ABSTRACT

The development of large-scale underground energy storage technologies is necessary to enable integration of renewable energy into the global energy market. Siliciclastic reservoirs generally represent promising geological storage sites. However, some risks might be associated with storage in these reservoirs such as: mineralogic alteration and/or geomechanical failure due to the cyclic nature of injection/depletion associated with hydrogen or methane storage. Therefore, it is critical to understand the influence of mineralogy and texture on grain-scale deformation mechanisms that arise during cyclic injection/depletion.

Combining multiscale multidimensional imaging techniques provides a powerful tool for better understanding the physical response of subsurface reservoirs to multiple cycles of injection and depletion, particularly influences on reservoir quality and thereby storage efficiency. Accordingly, the aim of this paper is to couple the information from 2D and 3D imaging modalities, using a newly developed tool for multimodal/multidimensional image registration to obtain a multimineral 3D digital rock for investigating the impact of framework minerology, particle size, and cement volumes on the geomechanical response of sandstones to multiple injection/depletion cycles.

The paper investigates the deformation of a lower medium grained sandstone (The Boise Sandstone), which is classified as a lithic arkose. The mineralogy comprises 10% lithic fragments, 50% feldspars, and 40% quartz. In order to link mechanical response to microstructural attributes of the given sample, a multiscale workflow was used. First, the sample was imaged using Micro-CT at a resolution of 13 microns/pixel. To capture mineralogy and pore/matrix features below the image resolution of the micro-CT, a thin section was cut and scanned using optical microscopy. After scanning, a cyclic uniaxial-strain test was performed on the sample to simulate energy storage. The post-test sample was imaged again using micro-CT and optical microscopy. With the aid of the newly developed image registration tool, each thin section is registered to its corresponding slice in the CT-volume to develop a multimineral 3D digital rock.

By comparing pre and post-test images of the Boise Sandstone, we observe the presence of grain fractures predominately developed in k-feldspar grains. The grain size is reduced from 260 microns pre-test to 215 microns after cyclic strain test. The total porosity of the sample was reduced by 3%. However, some regions of the sample experienced compaction (decrease in pore body size) and others were dilated (local increase in pore body size). The progressive strain displayed by the Boise Sandstone during cyclic loading and unloading suggests that coarser grained, highly feldspathic sands are not suitable for seasonal hydrogen or methane storage because of the strain weakening response to stress cycling.

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