The Theory Behind Steam Assisted Gravity Drainage

Steam Assisted Gravity Drainage (SAGD) is an enhanced oil recovery method that is used to extract heavy oil or bitumen from underground. It is an advanced form of steam flooding which involves the drilling of two in the reservoir with one well located a few meters above the other. Low pressure steam is flowing through the upper well as heat source to heat the oil and reduce its viscosity, enabling the oil to become mobile and flow into the lower well for production.

As mentioned above, this steam flooding process involves two parallel horizontal wells drilled inside the reservoir formation with one well above the other. During production, the upper well is injected with steam, as steam pass inside the well, it flows upwards and forms a steam chamber, due to a thermal driving force (ie temperature difference between steam and the bitumen). The heat energy that is being transferred from steam to the bitumen causes the bitumen to heat up, subsequently decreasing its viscosity and as a consequence, the bitumen is able to flow from the formation to the lower production well. Hence the term gravity drainage. As oil is being produced, the pressure inside the reservoir decreases, solutions gases dissolved in bitumen are released and rise towards the surface due to their low density. As gases rise, they force some bitumen out of the pore space and bitumen is pulled by gravitational force. On the surface, a high compatible pump is installed on the production well to ensure crude oil production is continuous.

Figure 1: SAGD well layout

Figure 2: SAGD process illustration

Fouling in General

Fouling is defined as the accumulation of deposits on solid surfaces which most often occurs in a aquatic environment. These unwanted depositions may either of living organisms (biofouling) or non-living compound (organic or inorganic). Since fouling usually happens on the solid surfaces, the fouling process is therefore interfering with the functions of these solid surfaces.

Fouling phenomena are common and diverse, varying from fouling of ship hulls to fouling of heat exchanger components by the cooling liquids. There are two main types of fouling, micro and macro fouling. Macro fouling is caused by coarse matter of either biological or inorganic origin such as industrially produced waste. These matter enters into the cooling system through pump from places like sea, rivers or lakes. Such substances can foul the surfaces of heat exchangers and cause the deterioration of the overall heat transfer coefficient. Other problems such as flow blockages, redistribute the flow inside the components or fretting damage can also happen. As to micro fouling, clear distinctions are made between different phenomena, these include: scaling or precipitation fouling (crystallization of certain substances); particulate fouling (accumulation of particles on a surface); corrosion fouling (corrosion deposits); chemical reaction fouling (decomposition or polymerization of organic matter on heating surfaces); solidification fouling (flowing fluids with high melting point freeze on a sub-cooled surface); bio-fouling (settlement of bacteria or algae) and composite fouling (fouling when more than one mechanisms are involved).

Figure 3: Crude oil fouling on heat exchanger tubes

Figure 4: macro fouling inside heat exchanger

Figure 5: Corrosion Fouling in heat exchanger tubes
Figure 5: Corrosion Fouling in heat exchanger tubes

Some fouling-related costs for industiral process involve heat exchangers include:
  • Increased capital investment, e.g. anti-fouling materials maybe used when builing the heat exchangers;
  • Additional operational costs, e.g. fouling cause higher engery consumption due increased thermal resistance induced by deposits;
  • Maintance cost, e.g. periodic cleaning is necessary secure heat exchanger performace;

Fouling is generally classified into five areas according to the principal processes: crystallization, particulate, cheamical fouling, corrosion and biofouling.