Subsurface usage (1)

Characterizing the physical and mechanical properties of fractured porous rocks and their interactions with the injected fluid at representative geo-reservoir conditions is crucial to assess the feasibility of any geothermal system and to minimize the risks (particularly the induced seismicity) associated with any geothermal exploitation activity.
At the laboratory scale, a novel Geo-Reservoir Experimental Analogue Technology (GREAT) cell has been developed by the collaboration between the University of Göttingen (UoG), the University of Edinburgh (UoE) and Heriot Watt University. This polyaxial apparatus is capable of testing large (200x200 mm) cylindrical samples at conditions equivalent of depths of 3-4 km, with the unique capability of applying a rotatable stress field.
A big limitation of the UoE GREAT cell is the absence of an acoustic emissions (AEs) and ultrasonic monitoring system to capture the nucleation and coalescence of microcracks during hydraulic fracturing experiments. To overcome this, a second poliaxial apparatus, equipped with an array of 20 (but expandable to 32) AEs sensors, has been recently delivered at UoG. In addition, the UoG GREAT cell is capable of simulating deeper conditions (vertical stress representative of depths of 6-7 km) or larger differential stresses (σ1 - σ3 ≈ 200 MPa), larger injection pressure (100 MPa), higher temperature conditions (120°C, compared to 100°C at UoE) on a larger sample (250x250 mm).
Preliminary experiments focus on comparing the results of the two apparatuses and to study the scale effect between the two samples, by recreating the same stress field, as in previous tests at UoE, on the same rock material, without generating any fracture. 
The scale effect is also investigated during hydraulic fracturing and subsequent fracture permeability (with horizontal stress rotation) experiments on the same rock but tested in both the UoE and UoG GREAT cells, and in a conventional triaxial cell (sample size 250x100 mm), also available at UoG. The latter is fitted in the same rig, as this and the GREAT cell are interchangeable, but does not allow stress rotation.
The aim is not only to confirm both numerical simulations and previous laboratory tests with the result acquired with the new apparatus but also to show the importance of the unique capability of the rotatable stress field. In particular because direct comparisons could be done between samples of the same rock tested in the same laboratory and rig.

The interaction of different gases, effective stress, and complex pore structures impacts gas flow in unconventional reservoirs. However, our understanding of these petrophysical properties in organic-rich shale with fine pores and high matrix compressibility is limited, especially in depleted reservoir conditions. This study examined the effects of various gases (N2, CO2, He, Ar) on slippage (b), intrinsic permeability (k∞), and effective stress coefficient (χ) in anisotropic shale samples from the Krishna-Godavari basin, India. We also compared these results with nanopore characterization using low-pressure sorption (N2, CO2) and He pycnometer analysis. The apparent permeability (kapp) was estimated using a pulse decay permeameter at low pore pressure (Pm: 0.72 to 3.07 MPa) with intervals of 0.34 MPa, at different confining pressure (Pc: 5, 10, 15, and 20 MPa). It was observed that kapp decreases with increasing Pm, and conversely, increases with Terzaghi effective stress (Pc - Pm), indicating an increase in gas-wall collisions due to the reduced width of pore conduits at higher effective stress levels.
This comprehensive study provides valuable insights into various aspects of shale formations, including anisotropy, permeability, gas influence, pore width, stress-dependent behavior, and their implications for reservoir engineering and hydrocarbon production. The key findings reveal that permeability anisotropy in shale is direction-dependent, with higher permeability observed parallel to bedding than perpendicular. However, increased effective stress leads to a shift, resulting in higher permeability perpendicular to bedding. Different gases exhibit varying permeabilities in shale, with helium demonstrating the highest permeability, followed by argon, nitrogen, and carbon dioxide. This difference is attributed to variations in adsorption affinity, with noble gases showing higher permeability than sorbing gases. Larger pore widths in shale lead to reduced permeability due to decreased pore throat connectivity. Additionally, gas type affects permeability due to differing adsorption affinities. Increased effective stress reduces permeability in shale, with the exponential function offering a better fit to describe this behavior compared to the power law function. The study also determines the stress sensitivity coefficient and permeability at zero effective stress for different flow directions and sample types, highlighting the higher sensitivity of permeability to effective stress in parallel flow samples compared to perpendicular flow samples. Finally, based on the findings, supercritical CO2 shows potential as an alternative fracking technique to enhance permeability. It could serve as a long-term carbon capture storage (CCS) solution in depleted reservoirs, utilizing their ample space for carbon storage.

