Geochemical Model Assisted Analysis of Solid Mineral— Aqueus Phase Interactions and Construction of Charts
Because of the highly complicated nature of the interactions of solid minerals and aqueus solution in geological porous media, it is most convenient to facilitate appropriate geochemical models for the analysis of the potential chemical interactions and formation damage affects. A typical example of such studies has been carried out by Schneider (1997) in an effort to quantify the potential formation damage problems, which would result from the invasion of incompatible foreign water, such as by drilling and workover fluids and water flooding, into the Lower Spraberry sandstone reservoir formation of Texaco's Jo Mill Unit (JMU) field.
Mineral Characterization and Analyses
Based on the analyses using a scanning electron microscope, electron microprobe, and x-ray diffraction, Schneider (1997) determined the properties of the sandstone as: "Fine-grained, immature sandstones that contain considerable detrital clay minerals and carbonate clasts, along with quartz, plagioclase, and minor volcanic rock fragments and K-feldspar grains. Observed accessory minerals were muscovite, glauconite, hornblende, zircon, and pyrite. Authigenic minerals are dominated by carbonate cements and filamentous lathes of pore-lining and pore-filling illite. Some authigenic chlorite and overgrowths of quartz and feldspar are also present." He reports that the formation porosities and permeabilities are in the ranges of 10-20% and 0.5 to several mD, having considerable natural fracture permeability in certain regions and possibly some systematic fractures. He reports that this sandstone formation contains 6-10 volume % illite, 1-2 volume % chlorite, and negligible amounts of kaolinite (Scott, 1988).
Typical minerals present in porous rocks and subject to dissolution in contact with aqueous phase include the various types of carbonates such as calcite, CaCO3, magnesite, MgCO3, dolomite, CaMg(CO2)2, strontianite, SrCO3, witherite, BaCO3, and siderite, FeCO3, and various types of sulfates such as anhydrite, CaSO4, gypsum, CaSO4 • 2H2O, celestine, SrSO4, and barite, BaSO4 (Schneider, 1997). Schneider (1997) points out that the kaolinite compositions remain close to the Al2Si2O5(OH)4 formula, but the illite and chlorite formulae may vary as indicated by Aja et al. (1991a,b). He considered the typical mean compositions of the Bothamsall (Pennsylvanian), Rotliegendes (Permian), and Gulf Coast illites given, respectively, by (Warren and Curtis, 1989; Kaiser, 1984):
Schneider considered the typical mean compositions of the Gulf Coast (Kaiser, 1984) and the North Sea (Curtis et al., 1984, 1985) chlorites given, respectively, by
Although the analyses of the various Jo Mill Unit produced waters were available from the Inorganic Laboratory of Texaco EPTD, Schneider (1997) considered only the analyses of waters from the wells at five locations that did not make any appreciable amount of water. Therefore, for all practical purposes, these locations preserved their original water compositions. He also considered the analyses of the Mule Shoe Ranch and Canyon Reef waters that can be used for drilling and waterflooding operations. The analyses of these waters are presented in this article by Schneider (1997). Schneider (1997) used the SOLMINEQ.88 software to simulate the potential interactions and adverse affects of the formation minerals and aqueous phase. He assumed equilibrium conditions for conservative predictions of the rock-fluid interactions and water compatibility.
Saturation Index Charts
The compatibility of foreign water with the JMU reservoir connate water is investigated in this section using the saturation index charts.
Saturation Indices of the JMU Reservoir Waters
Schneider (1997) determined the saturation indices of the carbonates and sulfates, given in Table 13-2 for the JMU #7231 water using the SOLMINEQ.88 program. The saturation index values reported with positive and negative signs in Table 13-2 indicate conditions of supersaturation and under-saturation, respectively, for various minerals. The only unexpected result is the unusually low predicted siderite undersaturation of the water. However, in general, the saturation indices of the various minerals calculated by the SOLMINEQ.88 program is consistent with the mineralogy of the JMU sandstone formation. This is a further confirmation of the accuracy of the geochemical model (Schneider, 1997).
