Downhole catalyst upgrades heavy oil

March 18, 2002
Experiments in the Liaohe oil fields, in northeastern China, indicate that placement of a catalyst downhole can improve cyclic steam, heavy-oil recovery.

Experiments in the Liaohe oil fields, in northeastern China, indicate that placement of a catalyst downhole can improve cyclic steam, heavy-oil recovery.

Results from seven wells showed that the technique upgraded the produced oil downhole by increasing the saturate and aromatic components and reducing the resin and asphaltene components.

In the tests, average oil molecular weight and viscosity decreased.

Aquathermolysis

Thermal oil-recovery processes are effective for enhancing heavy-oil recovery. Steam injection reduces the oil viscosity and chemical reactions between steam and heavy oil have been observed to play an active role in the recovery process. These reactions induce the formation of gaseous components such as carbon dioxide, hydrogen sulfide, and hydrogen during steam injection.

Hyne used the term "aquathermolysis" to describe the chemical interaction of high temperature, high-pressure water with the reactive components of heavy oil and bitumen.1 This term distinguishes this process from hydrothermolysis that is associated with the interaction of oil with hydrogen at elevated temperature and pressure.

In aquathermolysis, the metal species added to steam interact with organic sulfur compounds.

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To investigate the effectiveness of this technique, the experiments added aquathermolytic catalysts, prepared in the laboratory, to steam injected in seven wells, producing heavy oil.2-5

The results show that in a huff-and-puff operation the aquathermolysis catalyst reduced the oil viscosity downhole by more than 60% and oil production for the cycle increased by 280-699 tons/well (1,760-4,400 bbl/well).

Reservoir characteristics

The experimental wells are in the Shu 1-7-5, Du 66, and Du 33 blocks of the western part of Liaohe oil fields. These blocks border each other and have similar reservoir properties.

The reservoir lithology is a mixture of sand, clay, and gravel that is mostly pebbly sandstone and unequigranular sands. The rock has a high percentage of matrix with a high montmorillonite content.

Reservoir permeabilities are between 0.5 and 2.0 darcies with pore throat radii of between 10 to 22 μm.

The reservoir homogeneity coefficient is less than 0.25.

Catalysts

The catalyst prepared in the laboratory contained VO2+, Ni2+, Fe3+ and other additives.

The process for preparing VO2+ involved adding 1 g of V2O5 into 10 ml of N2H4 solution, then stirring the solution until it became yellow. At that point, 20 ml of 20% H2SO4 were added to the solution while it was stirred slowly until the color of the solution becomes green, indicating that it only contained VO2+ and SO42-. One obtains vanadyl sulfate after drying the solution.

The metal species in the experimental catalyst contained a 1:1:5 molar ratio of VO2+, Ni2+, and Fe3+, and VO2+ concentration was 0.001 mol/l.

The vanadyl sulfate and nickel sulfate are the catalysts for the aquathermolysis of heavy oils, and ferric sulfate is the catalyst for the water-gas shift reaction. At the end of aquathermolysis, the water-gas shift reaction is a major reaction for forming CO2 and H2.

Field experiments

The procedures in the field were as follows:

  • Inject 400 cu m of steam into the well to preheat the reservoir.
  • Inject the catalytic solution into the well.
  • Inject about 1,600 cu m of steam into the well.
  • Start producing the well after a 5-7 day shut-in.

Table 1 compares the production from the wells before and after the catalyst injection.

A high-performance liquid chromatographic (HPLC) analysis determined the composition of heavy oil samples from Well Du 67. The analysis consisted of:

  • Removing the asphaltenes by precipitation with the addition of a 40-volume excess of dry hexane to the solution of the oil in dichloromethane. The HPLC analysis was carried out on a semi-preparative basis with a Whatman Magnum-9, 10-μm silica column and ultra violet (UV) and refractive index (RI) detectors in series.
  • Activating the silica column by overnight flushing (2 ml/min) with dry hexane.
  • Obtaining the saturate and aromatic fractions by elution with hexane.
  • Collecting the aromatic fraction and then obtaining the resins by back-flushing the column with tetrahydrofuran (distilled from LiAlH4).
  • Quantifying the fractions gravimetrically after removal of solvent. The tests considered a mass balance in the range 99-101% as acceptable.
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From Table 2, it is clear that the oil composition changed. After the catalytic treatment, the oil has more saturate and aromatic components, which are lighter, and less resin and asphaltene components, which are heavier.

The results indicate that during aromatization some cyclic hydrocarbons were converted into aromatics. Some normal and iso-alkyl side chains, which are at the edge of the condensed aromatic core in resin and asphaltene molecules, broke off from the condensed aromatic and then converted into alkyl hydrocarbons.

The alkyl chain, which links two condensed aromatics in large molecular structures of resin and asphaltene, may also have broken off, and thus the amount of resin and asphaltene decreased and the amount of aromatics increased.

Generation of some low-molecular-weight components will dramatically reduce the viscosity of heavy oils.

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Table 3 shows the elemental analysis and average molecular weight of heavy oil from Du 67 before and after the downhole catalytic treatment. The results indicate that both the sulfur and oxygen content decreased. This is because the aquathermolysis usually occurs in the hetro-atom compounds in heavy oil.

