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Sep 1, 2007 - Lithium/tungsten oxide. 99.9. 742. 3,710 .... at an input power of 6 kW and 4,030 (3,757ıC) for an input power of 11 kW as shown in Table 2.
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Plasma Heating of Carbonate Formations

M. H. El-Naas a; R. J. Munz b; J. Rossi b; A. Y. Zekri a a Department of Chemical and Petroleum Engineering, United Arab Emirates University, UAE b Department of Chemical Engineering, McGill University, Montreal, Quebec, Canada Online Publication Date: 01 September 2007 To cite this Article: El-Naas, M. H., Munz, R. J., Rossi, J. and Zekri, A. Y. (2007) 'Plasma Heating of Carbonate Formations', Petroleum Science and Technology, 25:9, 1143 - 1161 To link to this article: DOI: 10.1080/10916460500527021 URL: http://dx.doi.org/10.1080/10916460500527021

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Petroleum Science and Technology, 25:1143–1161, 2007 Copyright © Taylor & Francis Group, LLC ISSN: 1091-6466 print/1532-2459 online DOI: 10.1080/10916460500527021

Plasma Heating of Carbonate Formations M. H. El-Naas Department of Chemical and Petroleum Engineering, United Arab Emirates University, UAE

R. J. Munz and J. Rossi Department of Chemical Engineering, McGill University, Montreal, Quebec, Canada

A. Y. Zekri Department of Chemical and Petroleum Engineering, United Arab Emirates University, UAE

Abstract: Calcium carbonate rock samples were exposed to high temperature argon plasma to investigate the efficiency of plasma heating for fracturing carbonate formations. Plasma temperature can change the basic properties of the rock through fracture and calcination, which in turn can result in a significant increase in its porosity and permeability. Several calcium carbonate cores with a diameter of 25.4 mm and a height of 29 mm were subjected to axial plasma heating at different temperatures and for different periods of time. The experimental results indicated that the top surface temperatures of the samples ranged from 800ıC to 900ıC and the temperature reached steady state within 4 min. The scanning electron microscope (SEM) and porosity analysis of the treated samples indicated significant changes in the basic structure of the rocks and a substantial increase in both porosity and permeability. A mathematical model was developed to predict the temperature distribution along the axis of the heated sample and to estimate the time needed to achieve steady state. The model predictions were in good agreement with experimental results. Keywords: carbonate formations, fracturing, modeling, plasma heating, stimulation

1. INTRODUCTION Thermal recovery processes used by the oil industry nowadays can either involve the injection of hot fluid into the reservoir or the generation of heat Address correspondence to M. H. El-Naas, UAE University, Department of Chemical & Petroleum Engineering, P.O. Box 17555, Al-Ain, UAE. E-mail: muftah@ uaeu.ac.ae 1143

