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Use Of Fired Heaters For Hydrocarbon Recovery

7.1 Introduction

In this method, a system of fired heaters is used for hydrocarbon recovery. The design used for this process involves the use of tubes arranged in concentric pattern and positioned inside the deposit. These are linked to the burner at the ground level by the wellhead. The gases are burned in the burner and then it travels to the lower end of the tube placed in the centre. After that the gas travels back upwards trough the space between the concentric tubes. If a particular zone in the formation does not require heating, then the portion of the tube passing through that region is insulated. In order to increase effectiveness of the system, a number of these heaters can be co-joined in a manner such that the gas output from one can be fed to the second as input air. Once the heat from the gas is transmitted to the surrounding deposit by thermal conduction, the gas losses some of its heat and the temperature fall as a consequence. This gas, now having a much lower temperature is then fed to the next wellbore in line as air. This procedure is however limited to the quantity of oxygen present in the gas and can be continued for a number of wells before the oxygen content reads nil. Fuel is burned also at the burners of the consecutive wellbores thus providing a constant flow of fuel gas through all the wells. A heat exchanger can also be applied to pre-heat the intake air of the first heater by using the combusted gases from the last heater of the system. The connection between all the wells should be insulated piping. It is essential to maintain an even range of temperature across the entire system and this is achieved by ensuring proper and unhinged flow of high quantity fuel gases in all the wells. Positive point about this system is that it is very easy to maintain. Moreover the concentric tubes used for this process can be utilized again and there is no need to dismantle the whole system for servicing of the gas burners as they are positioned on the surface rather than inside the wellbores. To fix one burner, it is not required to close the entire system and the other fired heaters may keep on working.

7.2 Background and Literature Review

Heating of formations has many uses in the petroleum industry like enhancing hydrocarbon recovery and remediation of contaminated soil. World has vast reserves of heavy and viscous oil and to be able to extract oil from these reserves is a very lucrative proposal. As previously discussed in this report, there are many thermal stimulation processes but advantage of using fired heaters is that it is able to provide uniform heating of the reservoir unlike steam and is much cheaper when compared to methods like electrical heating. Obtaining heat by burning natural gas is a much cheaper option than using electrical energy.

In the past a lot of research and field tests have been done in this field and many papers have been presented over the years. If we pay attention to these we will be able to notice many points worth noting:

“U.S. Patent Nos. 3,095,031 and 2,902,270 reveal details about burners having flames within the wellbores.” [ ]

By studying those papers one can conclude that they had certain drawbacks which prevented their practical use and hence diminished their commercial viability. For example the difference in temperature between the higher and lower ends of the well were very high and hence not acceptable as this leads to non-uniform heating of the formation. Moreover some of the previous designs had provisions for use of certain materials which are very expensive and thus raising the capital cost of the entire project. The use of down hole burners sometimes lead to rise in temperature of the casing in the vicinity of the burners beyond acceptable margins. This leads to the malfunction of the casings as well as the burners. Such problems normally aggravate as the duration of application of heat for thermal recovery processes can be up to a couple of years or even longer. The ideal case scenario for using fired heaters is that the burners should be low maintenance and if necessary should be easily accessible for quick fixing. Another major obstacle is the occurrence of coke and other impurities inside the fuel gas channels.

“U.S. Patent No. 3,181,613 proposes the use of an ignition propagation rod so that the flame can be extended over longer distances.” [ ]

Though it is useful in covering a greater distance, it has a fatal flaw. It is very hard to control the flame as the limitations in terms of heat transmission and temperature rise is very constrained. Moreover this design may very well lead to an accident if by any chance the gas travels back upward leading to ignition. Hence there is severe safety concern for these previous designs.

Therefore it is clear that a design is required which is free from the downsides of all its predecessors. Recent research works and improvement in engineering designs have made sure that a safe and commercially viable model could be developed. This report provides a detailed explanation of embodiment of the design and its operation.

7.3 Description of the Figures

Figure 7.1: Illustrates the schematic of a fired heater in application

Figure 7.2: Shows the cross section of the fired heater

Figure 7.3: This shows another cross section of the gas fired heater

Figure 7.4: Demonstrates the arrangement of a number of fired heaters connected together along with a heat exchanger.

Figure 7.5: Depicts the graph showing plots of temperature with respect to a depth of 200 ft which is heated.

Figure 7.6: Depicts the graph showing plots of heat injection with respect to a depth of 200 ft which is heated.

