Conclusiones y líneas futuras

A summary of the Eh and pH for various subsurface fluids, compared to seawater and rainwater, is given in Figure 96, which is also known as a Pourbaix diagram.

Rainwater is both acidic (i.e., low pH) and oxidizing (i.e., high Eh). Upon contact, rainwater oxidizes organic matter (i.e., humic acids) and is in turn reduced (lower Eh). Aerobic bacteria feed upon organic matter, deriving energy through the oxidation of carbon. In the presence of oxygen, the 慹lectron sink? is the reduction of oxygen, which in turn lowers the Eh. Anaerobic bacteria utilize the sulfate ion, which is reduced to sulfite and finally sulfide, in doing so the Eh decreases to -0.6 V (Collins, 1975; Walther, 2005). The percolating water will also acquire dissolved solids, which changes the pH from 慳cidic? to neutral/slightly alkaline. However, as a general rules the pH of oilfield water is typically controlled by the carbon dioxide-bicarbonate system (Collins, 1975).

Connate waters show a broad range of Eh and pH values (Figure 96) due to their variation in composition from basin to basin and formation to formation, although, in general, oilfield brines tend to be both alkaline and strongly reducing.

Figure 96. Eh-pH plot (Pourbaix diagram) showing the stability of water at STP. Also shown are the generalized Eh-pH characteristics of rainwater, meteoric water, modern sea water, connate water and the stability limits of water (after Pourbaix, 1966; Collins, 1975; Walther, 2005; and others).

Chapter 5—Water, Pressure, and Temperature

72 Chemical composition

There are a number of reasons for analyzing the composition and hydrochemistry of oilfield water. It has been stated that within the U.S.A., 7 bbl of water is produced for every 1 bbl of oil³. Therefore, understanding the characteristics of produced (i.e., oilfield) water can help operators increase production and facilitate water disposal, as well as identify potential well-bore or reservoir problem areas (Breit e. al., 2000, 2001). Knowledge of the TDS content can help define pay zones when coupled with conductivity and or resistivity measurements (Breit et al., 2000). Furthermore, temperature and pH influence the solubility of organic compounds (Collins, 1975; Price, 1976; McFarlane et. al., 2002;

Walther, 2005) with the result that oilfield water may contain organic acids, polycyclic aromatic hydrocarbons (PAH), phenols, and dissolved light hydrocarbons.

The hydrochemistry of water is typically defined using a suite of ions in solution (e.g., HCO3? SO42

-, Cl--, Na⁺, K⁺, Ca²⁺ and Mg²⁺), augmented by other analyses, such as the redox potential (Eh) and pH (-log[H+]), or the assessment of electrical conductivity (S- m^-1) to identify the specific origin for a given water. The acid-neutralizing capacity of a water sample is expressed by total alkalinity (mg/L CaCO3). The analysis of oilfield water may also include an analysis of organic constituents, such as humic acids, fulvic acids, carbohydrates and hydrocarbons. However, the most routine analysis generally involves the analysis of the TDS. Consequently subsurface waters have been 慸escribed? using such characteristics.

Classification

慛ormal? seawater contains approximately 35,000 ppm (3.5%) total dissolved solids (TDS), mostly as NaCl. In contrast, most connate waters contain up to 300,000 ppm TDS. Subsurface water containing more than 100,000 ppm TDS is considered a brine. The most concentrated brines occur

in undisturbed deep basins, although brine can become more concentrated if the stratigraphic sequence includes evaporites.

Meteoric waters tend to have higher concentrations of HCO3¯ and SO42

-, and relatively low amounts of Ca²⁺ and Mg²⁺. In contrast, connate waters differ from seawater because they contain lower concentrations of SO42-, Ca²⁺ and Mg²⁺, and higher proportions of Cl-, Na⁺, and K⁺. Therefore, most classification schemes reflect the dominant mineral ion, or ions, in solution, e.g., Cl-, SO4

2-, HCO3? Ca²⁺, Mg²⁺, and Na⁺. Sulin scheme

Proposed by the Russian hydrologist V.A. Sulin (1946), the Sulin classification (Figure 97) recognizes four hydrochemical water classes or types based upon the relative dominant anion (e.g., HCO3? ), and cation (e.g., Ca²⁺). These are:

a) Sulfate - sodium waters b) Bicarbonate - sodium waters c) Chloride - magnesium waters d) Chloride - calcium waters Most oilfield waters are Chloride - calcium waters because they typically contain almost no SO4

2- or HCO3

-, water associated with evaporitic sequences are characterized as Chloride - magnesium waters, whereas meteoric waters are typically Sulfate ? sodium and Bicarbonate - sodium waters. Data for a given water analysis can be plotted graphically (Figure 97) on a diagram consisting of two axes comprised of the ion pairs, Cl- and Na⁺, and SO4

2- and Mg²⁺. Ion concentrations in milligrams per liter (mg/L) are converted to millequivalents per liter (meq/L) and plotted using the appropriate axis. Regions within the diagram correspond to the four hydrochemical water types.