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within the reservoir itself. Thermal stimulation decreases resistance to oil flow and, thus, improves crude oil production very quickly due to the fact that its effects are confined to the neighborhood of the wellbore. In hot fluid injection, steam is injected into the formation for a few weeks to let the heat soak in and allow the steam to condense, and then put the well on production (Hong and Jensen, 1969; Rivero and Heintz, 1975). Other fluids can be used instead of steam, but none have been found as effective. White and Moss (1965) reported the use of high temperature to stimulate oil recovery. The wellbore is usually heated either by using a gas-fired downhole burner or a downhole electrical heater or by circulating fluids heated at the surface. Albernethy (1976) proposed to use electrical current as a source of energy to generate heat. DePriester and Pantaleo (1963) reported an increase in the oil production rate obtained with a downhole gas burner. Chakma and Jha (1992) introduced heat to the reservoir through electromagnetic heating. The effect of ultra-high temperature on the rock properties has not been reported in the literature. In many parts of the world, the petroleum industry employs hydraulic fracturing to handle reservoirs with extremely low permeability. The fractures are propagated by forcing a fluid into the geologic formation. McDaniel et al. (2002) proposed to use a small jetting tool constructed for the specific well being stimulated and placed on the end of the treating string. Jiang et al. (2003) presented a new comprehensive hydraulic fracturing technology to minimize formation damage during hydraulic fracturing. Fracture toughness, known as the critical stress intensifying factor (SIF), is one of the key parameters in fractured body analysis, and it is also an indication of the degree of elastic field singularity around the fracture tip. Chen and Zhang (2004) summarized aspects of fracture toughness, including rock sample preparation, experiment flowchart, and sample shapes after failure. They found the impact of confining pressure on toughness to be linear. The majority of today’s hydraulic fracture stimulations are performed in North America, with a good number of these in onshore U.S. tight-formation gas reservoirs (Britt, 2003). The optimization and conducting of the fracture in a correct way is essential for the success of the fracturing job. Britt (2003) pointed out that nearly $3 billion (U.S.) is invested annually in hydraulic fracturing, and the petroleum industry doesn’t optimize its treatments. He also mentioned that in a recent presentation, the SPE president stated that 2/3 of the wells fracture-stimulated in the U.S. do not perform as planned. Therefore, there is an urgent need for the development of a new, more reliable and successful fracturing technique. In the present study, the effect of plasma heating on the basic rock properties was investigated. The main purpose of using ultra-high temperature is to stimulate the area around the wellbore, and hence improve the permeability. This technique could be used in the case of tight carbonate formation instead of other normally used fracturing techniques. The intensity of heat could be easily controlled and treatment location could be precisely targeted,

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leading to a better control of the degree of improvement in permeability. A high temperature plasma jet can insure rapid and efficient heat transfer to the surrounding wellbore rocks. Argon plasma is used in this study for experimentation. However, in a commercial application, a less expensive and more readily available gas such as air, carbon dioxide, or nitrogen will be used. Plasma is defined as an ionized gas that contains a mixture of molecules, atoms, ions, electrons, and photons (Fauchais et al., 1987). Over the past decade, thermal plasma technology has seen considerable advances and growth in industrial applications and in scientific research. It has been applied to numerous processes, including spraying, extractive metallurgy, chemical vapor deposition, thermal decomposition, and synthesis of fine powders as well as spheroidization and sintering of powders (Boulos et al., 1994). Several studies have been carried out on the use of plasma in the pyrolysis of coal (Baumann et al., 1988), vitrification of nuclear waste (Munz and Chen, 1989), pyrolysis of medical waste (Camacho, 1990), pyrolysis of heavy oil residues (El-Naas et al., 1993), production of fumed silica (Addona and Munz, 1994), and synthesis of calcium carbide (El-Naas et al., 1998). In a previous study (El-Naas and Zekri, 2002), the effect of regular heating on the structure, porosity, and permeability of calcium carbonate rocks was examined. The study showed considerable increase in both porosity and permeability of carbonate rocks due to pore formation at the calcination temperature. The prime objective of the present work is to assess the efficiency of plasma heating for fracturing carbonate rocks and to model the temperature distribution along the heated cores.

2. MATERIALS AND METHODS 2.1. Carbonate Samples Limestone cores, obtained from outcrops at Hafeet Mountains (Al-Ain, UAE), were used to prepare calcium carbonate samples for the plasma heating experiments. The carbonate rocks were cut into 12 small cylinders, 25.4 mm in diameter and 29 mm in height. These samples were exposed to an induction plasma tailflame. They were positioned 25 mm below the plasma torch and supported by a 1/4-inch alumina (Al2 O3 ) rod inserted into a 10-mm deep hole drilled into the bottom surface of the sample. The support was designed to minimize heat losses. Since alumina is a poor thermal conductor and the rod had a small diameter, the conductive heat losses were kept low compared to the primary heat loss due to radiation. The alumina rod was supported by an electrically insulated metal clamp attached to a lab jack which allowed a precise and quick positioning of the sample below the torch. The torch was adjusted to the desired operating conditions and then the sample moved into place to start an experimental run. The samples were heat-treated with argon plasma at two input powers (6 and 11 kW) and for two time intervals (5 and

