Donald B. MacGowan

Difunctional carboxylic acid anions in oilfield waters

Abstract Recent models of porosity enhancement during sandstone diagenesis have called upon the metal complexing ability of difunctional carboxylic acid anions in subsurface waters to explain aluminosilicate dissolution. Although carboxylic acid anions have been known to exist in oilfield waters since the turn of the century, until now the existence of significant concentrations of difunctional carboxylic acid anions has not been documented. Data from this study show that difunctional carboxylic acid anions can exist in concentrations up to 2640 ppm, and can account for nearly 100% of the organic acid anions in some oilfield waters. Formation water samples with exceptionally high concentrations of difunctional carboxylic acid anions are found in reservoirs which are at maximum levels of thermal exposure, and which are presently in the 80–100°C thermal window. Plagioclase dissolution experiments performed with natural oilfield waters and artificial solutions indicate that waters with high difunctional acid anion concentrations are capable, by organo-metallic complexation, of being “apparently oversaturated” with respect to total aluminum concentrations compared to the inorganic solubility of kaolinite by several orders of magnitude. Dissolution experiments simulating a specific geologic environment (Stevens Sandstone, southern San Joaquin Basin, California; using natural oilfield waters and Stevens Sandstone core samples), produced plagioclase and calcite dissolution textures similar to those noted in well cores from the Stevens Sandstone, as well as raising total aluminum concentrations in these experimental solutions several orders of magnitude over the solubility of kaolinite. Bactericidal preservation of oilfield waters is essential for meaningful analyses of organic acid anions; bacterial degradation of waters can cause dramatic changes in both concentration and distribution of carboxylic acid anions, especially the difunctional acid anions. In addition, elevated temperatures must be avoided (i.e. gas chromatography with FID), for many of the carboxylic acid anions are unstable at temperatures above 100°C. Some oilfied waters have a much higher concentration of difunctinal carboxylic acids anions than was previously thought, and hence a greater capacity to dissolve aluminosilicate framework grains and transport aluminum.

Carboxylic acid anions in formation waters, San Joaquin Basin and Louisiana Gulf Coast, U.S.A.—Implications for clastic diagenesis

Abstract Carboxylic acid anions (CAA) in formation waters are of interest to studies of clastic diagenesis because of their ability to buffer formation water Eh and pH (and thus substantially contribute to controls on carbonate mineral stability), and their ability to complex and transport Al and Si from the site of aluminosilicate mineral dissolution during diagenesis. Carboxylic acid anions are also extremely important to the aqueous geochemistry of Ca, Fe, Mn, Pb and Zn. Some formation waters from sedimentary basins contain high concentrations of CAAs. The analyses of 20 formation waters from the San Joaquin Basin, California and 20 formation waters from the Louisiana Gulf Coast Basin presented in this study, show concentrations as high as 8100 ppm monofunctional and 370 ppm difunctional CCA; by comparison, previously reported analyses indicate monofunctional CAA occir in concentrations up to 10,000 ppm and difunctional CAA may occur in concentrations up to 2610 ppm. Analyses of drilling muds and scale soaps presented in this study show that few if any difunctional CAA in the study area can be attributed to contamination from these sources. Additionally, aqueous extracts of crude oils contain both mono- and difunctional acid anions, as do the aqueous and petroleum phases of hydrous pyrolysates. Previously unreported dissolution experiments, equilibrium computer simulations, and hydrous pyrolysis experiments support those already published and suggest that CAA are generated during thermal maturation of kerogen and expelled from the shale along oil-wet microfractures. Upon entering the water-wet sandstone pore, the hydrophilic CAA partition into the aqueous phase. Organic-inorganic reactions may occur which from CAA-metal complexes. Because the complexes are hydrophobic, they partition in the petroleum phase, where present. Carboxylic acid anions are of great importance to clastic diagenesis over the temperature range in which they dominate fluid alkalinity; certainly, no other viable mechanism has been advanced which adequately explains the observed aluminosilicate mineral dissolution with subsequent mass transfer of A1, as well as the carbonate mineral diagenetic successions observed in sand-shale systems world-wide. The utility of modeling these observations of organic-inorganic diagenesis is limited only by the ability to model the concentration and distribution of CAA through space and time.