Fig 7.1: Fired heater

Source: Reproduced from “John Michael; Scott Lee Wellington; Houston, Tex; May 2, 2000”

Fig 7.2: Cross Section of the fired heater

Source: Reproduced from “John Michael; Scott Lee Wellington; Houston, Tex; May 2, 2000”

Fig 7.3: Another cross Section of the fired heater

Source: Reproduced from “John Michael; Scott Lee Wellington; Houston, Tex; May 2, 2000”

Fig 7.4: Six fired heaters and a heat exchanger working together

Source: Reproduced from “John Michael; Scott Lee Wellington; Houston, Tex; May 2, 2000”

7.4 Description of the Design and Operation

In reference to figure 7.1, It depicts a tube casing (1) placed inside the well (10). The casing is closed at the lower end by using a plug (3). The well passes through the overburden (6) and the deposit (5). As seen in the schematic a flow-path (12) is clearly marked showing the path that will be taken by the gas once it is introduced into the system. It will travel from the burner straight down and extend up to the lower end of the well. The lower end of the flow-path is kept open so that the gas may continue to flow and hence enter the space between the two concentric tubes and move back upwards. According to this design a burner (4) which is used to burn the fuel is fixed to the wellhead (13). Air is supplied to the burner by the blower (14). The temperature of the gas is in the range of about 2100. The loss of heat from the gas while it passes through the overburden is minimal as it is covered with insulator (11). As the gas travels downwards into the formation, its temperature decreases as some of its heat is transmitted to the surrounding casing, this transmits the heat to the formation by thermal conduction. The temperature of the gas at the lower end of well is to about 1700. The temperature of the gas coming out of the well exhaust (12) is about 1600. To keep the heat losses to the bare minimum, the design has provisions for using cement in the region of overburden. One important design consideration is to make sure that while drilling the borehole, the radius in the overburden region should be greater than the radius of the region of the main deposit. This allows the width of the cement being used in the overburden region to be larger and hence more effective as an insulator. If the formation is not deep enough, for example less than 500m, then the casing do not need any cement to sustain itself. Under such circumstances the casing must have outer radii of at least 3cm in order to maximize the amount of heat being transmitted. Generally the casings experience high temperatures in this process and hence the casings must be made from materials resistant to high temperatures. For example stainless steel of the grade 304 may be used or alloys can also be used for this purpose. The parameters of the casings that have to be used are determined by the factors like the depth of the reservoir. For instance if the depth of a particular well is 60 ft. then a stainless steel casing can be used with an outer radii of 2 inch and minimum of 0.190 inch thick. In order to keep a constant tab on the temperature, a number of devices for measuring temperatures is utilized by placing them at regular intervals. The burner is supplied with fuel from the fuel bay (8) and is fed with fresh air through the air bay (9).

In reference to figure 7.2, it depicts tree tubes arranged in a concentric manner. The first tube (1) stretches via the overburden. The tube placed in the centre of the arrangement forms the flow-path (4) of the fuel gas. And it stretches up to the lower end of the well. The second tube is utilized to sustain the insulator, which is used to protect the casing. The second tube is provided with insulation (2) generally made out of fiber. A metal covering (3) normally made of steel is present in the exterior region of the insulation. The tubes being used in this process can be re-used in some different well, once the required amount of heating of the present deposit is achieved. Insulator (5) inside the flow-path of the gas can be stretched in order to achieve better transmission of heat to the deposit.

In reference to figure 7.3, the fired heater (3) is using a different arrangement as compared to that of figure 7.2. It has tubes arranged in side by side manner. The first tube (1) stretches via the overburden. The second tube (2) goes up to the lower end of the well. The interior of the casing is closed from the exterior by using catcher (5). Generally, the two tubes utilized for this process have the same radii. Once the two tubes have been put into position, the insulation material (4) can be filled into the casing above the catcher. Sometimes the catcher is not used and instead, to prevent the flow between the two tubes, packing is used which is generally made up of fiber.

In reference to figure 7.4, it depicts the use of more than one heating well (3). Air is introduced into the system by utilizing a blower (6). The system is provided with a heat exchanger (1) via which the gas obtained from the last heater well is passed. This undergoes the process of pre-heating before it is fed to the first well. The connection between the heater wells is made by the insulated tubing (2). This connection acts as the source of air for the next heater well. All the wells have their own individual burners (4) and they are fixed to the wellheads (5). By the time the gas reaches the last well, the amount of O2 left in the gas is less than 3% and is then fed to the heat exchanger for pre-heating. This cyclic process helps in maintain a highly efficient system. The connecting tubes need to be properly insulated in order to prevent loss of heat. This insulation can either be provided in the inside or at the exterior of the tube. In order to keep the capital costs of the whole system as minimum as possible, it is better to provide the insulation for the connecting tubes from the inside. Otherwise if the insulation is provided at the exterior of the tube then the tubes have to made up of high temperature resistant metal which will be very expensive and add to the cost of the system.

Fig 7.5: Graph showing plot of temperature with respect to a depth of 200 ft which is heated. Source: Data from “John Michael; Scott Lee Wellington; Houston, Tex; May 2, 2000”

In reference to the graph in figure 7.5 and figure 7.6, a stands for heat injection at a range of 420 watts/ft, b stands for the temperature of the input gas, c stands for temperature of the gas coming back, d stands for temperature of the tube and e stands for the temperature of the casing.

Fig 7.6: Graph showing plot of Heat injected with respect to a depth of 200 ft which is heated. Source: Data from “John Michael; Scott Lee Wellington; Houston, Tex; May 2, 2000”

During its operation, the engineer may check all the burners by lighting and testing them individually. These burners are now days provided with sensors which detect the level of O2 and CO. The pre-determined temperature has to be obtained and this is done by the operator who increases or decreases the flow of gas until the required temperature is acquired. If under any circumstances, one of the burners fail to work, then in that scenario the operator gets a warning but the others will remain online.

7.5 Inference

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