title: An experimental study of porosity collapse and deformation band formation in high-porosity sandstones
Creators: name: Rice-Birchall, Elliot
Publisher: name: University of Liverpool Repository
Year: 2023

description: Compaction bands are millimetre to several centimetre thick sub-seismic bands of localised deformation in sandstones, which form approximately perpendicular to the maximum principal stress. They are associated with intense grain crushing and pore collapse, which generally results in an intra-band reduction in porosity and permeability. Consequently, these structures can exert a significant control on fluid flow, with potential implications for industrial processes such as fluid extraction during petroleum or groundwater production, or fluid injection during geothermal or CO2 sequestration projects. However, due to the heterogeneous nature of sandstones, determining the role that specific microstructural properties have on band formation is extremely challenging and consequently, much is still unknown regarding compaction band formation. The aim of this study is to attempt to better understand the microstructural properties and external factors which promote and govern the formation of compaction bands in sandstones. A new methodology has been developed which enables the production of high-porosity sandstone samples for laboratory testing which have reproducible petrophysical properties that can be systematically controlled. The technique uses the chemical reaction between sodium silicate and hydrochloric acid to precipitate cementing amorphous quartz between initially incohesive sand grains. This enables the production of sandstone samples for laboratory testing which have reproducible petrophysical properties that can be systematically controlled. Microstructural and mechanical analysis of the synthetic sandstones shows them to have realistic and reproducible uniaxial compressive, tensile and hydrostatic yield strengths, and to also exhibit yield curves with comparable geometries to natural sandstones of similar porosity and grain size. They also display elastic moduli within the expected range for natural sandstones. The effect of porosity and grain size on compaction localisation is investigated using synthetic sandstones produced using the new methodology developed. Twelve sandstones are produced with 3 different starting porosities (27, 32, 37%) and 4 different mean grain sizes (314, 411, 747 and 987 µm). The samples are each shortened by 5% axial strain at an effective stress equivalent to 85% of their grain crushing pressure (P*). Discrete compaction bands (≤3 grain diameters in width) oriented normal to the axial loading direction are only observed in the sample with the lowest starting porosity (27%) and smallest grain size (314 µm), while diffuse bands (&gt;3 grain diameters in width) are observed for the same porosity at a larger grain size of 411 µm. No compaction bands develop for any grain size in either the 32% or 37% starting porosity samples. Porosity analysis indicates grain size reduction does not necessarily ii correspond to porosity reduction indicating that compaction by grain rearrangement is as effective as localisation through comminution for these high-porosity synthetic sandstones. The role of cement in compaction band formation is examined using three sandstones, Bentheim, Castlegate and a synthetic sandstone that each possess similar porosities (~26-29%) and grain sizes (~230-300 µm), but which are cemented differently, with syntaxial quartz overgrowths, clay, and amorphous quartz cement respectively. Each sample forms discrete compaction bands when taken to 5% axial strain at a starting effective stress equivalent to 85% of its hydrostatic yield (P*) value. The compaction bands are only located at the sample ends in Bentheim Sandstone, whereas, in Castlegate Sandstone they are distributed throughout the whole sample and in the synthetic sample, the bands are only located within the sample centre. The results suggest that cement type plays a significant role in the mechanics of deformation within each of the samples, which in turn, determines where the compaction bands nucleate and develop. Since all the compaction bands identified are discrete, cement is not the primary control regarding the preference for the formation of diffuse or discrete compaction bands. The nature of strain localisation with increasing effective confining pressure is examined in Castlegate Sandstone. At low effective pressures, deformation localises into sets of dilational conjugate shear bands orientated ~30° to the maximum compressive stress (