Impact of Lithological Variation and Topology on the Connectivity and Intensity of Fracture Networks in Carbonates

Authors

Fahad Qassim, University of Nebraska-LincolnFollow

First Advisor

Caroline M. Burberry

Date of this Version

Spring 5-2-2023

Comments

A THESIS Presented to the Faculty of The Graduate College at the University of Nebraska In Partial Fulfillment of Requirements For the Degree of Master of Science, Major: Earth and Atmospheric Sciences, Under the Supervision of Professor Caroline M. Burberry. Lincoln, Nebraska: May 2023

Copyright © 2023 Fahad Qassim

Abstract

Assessing fracture network connectivity in reservoirs remains a challenging task because of the complex nature of fracture networks at various length scales, which significantly impacts fluid flow behavior. Outcrop studies are essential for understanding subsurface fracture networks because intersecting fracture sheets' length, orientation, height distribution, and, therefore, network connectivity and intensity are difficult to measure from wellbores and are typically below seismic resolution. Nevertheless, fracture network connectivity is a crucial parameter and a critical factor in understanding, evaluating, and predicting fluid flow behavior in hydrocarbon reservoirs, aquifers, hazardous waste and CO₂ storage systems, and geothermal energy exploitation. Moreover, fracture connectivity studies are critical to predicting reservoir performance and hydraulic fracturing. Although using topology to assess the connectivity of fracture networks has recently gained popularity in structural geology, no study has been conducted to investigate the impact of lithological variation on the connectivity of fractured carbonates. This contribution aims to use the fracture network topology (nodes, branches, and lines) at outcrops to assess the connectivity and intensity of fracture networks in different carbonate lithologies from the Ozark Plateau, Arkansas, USA. Thin sections were obtained to determine the type of carbonate under the Dunham classification and to compare the connectivity and intensity of fracture networks to the carbonate lithology. Results show that mud-supported carbonates (mudstone and wackestone) have higher connectivity and intensity than grain-supported carbonates (packstone and grainstone). Mudstones exhibit the highest average connectivity and intensity, whereas grainstones show the lowest average connectivity and intensity. The variation in connectivity is due to the variation in the percentage of connected nodes and branches in the carbonate type. Packstones show a wide range of connectivity and intensity due to mechanical and chemical compaction processes. Further research is needed to investigate the variation in packstones and the natural factors that control the connectivity in carbonate type. Results of this study show that topology is an effective method to assess the impact of lithological variation on the connectivity and intensity of fractured carbonates.

Advisor: Caroline M. Burberry