting the coke formation proceeded at faster rates than those of the HTO reactions consuming the coke, as a result of increasing accumulated coke deposition. However, as the temperature continued to increase toward 300 °C, more oxygen was consumed by the HTO reactions resulting in less gross coke formation rate and less coke deposition. An additional experiment was designed to verify this interpretation using pressurized differential scanning calorimetry (PDSC) measurements of the coked sand formed at 225 °C in the LTO runs. The heat flow of the filtered coked sand without SARA fractions was measured with linear heating of 5 °C/min and 5 MPa. The heat flow and the differential heat flow are shown in

reactions did not begin until 225 °C, as seen by the increasing heat flow and the positive differential heat flow. Therefore, the PDSC result confirms that HTO of the coke produced in LTO runs with the Xinjiang oil occurs below 300 °C which differs from the traditional definition of HTO occurrence at the temperature higher than 300 °C. In addition, the oil was distilled more and displaced faster as the temperature increased due to the decreasing viscosity and increasing oil relative permeability.26 Therefore, less residual oil was left in the porous medium to participate in the reactions, further reducing the coke deposition in the LTO runs. 3.2. Effect of LTO Reaction Time on the Coke Deposition. The effect of the LTO reaction time on the coke deposition was investigated using an experimental procedure similar to that in the LTO runs at 300 °C with different constant temperature times. Three runs were conducted with constant temperature times of 2, 5, and 8 h. Figure 6 indicated the amount of coke deposition increased from 0.142 g coke/g oil to 0.313 g coke/g oil when the constant temperature time was increased due to the LTO effects. 3.3. Effect of Coke Deposition on the Pore Structure and Permeability Change. The effects study on coke deposition provided a basis to study the coupled effect of coke on permeability change which was conducted at operating conditions with Table 3 listing the conditions with a decreasing coke deposition rate. The LTO runs, at 250 °C for 5 h, were conducted twice to check the reproducibility. The total transmissibility, Tg, measured with air was used to easily represent the total gas resistance through the samples and tubing instead of the total permeability. The air-based total transmissibility with the gas compression was calculated as Tg =

2p0 μQ 0 (p22

−

p12 )

=

2p0 μQ 0 (2p2 + Δp)Δp

(1)

where p0 is the atmospheric pressure, μ is the gas viscosity as a function of the gas pressure and temperature, Q0 is the standard gas flow rate, p1 and p2 are the inlet and outlet gas pressures, and Δp is the measured inlet to outlet differential pressure. The 937

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Figure 6. Coke deposition with time in the LTO runs.

Figure 7. Air-based total transmissibility due to measurement uncertainties.

Table 3. Reaction Conditions for the Permeability Change Study no.

reaction type

1 2 3 4

LTO Run 4 LTO Runs 2 and 3 LTO Run 1 Coking Run 1

temp 250 250 300 425

uncertainty in the Klinkenberg-corrected transmissibility as the standard deviation of all the intercepts of all the possible fits. Two blank experiments were conducted to measure the gas flow resistances of the other zones involving the tubing and the samples in the kinetics cell besides the coked sand as shown in Figure 8. The Klinkenberg effect also affects the other zones.

reaction time

°C °C °C °C

8 5 5 5

h h h h

differential pressure directly measured by the differential pressure transmitters was more accurate than the calculated difference between p1 and p2. Therefore, p22 − p21 was replaced by (2p2 + Δp)Δp in eq 1. According to error propagation theory, the relative uncertainty in the air-based total transmissibility, Tg, at each pressure from eq 1 is ΔTg Tg

=

⎛ 1 1 ⎞ ⎟Δ(Δp) + ⎜⎜ + 2p2 + Δp Δp ⎟⎠ ⎝ 2p2 + Δp 2Δp2

(2)

where Δp2 is the measurement uncertainty in the outlet pressure, 1 PSI, and Δ(Δp) is the measurement uncertainty in the differential pressure, 0.02 PSI. Measurements were conducted for four different pressures. The air-based total transmissibility, Tg, is plotted versus the reciprocal of the average pressure in Figure 7,where the average pressure was calculated as p̅ =

2p2 + Δp

Figure 8. Air-based transmissibility of the other zones due to measurement uncertainties.

(3) 2 The transmissibility data was then correlated with linear fits. The data is fit well by the linear fits with the smallest square of the adjusted correlation coefficient being 0.931. Furthermore, the F-variance showed that Tg vs 1/p ̅ was significantly linear at the 0.05 level. Therefore, the Klinkenberg effect existed which is induced by gas slippage in the porous medium when the pore space approaches the mean free path of the gas. The corrected total transmissibility, Tc, with the Klinkenberg effect was determined as the intercept of the best fit line for pressures approaching infinitely as shown in Figure 7. Since the real airbased total transmissibility at each pressure could be any value within the error bar due to the Tg uncertainties, different fits of the data for the possible real air-based total transmissibility resulted in different intercepts of the fits. These gave the

