Concerns of Fouling in SAGD Operations




Steam assisted gravity drainage (SAGD) has now been extensively integrated into commercial production of bitumen in the Athabasca and Cold Lake regions of Alberta.( Ian G, Neilin, C. 2006).This is due to the highly viscous nature of the oil in these oil sands deposits, necessitating the use of non conventional oil recovery techniques such as SAGD, which has proven to be more economically viable than other in situ recovery processes. The Figure given below is a simplified flow chart of the SAGD process

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Figure 1 : Simplified process diagram of typical SAGD operations


The bitumen produced from SAGD operations is not produced as crude oil alone, the high amounts of condensed steam as well as other factors cause the oil to be produced in the form of an oil in water emulsion. Hence there is the need for the application of a reverse emulsion breaker to the emulsion before it enters into the primary separation vessel as outlined in Figure 1. Impurities in the produced water leaving the primary separation vessel that have not been separated by the reverse emulsion breaker is then sent along with the water through a heat exchanger to the skim tank and then to other mechanical separators (such as an induced air flotation, IGF device).

The higher the concentration of the impurities, the more unwanted depositions occur on the surface of the heat exchanger, thereby creating problems of fouling. Common practice in attempt to lessen the impact of this fouling include using two interchangeable heat exchangers, where one can be used while the other is being cleaned, although this fouling upset is still of grave concern due to the required economic expenditure of clean out and labor.

As such, latter parts of this investigation will place an emphasis on the formation, nature and breaking of reverse emulsions, which can result in produced water with lower impurities which can not only lessen the impact of fouling but also be reused for steam regeneration.

1. Change of Thermal Conductivity.




As mentioned above, bitumen is a fluid with high viscosity. The viscosity of this bitumen decreases as a function of increasing temperature, as plotted in Figure 2 given below.

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Figure 2: Viscosity temperature relationship of bitumen at Athabasca and Cold Lake

All the heat transferred to bitumen is from the steam injected on the upper tube, and the rate of heat transfer depends on the thermal conductivity (K=W/m K) of the tube. However, due to fouling inside the tube, the deposit has relatively low thermal conductivity and it will change the rate of heat transfer.

Table 1. Thermal conductivity for various deposits. (Hans, M. 2000)
Sodium Aluminum Silicate
0.2-0.4W/mK
Milk Components
0.5-0./mK
Hematite(Boiler Deposit)
0.6W/mK
Biofilm
0.7W/mK
Calcium Sulphate(boiler)
0.8-2.2W/mK
Calcite(Boiler Deposit)
0.9W/mK
Serpentine(Boiler Deposit)
1.0W/mK
Gypsum(Boiler Deposit)
1.3W/mK
Calcium Sulphate
2.3W/mK
Magnesium Phosphate
2.3W/mK
Calcium Phosphate
2.6W/mK
Calcium Carbonate
29W/mK
Magnetite Iron Oxide
2.9W/mK

2. Change of Pressure Drop




During the SAGD process, the most expensive part is generation of steam, and the effectiveness of thermal usage is measured by cumulative steam-to-oil ratio (cSOR).The higher steam usage with the higher cSOR value. However, if the SAGD operates with ideal cSOR, the optimized injection steam has to be constant and operated under 1000Kpa and 2000Kpa. (Ian G, Neilin, C. 2006) According to the figure(2), the formation of deposits around the heat exchanger will decrease the cross-sectional area, and therefore it has restricted the steam flow ( Hans, M. (2000). Assume the deposits distributed evenly inside the tube, the pressure drop will increase due to the fouling. As the results, the cSOR value will smaller and influences the thermal usage.

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Figure 3: Fouling reduces cross-sectional area (Pichlmayrgut, H. 2009)

3. Tube Failure




Since the fouling will affect heat transfer efficiency between steam and bitumen, the heat from the steam will gradually generated inside the tube and increases the wall temperature. Eventually, high heat fluxes result in tube failures. (Ganapathy,V)


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Figure 4: Tube Failure (Testex)

Tube failure may be the most serious problem caused by fouling. It will make production process shuts down and loss of production.

4 Extra cost




There are some extra costs due to fouling in heat exchangers

  • Capital Expenditure
In the case of SAGD, additional capital expenditure results from the consequence of having to purchase two heat exchangers for the function of using one while the other is being cleaned. As well other design criteria of the heat exchanger increases capital cost, for example, when designing a heat exchanger tube, extra surface area has to be considered, resulting in higher capital expenditure.

  • Fuel Costs
Because of fouling, rate of heat transfer rate will be lower than expected. In order to maintain the same production rate, extra costs on fuel is necessary for the steam injection.

  • Maintenance Costs
For removing the fouling deposits, more money has to be spent on maintenance such as antifouling measures such as clean up chemicals and labor.

  • Production Loss
The production loss is the main cost of fouling. It includes loss of production; although a common solution is to use two heat exchangers, this increases the capital expenditure of the project.