POROSITY SIZE DISTRIBUTION IN CALCITE-MINERALIZED FRACTURES WITHIN A SHALE FORMATION

Shale formations are both source rocks for unconventional oil and gas production and cap rocks for carbon dioxide sequestration and industrial waste water disposal. Mineralized fractures in these formations may act as both a barrier and a conduit for fluids, depending on flow direction, fracture orientation, and fracture porosity. In oil and gas production, variations in fracture characteristics can manifest as well production that is inconsistent with expectations. Likewise, mineralized fractures in cap rocks (for carbon dioxide sequestration and waste water disposal) can alter physical and chemical properties of the formation, resulting in changes to the sealing capacity. Additionally, porosity in fractures can provide pathways for fluids to migrate into overlying formations.

A mechanistic understanding of fracture mineralization and porosity is vital to predicting behavior of shale formations. Fractures that appear to be completely occluded by calcite precipitation under standard petrographic analysis may provide connected flow pathways at nanometer to micron scales. The current research examines the following hypotheses: (A) Pores in the fracture material will span orders of magnitude in length scale from nm to mm. (B) Small-scale pores (< micron) provide connected porosity for fluid flow. Core samples from a well bore were analyzed using small-angle neutron scattering (SANS) and scanning electron microscopy (SEM) to identify the size distribution of fracture porosity. Initial SANS analysis of the connected porosity was performed on two of the samples by adding a deuterated water mixture, a technique called contrast matching. The poly-disperse hard sphere model PRINSAS is being used to interpret the intensity data of scattered neutrons as a function of length-scale to produce the both the total porosity and connected porosity for each fracture analyzed. Continuing research will include further analysis of connected porosity using SANS and chemical analysis using cathode-luminescence microscopy to identify multiple mineralization events during fracture diagenesis.