Wolfgang van Berk

Controls on CO2 fate and behavior in the Gullfaks oil field (Norway): How hydrogeochemical modeling can help decipher organic-inorganic interactions

Oil degradation in the Gullfaks field led to hydrogeochemical processes that caused high CO2 partial pressure and a massive release of sodium into the formation water. Hydrogeochemical modeling of the inorganic equilibrium reactions of water-rock-gas interactions allows us to quantitatively analyze the pathways and consequences of these complex interconnected reactions. This approach considers interactions among mineral assemblages (anorthite, albite, K-feldspar, quartz, kaolinite, goethite, calcite, dolomite, siderite, dawsonite, and nahcolite), various aqueous solutions, and a multicomponent fixed-pressure gas phase (CO2, CH4, and H2) at 4496-psi (31-mPa) reservoir pressure. The modeling concept is based on the anoxic degradation of crude oil (irreversible conversion of n-alkanes to CO2, CH4, H2, and acetic acid) at oil-water contacts. These water-soluble degradation products are the driving forces for inorganic reactions among mineral assemblages, components dissolved in the formation water, and a coexisting gas at equilibrium conditions. The modeling results quantitatively reproduce the proven alteration of mineral assemblages in the reservoir triggered by oil degradation, showing (1) nearly complete dissolution of plagioclase; (2) stability of K-feldspar; (3) massive precipitation of kaolinite and, to a lesser degree, of Ca-Mg-Fe carbonate; and (4) observed uncommonly high CO2 partial pressure (61 psi [0.42 mPa] at maximum). The evolving composition of coexisting formation water is strongly influenced by the uptake of carbonate carbon from oil degradation and sodium released from dissolving albitic plagioclase. This causes supersaturation with regard to thermodynamically stable dawsonite. The modeling results also indicate that nahcolite may form as a CO2-sequestering sodium carbonate instead of dawsonite, likely controlling CO2 partial pressure.

Retracing the development of raw water quality in water works applying reactive controlled material flux analyses

Abstract.The Fuhrberger Feld aquifer in northern Germany provides the majority of water supply for the city of Hannover. Although parts of the recharge area have received a strong intake of nitrate from agricultural activities during the past 40 years, the extracted raw waters show little sign of nitrate contamination due to microbial nitrate reduction coupled with iron sulfide oxidation in the aquifer. Accordingly, iron and sulfate concentrations, in particular, are temporarily objectively high. To evaluate the contribution of quantitatively important processes to the development of raw water quality, all relevant natural and anthropogenic induced processes were identified, quantified and stepwise integrated in a reactive controlled material flux model using PhreeqC.Comparing measured and modeled data, long-term trends in raw water quality could be retraced. As a result, anthropogenic initiated processes such as varying nitrate influxes, conversion of wet grasslands into arable land coupled with geochemical processes (biomass degradation, oxidation of iron monosulfides in hydromorphic soils) and a partial nitrate breakthrough to the wells due to local complete consumption of iron disulfides were responsible for observed quality changes.

From shale oil to biogenic shale gas: Retracing organic–inorganic interactions in the Alum Shale (Furongian–Lower Ordovician) in southern Sweden

Methane-rich gas occurs in the total organic carbon–rich Alum Shale (Furongian to Lower Ordovician) in southern Sweden. The lower part of the thermally immature Alum Shale was impregnated by bitumen locally generated by heating from magmatic intrusions from the Carboniferous to the Permian. Organic geochemical data indicate that the migrated bitumen is slightly degraded. In the upper Alum Shale, where methane is the main hydrocarbon in thermovaporization experiments, centimeter-size calcite crystals occur that contain fluid inclusions filled with oil, gas, or water. The Alum Shale is thus considered a mixed shale oil–biogenic shale gas play. The presented working hypothesis to explain the biogenic methane occurrence considers that water-soluble bitumen components of the Alum Shale were converted to methane. A hydrogeochemical modeling approach allows the quantitative retracing of inorganic reactions triggered by oil degradation. The modeling results reproduce the present-day gas and mineralogical composition. The conceptual model applied to explain the methane occurrence in the Alum Shale in southern Sweden resembles the formation of biogenic methane in the Antrim Shale (Michigan Basin, United States). In both models, melting water after the Pleistocene glaciation and modern meteoric water may have diluted the contents of total dissolved solids (TDS) in basinal brines. Such pore waters with low TDS contents create a subsurface aqueous environment favorable for microbes that have the potential to form biogenic methane. Today, biogenic methane production rates, with shale as the substrate using different hydrocarbon-degrading microbial enrichment cultures in incubation experiments, range from 10 to 620 nmol per gram and per day.

