Experimental Investigation on the Mechanics of Fluid-induced Tensile Fracturing in Sandstones with Contrasting Porosity and Permeability

Abstract

Tensile fractures in rock masses are the subject of interest in various fields such as unconventional oil and gas exploitation and geothermal exploration, while they are considered undesirable in engineering disciplines such as construction. Managing these fractures efficiently requires knowledge not only of the mechanical-driven fracture mechanics, but also of how geological factors such as regional stresses and properties of the reservoir (e.g., porosity and permeability) control fracture mechanics. Although significant progress has been made in understanding the mechanics of tensile fractures, fluid-solid mechanic effects with respect to rock physical properties (porosity and permeability) are not fully understood. To bridge this knowledge gap, this study utilizes new laboratory techniques to produce a fluid-driven tensile fractures in two different sandstone samples, the low-porosity/ low-permeability Crab Orchard Sandstone (COS) and the high-porosity/ high-permeability Darley Dale Sandstone (DDS). The mechanical, elastic, and flow properties of these samples were obtained through a series of laboratory sample characterization tests. Subsequently, fluid-driven tensile fracture experiments were conducted in a conventional triaxial cell (simulating burial depth) under varying conditions (dry/saturated samples, type of injection fluid, injection pressure) with rock physics data (e.g., acoustic emission, radial strain) recorded at high speed and high temporal resolution to capture the tensile fracture events which are known to have a fast mechanism.
By linking the seismic and mechanical behaviours recorded in the fluid-driven tensile fracture experiments, distinct fracture development stages were observed: (i) fracture initiation pressure, (ii) stable fracture propagation, and (iii) unstable fracture propagation or sample breakdown. The role of fluid-solid interaction in controlling the tensile fracture mechanism was investigated under dry and saturated conditions. The results from this study showed that the fluid-solid interaction in fluid-driven tensile fracture mechanism is governed by
permeability (which promotes infiltration) and porosity (which facilitates pore fluid effect via saturation) of the sample, which play a critical role in determining the duration of the stable fracture propagation and the fracture initiation pressure, respectively. XCT scanning analysis further revealed that fracture characteristics, such as aperture and tortuosity, varied in different rock samples.
Additionally, this study introduces a novel approach that measures the in-situ fluid flow through the tensile fracture, enabling a more accurate assessment of how fractured rocks respond to regional stress through fracture closure. The results presented in this study demonstrated that fractured rocks are more compliant to confining pressure increase compared to intact rocks.
In view of the limitation of experimental method applied in this study to predict pore pressure, the scope of this study was expanded to compare laboratory data with selected theoretical models (linear elastic and poro-elastic model) to predict the pore pressure in the sample during sample breakdown. The linear elastic model provided the best agreement with the experimental data, and it provided better estimation of pore pressure in both the COS and DDS samples. This revealed a degree of connectivity between the wellbore and the pores.
Although the techniques used in this study were conducted on a laboratory scale, the findings have the potential to enhance our understanding of tensile fracture mechanics in the subsurface. Additionally, they can contribute to the improvement of hydraulic fracture and engineering designs, thereby optimizing hydrocarbon extraction from unconventional reservoirs by reducing costs and environmental impacts and minimizing risks associated with engineering structures.

Date of Award	7 May 2026
Original language	English
Awarding Institution	University of Portsmouth
Supervisor	Sabine Wulf (Supervisor), Philip Benson (Supervisor) & Nicholas Paul Koor (Supervisor)