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Oil Degradation in the Gullfaks Field (Norway): How Hydrogeochemical Modeling can Help to Decipher Organic-Inorganic Interactions Controlling CO2 Fate and Behavior

Abstract

Oil degradation in the Gullfaks Oilfield led to hydrogeochemical processes that caused (1) intense in-situ gas formation resulting in high CH4 and CO2 partial pressures, (2) an intense reservoir rock matrix alteration, and (3) a massive release of carbonate carbon and sodium into the formation water. We aim to quantitatively analyze the pathways of the complex, interconnected reactions and to retrace the consequences of these reactions by applying hydrogeochemical modeling. Our approach considers interactions among mineral assemblages (anorthite, albite, K-feldspar, quartz, kaolinite, goethite, calcite, dolomite, siderite, dawsonite, and nahcolite), aqueous solutions, and a multi-component fixed pressure gas phase (CO2, CH4, H2). 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 to finally reaching equilibrium conditions. By using the USGS's computer code PHREEQC, the modeling results quantitatively reproduce the proven reservoir rock matrix alteration triggered by oil degradation showing: (1) a nearly complete dissolution of plagioclase, (2) the stability of K-feldspar, (3) a massive formation of kaolinite, and to a lesser degree of Ca-Mg-Fe carbonate, as well as (4) an observed unusually high CO2 partial pressure (61 psi at maximum). In addition, this modeling reveals a specific sequence of alteration reactions which are coupled to the release of CO2. The evolving composition of co-existing formation water is strongly influenced by the uptake of carbonate carbon from oil degradation and sodium released from dissolving albitic plagioclase. Nahcolite (NaHCO3; instead of thermodynamically stable dawsonite) forms as a CO2 sequestering sodium carbonate, likely controlling CO2 partial pressure. High degrees of modeled oil degradation, resulting rock matrix alteration and nahcolite formation lead to CO2 and CH4 partial pressures similar to those observed in the areas of the Gullfaks field with strong degradation. The illustrated and quantitatively retraced diagenetic features can be taken as proxies for intense oil degradation; the degree of oil degradation and the described coupled inorganic processes including potential nahcolite formation are key factors affecting the amount and composition of gas generated in the Gullfaks oil reservoir.