The presence of acetic and formic acids in the produced water of gas condensate wells has been known for some time by the industry. In traditional water analysis, it has been titrated and reported as alkalinity. The calculation of accurate downhole pH values requires that these ions be analyzed separately in the water and that an organic acid material balance be performed on all three phases in the separator. In this manner, it is then possible to use phase distribution coefficients involving ionic equilibrium to determine how these acids distribute themselves between phases as the pH calculation proceeds downhole.
In this paper, the above method of calculation of pH and ?pH is used to examine the effect that various concentrations of these acids have on the downhole pH. various concentrations of acids are examined, and two cases are calculated in which the effect of condensate on the pH is examined.
A number of investigators1,2,3,4,5,6 have presented models for calculating in situ pH in gas condensate wells. In their work they have examined the effect of temperature, pressure and partial pressure of carbon dioxide on PH. Most do not correct for partial pressure of carbon dioxide with the fugacity coefficient and only one of these models does identify organic acids as being important. However, it does not use phase distribution coefficients involving ionic equilibrium to determine how these acids partition between the various phases at downhole conditions. It is this distribution that can affect the final PH. The Corrosion Research Center at USL presented a method for calculating the downhole pH and ApH inside a producing gas condensate well containing carbon dioxide and organic acids7. This method does correct for all of the above mentioned deficiencies that have historically plagued this rather complex calculation.
As early as 1953 investigators8,9,10 have shown that organic acids played an important role in corrosion since they have an affect on pH. Crolet and Bonis6 proposed that condensed water having acetic acid concentrations of more than 0.1 meq/l can be considered to produce a severely corrosive medium. These organic acids also interfere with the standard acid titration methods of determining alkalinity. When these acids are reported as alkalinity, the calculated pH is even further removed from the true value.
Recent organic acid measurements made at the USL Corrosion Research Center7,11 have shown the presence of formate and acetate ions in Gulf Coast gas condensate wells. A total of 28 wells having zero or very low levels of formation water have shown a range of acetic acid from 0 to 381 mg/1 with an average level of 125 mg/1. The level of formates ranged from 0 to 88 mg/1 with an average value of 15 mg/l. It was found that 93% of these wells had acetates and 54% had formates. Therefore, if gas condensate wells are to be modeled for pH, it is necessary to measure these organic acids in the produced water, and to include them in the pH calculations to get accurate values.