Redox Reactions Involving Hydrocarbons and Mineral Oxidants: A Mechanism for Significant Porosity Enhancement in Sandstones

Hydrocarbon invasion into a sandstone containing mineral oxidants and carbonate or sulfate intergranular cements may result in redox reactions and significantly enhanced porosity. For years, geologists have noted that when hydrocarbons invade red sandstones, significant bleaching (i.e., iron reduction) takes place. The reactions responsible for the color distribution in the red (oxidized) and white (reduced) zones are reactions of iron oxides (± sulfate) with hydrocarbons. The iron oxides (± sulfate) oxidize the hydrocarbons (reductant) to oxygenated organic compounds; the Fe2O3 (oxidant) is reduced by hydrocarbons to pyrite (± chlorite). Commonly, the red sandstones are tight due to carbonate and sulfate cements, whereas the white zones within them are more porous. These redox reactions are of three types: [EQUATION] or [EQUATION] or [EQUATION] The produced organic acids are available to dissolve carbonate cements via the reaction [Equation]. Volumetric calculations demonstrate that if a hematite-stained sandstone (1.5% Fe2O3) is invaded by a fluid containing a 50/50 mixture of water and hydrocarbons, and redox reactions result, enough organic acid and consequent carbonate dissolution could occur to generate 8-14% additional porosity. More subtle redox reactions involving hydrocarbons and mineral oxidants have the potential to significantly enhance porosity in any sandstone. These redox reactions may explain why hydrocarbon accumulations appear to have created porosity in some cases.

Oilfield waters and sandstone diagenesis

Abstract The evolution of the organic geochemistry and carbonate alkalinity of oilfield waters is apparently regular and predictable; this evolution can be typified by five generalizations (1) at or near 80°C there appears to be an exponential rise in the concentration of organic acid anions; (2) the maximum concentration of organic acid anions occurs over approximately the 80–100°C temperature interval; (3) the highest concentrations of difunctional acid anions are associated with the other organic species maxima; (4) difunctional acid anions are the first to be decarboxylated, typically at temperatures of 100–110°C; (5) with increasing temperature (110–130°C) monofunctional acid anions begin to become decarboxylated, resulting in a fluid alkalinity dominated by bicarbonate. Dissolution experiments using artificial and natural oilfield waters demonstrate that mono- and difunctional carboxylic acid anions and hydroxybenzoic acid anions (present in both oilfield waters and the aqueous phase of hydrous pyrolysates) are capable of greatly enhancing Al, Si, Fe and Ca concentrations in solutions from dissolution of minerals by organometallic complexation. This enhancement of mineral solubility has been called upon to explain aluminosilicate dissolution porosity which is quantitatively important in many subsurface reservoirs; certainly, no other viable mechanism has been proposed to explain aluminum transport in the subsurface. When integrated into basin models, the regular evolution of organic and carbonate alkalinity in oilfield waters and the changing mineral stabilities that accompany that evolution help explain commonly observed diagenetic sequences in clastic systems.

Solid-state NMR characterization of Mowry shale from the Powder River Basin

Abstract Solid-state 13 C NMR measurements were carried out on a suite of petroleum source rocks from the Mowry shale of the Powder River Basin in Wyoming. The objectives of this study were to use CP/MAS 13 C NMR measurements to monitor changes in the carbon structure of the kerogen that result from depth of burial. NMR measurements were made on a suite of samples covering a present-day depth interval of 3000–11,500 ft. Because total organic carbon values were mainly in the range of 1–2 wt%, a large-volume sample spinner was used for the 13 C NMR measurements. Washing the samples with HCl was found to improve the quality of the 13 C spectra. The carbon aromaticity of the kerogen increased with depth of burial, and at depths greater than ∼10,000 ft the kerogen showed little capacity to generate additional oil because of the small fraction of residual aliphatic carbon remaining in the kerogen. By combining NMR and Rock-Eval measurements, an estimate of the hydrogen budget was obtained. The calculations indicated that ∼20% of the kerogen was converted to hydrocarbons, and that sufficient hydrogen was liberated from aromatization and condensation reactions to stabilize the generated products.

Predictive models for sandstone diagenesis

Abstract Factors governing the evolutionary path that sandstone and shale sequences follow during burial diagenesis include: provenance and depositionally-controlled compositional and textural elements; near-surface redox reactions; organic-inorganic interactions within the zone of intermediate burial diagenesis; deep diagenetic reactions, which include abiotic, thermal sulfate reduction, carbonate mineral alteration and quartz cementation; and, the potential for meteoric influx and renewal of near-surface processes due to uplift, exposure and/or base level fluctuation. As a consequence, the use of thermal exposure indicators to describe the extent of diagenesis, such as R0, or time-temperature history indicators, such as TTI, must be used with caution. Though useful in predicting general trends in porosity loss due to simple compaction, they have little utility in making forward predictions of anomalously-high porosity (e.g. preserved and/or enhanced porosity), or anomalously-low porosity. A more useful approach, whereby the processes controlling porosity evolution are evaluated in terms of the depositional, hydrologic, burial and thermal histories, is presented here.

The effect of carboxylic acid anions on the stability of framework mineral grains in petroleum reservoirs

This paper presents experimental and empirical evidence to show that carboxylic acid anions (CAAs) are a major diagenetic control on first-cycle basins in Jurassic-to-Pleistocene reservoirs in the 80-to-120{degrees}C thermal window.

