Summary
The prediction and prevention of both sodium and calcium naphthenate
"scales" is an important issue in oil production. A broad description of how
these scales form has been available for some time, although most experimental
findings are still of a qualitative nature. In this paper, an equilibrium
thermodynamic model is presented for predicting naphthenate partitioning and
precipitation in an oil/brine immiscible system from some chosen initial
conditions (i.e., naphthenate initial concentration in oil, brine pH,
[Ca2+], etc.). This model has, with some assumptions, been applied
to both model and real naphthenate system.
This model describes two types of naphthenate experiment: 1) full
naphthenate precipitation, and 2) simpler "pH change" experiments in which no
precipitation occurs. To predict naphthenate precipitation, the theory suggests
knowing: 1) the partition coefficient of the naphthenic acid, HA, between the
oil and the water phases, Kow; 2) the pKa of the
naphthenic acid in water; and 3) the solubility product, KCaA2 (or
other similar solubility parameter), of the naphthenate deposit. In the simpler
pH change experiments, only the first two of these parameters (i.e.,
Kow and pKa) are required. Using the naphthenate model
without precipitation, the effect of varying parameters on the degree of pH
change predicted at equilibrium in the oil/naphthenic acid/brine system was
studied. Also, the model was used to examine the sensitivities of the various
parameters on the final pH was also applied. The comparison between the model
predictions and experiment at a higher brine pH value is overall
satisfactory.
© 2009. Society of Petroleum Engineers
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History
- Original manuscript received:
7 March 2008
- Meeting paper published:
28 May 2008
- Revised manuscript received:
4 December 2008
- Manuscript approved:
20 December 2008
- Published online:
30 July 2009
- Version of record:
8 September 2009
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