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10 min); all experiments were carried out in triplicate. After treatment, the samples were removed and placed in desiccators to cool. They were then taken for porosity and SEM analysis. 2.2. Plasma Generating Device Plasma is usually generated by applying a high frequency, high voltage electric field to a gas. This can be achieved through generating devices that have electrodes, which are characterized by the presence of a cathode and an anode like plasma arcs or electrodeless devices, which use inductive or capacitive coupling. The plasma device used in this study was an RF induction plasma torch, which generates plasma by inducing a current within the gas. An electric current is passed through a copper coil, which is wrapped around a water-cooled quartz tube (38 mm internal diameter and 3 mm thickness). The current induces a time-varying magnetic field, which in turn induces an electric field inside the tube. The tube was internally coated with gold to facilitate ignition of the plasma. Argon at about 1.39E-03 kg/s (50 slpm) and a pressure of 345 kPa (50 psi) is injected into the plasma torch, where it is heated and ionized by the induced current, resulting in high enthalpy plasma. The ionized gas exited the torch through a water-cooled copper nozzle 25.4 mm in diameter. 2.3. Temperature Measurements The top surface temperature of the heated samples during plasma treatment was measured using a Micro-Optical Pyrometer Model M-6533 (Luxtron Corporation, Beaverton, Oregon) focused on the side of the sample at a location about 3 mm from the upper surface. The instrument is capable of measuring blackbody temperatures from 704 to 3,204ıC and is of the disappearing filament type. Since plasma could affect the readings of the optical pyrometer, comparisons were also made with the heating of pellets made from materials with known melting points. The pellets were prepared and placed in the plasma tailflame at different plasma powers. The dimensions of the pellets and their physical properties such as emissivity have important effects on heat transfer. It was therefore necessary to make pellets out of materials that had heat transfer properties similar to those of calcium carbonate with approximately the same dimensions. In addition, the materials used should not be hazardous and should have melting points ranging from 750 to 1,250ıC. Four metal oxides were chosen as shown in Table 1. The pellets were exposed to the induction plasma tailflame, while varying the input current from 1.6 to 2.7 A at constant argon gas flow rate. If the pellet melted, then the temperature reached at least the melting point of the material. At the same power setting, a higher melting point material was placed under the torch. If it did not melt, then the temperature range at this

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Table 1. Properties of pellets of known melting points

Material

Purity %, metal basis

Melting point, ı C

Density, kg/m3

Lithium/tungsten oxide Cobalt (II,III) oxide Tin (II) oxide Copper (I) oxide

99.9 99.7 99.9 99.0

742 900 1,080 1,235

3,710 6,110 6,450 6,000

particular power is between the melting point of the first and the second material. If it did melt then the temperature is greater than the melting point of the second material and the next higher melting point material was placed under the flame and the procedure was repeated.

3. MODELING OF THE HEATING PROCESS A one-dimensional mathematical model was developed to predict the temperature profile along the axis of the heated carbonate sample as a function of time. The following assumptions were made. 1. The sample receives heat only at the top surface by convection. Radiative heat transfer to the sample from within the torch could be neglected because of the small shape factor and low emissivity of argon. 2. The sample loses heat from the sides and bottom by radiation. Conductive heat losses through the alumina rod are negligible since ceramic is a poor thermal conductor and the rod cross-sectional area is small. 3. Heat transfer within the carbonate sample is only by conduction. 4. Temperature varies only in the axial direction. This is an approximation that can be justified by the fact that the top surface of the sample is completely immersed in the plasma flame as shown in Figure 1. 5. The properties of calcium carbonate remain constant. The plasma temperature was estimated through a simple heat balance on the gas in the torch, Net electrical energy to gas D enthalpy change of gas (1) P D m.H P Plasma

Ho /

where P is the power supplied to the torch  is the torch efficiency, HPlasma is the plasma enthalpy, Ho is the enthalpy of argon at 300 K, and m P is argon mass flow rate 1:39  10 3 kg/s. The torch efficiency was previously measured by calorimetry and found to remain constant at a value of 25%

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Figure 1. A photograph of a calcium carbonate sample in an argon plasma flame at 11 kW about 60 s after insertion into the flame.

