ROLE OF MINERALOGY IN CONTROLLING FRACTURE FORMATION

Abstract

In subsurface CO2 storage, impermeable caprocks, often shale, are relied upon to maintain pressure and prevent leakages. Natural or induced fractures in caprocks can threaten the system integrity by allowing CO2 migration into the caprock. The evolution of a fracture’s aperture and permeability over time is dependent on dissolution and precipitation reactions caused by interactions between the injected fluid and the mineralogy exposed at the fracture surface. The rate and direction of these reactions can be predicted based on bulk mineralogy of the formation. However, it is unclear whether the bulk mineralogy is representative of the mineralogy present along the fracture surface. To explore this, shale cores from the Mancos and Marcellus shale formations were subject to unconfined compression until an initial fracture formed. High resolution images were taking using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). From these images, color coded mineral maps representative of the surface were created to quantify the mineralogy on each side of the fracture surface. Parallel and perpendicular thin sections were made of the same cores, and images were taken to examine the near fracture rock matrix. The remaining pieces of the cores were examined using x-ray powder diffraction (XRD) to obtain the bulk mineralogy of the samples. Results show that the Marcellus sample is relatively homogenous such that the mineralogy of the bulk sample, and at the fracture surface and rock matrix, is around 97% calcite. However, the Mancos sample contained two distinct strata. The majority of the fracture surface was made up of a darker, kaolinite-rich layer, with the rest being a lighter, calcite-rich layer, with respect to the XRD data. The near fracture rock matrices had higher kaolinite and smectite content relative to XRD. This data suggests that the mineral distribution at the fracture surface is unique to the rest of the sample and could impact the estimation of a fracture’s reactivity and permeability over time.