Information about Fouling in SAGD

During a SAGD process, the recycling of produced water (water from that was divided from oil) is needed for two different reasons. The first one is that in order to protect the environment, discharging of the produced water must be minimized, which leads us to recycling the water [1]. The second reason is purely economic, as the SAGD process requires heavy use of steam, recycling the water would be cheaper for the oil and gas company [2]. Improvements to this process will give the added benefit of lesser effects of fouling on the heat exchanger involved.

The boiler feed water from bitumen recovery operations is the main reason in which fouling occurs throughout the SAGD process. This boiler feed water contains “metastable, submicron dispersions and /or micellar Fe and Ca salts of bituminous material” [1] which are created and captured during the recycling process. This boiler feed water will eventually precipitate those salts and submicron dispersion when heated, subsequently depositing into the tubes and pyrolyzing into coke and silica precipitating ash. Since there are many of these materials within the system, substantial fouling can occur in heat exchangers. Heat exchangers that are especially affected by this are the cooling exchangers following the high temperature separator and the heat exchangers entering the boiler. From [2] a certain fouling deposit contained 68% oxygen, 23% silicone, and 4% aluminum (excluding carbon). This composition tells us that there are silicates (or clay) particles in many fouling deposits.

Precipitation Fouling

The primary type of fouling in the heat exchanger is precipitation fouling. The heat exchanger involves crystallization of solid salts, oxides and hydroxides from mixture of water and bitumen. During the SAGD process, water mixture enters the heat exchanger, and the concentration of salts may exceed the saturation and lead to a precipitation of crystals because of changes in temperature. To demonstrate an example of precipitation fouling, the equilibrium chemical reaction between readily soluble calcium bicarbonate and poorly soluble calcium carbonate may be used,

external image 733fd803e18077e6a13674a29d669844.png

The product of this reaction, calcium carbonate that is produced in the heat exchanger precipitates. The amount of calcium carbonate at the hotter inlet is actually higher than the one at the cooler outlet because of the temperature dependence of the reaction, and increasing solution CO2 with increasing temperature.

Because the solubility of salt is dependent on temperature or presence of evaporation, the dependence can be considered the driving force for precipitation fouling. There are two types of dependencies of solubility on temperature. One is the ''normal'' dependence of solubility, and the other is ''retrograde'' dependence of solubility. The salts with the ''normal'' solubility decrease their solubility with decreasing temperature, so they are most likely the fouling at the cooler surfaces. On the other hand, the salts with the ''retrograde'' solubility are most likely the fouling at the hotter surfaces. Calcium sulfate, which is one of the common precipitation foulant in the heat exchanger, is used as an example of illustrating the ''retrograde'' dependence of solubility on temperature, and the graph of solubility of calcium sulfate versus temperature is shown below.

Figure 1: Temperature dependence of the solubility of calcium sulfate (3 phases) in pure water.
Figure 1: Temperature dependence of the solubility of calcium sulfate (3 phases) in pure water.

Specifically, applied to SAGD, an analysis of various ion concentrations at SAGD plant without the slop tank recycling was analyzed. Results are synthesized below

Table 1. Properties of Produced Water from the Primary
Separation Vessel

314 ppm
7 ppm
0.8 ppm
150 ppm
200 ppm
107 ppm

From Table 1, a relatively moderate of sulfate is observed, it is predicted that since the heat exchanger is handling high temperature streams, the precipitation of calcium sulfate and may be a cause of the fouling in the heat exchanger