(Munz, 1974). The plasma temperature was then calculated using Eq. (1) and thermodynamic tables (Boulos et al., 1994). The computer program divided the cylindrical sample into thin discs and considered three different types of discs: the top, bottom, and interior discs. Heat balance can then be carried around the three types of discs to solve the temperature distribution along the axis of the sample as a function of time. The top disc, which is in direct contact with the plasma tailflame, receives heat by convection and loses heat by radiation and conduction. The heat balance equation can be set up as follows: hc A2 .TPlasma

Tn /

"A1 .Tn4

4 T1 / C kA2

@T @T D VCP : @z @t

(2)

For an incremental change in time, t, Eq. (2) becomes hc A2 .TPlasma

Tn /

"A1 .TnP

4

4

P P T1 / C kA2 .TnC1

TnP /=L (3)

D

VCP .TnP C1

TnP /=t

solving for TnP C1 , the temperature at the next time increment, TnP C1 D

t Œhc A2 .TPlasma VCP C

P kA2 .TnC1

Tn /

TnP /=L

"A1 .TnP

4

4

P T1 /

(4) C

TnP :

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The interior discs receive heat by conduction and lose heat by radiation and conduction. The heat balance equation can be set up as follows: kA2

@T @z

"A1 .Tn4

4 T1 / C kA2

@T @T D VCP : @z @t

(5)

For an incremental change in time, t, Eq. (6) becomes, kA2 .TnP

TnP 1 /=L

D VCP .TnP C1

"A1 .TnP

4

4

P P T1 / C kA2 .TnC1

TnP /=L (6)

TnP /t

solving for TnP C1 , TnP C1 D

t Œ kA2 .TnP VCP C

P kA2 .TnC1

TnP 1 /=L

"A1 .TnP

4

4

P T1 /

(7)

TnP /=L

C

TnP :

The bottom disc receives heat by conduction and loses heat only by radiation. The heat balance equation can be set up as follows: kA2

@T @z

".A1 C A2 /.Tn4

4 T1 / D VCP

@T : @t

(8)

After an incremental change in time, kA2 .TnP

TnP 1 /=L

".A1 C A2 /.TnP

4

4

P T1 /

(9) D VCP .TnP C1

TnP /=t

solving for TnP C1 , TnP C1 D

t Œ kA2 .TnP VCP ".A1 C

4 A2 /.TnP

TnP 1 /=L (10) P4 T1 /

C

TnP :

Equations 4, 7, and 10 were solved using a MATLAB program. The convective heat transfer coefficient, hc , could not be found in the literature because of the extreme experimental conditions (very high plasma temperatures). Therefore, the computer program was designed to predict the heat transfer coefficient from top surface temperature, which was determined experimentally in section 2.3. The program assumes that hc does not change with time. Inputs to the program include: the top surface temperature, Ttop ; the plasma temperature, TPlasma ; and the limits of hC , hC1 , and hC 2 . The computer model produces a heating curve that shows the temperature profile

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of each node as a function of time. The sample was divided into 30 nodes; the height of each node (L) is simply the height of the sample (29.0 mm) divided by the number of nodes. The outputs of the program include: the time to reach steady state, S ; the convective heat transfer coefficient, hC ; 0 and the calculated top surface temperature, Ttop . Steady state was achieved when the initial and final temperatures were within a certain preset error.

4. RESULTS AND DISCUSSION 4.1. Heating Temperatures Although high temperature is one of the main advantages of plasma processes, the accurate measurement of temperature is one of the major difficulties associated with plasma. Two temperatures were estimated in this study: the plasma temperature and the top surface temperature of the heated samples. The first was calculated through combining energy balance and thermodynamic tables. The plasma enthalpy (HPlasma ) was calculated from Eq. (1) and the corresponding plasma temperature was obtained from thermodynamic tables for argon plasma. The calculated temperatures were 2,530 K (2257ıC) at an input power of 6 kW and 4,030 (3,757ı C) for an input power of 11 kW as shown in Table 2. The temperature of the top surface was estimated through heating of materials with known melting points and measured using a micro-optical pyrometer. For the low input power (6 kW), the estimated temperature using the first method was approximately 850ı C. This temperature was confirmed by the pyrometer reading, where the average steady state temperature was measured to be 846ıC. In spite of this good agreement, the two methods gave different results for surface temperature at high power input. At the 11 kW plasma power, the estimated temperature was about 900ıC, while the average measured steady state temperature was 958ıC. This discrepancy could be attributed to the interference of the high temperature plasma on the sample temperature measurements. For this reason, the temperature estimated by the melting point method is expected to be closer to the actual surface temperature; the estimated temperature for both power inputs was used as an input for the Ttop in the computer model.

