Experimental Investigation of Damage Induced Creep in Organic Rich Shales: Insights from Beetaloo Shale, Australia

The rapid advancement of shale gas development has revolutionized the energy landscape, providing a plentiful and cleaner-burning energy resource. The gas production from such unconventional resources is controlled by several processes amongst which the time-dependent deformation (creep for constant stress boundary condition) is an important contributor. To shed light on the macro-scale creep behaviour of organic-rich shales and micro-scale processes controlling such macro-scale behavior, we conducted a series of multi-scale (micro and macro) creep experiments on the Beetaloo shale samples from Northen Territory, Australia. The macro-scale triaxial creep tests on shale samples were conducted at differential stresses of 10, 15, and 20 MPa, followed by unloading to quantify permanent deformations. Furthermore, micro-scale shear creep tests on small samples utilizing a novel in-house micro-shear cell with imaging systems were performed. We investigated the coupling effect of damage and creep at micro-scale via digital image correlation, revealing that softer minerals creep within a hard rock fabric only when the fabric is damaged resulting in stress transfer. The combination of the multi-scale results obtained from micro and macro-scale creep experiments show that the creep in organic-rich shale is triggered by the initial skeletal damage i.e., absence of fabric damage leads to no creep in the samples. 1. INTRODUCTION Shale gas has emerged as a promising additional energy source amidst the rapid depletion of fossil fuels. Understanding the gas flow mechanism is crucial for predicting gas production in shale reservoirs. Shale rock undergoes various processes, including mechanical, sorption, and hydraulic processes, collectively known as multiphysics phenomena, during gas production or injection. Due to the inherently low permeability of shales (Patzek et al., 2013), hydraulic fracturing plays a crucial role in stimulating reservoirs, boosting productivity, and facilitating cost-effective resource extraction (Li & Ghasemi 2012). The process of proppant embedment is widely acknowledged as a significant factor that contributes to the decrease in fracture aperture and conductivity during the production phase of shale gas (Li et al., 2017). Fracture closure in shale formations is influenced by the inherent elastic, plastic, and viscous properties, which evolve through sediment compaction over time (Patzek et al., 2014; Wang, 2018). Among the viscous properties, creep becomes an important mechanism depending on the loading conditions over time. Creep, a phenomenon inherent to time-dependent deformation, significantly influences the mechanical integrity of shale formations. When rocks are consistently subjected to stress, they undergo deformation at a rate that changes over time. The behaviour of creep influences the effectiveness of fracking operations and the long-term integrity of fractures created during stimulation (Du et al., 2018; Liu et al., 2021).