Bio-Geochemical Processes in Subsurface: From Produced Water Reinjection (PWRI) to Geological CO₂ Storage

Typically, subsurface operations such as produced water reinjection (PWRI) and geological CO₂ storage are influenced by bio-geochemical interactions that can disturb the process as well as underground existing equilibrium conditions. Many mature hydrocarbon fields, such as those in Denmark, face rising volumes of wastewater whose offshore discharge impacts marine ecosystems. Besides, the continuous release of CO₂ exacerbates climate change globally. Injection (water) or storage (CO₂) of these streams underground offers a sustainable solution but introduces obstacles and uncertainties, including injectivity impairment in PWRI settings and long-term containment in CO₂ storage applications. As clarified in Chapter 1, despite various batch and dynamic laboratory and modelling studies, knowledge gaps persist in developing predictive tools that integrate bio-physico-chemical processes to predict formation damage and test mitigation strategies. Besides, there is a lack of reactive transport models that consider the gas-brine-rock interactions to ensure secure and permanent storage of CO₂ stream with reactive impurities. In 5 Chapters, this thesis tries to address these missing aspects through proposing predictive models that help characterize subsurface disposal strategies for both CO₂ and produced water.

Chapter 2evaluates mineral scale formation and prevention during PWRI in the Danishoffshore fields. Considering the actual compositions of the produced water andseawater, it is first identified (through modelling) which mineral phases(e.g., sulfate, carbonate, iron scales) are most likely to form under differenttemperature, pH, and mixing ratio conditions. Then, the efficiency of acommercial scale inhibitor against carbonate and sulfate scaling is testedexperimentally. Through a batch chemical model, the equilibrium constants ofthe scale inhibitor reaction with scale forming species (Ca²⁺, Ba²⁺,and Sr²⁺) are optimized. The efficiency of the scale inhibitor inlowering the carbonate and sulfate scaling is explored, showing that efficiencyagainst carbonate scaling decreases as pH increases (induced by using H₂Sspent scavenger). Through a general chemical model, the impact of combined mitigatingchemicals (scale inhibitor, acid, oxidizing and chelating agents) in limitingthe overall scaling load is assessed and chemical dosages required for scalinginhibition are predicted. It is theoretically shown that using chemical recipescan help handling the precipitation of integrated Fe-, CO₃ ^2--,and SO₄ ²-bearing scales. Chapter 3 proposes a newapproach to estimate the potential for colloids’ (oil and CaCO₃)attachment to or detachment from the chalk surface during water injection. Theproposed model combines DLVO theory, hydrodynamic considerations, and surfacecomplexation models adapted for calcite and oil. The performance of chemicalmodels for both colloid types is confirmed using relevant measurements. In highsaline produced water injection, it is shown that regions near reservoirsurface are chemically favourable for oil and calcite deposition underdifferent pH and temperature conditions, implying a potential for formationdamage. Reducing the salinity (50 times lower) and increasing pH simultaneouslydiminish the tendency for attachment to the chalk surface at the assumedreservoir temperature of 80 °C. Considering typical flow velocities in porousmedia, it is concluded that chemical processes govern the colloid-aqueous-rockinteractions close to the surface, whereas hydrodynamics control suchinteractions farther from the surface. Chapter 4 expands upon these findings byproviding a mechanistic reactive transport model designed to capture theinjectivity impairment caused by potential damaging components (e.g., solid andoil colloids, biomass, and inorganic scales) during water injection. A fewbatch and core flooding data are used to tune/verify the clogging models, whichsimulate oil/solid particle retention, mineral scale precipitation, and biomassgrowth in porous media. The general model can also be employed to assess thedamage mitigation through physico-chemical treatments strategies (the findingsfrom Chapter 1 can be applied). Additional lab-scale measurements under certaincontrolled conditions could help refine adsorption rates, leading to the developmentof more generalized models applicable to broader conditions. Chapters 5 and 6 switchfocus to geological CO₂ storage, elaborating on the diffusion of CO₂and reactive impurities (H₂S, SO₂, NO₂) intocaprocks over geological periods (10,000 years). Through 1D reactive transport simulations,these chapters assess how dissolution/precipitation processes affect C-S-Nchemical trapping or aqueous speciation, diffusion retardation, and overallcaprock integrity. Considering the mineralogy of three shale caprocks fromfields in the North Sea, it is indicated that the macroscale CO₂ diffusiondiminishes by almost 1.5-2 orders of magnitude due to the involved CO₂-brine-rockreactions. In addition, it is shown how impurity-laden CO₂ potentially“seal” or “heal” the caprock via newly formed minerals.

This work provides valuable insights into the coupled processes governing the formation damage in PWRI and long-term caprock considerations in geological CO2 storage, ultimately contributing to more sustainable subsurface management. The thesis offers workflows that help interpret the results of batch or core-scale experiments and design laboratory and field analyses for subsurface water injection or CO₂ storage. The models and outcomes from this thesis have broader relevance to other subsurface applications, such as pollutant transport, geothermal energy production, soil bio-clogging, etc.