Table 2. Calculated plasma temperature at different powers Experimental set

Current, A

Power, kW

Plasma temperature, K

1 2

1.9 2.7

6 11

2,530 4,030

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4.2. Modeling Results The main objectives of the mathematical modeling were to estimate the time needed to reach steady state, to estimate the top surface temperature, and to determine the temperature profile along the axis of the heated samples as a function of heating time. Although the model was simplified, it gave useful predictions that were within the experimental observations. The modeling predictions for the Ttop and the s are compared with the experimental values in Table 3. For the low plasma power (6 kW), the predicted values for hC , Ttop , and s were 17.5 W/m2 K, 857ıC, and 246 s, respectively. The values predicted at the high power (11 kW) were 10 W/m2 K, 907ıC, and 248 s. The measured and predicted top surface temperatures are plotted versus time in Figure 2 for the low power and in Figure 3 for the high power. Both figures clearly show that the model gave good predictions for the sample surface temperature and the time needed to reach steady state. However, the mathematical model seems to predict initial heating rates that are lower than those observed experimentally as shown in the figures. This could be attributed to the possibility of convective heating from the sides near the top of the samples. The contribution of such heating was assumed to be negligible in the model. It is also important to note that temperature measurements using the optical pyrometer were rather difficult during the first few seconds (less than 100 seconds); therefore, values recorded during this time period may not be reasonably accurate. Another important parameter plotted in the figures is model prediction for the temperature at the bottom of the treated samples as a function of time. At steady state, the bottom temperature Tbtm was 707ıC and 727ıC for input powers of 6 kW and 11 kW, respectively. Clearly, both top and bottom surface temperatures reached steady state in a relatively short period of about 4 min. The fact that the bottom surface, which was 29 mm away from the exposed surface, could reach temperature greater than 700ıC within 4 min in spite of the huge heat loss by radiation is a clear indication of the effectiveness of the plasma heating process. Although reasonable amounts of calcination may not be achieved for temperatures less than 800ıC (El-Naas and Zekri, 2002), some fracture may take place due to the sharp increase in temperature.

Table 3. Summary of modeling results

Power, kW

hC , W/m2 K

Ttop measured, ıC

Ttop , model, ıC

Tbtm model, ıC

S model, sec

S experimental, sec

6 11

17.5 10.0

850 900

857 907

707 727

246 248

300 260

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Figure 2. A comparison between measured and predicted top surface temperature, Ttop . Power input is 6 kW.

Figure 3. A comparison between measured and predicted top surface temperature, Ttop . Power input is 11 kW.

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4.3. Calcination and Porosity Calcium carbonate decomposes at atmospheric pressure and high temperatures to form calcium oxide (quick lime) and carbon dioxide according to the following calcination reaction: CaCO3 , CaO C CO2 : Thermodynamic analysis of the reaction indicated that it commences at about 650ı C and reaches completion at 1,000ıC. At atmospheric pressure, the equilibrium molar fraction of carbon dioxide produced (x), which is also the fraction of carbonate decomposed, is given by the following equation (El-Naas and Zekri, 2002): x.T / D

