Reducing Greenhouse Gas Emissions in SAGD Operations via Pinch Analysis

What is a Pinch Analysis? 

Steam-assisted gravity drainage (SAGD) is a common in-situ technique for oil production in Alberta. In this process, steam is injected downhole to reduce the viscosity of bitumen which is then brought to the surface via a production well. SAGD operations require significant amounts of high-pressure steam. Pinch analysis is a methodology for minimizing the utility requirements of a chemical process by calculating thermodynamically achievable targets as well as design rules for heat exchanger network retrofitting. Ultimately, minimizing the utility requirements of the process translates to greenhouse gas (GHG) emissions reductions and cost savings stemming from reductions in the fuel required to create steam. Pinch analysis can often lead to saving potentials in the thermal energy supply of 10-30% [1-4].

Why Perform a Pinch Analysis?

To perform a pinch analysis, all heating and cooling loads in the process need to be identified (i.e. stream inlet and target temperatures). Once identified, the heating and cooling loads for the system can be graphed in a load vs temperature graph for analysis. The Composite Curves such as those presented in Figure 1 are useful to determine the minimum cooling and heating requirements of the process as well as the maximum possible heat recovery. The actual heat recovery achieved by the process depends on the heat exchanger network configuration used for heat integration. The heat exchanger area required to perform heat integration in the process is a function of the temperature difference between the heating and cooling curves, the greater the temperature difference, the smaller the required area. Note that theoretically, at an infinite heat exchanger area, the maximum heat recovery is achievable for any process. However, there is a trade-off between energy recovery and heat transfer area that can be incorporated through the definition of a minimum temperature approach parameter.  If at any point the temperature difference is below the minimum approach, then heat recovery is deemed too costly as a large heat exchanger surface area would be required.

Figure 1: Example of heating and cooling loads versus temperature graph [1].

Common Features of SAGD Processes

Figure 2 outlines the main unit operations typically found in a SAGD operation. The SAGD process consists of two horizontal wells in the well pad that are drilled into the oil reservoir. The injection well increases the temperature and pressure of the reservoir through high-pressure steam injection and the production well brings the oil emulsion to the surface. The oil emulsion coming up from the production well is then separated, often with the help of a diluent. While the oil is sent to an upgrading facility, most of the water is recycled in the process. The water stream undergoes treatment before being sent to the boilers to produce steam. Note that some makeup water is required as not all recycled water is fit for reuse. The steam is then sent back into the reservoir through the injection well.

Figure 2: Typical SAGD operation using COSIA’s InsituSim simulator.

The main heating and cooling streams identified in a typical SAGD process are:

• The bitumen emulsion leaving the production well needs to be cooled to around 130 °C in the emulsion treatment train to:
  1. Help with oil-water separation
  2. Minimize diluent flashing in the emulsion treatment
  3. Meet pipeline vapor pressure specification
• Produced gas (PG) from the inlet separators needs to be cooled before it can be used as fuel gas in steam generation.  
• The diluent that is mixed in the emulsion needs to be heated to achieve thermal equilibrium in the separators.
• The produced water from emulsion treatment needs to be cooled down before being sent to the deoiling process which typically runs under atmospheric conditions at around 85-90 °C.
• If the water treatment includes hot lime softening (HLS), then deoiled water from the deoiling process needs to be heated to around 100 °C. 
• Recycled blowdown water needs to be cooled before being recycled to the water treatment.
• The makeup water needs to be heated before being sent to the water treatment tanks.
• Boiler feed water (BFW) needs to be heated for steam generation.
• Air used for fuel combustion in the boilers needs to be heated.
• Disposal water needs to be cooled prior to being sent to disposal.

Common Heat Exchanger Configurations

A common heat exchanger network (HEN) in a SAGD facility comprises the following strategies:

• Heating BFW with incoming hot emulsion and produced gas.
• Heating BFW with the boiler blowdown. 
  • Note that the boilers typically run at high pressure of ~10 Mpa (corresponding to the water saturation temperature of ~290-300 °C). The high-pressure blowdown is then typically sent to a pressure letdown operation where the blowdown pressure is reduced. Due to the pressure reduction, a portion of the blowdown is flashed as steam at lower pressure. The recovered steam at lower pressure could be used for process reasons (e.g., sludge agitation and deaeration) and a portion of it could be used for heating up the BFW and recycled back to process as clean water.
• Heating the deoiled water with the produced water.
• Heating make-up water with produced water.
• Mixing diluent into the liquid emulsion stream as a way of both cooling the emulsion and heating the diluent.

There are usually several options to improve heat integration that should be investigated in detail:

• Arrangement of BFW-Emulsion and BFW-PG exchangers.
• Heat recovery from produced water. Produced water exchangers are susceptible to fouling and their performance needs to be monitored.
• The pressure at which the separator operates and the sequence of heat integration (direct heat recovery from HP Blowdown with lower pressure in separator OR heat recovery after the pressure reduction).
• Possibility of high-temperature deoiling operations.

How Can We Help?

Our team of talented process engineers is ready to help you determine the heating and cooling requirements of your process and come up with the optimal retrofit strategy for the existing heat exchanger network or alternatively design an optimum heat exchanger network for a greenfield facility. Our focus is not only on reducing energy demand as a proxy for cost and GHG emission reductions but to support the projects with relevant regulatory knowledge to achieve maximum savings. Feel free to ask us a question at info@processecology.com!

References
 [1] Brunner, Florian; Krummenacher, Pierre: Einführung in die Prozessintegration mit der Pinch-Methode. Bern: Bundesamt für Energie, 2015.

[2] Dadashi Forshomi, Zainab, Alva-Argaez, Alberto & Bergerson, Joule A, 2017. Optimal design of distributed effluent treatment systems in steam assisted gravity drainage oil sands operations. Journal of cleaner production, 149, pp.1233–1248.

[3] Natural Resources Canada, 2003. Pinch Analysis: For the efficient use of energy, water & hydrogen.

[4] Smith, Robin, 2016. Chemical Process Design and Integration 2nd ed., Wiley.