MINERAL PRECIPITATION IN FRACTURES FOR SUBSURFACE ENERGY APPLICATIONS: MULTISCALE IMAGING, GEOCHEMICAL REACTIVE TRANSPORT MODELING, AND LABORATORY EXPERIMENTS

Abstract

In subsurface energy applications such as geologic CO2 storage, fractures can serve as conductive pathways for CO2 to leak from the storage location. It is crucial to have strategies to seal underground fractures so that they cannot serve as conduits. The goal of this dissertation is to understand the hydrodynamic and geochemical conditions that lead to precipitation of minerals in fractures and to propose strategies to seal them. In chapter 2, I examine a natural analogue of this process by studying a shale specimen with existing sealed fractures, using an array of imaging and characterization methods to describe mineralogy and porosity at several spatial scales. Collectively, these methods reveal crystals of dolomite as large as 900 microns in length overlaid with a heterogeneous mixture of carbonate minerals including calcite, ferroan dolomite, and iron-rich ferroan dolomite, interspersed at spatial scales as small as 5 microns. In chapter 3, I introduce a new approach to seal highly permeable fractures by inducing mineral precipitation in them using magnetite nanoparticles. Through thermodynamic modeling and laboratory experiments on fractured rock samples, the observations confirm that under the high-pressure conditions relevant to CO2 storage, magnetite reacts with CO2 acidified brine and produces hematite and siderite as sealant products. In addition, a detailed quantitative analysis is performed in Chapter 4 to show that in addition to the thermodynamic feasibility of the process, it is economically viable to connect acid mine drainage with geologic carbon storage. In chapter 5, to address and incorporate the critical impact of spatially random nature of nucleation on the kinetics of carbonate precipitation at the continuum scale, I develop a stochastic reactive transport model to explore a novel approach that treats water-mineral precipitation reaction rates as probabilistic. The model mimics the uneven and distributed profile of carbonate mineral precipitation along a fracture length and estimates the reduction in fracture permeability. Chapter 6 supplements Chapter 5 and includes additional sensitivity analysis on the impact of geochemical and hydrodynamic conditions on the fracture sealing potential and permeability reduction.