Therefore, the transmissibility in the other zones had to be corrected for the Klinkenberg effect. The corrected transmissibility of the two blank experiments, T2,c, differed by only 0.001, showing very good repeatability. Since the gas flowed through the zones in series, the corrected total transmissibility, Tc, is related to the corrected transmissibility of the other zones and that of the coked porous medium as the harmonic average. The average corrected transmissibility of the other zones, T2,c, was subtracted to obtain the transmissibility, T1,c, and permeability, K, of the coked porous medium as 938

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1 1 1 = − T1,c Tc T2,c K=

corrected transmissibility of the other zones, T2,c, thereby amplifying the permeability uncertainty, ΔK, due to the propagation of the uncertainties in ΔTc and ΔT2,c in eq 5. This is the reason the present study did not use the same experimental procedure to measure the permeability of the clean porous medium. The permeability change with the coke deposition is described using a normalized reduced permeability R defined as K −K R= 0 K0 (6)

T1,cL A

(4)

where L is the length of the coked porous medium, 20 cm, and A is the annular sectional area of the coked porous medium, 15.0 cm2. The relative uncertainty of the permeability of the coked porous medium was ΔT1,c ΔK 1 1 1 1 = = + ΔTc + − ΔT2,c K T1,c Tc T2,c − Tc Tc T2,c − Tc

where K0 is the measured permeability of the clean glass bead porous medium. The Kozeny−Carman (K−C) model has long been used to relate the permeability of a clean porous medium and the porosity. The normalized reduced permeability, R, can be described by the K−C model as

(5)

where ΔTc and ΔT2,c are the uncertainties in the Klinkenbergcorrected transmissibilities of all the zones and the other zones. Thus, the uncertainty in the permeability of the coked porous medium was determined from the error propagation via eq 2, linear fitting with variable uncertainties, and from eq 5 from the measurement uncertainties Δp2 and Δ(Δp). The permeability of a porous medium packed with 160−180 mesh clean glass beads was also measured with water using the steady-state method according to Darcy’s law. The permeability of the clean glass bead porous medium was measured at water flow rates of 8 mL/min, 12 mL/min, and 16 mL/min. The average permeability was 5.29 D with a standard deviation of 0.0557 D. The permeabilities of the coked and clean porous medium are plotted with their absolute uncertainties versus the coke deposition per unit reservoir volume in Figure 9.

R=1−

2 ϕ3 ⎛ 1 − ϕ0 ⎞ ⎜ ⎟ ϕ03 ⎝ 1 − ϕ ⎠

(7)

where ϕ0 is the original porosity, and ϕ is the porosity after coke deposition which depends on the amount of coke deposition and the coke density. Figure 10 shows the measured

Figure 10. Measured normalized reduced permeability and K−C predictions.

and predicted normalized reduced permeability. The uncertainty in R predicted by the K−C model was due to the uncertain coke density, which ranged from 0.9 kg/m3 to 1.1 kg/ m3. As shown in Figure 10, the experimentally measured R is close to those predicted by the K−C model at the smallest coke deposition of 18.5 kg coke/m3 beadpack, while R increases more rapidly with coke deposition than predictions by the K−C model. Two reasons can be used to explain the different relationship between R and coke deposition related to the Darcy scale and the reservoir scale. The Darcy scale refers to the scale of a representative elementary volume without great spatial variability of porous medium properties such as porosity and permeability. The properties in the reservoir scale are more heterogeneous with greater variations than those in the Darcy scale. The K−C relationship or other models have been used to relate the properties based on the Darcy scale. The first reason is related to the complicated pore structure at the Darcy scale. An SEM and a mercury porosimeter were used to analyze the

Figure 9. Permeabilities of the coked and clean porous medium.

As shown in Figure 9, the permeability decreased as the coke deposition increased. The coke deposition varied from 18.5 kg coke/m3 beadpack to 53.3 kg coke/m3 beadpack for the five experiments conducted at four operating conditions. LTO Run 2 and LTO Run 3 were conducted at the same condition, resulting in 33 kg coke/m3 beadpack in LTO Run 2 and 40 kg coke/m3 beadpack in LTO Run 3. The different amounts of the coke deposition then led to different permeabilities with the same qualitative relationship as the general trend seen in all the experiments. The maximum permeability uncertainty occurred at the smallest coke deposition of 18.5 kg coke/m3 beadpack where the corrected total transmissibility, Tc, approached the 939

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Figure 11. SEM micrograph of the coked grain clusters.