0:98 T 1168 1 C e Œ 65 

(11)

where, T is the reaction temperature in Kelvin. According to Eq. (11), the fraction of calcium carbonate calcinated (decomposed) for the plasma treatment temperatures, 850ıC and 900ıC, are 0.33 and .51, respectively. These fractions would be achieved if the whole carbonate sample reached these temperatures. However, experimental and modeling results indicated that only the top part of the samples reached these temperatures; therefore, the actual fraction of carbonate decomposed was found to be much less as shown in Table 4. This was estimated from the weight loss after treatment, assuming that this loss in mass represented the amount of carbon dioxide produced. The changes in porosity and permeability due to calcination and the release of CO2 were measured for regular oven heating and correlated to temperature (El-Naas and Zekri, 2002). The increases in porosity (˛) due to pore formation for oven heating were 41% and 61% at 850ıC and 900ı C, respectively. The change in carbonate porosity due plasma treatment was also measured in the present study. The average increases in porosity (˛) were found to be 303% and 470% for 850ıC and 900ıC, respectively. These

Table 4. A comparison of changes in porosity, permeability, and calcination between oven heating and plasma heating Calcination fraction, x

Increase in porosity, ˛

Increase in permeability, 

Temp., ıC

Eq. (11)

Plasma heating

Oven heating

Plasma heating

Oven heating

Plasma heating

850 900

0.33 0.51

0.14 0.28

41% 61%

303% 470%

45% 48%

88% 130%

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significant increases in porosity can only be attributed to fracture caused by the plasma heating. It is important to note here that the initial porosity of the carbonate was found to have considerable effect on the percent increase; the lower the initial porosity the higher the increase. This could be expected since less porous formations can have better conductive heat transfer. The increases in permeability () due to pore formation for oven heating were 45% and 48% at 850ıC and 900ı C, respectively. The change in the carbonate rock permeability due to plasma treatment was estimated in this study based on the measured change (due to plasma exposure) in porosity. The average increases in permeability () were estimated to be 88% and 130% for 850ıC and 900ı C, respectively. This substantial improvement in permeability is attributed to the creation of micro-fractures in addition to pore formation. The changes in porosity and permeability are summarized in Table 4. 4.4. Fracture and Pore Formation One of the most important objectives of this study is to assess the efficiency of plasma heating in fracturing calcium carbonates rocks. It is expected that plasma heating will not only lead to calcination, but also to fracture due to sudden and intense heating. Therefore, it was essential to examine the effect of plasma heating on the microstructure of the treated carbonate samples. Several samples, treated at different plasma conditions, were examined under a scanning electron microscope (SEM). An untreated sample was also examined and its SEM image is presented in Figure 4 for comparison. Images for

Figure 4. An SEM image of an untreated calcium carbonate sample.

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the treated samples were taken for different positions along the axis of each sample. Figure 5 presents a micro image of the bottom of a sample exposed to argon plasma at an input power of 6 kW for a period of 5 min. The heating temperature is estimated to be around 707ıC. It is clear that there is no calcination but some signs of fracture are evident. It is worth noting at this point that estimation of fracture gradient required to induce fracturing of subsurface formation is one of the major problems facing the petroleum industry when applying the current hydraulic technique. A lost-circulation problem will be created if the engineer fails to accurately predict the pressure gradient required to create the fracture and that could be very costly for the company. Lost-circulation is loss of drilling fluids to the formation due to the high permeability section of the zone and pressure against the formation is much higher than formation pressure. In the proposed plasma technique, pressure does not play any role in fracturing the formation, and the safety aspects of the operation are quite high if compared to the current technique. Figure 6a shows an image for the bottom of a sample treated at the high power (11 kW) for 10 min. There are clear signs of fractures and the start of calcination. It is obvious that fracture occurred due to thermal shock even before reaching the calcination temperature, since the estimated surface temperature is 727ı C. It is clear that the plasma heat affected those parts of the core relatively far away (29 mm) from the exposed surface keeping in mind that the system was exposed to a very short time of 10 min. This indicates that the fractures are continuous, which was confirmed by visual

Figure 5. An SEM image of the bottom of a sample treated at the lowest power (6 kW) for 5 min. Estimated temperature (707ıC).

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(a)

(b) Figure 6. (a) An SEM image of the bottom of a sample treated at high power input (11 kW) for 10 min. Estimated temperature (727ı C). (b) Another SEM image from a different angle of the bottom of a sample treated at high power (11 kW) for 10 min. Estimated temperature (727ıC).

