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Propiedades De Los Fluidos Petroleros


Enviado por   •  1 de Abril de 2014  •  1.423 Palabras (6 Páginas)  •  301 Visitas

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Water Geochemistry of Oil Field Brines

Water geochemistry provides a series of powerful tools for solving various oil field development and production problems. Specifically, naturally occurring chemical "tracers" in water can be used to identify the origin and track the movement of water in oil fields, as well as predict the precipitation of scales. These naturally occurring tracers include the absolute and relative abundance of the various dissolved salts (anions and cations) as well as the isotopic composition of certain cations (e.g., 86Sr/87SR) and the hydrogen and oxygen stable isotopic composition of the water itself.

Which formation is responsible for water moving behind casing can be determined by comparing formation water geochemistry data with data from produced waters. Similarly, the progression of water floods can be monitored, as can the relative progression of water that is flooding a group of discrete sands. Additionally, water geochemistry can be used to diagnose the cause of precipitation of mineral scales (e.g., barite, calcite, silica, iron oxide, halite) in flow-lines, valves, gauges and other surface equipment by identifying the mixing of geochemically incompatible formation fluids at surface facilities.

The rest of this article describes why water geochemistry is able to distinguish different waters from one another. The types of processes described below provide the geochemical signals that we monitor to track the origin and movement of oil field waters and brines. Although this article gives particular emphasis to Gulf of Mexico water geochemistry, the concepts and tools described here can be applied in any basin to any oil field.

Figures 1 through 6 are cross-plots of water geochemistry data for various cation and anion constituents. Common to each of these figures are: the seawater evaporation trajectory showing seawater composition, and the gypsum and halite precipitation points (McCaffrey et al., 1987; Carpenter, 1978). Each figure also highlights regions showing the range of compositions of formation water produced from US Gulf Coast Cenozoic and Mesozoic reservoirs (from published and unpublished data: Graf et al., 1966; Carpenter et al., 1974; Carpenter and Trout, 1978; Land and Prezbindowski, 1981; Stoessel and Moore, 1983; Grossman et al., 1986; Kharaka et al., 1987; Morton and Land, 1987; Land et al., 1988; Land and Macpherson, 1989; Land and Macpherson, 1992; Moldovanyi and Walter, 1992; Moldovanyi et al., 1993; Macpherson, 1992; A. B. Carpenter personal communication to M. A. Beeunas, 1996).

Figure 1: Chloride versus Sodium Evaporation Trajectory

Figure 1: On this cross plot (and the following) of chloride versus sodium concentration (mg/L) are shown the composition of normal seawater with an evaporation trajectory through the gypsum and halite precipitation points (McCaffrey et al., 1987; Carpenter, 1978). Also are highlighted regions showing the compositional ranges of formation water from Gulf of Mexico offshore Cenozoic reservoirs, and offshore/onshore Mesozoic reservoirs (from published and unpublished data: see References). A significant number of the formation brines from Mesozoic reservoirs have both sodium and chloride concentrations less than that defined by the evaporation trajectory and are most likely the result of dilution with fresher waters during the initial evaporation stage or shortly after burial.

Figure 2: Chloride versus Calcium Evaporation Trajectory

Figure 2: Cross plot of chloride versus calcium concentration (mg/L). The affects of sulfate lost due to bacterially mediated sulfate reduction during early burial, the greater association of calcium carbonate containing lithologies during the Mesozoic and the generally greater reservoir temperatures result in the calcium concentration of Mesozoic age formation water to depart from the evaporation trajectory to higher concentrations.

Figure 3: Chloride versus Magnesium Evaporation Trajectory

Figure 3: Cross plot of chloride versus magnesium concentration (mg/L). The range of magnesium concentrations of formation water from Gulf Coast Cenozoic and Mesozoic reservoirs plot well below the evaporation trajectory due to magnesium removal during the formation of dolomite.

Figure 4: Chloride versus Potassium

Figure 4: Cross plot of chloride versus potassium concentration (mg/L). The range of potassium concentrations of formation water from Gulf Coast Cenozoic and Mesozoic reservoirs plots below the evaporation trajectory due to potassium removal during the formation of kaolinite and or illite. The greater concentration of potassium in formation water from Smackover reservoirs is due to the greater degree to which the Smackover brines were evaporated and from the albitization of K-feldspar.

Figure 5: Chloride versus Specific Gravity

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