The decrease in average molecular weight indicates that cleavage takes place in the treatment.

Process mechanism

In the process, the heavy oil undergoes aquathermolysis during steam injection, and the catalyst injected with steam can accelerate the reaction, resulting in a reduced viscosity and a changed composition of the produced oil.

According to the theory of chemical valence, among C-O, C-S, and C-N chemical bonds, the C-S bond energy is the least. At the same time, the sulfur atom electronegativity is greater than the carbon atom, so that in organic sulfur compounds, the sulfur atom has a negative charge while the carbon atom has a positive charge.

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With a catalyst added to the steam, the metallic ions can interact with water. The proton from the complex molecule can attack the sulfur atom, and the hydroxide ion can attack the carbon atom. This results in the electronic cloud excursion and leads to a further decrease in bond energy.

Because of this, the C-S bond will break in the process of aquathermolysis and result in a low amount of sulfur and heavy components such as resin and asphaltene.

In the aquathermolysis process, H2S will be produced because of the desulfurization of heavy oil. Recently it has been suggested that gaseous H2S may promote the water-gas shift reaction through the intermediate formation of carbonyl sulfide (COS),6 as shown in Equations 1 and 2 (see equation box).

At the same time, H2S will react with the metal ions and produce metal sulfides. It is well known that metal sulfides are useful catalysts for hydrode- sulfuriztion of heavy oil.

The analysis found that all transition metal species have the ability to accelerate the decomposition of the sulfur compounds regardless of whether the sulfur was in an aromatics or an aliphatic environment. Among all the transition metal species, VO2+ and Ni2+ are the most effective for aquathermolysis of heavy oil.

Oil reservoirs are large porous medium that contain sands, clay minerals, and non-clay minerals. The clay mineral surface has a negative charge. When the catalyst solution is injected into the oil reservoir, the metal ions, such as VO2+ and Ni2+, can be adsorbed on the surface of clay minerals via the electrostatic force. Under this circumstance, the minerals support the catalyst in a similar manner as in a typical refinery process.

At the same time, the steam injected into the oil reservoir reacts with most of the rock minerals and clay minerals. Clay minerals are silica-aluminate compounds that under high temperature can react with steam. For example, Equations 3 and 4 show the reaction of montmorillonite and feldspar.7

When the H4SiO4 forms, the Al3+ can adhere to the surface of the H4SiO4 and produce a surface hydroxyl group with strong acidity. The process yields a proton acid because of water dissociation and adsorption on the surface of Al3+.

H+ from the water combines with the oxygen Si4+ bond and leads to the formation of an acidic hydroxyl group. The hydroxyl releases H+ easily and has the characteristics of a Bronsted acid.8

Because of the electrophilic property of Al3+, it can lose an electron and form an electronic field to produce the hydroxyl group from water. The SiOOHAl group has a strong acidity.

As this article discusses, mineral reaction with steam can yield products with the structure and properties similar to amorphous silica-alumina catalysts that commonly are used for catalytic cracking in an oil refinery.

References

  1. Hyne, J.B., et al., "Steam-Oil Chemical Reaction: Mechanism for the Aquathermolysis of Heavy Oils," AOSTRA Journal of Research, Vol. 1 (1984), pp. 15-21.
  2. Rivas, O.R., et al., "Experimental Evaluation of Transition Metals Salt Solutions as Additives in Steam Recovery Processes," SPE Paper No. 18076, 1988.
  3. Fan, Hongfu, Liu, Yongjian, and Zhao, Xiaofei, "Downhole aquathermolsyis catalytic upgrade heavy oils," Oilfield Chemistry (Chinese), Vol. 18, No. 1, 2001 (1), 2001, pp. 13-16.
  4. Fan, Hongfu, Liu, Yongjian, and Zhao Xiaofei, "The studies on composition changes of heavy oils under stream treatment," Journal of Fuel Chemistry and Technology (Chinese), Vol. 29, No. 2, 2001, pp. 269-73.
  5. Monin, J.C., and Audlbert, A., Thermal cracking of heavy-oil/mineral matrix system," SPE Reservoir Engineering, November 1988, pp. 1243-50.
  6. Clark, P.D., et al., "Studies on the Effect of Metal Species on Oil Sands Undergoing Steam Treatments," AOSTRA Journal of Research," Vol. 6, 1990, pp. 53-64.
  7. Zhu, Tingyu, Liu, Lipeng, and Wang, Yang, "Studies on coal mild gasification with CaO catalyst," Journal of Fuel Chemistry and Technology (Chinese), Vol. 28, No. 1, 2000, pp. 36-39.
  8. Zhi, Gao, Zeolite catalyst and separation, Sino-Petrochemical press, Beijing, 1999, pp. 44-62.

The authors

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Hongfu Fan is an associate professor at the Daqing Petroleum Institute, Daqing, China. He has undergraduate and a masters degree in applied chemistry from Southwestern Petroleum Institute and will receive a doctorate from the Daqing Petroleum Institute in April 2002.

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Yongjian Liu is a professor and vice-president at Daqing Petroleum Institute. His research interests are in oil field chemistry and technology. Liu is a graduate of the Daqing Petroleum Institute and received a doctorate from Zhejiang University.