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Figure 7. An SEM image of the top of a sample treated at the lower power (6 kW) for 10 min. Estimated temperature (850ıC).

observation. Fracture interconnection is very important for the fluid flow and transport to take place in the void space created by these fractures. Figure 6b shows another image of the bottom of a sample exposed to a power of 11 kW for 10 min; again there are a good number of fractures with 10 m of length and around 2 m width. The image exhibits fractures that are capable of conducting fluids from the matrix and/or through the fracture. An image of the top surface of a sample treated at an input power of 6 kW for 10 min is shown in Figure 7. Since the surface was directly exposed to the plasma heat (estimated temp 850ı C), the image shows clear fracture at the grain level with obvious calcination. The fractures are interconnected at the exposed surface which will accelerate the flow of fluids and increase fractures/matrix communication. It is clear from the image that the density of the fracture (number of fractures per unit area) is quite high around 70–80%, which significantly increases the conductivity of the system and consequently improves productivity. Figure 8a shows an image of the top of a sample treated at 11 kW for 10 min, where the final surface temperature is estimated at 900ıC. The image shows a clear fracture at the grain level with obvious calcination. Increasing the temperature and exposing the surface directly to plasma heat resulted in spreading the fractures on the exposed surface (100% fracture density) with a variation with respect to fracture width as shown in the figure. The width of the fracture reached up to around 3 m in some areas of the exposed surface. This means that the fracture will be very conductive, acting as an open channel for the flow.

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(a)

(b) Figure 8. (a) An SEM image of the top of a sample treated at the high power (11 kW) for 10 min. Estimated temperature (900ı C). (b) An SEM image of the top fragment of a sample treated at the high power (11 kW) for 10 min. Estimated temperature (900ı C).

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Another image for the top surface that was directly exposed to the plasma at the same conditions (11 kW) is shown in Figure 8b. This part of the sample was cracked into fragments due to the thermal shock. It is believed that radial propagation of the high density fracture has resulted in cracking the top part of the sample, which has a small diameter (25.4 mm). Cracking will not be expected for an actual application since the fracture will have room for continuity and propagation. The experiments were carried out in triplicate to ensure that the findings previously described are not isolated cases and that plasma treatment is capable of fracturing the carbonate rocks.

5. CONCLUSIONS Several calcium carbonate samples were treated with argon plasma at different conditions. The exposed surfaces reached steady state temperatures of up to 900ı C within 4 min. Microscopic analysis of the treated samples indicated clear fracture with high fracture density. The top layers, which were directly exposed to the plasma tailflame, exhibited pore formation due to calcination as well as considerable fracture, while the bottom layers did not have any pore formation but showed some fracture due to the thermal shock. Porosity and permeability analysis of heated samples showed substantial improvements in both porosity and permeability. The results provide evidence for the technical viability of plasma technology as a new tool for stimulating tight carbonate formations. Plasma technology could offer an effective, safe, and environmentally friendly process that would lead to significant improvements in overall oil recovery.

ACKNOWLEDGMENT The financial support of the Natural Science and Engineering Research Council of Canada is gratefully acknowledged.

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Plasma Heating of Carbonate Formations

NOMENCLATURE A1 A2 CP hC H0 HPlasma k L m P P t T1 Tbtm TnP TnP C1 TPlasma Ttop V x z

Side surface area ( DL), m2 Top and bottom surface area (D 2 =4/, m2 Heat capacity, kJ/kg K Convective heat transfer coefficient, W/m2 K Gas enthalpy at 300 K, kJ/kg Plasma enthalpy, kJ/kg Thermal conductivity, W/m. K Thickness of the thin disc, m Plasma gas flow rate kg/s Power, kW Time, s Ambient temperature, K Bottom surface temperature, K Temperature at node n at time t, K Temperature at time .t C t/, K Plasma flame temperature, K Top surface temperature, K Volume, m3 Fraction calcinated Axial distance away from the heated surface, m

Greek Symbols ˛   "    S

Porosity The Stefan-Boltzmann constant .5:67  10 8 ), W/m2 K4 Incremental change Emissivity Permeability Density of calcium carbonate, kg/m3 Torch efficiency Time to reach steady state, s

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