Deposition of diamondoids can be particularly problematic during production and transportation of natural gas, gas condensates, and light crude oils. These low-molecular-weight compounds which have diamond-like fused ring structures are common in reservoir fluids in general.
However, their presence in crude oils is usually ignored due to their low concentration in these fluids. Nevertheless, in certain cases existence of these compounds in natural gas and gas condensate sources could lead to severe complex system problems of deposition and eventual plugging of flow paths.
In reservoir fluids, at certain concentrations, which depend on the nqture of the fluid, these compounds may nucleate out of solution due to drastic changes in pressure and temperature during the production cycle.
These nuclei may promote interactions among other heavy organic species and serve as floculation sites. Thus, it is important to determine whether or not diamondoids are present in a reservoir fluid at a harmful level.
Diamondoid was first discovered and isolated from a Czechoslovakian petroleum in 1933. The isolated substance was named adamantane, from the Greek for diamond. This name was chosen because it has the same structure as the diamond lattice, highly symmetrical and strain free.
It is generally accompanied by small amounts of alkylated adamantanes: 2-methyl-; 1-ethyl-; and probably 1-methyl-; 1,3-dimethyl; and others.
The unique structure of adamantane is reflected in its highly unusual physical and chemical properties. The carbon skeleton of adamantane comprises a small cage structure. Because of this, adamantane and diamondoids in general are commonly known as cage hydrocarbons.
In a broader sense they may be described as saturated, polycyclic, cage-like hydrocarbons that are present in some reservoir fluids. The diamond-like term arises from the fact that their carbon atom structure can be superimposed upon a diamond lattice.
The simplest of these polycyclic diamondoids is adamantane, followed by its homologues diamantane, tria-, tetra-, penta- and hexamantane. The homologous polymantane series has the general molecular formula C4n+6H4n+12 where n=1,2,3,...(n=1 for adamantane).
Note that there are non-isomeric polymantanes which do not obey this chemical formula. The figure below shows the chemical structures of adamantane, diamantane and trimantane, The lower adamantologues, including adamantane, diamantane, and triamantane, each has only one isomer. Depending on the spatial arrangement of the adamantane units, higher polymantanes can have numerous isomers and non-isomeric equivalents.
There are three possible tetramantanes all of which are isomeric, respectively as iso-, anti- and skew-tetramantane.
Anti- and skew-tetramantanes each possess two quaternary carbon atoms, whereas iso-tetramantane has three. There are seven possible pentamantanes, six being isomeric (C26H32) obeying the molecular formula of the homologous series and one non-isomeric (C25H30).
For hexamantane, there are 24 possible structures, among them, 17 are regularly cata-condensed isomers with the chemical formula (C30H36), six are irregularly cata-condensed isomers with the chemical formula (C29H34) and one is peri-condensed with the chemical formula (C26H30).
It has been found that adamantane crystallizes in a face-centered cubic lattice. This is extremely unusual for an organic compound. The molecule therefore should be completely free from both angle and torsional strain.
At the beginning of growth, crystals of adamantane show only cubic and octahedral faces. The effects of this unusual structure upon physical properties are striking. Adamantane is one of the highest melting hydrocarbons known (m.p. 269 oC), yet it sublimes easily, even at atmospheric pressure and room temperature.
Because of this, its boiling point cannot be determined directly. However, adamantane, present in a mixture of hydrocarbons being fractionally distilled, is found in the cuts of b.p. near 190 oC (boiling point of n-C11 is 195.95 oC). The phase transition boundaries (envelop) of adamaentane in a reservoir fluid can be constructed having the fluid characterization data.
Diamondoids are saturated hydrocarbons and therefore analysis to determine their presence in reservoir fluids must be performed in their saturated fraction, for example in SARA analysis. All diamond hydrocarbons display high thermal stability and are resistant towards oxidation and microbial degradation.
It has been observed that phase segregation of diamondoids from dry petroleum streams takes place upon reduction of pressure and/or temperature of the system. It has also been observed that in wet streams diamondoids partition themselves among the existing phases (vapor, liquid, solid).
Furthermore, these compounds may nucleate out of solution upon drastic changes of pressure and temperature. Instabilities of this sort in crude oils may potentially initiate a sudden precipitation of other heavy organic compounds on such nuclei.
Therefore, solubility behavior of diamondoids in organic solvents and dense gases is important. Such solubility calculations can be performed knowing appropriate characterization data for the reservoir fluid.
The danger of solid diamondoid phase segregation during the production cycle stems from the solubility properties of diamondoids in gases and liquids. Although diamondoids will preferentially partition into the liquid phase there remains a fraction of these species which partitions into the gas phase.
Bear in mind that diamondoids will segregate out of the gas phase upon a decrease in temperature and pressure. Thus, phase segregation of diamondoids is possible whenever instabilities, leading to the appearance of a gas phase, occur during the production cycle.
Note that even though diamondoid crystals will redissolve in the liquid phase their appearance will cause a momentary imbalance at the vapor-liquid interface where asphaltenes tend to adsorb.
This could potentially lead to further interactions with asphaltene and other surface active species present at the interface. This momentary imbalance and further interaction could, in principle, lead to subsequent phase segregation of heavy organics concentrated at the interface.
Diamantane, triamantane and their alkyl-substituted compounds, just as adamantane, are also present in certain petroleum fluids. Their concentration in these fluids are generally lower than that of adamantane and its alkyl-substituted compounds. In rare cases, tetra, penta, and hexamantanes are also found in petroleum fluids.
Deposition of adamantane from petroleum streams is associated with phase transitions resulting from changes in temperature, pressure, and/or composition of reservoir fluid. Generally, these phase transitions result in solid-gas or solid-liquid equilibrium. Deposition problems are particularly cumbersome when the fluid stream is dry (i.e. low LPG content in the stream).
Phase segregation of solids takes place when the fluid is cooled and/or depressurized. In a wet reservoir fluid (i.e. high LPG content in the stream) the diamondoids partition into the LPG-rich phase and the gas phase. Deposition of diamondoids from a wet reservoir fluid is not as problematic as in the case of dry streams.
Some useful references:
Baroody, E.E.; Carpenter, G.A., Rpt. Naval Ordnance Systems Command Task No. 331-003/067-1/UR2402-001 for Naval Ordnance Station, Indian Head, MD, 1972, 1-9.
Boyd, R.H.; Sanwal, S.N.; Shary-Tehrany, S.; McNally, D., J. Phys. Chem., 1971, 75, 1264-1271.
Clark, T.; Knox, T.M.O.; McKervey, M.A.; Mackle, H.; Rooney, J.J. J. Am. Chem. Soc., 1979, 101, 2404-2410.
Jochems, R.; Dekker, H.; Mosselman, C.; Somesen, G., J. Chem. Thermodyn., 1982, 14, 395-398.
Mansoori, G.A., J. Petrol. Science & Eng. 1997, 17, 101-111.
Vazquez Gurrola, D., Escobedo, J., Mansoori, G.A. "Chearacterization of Crude Oils from Southern Mexican Oil Fields", in EXITEP 1998 Proceedings, Mexico City, Mexico.
Westrum, E.F., Jr., J. Phys. Chem. Solids, 1961, 18, 83-85.