Analysis of Depositions

As the demand for the heavy crude oil resources, bitumen tar sands, in Alberta continues to increase, the important of SAGD operations, and their success, prove to be increasingly vital. As such, leading operational problems, in this case fouling in heat-exchangers, must be analyzed closely and carefully. When tests ,such as carbon, hydrogen, and nitrogen analyses, X-ray fluorescence, X-ray diffraction, and Fourier transform infrared spectroscopy, were conducted on various heat-exchanger banks, they revealed that certain compositional species are preferentially depositional. These heat-exchanger banks are used to process streams containing a reverse emulsion of oil and water, as well as produced water that is to be re-used. Deposits exiting these streams will be critically analyzed in Inverted and diluted SAGD processes. It should be noted that due to the varying nature of the bitumen extracted that is dependent on varying reservoir natures, the analysis below should not be taken as a generalization for all cases.

In inverted SAGD processes, it is found that a significant portion of fouling in heat-exchangers can be attributed to the practice of using recycled streams in the slop tank and the main reverse emulsion feed. These recycled streams were analyzed and found to contain Mg2SiO4 and Mg2+ silicate compounds. Fouling has been found to occur shell-side, site of reverse emulsion, and tube side, site of produced water. However, via results from multiple exchangers, it is found that tube side is more predominant in being responsible for exchanger fouling, regardless of the stream that is being processed. In further investigations, deposits of each individual flow stream will be analyzed.

Reverse Emulsion Flow Stream

Shell side analysis revealed high concentrations of inorganic deposits. Si+4 concentrations ranged from 11 to 18 wt %, and Mg+2 concentrations were 5-8 wt %. Analyses yielded a 50-75% weight loss upon heating to 700 °C, indicating approximately 25-50% non-volatile inorganic material. The total C, H, and N content of the samples ranged from 48 to 70 wt %.

The magnesium concentrations present in the deposit is a further indication of the impact of the recycled slop tank stream. While, magnesium ions and magnesium silicates are by-products of reservoir production, the SAGD process normally contains magnesium ions at a concentration of approximately 0.9 ppm, as indicated from produced water streams. Furthermore, produced waters contain calcium ions 4-5 times in excess to that of magnesium ions. However, these calcium ions were not present in the deposit analysis. As such, the dramatic increase in the magnesium ions can be attributed to the warm lime softener sludge being dumped into slop tank.

In addition, an excess concentration of Si4+ ions was found in the analyzed streams. While some of this can be due to the warm lime softener sludge, it does not account for the entirety of the concentration. The source is likely to be due to the sand and other inorganic silicates produced during the lifetime of the reservoir.

On the other hand, of the organic species present in the exchanger, analysis indicated the present of paraffinic and asphaltenic hydrocarbons along with organic acids and organic salts. Another significant portion of organic material was found. These organic materials were categorized by their insoluble properties in chloroform, toluene, or methanol. These insoluble organic materials include a significant portion of organic acid salts. This may be attributed to the reverse emulsion not effectively breaking the water and oil contact, emulsions are further discussed in here.

Figure 1: Shell side deposition from produced reverse emulsion flow

Produced Water Flow Stream

Similar to the reverse emulsion flow stream, the produced water stream deposits were also found to contain a relatively high inorganic composition. Si+4 concentrations ranged from 7 to 23 wt %, and Mg+2 concentrations were from 5 to 12 wt %. Analyses yielded a 43-73% weight loss upon heating to 700 °C, indicating approximately 27-57% non-volatile inorganic material. The total C, H, and N content of the samples ranged from 34 to 66 wt %.(reference this page 7).

Once again the dumping of the warm lime softener sludge into the slop tank was believed to be the primary cause behind the excess Mg2+ and Si4+ ions in the stream. This is additionally supported by the fact that in heat-exchangers downstream from the initial dump of the softener sludge, Mg2+ and Si4+ ion concentrations continued to diminish.
In addition, hydrocarbon, asphaltene, carbonyl, organic acid and organic acid salt structures were also found in analyzed deposits. One of the tube deposit samples was found to contain a high concentration of iron, 13-14 wt %. The excess iron concentration is very likely due to corrosion and consequent by-products.

Figure 2: Deposition on tube bundle heater from produced water