Mixing Paths on the Mineral Stability Charts
Effects of mixing foreign and reservoir waters on mineral stability are best realized by constructing mixing paths on the mineral stability charts. Schneider (1997) investigated the compatibility of the JMU #7231 well connate water with the Mule Shoe Ranch and Canyon Reef waters considered for potential use in drilling and/or water flooding operations. The analyses of these waters are given by Schneider (1997) in Table 13-1. It is apparent that the Mule Shoe water has a higher CO2-21 and
HCO-3 content and, therefore, higher alkalinity than the other waters reported in Table 13-1. He used the SOLMINEQ.88 program and simulated the consequences of mixing the JMU #7231 connate water with 10% volume increments of the Mule Shoe Ranch and Canyon Reef waters. Schneider (1997) constructed an illite-chlorite mineral stability chart based on the following reaction equation with proper stoichiometric coefficients for the Rotliegendes illite formula:
He then plotted the curves for mixing the JMU connate water with the Mule Shoe water on this chart for the 8.4, 9.5, and 10.5 pH values He also investigated the effect of the K+ activity on the illite stability. The mixing curves for 0, 2 and 5 weight % KCl solutions at the 10.5 pH level are shown in this article, B, and C, respectively. Clearly, adding KCl increases the illite stability. However, K+ activity has a relatively smaller effect than pH.
Saturation Index Charts for Clay Minerals
Schneider (1997) constructed the illite saturation index curves for mixing the Mule Shoe water with the JMU connate water as a function of the volume percent Mule Shoe water in the mixture. For this purpose, he considered the following dissolution/precipitation reaction for the Rotliegendes illite formula:
He explains that the Mule Shoe Ranch water is usually treated with lime and therefore its pH is above 10. A comparison of A and B reveals that injecting a high pH Mule Shoe Ranch water into the reservoir reduces the illite stability of the JMU connate water upon mixing. Schneider (1997) indicate that adding KCl into the Mule Shoe Ranch water improves the illite stability.
Schneider (1997) equilibrated the activities of the five connate water compositions given in Table 13-1 to the 135°F temperature of the Jo Mill Unit reservoir using the SOLMINEQ.88 program. He then plotted these activity values on the activity-activity charts. All points appear inside the mineral stability fields of the types of clay minerals present in the sandstone formation of the Jo Mill Unit reservoir. Hence, this confirmed the validity of the geo-chemical model and the accuracy of the mineral stability field charts generated by the SOLMINEQ.88 program. Schneider (1997) explains that the formula
of the Rotliegendes illite is somewhat similar to the formula KA12 (AlSi3]Olo(OH)2 of the muscovite, which is an end-member composition illite. Because the JMU reservoir contains a high amount of illite (6-10 volume %), the JMU reservoir connate water compositions should appear within the muscovite stability region by Schneider (1997). Schneider (1997) constructed the illite-chlorite mineral stability charts shown in this article based on the following illite to chlorite incongruent reactions using the proper stoichiometric coefficients according to the compositional formulae of the illites and chlorites mentioned above:
Again, as indicated by the mineral stability charts shown in this article by Schneider (1997), all the JMU reservoir connate water composition appear inside the mineral stability regions of the illites. Schneider (1997) constructed the illite-kaolinite mineral stability charts shown in this article based on the following illite to kaolinite incongruent reactions using a proper set of stochiometric coefficients according to the compositional formulae of the illites and kaolinites considered for the study:
Because of the existence of a large quantity of illite (6-10 volume %) and a negligible amount of kaolinite in the JMU sandstone reservoir formations, all the JMU connate waters appear inside the illite stability region. Schneider (1997) constructed the chlorite-kaolinite mineral stability charts shown in this article the following chlorite to kaolinite incongruent reactions using the proper set of stoichiometric coefficients according to the compositional formulae of the chlorites considered for the study
Because of a relatively larger quantity of the chlorite (1-2 volume %) compared to the negligible amount of kaolinite present in the JMU sandstone formation, all the JMU connate waters appear inside the chlorite stability region.
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