3Sourcehttp://www.energy.gov/

Figure 98. Stiff (1951) diagrams, showing the plotting axes and example plots (after Stiff, 1951; Collins, 1975; and others).

Figure 97. Sulin (1946) classification plot. Values are in milliequivalent percent (meq/L %) and regions for the four hydrochemical water classes or types are indicted (after Sulin, 1946; Collins, 1975; Walther, 2005; and others).

Chapter 5—Water, Pressure, and Temperature

Stiff diagrams

Stiff diagrams (Figure 98) are another visual method to compare and contrast the relative proportions of ions in water (Stiff, 1951). Like the Sulin diagram, ion concentrations are plotted in millequivalents per liter (meq/L), in which cations (e.g., Na⁺, Ca²⁺, Mg²⁺, and Fe²⁺) are plotted on the left and anions (e.g. Cl-, SO4

2-, HCO3

-, and CO3) on the right (Figure 98). The shape of each Stiff plot visually conveys the relative abundance and dominance of specific ions in a given water, since the distance of each vertex from the medial line is proportional to ionic content and abundance. Stiff plots are somewhat flexible, in that different ion combinations can be plotted depending on aqueous geochemistry. For example, plots could incorporate sodium plus potassium (Na⁺+K⁺), calcium (Ca²⁺), and magnesium (Mg²⁺) on the left, with chloride (Cl-), bicarbonate plus carbonate (HCO3? + CO3), and sulfate (SO4

2-) on the right.

Piper diagrams

The Piper diagram, also known as a Trilinear diagram (Piper, 1944), is a plot of the major ions as percentages of milli-equivalents in two base triangles, in which cations plot on the left plot and anions on the right (Figure 99). For each triangular plot the values to be plotted are derived by taking the value for each ion and dividing that value by the sum of all three values, to produce a percentage.

For example, if Ca²⁺ = 0.26 meq, Mg²⁺ = 0.89 meq and Na⁺ + K⁺ = 0.80 meq, then Ca²⁺ = 13%, Mg²⁺ = 45%, and (Na⁺ + K⁺) = 40%.

The data points in the central (diamond-shaped) field represent points of intersection, located by projecting points from each of the two lower triangular plots to a point of intersection in the center field. The main purpose of the Piper diagram is to show the clustering of data points to identify and indicate water samples that have similar compositions.

Isotopes

Isotopes are increasingly used in the analysis of oilfield water and ground water in general. Isotope analysis is based on the concept that the relative mass abundance of light to heavy isotopes is large and that some natural process modifies the relative abundance of a given isotope within a system (Turan, 1982). The most commonly used isotopes include the stable isotopes deuterium (an isotope of hydrogen), oxygen, sulfur, and carbon (D, ¹⁸O,³⁴S, and ¹³C respectively), and the radiogenic isotopes chlorine (³⁶Cl), carbon (¹⁴C), and strontium (Sr⁸⁷) (Chaudhuri, 1978; Connolly et al., 1990).

For stable isotopes, usually one isotope (e.g., oxygen ¹⁶O, ¹⁸O) has a greater relative abundance, and differences due to fractionation are often small, isotopic abundance is therefore calculated as a ratio and expressed as positive or negative deviations from a standard (Fritz and Fontes, 1980).

For example, the relative abundance of ¹⁸O/¹⁶O within a sample is compared to the ¹⁸O/¹⁶O of VSMOW (Vienna Standard Mean Ocean Water as established by the International Atomic Energy Agency). Relative abundance is therefore represented by the formula:

where: represents the deviation from the standard, reported as per mil ( ) and R is the isotopic ratio: e.g. (¹⁸O/¹⁶O).

The isotopic abundance of D or ³⁴S is defined in a similar way, although both ¹⁸O and D are compared to SMOW, whereas ³⁴S is compared to the Canyon Diablo meteorite (Nielson, 1979).

Radioactive isotopes are reported in different ways. Measurements of ³⁶Cl are reported as a ratio of ³⁶Cl/Cl, whereas

87Sr is given as a normalized ratio, checked against a standard (e.g., SRM-987). Analyses of ⁸⁷Sr/⁸⁶Sr isotopic ratios, in conjunction with stable isotopes, can help determine the extent of water/mineral interactions and water mixing, and help identify migration pathways (e.g., Chaudhuri, 1978; Stueber et al., 1987). Oxygen and deuterium values are useful

(5) Figure 99. Piper diagram (Trilinear diagram) for the Great Plains and Cedar Hills Aquifers in Central Kansas, U.S.A. (from Macfarlane et al., 1988, Open-file Report 88-39).

Chapter 5—Water, Pressure, and Temperature

74

indicators of water origin, whereas strontium can provide information concerning the chemical evolution of water (Chaudhuri, 1978).