pore volume for the pore sizes between 1 μm and 10 μm accounted for 34.5% of the entire pore volume. Since the air mean-free path at standard conditions is about 0.1 μm, slip flow then appeared in the coked porous medium in approximately 34.5% of the pore with the Knudsen numbers between 0.01 and 0.1. Furthermore, the cumulative volume where pore sizes were less than 1 μm occupied 5.3% of the pores. Therefore, transition flow occurred in 5.3% of the pores at standard conditions. Thus, the complicated pore structure and increasing percentages micropores at great coke depositions not only rapidly reduced the permeability but also tended to further promote the Klinkenberg effect. Another reason for the different relation between R and the coke deposition is related to the heterogeneous coke concentration distribution at the reservoir scale. The coked porous medium measured was 20 cm long with a heterogeneous structure. The coke mass fractions in 24 coked grain clusters all collected from one experiment were measured by TGA, with the results shown in Figure 13. The coke concentration was not homogeneous with a highest mass fraction of 8.84% and a lowest mass fraction of 2.90% in the same core. The oil distribution was speculated to become uneven during the experiments, which then affected the coke

microstructure of the coked porous medium. Figure 11 shows an SEM image of the grain cluster with coke deposition after removing the toluene soluble fractions. The SEM images showed that a portion of the coke was coated on the glass bead surface with the rest in pore bridges and in pores. The coke consolidated the grain clusters and complicated the pore structure. The pore structure is similar to Chen et al.17 on the permeability variations after ZrO2 colloid deposition. Chen et al.17 demonstrated that the K−C model could not well predict the permeability after colloidal deposition because of the increased spatial complexity, particularly for a high amount of colloid deposition. Chen et al.17 stated that the solid residue created discontinuous regions in the pore structure that developed heterogeneous preferential flow paths with scales of less than 1 mm. Therefore, the pore structure physically deviated from the modeling premise of the K−C model developed for clean porous medium.17 The pore size distribution of the coked grain clusters is shown in Figure 12

Figure 12. Pore distribution in the coked grain clusters with an average 4.28% coke deposition by weight.

with an average coke residue mass fraction of 4.28% as measured by TGA. The samples were prepared in the LTO runs using another Xinjiang super crude oil Z32. The coke clogged the medium and large pores, which increased the micropore volume and then destroyed primal normal distribution of pore size. For these samples, the cumulative

Figure 13. Coke concentrations in coked grain clusters from one core sample. 940

DOI: 10.1021/acs.energyfuels.5b02600 Energy Fuels 2016, 30, 933−942

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Energy & Fuels

underestimates of the global permeability predicted only in terms of the average coke concentration, particularly for cores with high average coke concentrations. Finally, a correlation of the permeability change with the amount of coke deposition was obtained, which is applicable to ISC or THAI processes in sandstone reservoirs composed of well sorted sand and with high original permeability.

concentration distribution. The permeability decreased more rapidly in the regions with more solid residue with a nonlinear decrease.17 Since the gas flowed through all the sections in the cores in series, local regions of low permeability would greatly decrease the global permeability based on the harmonic average. However, the average coke concentration does not reflect the floating local coke concentrations in the core samples. Therefore, the global permeability change in the 20 cm long core samples would be underestimated by the average coke concentration, regardless of the coke concentration heterogeneous distribution in the cores. The underestimates were aggravated, particularly in the cores with high average coke concentrations due to more heterogeneous structures. 3.4. Correlation of the Permeability Change with the Amount of Coke Deposition. The experimentally measured R was linearly correlated with the coke depositions for coke deposition from 18.5 kg coke/m3 beadpack to 53.3 kg coke/m3 beadpack as R = 0.118m _coke + 0.0123

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

The work was supported by the National Science and Technology Major Project (2011ZX05012) and the PetroChina Technology R&D Project on New Technology and Method for Oil & Gas Development (2014A-1006) and the Science Fund for Creative Research Groups (No. 51321002).

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The correlation coefficient was 0.991, showing a very good fit. The nonlinear relationship may occur outside this range. The coke concentrations in the coked zone in combustion tubes and actual reservoirs undergoing ISC are usually in this range according to the public literature.27 In addition, Yang and Gates compared data in the literature for the thickness of the combustion front with coke deposition during ISC with estimates from 1.5 to 10 cm.28 Thus, the combustion front thickness is close to the size of the coked cores in the present study. Therefore, the model is recommended for describing the permeability change in postdeposition sandstone composed of well sorted sand and with high original permeability.

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4. CONCLUSIONS The study presents an experimental study to physically simulate coke formation in the crude oil ISC process with in situ measurement of the postdeposition permeability. The studies measure the coke deposition with the Xinjiang crude oil with air or nitrogen flows. The critical coking temperatures differed significantly with at least 200 °C difference between the LTO runs with air and the coking runs with nitrogen. Many resins and small amounts of asphaltenes in the Xinjiang crude oil stabilized the colloidal dispersion structure in the crude oil, thereby prolonging the induction coking period in the coking runs. The coke deposition did not increase continuously with the temperature in the LTO runs but exhibited a maximum due to the balance of the coke generation by LTO and the coke consumption by HTO at various temperatures. Increasing oil displacement and distillation with temperature also reduced coke deposition after reaching the peak in the LTO runs. The study also found that coke deposition increased with time with the air flow due to the LTO effects. The understanding of the effects of the LTO and reaction time on the coke deposition was used to produce core samples with different amounts of coke deposition. The results indicated that the Klinkenberg effect occurred in the coked porous medium. The complicated pore structure and increasing numbers of micropores at large coke depositions not only rapidly reduced the permeability so that it deviated from the Kozeny−Carman relationship at the Darcy scale but also further promoted the Klinkenberg effect. The heterogeneous coke concentration distribution in the core samples led to 941

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