Developing Methods to Assess Changes in Mechanical Properties of Shale Modified by Engineered Mineral Precipitation

Abstract

ABSTRACT Fractures in subsurface shale formations serve multiple purposes, for example, in the recovery of resources in hydraulic fracturing or as potential harmful leakage passages through caprocks that may contribute undesired fluids to the atmosphere or functional groundwater aquifers. A proposed method to seal or influence fracture properties is Ureolysis-Induced Calcium Carbonate Precipitation (UICP), a bio-mineralization technology driven by the enzymatic hydrolysis of urea, resulting in the formation of calcium carbonate. The resulting calcium carbonate can bridge the gaps in fractured shale and reduce fluid flow through fractures. This study represents the first step toward determining the influence of UICP treatment on shale material and its subsequent mechanical strength properties. The goal of this preliminary work is twofold: first, we aim to identify a method to test tensile strength along a core axis and second, we seek to assess the effect of temperature on the tensile strength of intact, unfractured shale cores (2.54 cm (1 in) diameter, 5.08 cm (2 in) long for comparison with future fractured and UICP-treated cores. A modified Brazilian indirect tensile strength test successfully measured splitting tensile strength of shale cores from Eagle Ford and Wolfcamp formations at room temperature and 60°C. INTRODUCTION Hydraulic fracturing coupled with horizontal drilling has been an instrumental technology for extraction of natural gas reserves in subsurface rock formations such as sandstone and shale. Shale gas is the natural gas trapped within subsurface shale formations and has become a crucial unconventional energy source amid an evolving energy landscape. Hydraulic fracturing and horizontal drilling stimulated energy independence, lowered fuel prices, and promoted job creation in the United States (Q. Wang et al., 2014). While hydraulic fracturing created many positive economic impacts, it has also generated undesirable environmental effects. This research is aims at offsetting environmental costs by sealing fractured subsurface shale using a sustainable bio-mineralization technology known as Ureolysis-induced calcium carbonate precipitation (UICP) (Kirkland et al., 2021). If UICP can bridge the gap in fractured shale, and prove strong enough to seal fractures, UICP treatments may be used at the field scale to limit fracture propagation, better store carbon dioxide, and extend the life and efficiency of existing and future wells.