An experimental study of the confined compressive strength and ductility of a high-porosity sandstone

Abstract

Understanding the mechanical deformation of porous rocks is essential in many geological applications, including subsurface extraction or injection of liquids and gases for energy and waste disposal purposes. Such activities alter the local stress field, and thus may induce mechanical compaction and failure of the reservoir and surrounding rock. This thesis describes an experimental study aimed to augment our understanding of the mechanical compaction of a high-porosity (ca. 25%) sandstone, and thus to reduce the uncertainties in our predictions of the deformation behaviour of such rocks. The sample material (Hollington sandstone) used for this investigation is an excellent analogue for reservoir rocks that are targeted for conventional energy resources (oil and gas) as well as geothermal energy extraction, deep underground disposal of carbon dioxide, and potentially in the future for underground storage of hydrogen and compressed air to support decarbonisation. Detailed mechanical characterisation of the starting material revealed that the rock displays elastic anisotropy dominated by the grain fabric. The results of hydrostatic loading with concomitant pore volumometry and simultaneous P-wave velocity measurements demonstrated that the attainment of the critical pressure for the onset of grain cracking (P*) is characterised by a decrease in rock stiffness, which is immediately overridden by the stiffening effect of pore collapse. Mechanical data obtained from drained axisymmetric shortening experiments showed mechanical strength anisotropy in Hollington sandstone at elevated pressures in the ductile deformation field in agreement with velocity-derived stiffness anisotropy of the rock. Synthesis of the mechanical and microstructural data indicate that the onset of initial shear-enhanced compaction is related to the collapse of local large pores that develops at pressures (called C) lower than those required to induce widespread grain fracturing (C*). The careful application of X-ray micro-computed tomography and microscopy to examine failed samples revealed the development of compaction bands within the entire field of macroscopically ductile deformation. Microstructural observations revealed tortuous, irregular and non-continuous compaction bands in samples deformed by shortening parallel to bedding, in contrast to the continuous and fairly regular compaction bands observed in samples deformed by shortening normal to bedding, which appear to evolve in geometry with increasing confinement. The results of undrained axisymmetric compression experiments demonstrated that pore pressure injection under conditions leading to compactive failure in the ductile regime can lead to brittle faulting and unloading of the rock along the frictional sliding line, whereas pore pressure depletion induces immediate compactive deformation by reducing the differential stress and thus causing the stress path to track the yield surface in the shear-induced compactive failure field. These results illuminate the micromechanics that underpin mechanical compaction in porous sandstones and provide further insights into the nature and extent of the deformation and its localisation. They also provide immediate insight into the origins of deformation that can be induced in pre-stressed porous sandstones during engineering activities either by fluid injection or fluid withdrawal, and demonstrate the importance of full rock mechanics characterisation of reservoir rocks that are used in such applications.

Date of Award	31 Dec 2023
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Ernest Rutter (Supervisor) & Julian Mecklenburgh (Supervisor)