Time‐Dependent Consolidation in Porous Geomaterials at In Situ Conditions of Temperature and Pressure

Abstract

Analysis of quartz sandstones shows that grain‐scale crushing (fracture and rearrangement) and associated sealing of fractures contribute significantly to consolidation. The crushing strength (P*) for granular material is defined by laboratory experiments conducted at strain rates of 10−4 to 10−5 s−1 and room temperature. Based on experiments, many sandstones would require burial depths in excess of the actual maximum burial depth to create observed microstructure and density. We use experiments and soil mechanics principles to determine rate laws for brittle consolidation of fine‐grained quartz sand to better estimate in situ failure conditions of porous geomaterials. Experiments were conducted on St. Peter sand utilizing different isostatic consolidation and creep load paths at temperatures to 200 °C and at strain rates of 10−4 to 10−10 s−1. Experiment results are consistent with observed rate dependence of consolidation in soils, and P* for sand can be identified by the change in the dependence of consolidation rate with stress, allowing the extrapolation of P* determined in the laboratory to geologic rates and temperatures. Additionally, normalized P* values can be described by a polynomial function to quantify temperature, stress, and strain‐rate relationships for the consolidation of porous geomaterials by subcritical cracking. At geologic loading rates, P* for fine‐grained quartz sand is achieved within ~3‐km burial depth, and thus, shear‐enhanced compaction under nonisostatic stress can occur at even shallower depths. These results demonstrate that time and temperature effects must be considered for predicting the brittle consolidation of sediments in depositional basins, petroleum reservoirs, and engineering applications.

Creep Deformation in Vaca Muerta Shale From Nanoindentation to Triaxial Experiments

“…Experimental observations support this model. Yield surfaces that depend on strain rate have been observed in granular geomaterials such as soils and clays at both dynamic (Tong & Tuan, ) and slow (10 −6 to 10 −8 s −1 ; Augustesen et al, ; Boudali, ) loading rates, as well as in quartz sands (Choens & Chester, ). Dependence of yield stresses upon strain rate is also established during both brittle dilation and compaction of sandstones and limestones at low temperatures (Brantut et al, , , ; Heap, Baud, & Meredith, ).…”

“…The yield of porous rocks under compressive triaxial loading is often modeled using an elliptical yield surface in p (mean stress) versus q (deviatoric stress) space [for a review, see Wong and Baud ()]. Below the yield surface, deformation is sometimes modeled as elastic, but some studies (see section and Choens & Chester, ) suggest that the yield surface, in fact, depends on strain rate.…”

Strengthening of Calcite Assemblages Through Chemical Complexation Reactions

“…Samples deformed uniformly across the length of the sample, with no large fractures or deformation bands observed in the sample. As with granular sand and soils (Choens & Chester, 2018;Karner et al, 2003), the initial consolidation of calcite was gradual, with minor porosity loss (∼3%) occurring before the onset of P* (Figure 1a). After P*, porosity loss dramatically increased with a linear relationship with increasing log P E .…”

Compactive Deformation of Sandstone Under Crustal Pressure and Temperature Conditions

“…Overall, we interpret the weakening observed at elevated temperature in our data as a combination of a decrease in fracture toughness and, to some minor extent, an increase in subcritical crack growth rate. In any case, it is expected that subcritical crack growth becomes significant at lower strain rates, as demonstrated in the both the brittle and ductile regime in sandstones (e.g., Heap, Baud, Meredith, Bell, & Main, 2009;Heap et al, 2015) and sand (Karner et al, 2008;Choens & Chester, 2018). Elevated temperatures would tend to increase the threshold strain rate below which subcritical crack growth becomes significant; this remains to be tested in detail in long term creep tests.…”

“…This sandstone was also the sandstone with the highest percentage of quartz and the lowest amounts of feldspars and clays, which suggests that the higher quartz content may lower the weakening observed at high temperature. Nevertheless, even a sandstone of nearly pure quartz could be expected to show signs of weakening at elevated temperatures as experiments carried out on pure quartz sand by Karner et al (2008) and Choens & Chester (2018) demonstrated that the value of P* was reduced at 150°C by ∼10%. These authors also explored the combined effect of strain rate and temperature and noticed that at lower strain rates the weakening effect of elevated temperature was greater and this was attributed to Boise an increase in subcritical cracking at lower strain rates.…”