Techniques and Problems in Sampling and Analyzing Formation Waters for Carboxylic Acids and Anions

Carboxylic acids and anions (CAA) have been known to exist in sedimentary formation waters since before the turn of the century. They have been proposed as essential agents in various geochemical processes such as metal ore migration and deposition, as precursors to natural gas, and as the mediators of organic-inorganic diagenesis. If the importance of CAA in geochemical processes is to be critically evaluated, viable samples of formation waters must be taken, and accurate and precise measurements of the CAA in the samples made. Key among sampling procedures designed to ensure sample viability are: flushing all production lines, sample lines, and sample containers with the formation fluid before the sample is taken; filtration of the sample through 0.1 μm at the time of sampling; addition of preservatives; and protection of the sample from light and elevated temperature. Accurate and precise measurements of the concentration of CAA in formation waters may be made by various analytical techniques, the most popular of which is ion chromatography. However, in complex mixtures this analysis is nontrivial; adjustments to the analytical technique may have to be made, and multiple runs performed on a single sample. Generally, a single sample run is an insufficient basis for meaningful interpretation.

Carboxylic acid anions in formation waters, San Joaquin Basin and Louisiana Gulf Coast, U.S.A.—Implications for clastic diagenesis. Reply to discussion by P.D. Lundegard and L.S. Land

Abstract MacGowan and S urdam (1990a) suggested some modifications to the model of Lundegard and Land (1989) to make it more geologically and geochemically reasonable. The predictive power of such a geochemical model is wholly dependent on the species modeled and the constants used; any model that excludes important species or important thermodynamic data, or one that couples certain reactions in an unrealistic way, may produce results which are not geologically or geochemically reasonable (W. K. Harrison , pers. commun., 1988; Y. K. Kharaka , pers. commun., 1991). We have long recognized that, under early-burial diagenetic conditions, aluminosilicate hydrolysis generally controls formation water pH ( Surdam and Eugster , 1976 ; Mariner and Surdam , 1970 ; Taylor and Surdam , 1981 ) and that, during intermediate burial, either aqueous CO2 or CAA species (in the absence of aqueous S species or other weak conjugate acid-base pairs) will dominate formation water alkalinity and control pH ( Surdam et al., 1989c ). We reassert that the model of Lundegard and Land (1989) does not take into account the relative importance of PCO2 and of concentrations of both Ca2+ and CAA and their relative organic metal complexes to carbonate mineral stability in sandstones in the zone of intermediate burial clastic diagenesis (cf. the models of Surdam et al., 1984 and Surdam and Crossey , 1985 ). The usefulness of such models is predicted on the completeness of the model and the use of the best, most accurate thermodynamic data. Also, geologically realistic concentrations of critical species are required for reasonable modeling to be done. Although their model is vigorously defended in the discussion of Lundegard and Land, 1989 , Lundegard and Land, 1993 , we continue to disagree that their analysis of their model conditions are either geologically or geochemically satisfying. We agree with the fundamental approach and philosophy of Lundegard and Land, 1989 , Lundegard and Land, 1993 . It is of the utmost importance to determine from experimental, geochemical, petrographic, and geological data what the controls on pH and alkalinity in formation waters are, as well as the exact thermodynamic speciation of aqueous moieties and the stability of detrital and authigenic minerals. Lundegard and Land (1993) raise an additional point about CAA reaction with carbonate minerals in shales, although Fisher and Lewan (1989) , Lewan (1989) and MacGowan and Surdam (1990b) have demonstrated that CAA generated in shale likely migrate in the oil phase along incipient shale microfractures to the sandstone reservoir, and thus are likely to not react much with the shale. Finally, we agree with Lundegard and Land that these areas require much additional experimental and field analysis, and petrographic study.

Modeling clastic diagenetic systems during progressive burial

The volumetrically important diagenetic reactions that result in significant porosity and permeability enhancement include framework grain and carbonate cement dissolution. The distribution in space and time of many of these reactions is mediated by organic/inorganic interactions. These interactions can be modeled by using coupled pathway, kinetic, and water-rock equilibria models. Pathway models suggest which diagenetic reactions are likely to occur, and how pore water chemistry will change with progressive diagenesis. Kinetic models predict the position in space and time of kinetically driven reactions; they require detailed knowledge of the time-temperature history of the system and the reaction kinetics involved. Water-rock equilibria models predict the thermodynamic direction of diagenetic reactions; for basin modeling, knowledge of the thermodynamic properties for reactions of interest as well as the time-temperature history of the system is required. These data are available for a variety of geochemical reactions of interest in many sedimentary basins. Preliminary results demonstrate that these techniques neatly predict porosity/permeability distributions in sand/shale sequences in some basins. The techniques appear to work best in basins where source and reservoir rocks are intercalated (minimizing fluid migration distances), as well as basins characterized by sporadic hydrodynamic events (e.g. seismic pumping and/or pressure chamber rupture).