The global energy sector is transitioning to a more sustainable future. As a result, the use of depleted sandstone reservoirs for geo-engineering techniques including the storage of hydrogen gas, compressed air, and carbon dioxide will increase. These storage practises, as well as the production of geothermal energy, all involve the injection and potentially the removal of a pore fluid on a cyclic basis over a range of timescales. 
Recent experimental advancements have demonstrated that during the initial depletion of sandstone reservoirs, up to 40% of the total strain measured in the reservoir may be inelastic (i.e. permanent). However, uncertainties remain pertaining to how the portion of inelastic strain increases if the rate of depletion/deformation occurs over decadal time periods, or if repeated cycles of injection and depletion are observed. 
To answer these questions, a series of triaxial deformation experiments have been performed on typical reservoir sandstones. These experiments were designed to assess how the rate of depletion and cyclic loading affect the elastic parameters of the rock, as well as the evolution of inelastic strain. Experiments were carried out at the pressure, temperature, and chemical conditions of the Groningen gas field in the Netherlands and ranged from a few minutes to several weeks in duration.
We demonstrate that a non-negligible rate effect is observed, and that the amount of inelastic strain introduced into the samples at an axial strain rate of 10-9s-1 may be 1.5 times that measured at a strain rate of 10-5s-1. Additionally, we show that compared to the initial loading, further loading cycles to the stress levels relevant for typical storage sites, produce only very small amounts of further inelastic strain at realistic numbers of cycles (sub-100).  We observed that over 1000 further loading cycles are required to double the total amount of inelastic strain and no failure of the samples occurred. In contrast, at stress levels closer to the conventional yield stress, the effect of large numbers of cycles is much more pronounced, and eventually failure of the sample is observed through fatigue damage.  
We describe the rate dependence of inelastic strain generation and the small increase due to cyclic loading through a combination of mechanisms including intergranular clay compaction, stress corrosion cracking, and grain rearrangement and frictional controlled sliding. We then discuss what the implications of our results are in regard to current gas production and future storage techniques.

to mitigate climate change, carbon capture and storage (CCS) is considered to be one of the most promising technologies to meet CO2 emission reduction targets. During CCS, CO2 is captured at the source (e.g. power plant or CO2-heavy industrial plant), transported and injected at depth into the subsurface. In the Netherlands, re-use of depleted oil and gas fields is currently being considered for long-term CO2 storage through the Porthos and Aramis projects. However, injection of high-pressure CO2 into a low-pressure, depleted reservoir can affect mechanical, transport and thermal properties of the host reservoir and the overlying caprock, as it can lead to substantial, local, cooling. This could impact injectivity, near-well stability and well integrity, thereby impacting storage safety. Several studies have investigated the impact of thermal cycling on reservoir behavior, though most of these studies were performed under unconfined conditions. However, little is known of the effect of freeze-thaw cycles under elevated pressures representative for realistic in-situ conditions.
Therefore, we conducted a series of hydrostatic experiments (up to 30 MPa confinement) under cyclic temperature conditions (100°C down to 5 or -20°C) to assess the potential damage and volumetric response of an analogue reservoir sandstone (Bleurswiller sandstone, 24% porosity). We investigated the impact of saturation, number of thermal cycles, temperature amplitude, and confining pressure. Following the thermal treatment, triaxial compressive tests, coupled with permeametry, were performed to determine the impact of thermal cycling on mechanical properties and permeability.
Our results demonstrated that freeze-thaw cycling under confinement, leads to permanent bulk volume reduction, which decreases with increasing number of the cycles. However, no significant impact on peak strength and elastic moduli was observed, though a significant decrease in permeability was measured after the thermal treatments. Furthermore, we found that this impact varies depending on the specific conditions investigated. Notably, a greater reduction in permeability was observed when either the number of freeze/thaw cycles or the saturation level of the sample was increased. To assess the grain-scale mechanisms causing the damage induced by freeze-thaw cycling, detailed microstructural analysis will be performed. Eventually, based on the predominant damage mechanisms, a constitutive law will be formulated to establish the relationship between thermal cycling and the observed damage.