A Novel Experimental Protocol for Fault Generation and Reactivation Studies

ABSTRACT: In this paper, we present an innovative workflow that allows the performance of coupled direct shear tests in a large direct shear rig utilising whole core samples extracted from wellbores to enable the investigation of fault generation and reactivation processes using rock materials that more closely relate to a given underground study area, whilst minimising the need for large outcrop or synthetic rock samples in experimental campaigns. The methodology involves the preparation and shearing of a composite sample made from the intact (i.e., not fractured) core sample and cement paste and then sub-coring of the sheared composite sample to obtain a 38 mm diameter plug containing the fault zone for further investigation in axisymmetric triaxial setups. The examination of the sheared plug obtained in the first trial confirmed the generation of the fault zone at the predicated angle and location, evidencing the technique's success. It represents a significant advance in our capabilities to characterise sheared materials from diverse depth ranges and geological contexts, complementing field observations and helping to address several aspects relevant to industry and academia. 1. INTRODUCTION Tectonic and anthropogenic stress changes within and near an underground reservoir can significantly impact the integrity of faults and fractures, potentially leading to fault reactivation, with a follow-on impact on the reservoir's containment (vide, e.g., Wriput and Zoback, 2000). Therefore, it is crucial to assess the mechanical integrity of these discontinuities and understand the alterations in fluid flow along or across them in response to changes in the surrounding stress state. This knowledge is essential for the safe and effective storage or production of natural gas, CO2, and hydrogen accumulations. In this context, experiments that replicate fracture or fault generation and their reactivation while monitoring changes in their hydraulic conductivity and microseismic response can provide insightful information for the understanding of faults’ static and dynamic behaviour and better predict how geological events or injection or production strategies could affect the reservoir's overall stability and sealing potential.