Carboxylic acid anions in formation waters

MacGowan, D.B. Surdam, R.C. (1990) Carboxylic acid anions in formation waters, San Joaquin Basin and Lousiana Gulf Coast, USA—implications for clastic diagenesis. Appl. Geochem., 5, 687-701. 

Controls on Concentration and Distribution of Carboxylic Acid Anions in Formation Waters... 

MacGowan DB, Surdam RC (1987) The role of carboxylic acid anions in formation waters, sandstone diagenesis and petroleum reservoir modeling. Geol Soc Am Programs Abstr 19 753... 

Data for the concentrations of dicarboxylic acid anions in formation waters from sedimentary basins are much more limited than for mono-carboxylic anions. Some of the reported values are controversial (Hanor et al., 1993 Kharaka et al., 1993a). The total reported range is 0-2,640 mg L (for discussion and references... 

Table I. MAXIMUM REPORTED CONCENTRATIONS OF CARBOXYLIC ACID ANIONS IN PRODUCED FORMATION WATERS...

A carboxylic acid can be represented as R — CO2 H. Many different carboxylic acids participate in organic chemistry and biochemishy. Although carboxylic acids react in many different ways, breaking the C—OH bond is the only reaction that is important in polymer formation. A carboxylic acid is highly polar and can give up H to form a carboxylate anion, R — CO2. The carboxyl group (— CO2 H) also forms hydrogen bonds readily. These properties enhance the solubility of carboxylic acids in water, a particularly important property for biochemical macromolecules. 

Dissolved organic species have been known to exist in sedimentary basin formation waters since before the turn of the century (5.6.71. A host of aqueous organic species have been identified in sedimentary formation waters including hydrocarbons, mono-, di- and tri-carboxylic acid anions, keto and hydroxy-acids, amino acids, phenols, cresols, and hydroxybenzoic acids (8.9.10.11.121. 

Destruction of Carboxylic Acid Anions. Destruction of CAA has been postulated to occur by two processes bacterial degradation at low temperature and thermal decarboxylation at higher temperature (JJ). Little is known about the distribution of CAA substrate bacteria in formation waters, so their affect upon the distribution and concentration of CAA in formation waters is difficult to effectively model. However, bacterial degradation is important in many formation waters (7.43.45). Although they don t coexist, both anaerobic and aerobic bacteria have been reported in different petroliferous environments at temperatures in excess of 90 C (60.61) it should be noted that at temperatures in excess of about 45 °C they are metabolically inactive (2). At surface conditions, CAA are so readily metabolized by aerobic bacteria, produced formation water samples require immediate preservation at the time of sampling (35.45). 

Several methods have been employed by various researchers to quantify the short-chain carboxylic acids and anions in solutions of geochemical interest. Before the advent of gas chromatography, many researchers used titrimetric methods to quantify CAA in formation waters. However, in practice, formation waters that contain high concentrations or complex mixtures of CAAs yield titrimetric results that are uninterpretable (e.g., a Gran Mals plot that yields a line rather than a set of curves with clearly defined titration inflection points). 

Reed (1990) and Reed and Hajash (1990, 1992) concluded that geologically reasonable concentrations of both monofunctional and difunctional carboxylic acids can enhance substantially the solubility of aluminum at pH 4.5-4.7, although oxalate has a much greater effect than acetate. Observed elevated metal concentrations were attributed to formation of organic anion complexes and/or increased reaction kinetics, which could allow rapid dissolution of aluminosilicates and creation of secondary porosity through reaction with carboxylic acid anions. They also suggested that the acetate system may buffer the pH in formation waters. 

Boles and Franks 1979). However, it is our position that although these relatively slow aluminosilicate reactions contribute to the alkalinity, they do not buffer the alkalinity until all relatively fast-reacting minerals such as carbonates are eliminated from the fluid/rock system. It is the weak acids and bases (dominately carboxylic acid anions, HCO3, and HS ) in formation waters, coupled with rapidly reacting minerals, such as the carbonate minerals, that buffer formation water pH. 

As the formation temperature increases in the zone of intense diagenesis (80 to 130 °C), the concentration of organic acid anions in the formation water increases to the maximum concentration at which the carbonate system is externally buffered (by acetate). At temperatures greater than 100°C the carboxylic acids, particularly the difunctional species, begin to decarboxylate ... 

The evolution of formation water alkalinity, then, can be generalized in the following scenario. Over the 80 to 110°C interval, the carbonate system will be externally buffered and Pco2 be relatively low. As decarboxylation begins (at about 100 °C), the system remains externally buffered with carboxylic acid anions as Pco2 increases due to decarboxylation and carbonate dissolution. At a temperature somewhere between 120 and 160°C,... 

Table 7. Concentration (ppm) of carboxylic acid anions detected in produced formation waters from the Gippsland Basin...

In base the tetrahedral intermediate is formed m a manner analogous to that pro posed for ester saponification Steps 1 and 2 m Figure 20 8 show the formation of the tetrahedral intermediate m the basic hydrolysis of amides In step 3 the basic ammo group of the tetrahedral intermediate abstracts a proton from water and m step 4 the derived ammonium ion dissociates Conversion of the carboxylic acid to its corresponding carboxylate anion m step 5 completes the process and renders the overall reaction irreversible... 

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