Analysis of high‐resolution X‐ray computed tomography images of Bentheim sandstone under elevated confining pressures

Abstract

A sample of Bentheim sandstone was characterized using high‐resolution three‐dimensional X‐ray microscopy at two different confining pressures of 1 MPa and 20 MPa. The two recordings can be directly compared with each other because the same sample volume was imaged in either case. After image processing, a porosity reduction from 21.92% to 21.76% can be deduced from the segmented data. With voxel‐based numerical simulation techniques, we determined apparent hydraulic transport properties and effective elastic properties. These results were compared with laboratory measurements using reference samples. Laboratory and computed volumes, as well as hydraulic transport properties, agree fairly well. To achieve a reasonable agreement for the effective elastic properties, we define pressure‐dependent grain contact zones in addition to mineral phases in the digital rock images. From that, we derive a specific digital rock physics template resulting in a very good agreement between laboratory data and simulations. The digital rock physics template aims to contribute to a more standardized approach of X‐ray computed tomography data analysis as a tool to determine and predict elastic rock properties.

Digital carbonate rock physics

“…For example, Darcy permeability can be predicted by numerically simulating single-phase fluid flow through 3-D rock pore structure models, with the numerical results being in reasonable agreement with laboratory measurements (e.g. Osorno et al, 2015;Saenger et al, 2016). In this case, the resolution of the micro-XRCT technique is sufficient, because fluid pathways predominantly follow larger pores.…”

“…Even with the highest resolution currently available in micro-XRCT imaging there will be a significant amount of unresolved pore features which need to be treated in the DRP workflow (Saenger et al, 2016). On the grey-scale intensity level histograms of the low-and high-resolution images of the micro-CT scanning (Fig.…”

“…On the other hand, mechanical properties, such as the effective elastic moduli, strongly depend on the microstructural details of the rock, which may stay unresolved by the micro-XRCT technique (e.g. for Bentheim sandstone considered in Saenger et al, 2016). The inability to fully characterize the microstructural details of a rock sample can lead to a disagreement between numerical estimates of mechanical properties based on micro-XRCT images and laboratory data (Andrä et al, 2013b).…”

“…The porosity of these regions cannot be determined exactly, as some pores are below the resolution of the scans: typically, micrites exhibit pore sizes with a maximum diameter of 1 µm (Moshier, 1989;Cantrell and Hagerty, 1999), and pore sizes as low as 0.06 µm have been measured by mercury intrusion capillary pressure tests (MICP) for the samples under investigation (Vialle et al, 2013). To account for the unresolved pore space we perform a number of two-phase wave propagation simulations to estimate effec- tive elastic properties (Saenger et al, 2004(Saenger et al, , 2011(Saenger et al, , 2016. For these time-of-flight simulations we assign vacuum properties to the pore phase, while the rest of the digital image including the micritic phase will be assumed to be solid with the mineral properties of calcite (e.g.…”

Sorption‐Induced Deformation and Elastic Weakening of Bentheim Sandstone

“…The elastic moduli of the Bentheim sandstone are pressure dependent (e.g., Benson et al, ; Saenger et al, ). Thus, the change in the solvation pressure by 2–3 MPa should produce changes in the elastic moduli of the rock.…”