Hydrogen sulfide formation, fate, and behavior in anhydrite-sealed carbonate gas reservoirs: A three-dimensional reactive mass transport modeling approach

A novel hydrogeochemical modeling approach is developed to unravel thermochemical sulfate reduction (TSR) in hydrocarbon reservoirs. Our numerical model couples a web of interconnected hydrogeochemical reactions to three-dimensional (3-D) and reservoir-wide diffusive mass transport. Our modeling approach simulates a semigeneric gas reservoir sealed by anhydrite. The calculated diagenetic processes fit the observations in reservoirs affected by TSR: formation of water, precipitation of calcite, metal (di-)sulfides, and elemental sulfur as replacements of dissolved anhydrite at the expense of CH4(g), as well as formation of hydrogen sulfide (H2S). By varying input parameters, the crucial factors controlling TSR have been identified. Our results highlight that reservoir-wide diffusive mass transport is one prerequisite for TSR. An increase in the rate constant of abiotic sulfate reduction (ASR) and in diffusive mass fluxes, as well as lack of precursor minerals for metal (di-)sulfide precipitation, can increase the souring intensity and accelerate H2S outgassing. In contrast, precipitation of elemental sulfur, which is stable according to the chemical thermodynamics, weakens H2S formation. Our modeling shows that TSR is complex and cannot be represented by the single reaction ASR and by simple correlations between the rate constant of ASR and the H2S gas content. The application of 3-D reactive transport modeling presented here, despite its semigeneric nature, provides a good example of how such an approach can be used ahead of drilling. Our modeling helps to investigate TSR in time and space to quantify the mass conversion of all reactants involved within this web and to predict the souring level.

Creation of pre-oil-charging porosity by migration of source-rock-derived corrosive fluids through carbonate reservoirs: one-dimensional reactive mass transport modelling

Locally increased porosity of carbonate reservoir rocks may result from acidic fluids that migrated as a pre-oil phase through the reservoir. Here, hydrogeochemical modelling, which is based on the principles of chemical equilibrium thermodynamics, is performed to test such a hypothetical concept. Despite the generic nature of the model, the modelling results give basic and quantitative insights into the mechanisms of calcite dissolution in carbonate reservoirs induced by migrating acidic and corrosive aqueous fluids. The hydrogeochemical batch modelling considers pre-oil-phase aqueous fluids that form by kerogen maturation in siliciclastic source rocks underlying the carbonate reservoir rocks. Although saturated with respect to calcite, migration of such fluids through the carbonate reservoir triggers continuous calcite dissolution along their migration path following a decreasing pressure and temperature regime. One-dimensional reactive transport modelling reveals that thermodynamically controlled chemical re-equilibration among pre-oil-phase fluids, calcite and CO2(g) is the driving force for continuous calcite dissolution along this migration path. This reflects the increasing solubility of calcite in the system ‘pre-oil-phase fluids/calcite/CO2(g)’ with decreasing pressure and temperature. In consequence, such fluids can preserve their calcite-corrosive character, if they are exposed to continuously decreasing pressure and temperature along their migration path through the reservoir. Supplementary material: The modelling input files to ensure retraceability of our modelling approach and its results are available at http://www.geolsoc.org.uk/SUP18802.