Laboratory-scale analysis of natural rocks provides petrophysical properties such as density, porosity, pore diameter/pore-throat diameter distribution, and fluid accessibility, in addition to the size and shape of framework grains and their contact relationship with the rock matrix. Different types of laboratory approaches for petrophysical characterization involve the use of a range of sample sizes. While the sample sizes selected should aim to be representative of the rock body, there are inherent limitations imposed by the analytical principles and holding capacities of the different experimental apparatuses, with many instruments only able to accept samples at the μm–mm scale. Therefore, a total of nine (three limestones, three shales, two sandstones, and one dolomite) samples were collected from Texas to fill the knowledge gap of the sample size effect on the resultant petrophysical characteristics. The sample sizes ranged from 3 cm cubes to <75 μm particles. Using a combination of petrographic microscopy, helium expansion pycnometry, water immersion porosimetry, mercury intrusion porosimetry, and (ultra-) small-angle X-ray scattering, the impact of sample size on the petrophysical properties of these samples was systematically investigated here. The results suggest that the sample size effect is influenced by both pore structure changes during crushing and sample size-dependent fluid-to-pore connectivity.

No abstract

The fracturing behavior of enhanced geothermal system (EGS) reservoirs merits investigation under field-relevant temperature and stress conditions, in order to understand the creation of an extensive fracture network that helps achieve a high heat exchange efficiency. In this work, hydraulic fracturing tests were conducted on two 300 mm sized cubic granite samples at room (32°C) and field-relevant (250°C) temperatures under true triaxial compression conditions. The failure behavior and fracturing plane were studied using acoustic emission (AE) testing, μm-scale computed tomography (μm-CT), and scanning electronic microscopy (SEM), in addition to the monitoring and analyses of pressure-flow curve during fracturing. The results show that (1) at the macroscale, the strength of the granite sample was weakened at high (field-relevant) temperature, as shown by a decrease in the breakdown pressure and increase in closure pressure from the pressure-flow curve; (2) at the microscale, the failure pattern of grains during fracturing did not differ much at high and room temperatures for both intergranular and transgranular fractures; and (3) due to upscaling issues from the laboratory to field, however, the laboratory experiment will not directly provide some critical parameters (e.g., mud window pressure needed for fracture initiation and borehole failure avoidance) needed for an EGS field exploration.

Abstract. Nano-darcy level permeability measurements of porous media, such as nano-porous mudrocks, are only practically feasible with gas invasion methods into granular-sized samples with short diffusion lengths and thereby reduced experimental duration; however, these methods lack rigorous solutions and standardized experimental procedures. For the first time, we resolve this by providing an integrated technique (termed as gas permeability technique) with coupled theoretical development, experimental procedures, and data interpretation workflow. Three exact mathematical solutions for transient and slightly compressible spherical flow, along with their asymptotic solutions, are developed for early- and late-time responses. Critically, one late-time solution is for an ultra-small gas-invadable volume, important for a wide range of practical usages. Developed as applicable to different sample characteristics (permeability, porosity, and mass) in relation to the storage capacity of experimental systems, these three solutions are evaluated from essential considerations of error difference between exact and approximate solutions, optimal experimental conditions, and experimental demonstration of mudstone and molecular-sieve samples. Moreover, a practical workflow of solution selection and data reduction to determine permeability is presented by considering samples with different permeability and porosity under various granular sizes. Overall, this work establishes a rigorous, theory-based, rapid, and versatile gas permeability measurement technique for tight media at sub-nano darcy levels.