Maurice Shevalier

pH buffering by metastable mineral-fluid equilibria and evolution of carbon dioxide fugacity during burial diagenesis

Numerous potential pH buffers including reactions among aqueous organic acid and carbonate species carbonate and silicate minerals are typically present during burial diagenesis. Buffering of pH in natural systems is a function of mass action, mass balance kinetic constraints. In most sedimentary basins, carbonate and silicate minerals are present in amounts sufficient to buffer pH the activities of aqueous species are consistent with metastable equilibrium among observed diagenetic minerals. These observations indicate that mass balance and kinetic constraints are relatively less important than mass action constraints measured by the buffer index, β, here defined as follows: β = −dξdpH The buffer index ultimately dictates which buffer reaction controls pH under diagenetic conditions; buffer reactions with high β values are favoured over those with low values. Buffer indices for a number of potential diagenetic buffer reactions have been calculated by reaction path modeling. Heterogeneous equilibria among carbonate and silicate minerals and an aqueous phase have greater β values than those for homogeneous reactions among aqueous carbonate and organic acid species. This implies that pH, calcite dissolution ƒCO2 are strongly dependent on carbonate-silicate-fluid interactions during diagenesis. The role of carbonate-silicate reactions in controlling pH is tested by examining the evolution of CO2 fugacities with temperature during burial diagenesis. Carbon dioxide fugacities have been calculated by reaction path modeling of diagenetic carbonate-silicate equilibria for sedimentary and geothermal systems. Calculated CO2 fugacities are in general agreement with observed CO2 fugacities. The combination of high buffer index, apparent metastable equilibrium between diagenetic minerals and waters the relatively accurate prediction of z.hfl;CO2 trends with temperature suggest that carbonate-silicate reactions are important in determining the evolution of fluid compositions in sedimentary basins and influence the course of dissolution events in burial diagenesis.

- Geochemical monitoring of gas-water-rock interaction at the iea Weyburn CO 2 Monitoring and Storage Project, Saskatchewan, Canada

Publisher Summary The Weyburn Oil Field is the site of EnCanas C02-injection enhanced oil recovery (EOR) project in a carbonate reservoir in southern Saskatchewan. An IEA-sponsored program was initiated to evaluate the potential for geological storage of greenhouse gases. Geochemical monitoring and modeling plays an important role in assessing gas-fluid-rock interactions. The geochemical dataset is the primary input for quantifying the short- and long-term (years to thousands of years) CO2 storage potential in the basinal brine, non-recoverable oil, and in newly precipitated minerals. Three geochemical processes have been found occurring in the area of the reservoir as a result of CO2 injection: (1) rapid CO2 dissolution in the brine and oil; (2) the CO2 “sweep” previously assessing the inaccessible portions of the reservoir; and (3) carbonate mineral dissolution due to acidification of the basinal brine. Equilibrium relations among gases, brines, and liquid hydrocarbons in the Weyburn field demonstrated a close correlation among dissolved CO2, free gaseous CO2, and the amount of injected CO2.

Mineralogical characterization of the Weyburn reservoir, Saskatchewan, Canada: Are mineral reactions driving injected CO2 storage?

Publisher Summary The Weyburn Oil Field is the site of EnCanas large C02-injecfion enhanced oil recovery project in southern Saskatchewan, Canada. The Weyburn field is one of a number of large oilfields that lie along the Mississippian subcrop belt on the northern part of the Williston Basin. In close cooperation with the International Energy Agency, an international multi-disciplinary research initiative to study the short and long-term potential of geological storage of CO2 in a carbonate reservoir has been established. Although the Weyburn Midale reservoir is dominantly calcitic and dolomitic, significant amounts of potentially reactive silicate minerals are present to assist in CO2 storage have grouped CO2 trapping into three types. Type 1 capture is defined as occurring when a mineral is precipitated incorporating the anion formed by the dissolved gas. Type 2 capture is defined as occurring when an acid gas is neutralized in solution forming a nonvolatile soluble salt and subsequently leading to brine formation. Type 3 capture is characterized by CO2 trapping in both solid and aqueous phases. The detailed mineralogy of the Weyburn reservoir, with particular attention paid to EnCanas flow unit nomenclature, is described. Mineral reaction modelers, assessing the long-term fate of injected anthropogenic CO2, may use the mineral modes of flow units as input variables.

Chemical and isotopic examination of produced waters from the BP-Wolf lake in situ combustion pilot

Abstract The chemical and isotopic compositions of co-produced waters can be used to monitor the processes that take place during in situ combustion. Anticipated processes include mixing of waters, production of CO 2 , production of high concentrations of dissolved sulphate and variations in water chemistry associated with heated zones. Water sources include pore waters in oil-bearing strata, waters in overlying or underlying aquifers, water condensed from previously injected steam, and waters associated with combustion. Waters from all sources may mix during production and interpretation of the combustion process can be refined by an understanding of water sources. Produced fluids from the BP-Wolf Lake pilot site in Alberta have been examined to evaluate the effectiveness of the chemical composition of water and the isotopic compositions of aqueous species for monitoring in situ combustion. Produced waters do not show simple conservative mixing behaviour. This suggests that multiple sources of water and other processes, including water-rock reactions, act to modify water compositions. At least three sources of produced waters can be recognized and these are interpreted to be formation water, injected steam and waters that have low Cl and high HCO 3 due to combustion. It is not possible to distinguish waters in the oil-bearing formation from regional waters present in aquifers that underlie the stimulated intervals. Dissolved aqueous species, such as SiO 2 , Na, K (as Na/K) and Cl can be used to monitor the approach of the combustion front. Sulphate has been suggested as an indicator of approaching combustion and, although sulphate concentrations rise as combustion approaches a producing well, this indicator is not reliable in all cases. The use of all the above chemical parameters is recommended for detection of combustion zones during operation. The isotope composition of produced waters confirms that there has been significant water-rock interaction during combustion. Carbon isotope compositions of HCO 3 that range from −8 to −25% δ 13 C show that oil oxidation is a major contributor of CO 2 at high temperatures, but CO 2 produced by carbonate mineral dissolution becomes more significant as temperature decreases. Sulphate concentrations in waters produced during combustion can be an order of magnitude higher than those observed during steam stimulation. Both the oil (bitumen) and pyrite (FeS 2 ) are significant sulphur sources. Typically, the sulphur in both phases is in a reduced state and is available through oxidation associated with combustion. The δ 34 S of dissolved sulphate in produced waters does not unequivocally identify either of the two major sources of sulphur. However, the relatively depleted δ 34 values for SO 4 suggest that the high sulphate concentrations generally associated with the approach of the combustion front result from the oxidation of pyrite.