Micro-mechanics based numerical simulation of NaCl brine induced mechanical strength deterioration of sedimentary host-rock formations

doi:10.1016/j.enggeo.2018.05.005

Engineering Geology

Volume 242, 14 August 2018, Pages 55-69

https://doi.org/10.1016/j.enggeo.2018.05.005 Get rights and content

Highlights

•
Numerical simulation of salinity dependent mechanical properties of sandstone.
•
Bond-strength reduction method using PFC3D to simulate effect of salinity.
•
Accurate modelling of compressive and tensile strength of saturated sandstone.

Abstract

Artificial fracture stimulation in low-grade sedimentary ore deposits is one method to improve mineral extraction efficiency of In-Situ Leaching (ISL) process. Low to moderate saline environments are found in most sedimentary ore deposits that deteriorate intact rock strength over time. It is important to recognize the host rock strength properties prior to artificial fracture stimulation. Therefore, to identify the effect of salinity on the mechanical properties of brine-saturated sandstone, a series of uniaxial compressive strength (UCS) tests and Brazilian tensile strength tests were performed on specimens saturated with water, and with varying NaCl brine concentrations (5.0%, 7.5%,10% and 12.5%). Scanning electron microscopy and inductively coupled plasma mass spectrometry (ICP-MS) tests reveal that increasing salinity deteriorate the mechanical integrity of sandstone by accelerated clay mineral dissolution. Consequently, 12.5% saturation fluid salinity resulted in a 40% reduction in UCS, 22% reduction in Young's modulus and 33% reduction in tensile strength. The strength deterioration observed in the experimental study was then successfully simulated using Particle Flow Code 3D (PFC3D) in consideration of the bond strength deterioration mechanism for sandstone. The calibrated model was used to accurately replicate the damage mechanism of sandstone under the influence of brine saturation. The model forms an accurate intact rock assembly for numerical modelling of saline sedimentary host-rock formations.

Introduction

A global decline in ore-grades is leading conventional mining operations – such as open cut mining – to be uneconomical with the generation of excessive waste rock. Waste rock generation is one of the world's largest chronic waste concerns (Bian et al., 2012) and declining global ore grades further aggravate this issue. At present, over 220 MJ/t of ore (63% of total energy consumed) in mineral liberation is expelled in mineral crushing and grinding operations (US Department of Energy, 2007). The resultant CO₂ emissions from Iron, Bauxite, and Copper mining alone add up to 50 million tonnes per annum (Norgate and Haque, 2010). With diminishing grades of mineral ore, the energy consumption in mineral liberation using conventional mining techniques will further increase, and therefore, alternate mining technologies have become a necessity.

In-Situ Leaching (ISL) is becoming an economic option in relation to the current trend of declining ore grades. The complete elimination of excavation and grinding operations of rock significantly reduces the cost of production as large-scale excavations become uneconomical with low-grade ores in conventional open-pit and underground mining. Furthermore, leaching of minerals can be done with minimal ground disturbance and overburden removal (Yang et al., 2013; Brierley, 2010). Currently, the application of in-situ leaching is limited to Uranium deposits in sedimentary host rock with ore-grades ranging from 0.04% to 3% and a recovery rate of over 75% (Mudd, 2014). However, ISL has the potential to be extended for the recovery of other minerals such as Copper, of which over 25% of global copper is in the form of sediment-hosted Copper (Sinclair and Thompson, 2015). Copper ISL has been successfully performed in formations with porosities of 3%–21% and permeability of 20 mD - 6000 mD (Sinclair and Thompson, 2015).

Sediment-hosted Lead-Zinc deposits are another potential resource that can be exploited using ISL. These elastic dominated Lead-Zinc ore are hosted in shale, sandstone, and siltstone (Leach et al., 2010). Porous sedimentary Bauxite, containing a combination of Al₂O₃, SiO₂, Fe₂O₃, TiO₂ and accompanied by minerals such as kaolinite and quartz (Zhang et al., 2013) is also a potential candidate for ISL. The high porosity of these sedimentary deposits makes them ideal sources for ISL. The majority of these sedimentary deposits contain fluid inclusions that have a moderate salinity varying from 8%–19% NaCl equivalent (Wilkinson, 2010) which changes the strength characteristics of the rock (Rathnaweera et al., 2014).

The leaching efficiency of ISL is dependent on the porosity and permeability of the ore-bearing rock. When the permeability of the sedimentary rock deposit is not sufficient, artificially induced fracturing is used to improve the permeability of the rock reservoir (Sinclair and Thompson, 2015). Contamination of groundwater by leaching fluid migration is a fundamental issue that needs to be addressed during ISL (Klimkova et al., 2011) and artificial fracturing of rock increases the risk of contamination. Therefore, induced fracturing of the reservoir rock needs to be carried out with extreme caution in order to prevent the formation of undesirable fractures and flow paths for eventual fluid migration and groundwater contamination. One such method of artificial fracture stimulation is rock fracturing that uses soundless cracking demolition agents (SCDA) through quasi-static fracture propagation (De Silva et al., 2016). For this reason, understanding strength characteristics of rock are important before any fracturing process begins.

As most sedimentary ore deposits contain fluid inclusions with moderate salinity, it is important that strength deterioration of these deposits caused by chemical weathering (Chigira and Oyama, 2000) be identified beforehand. Salinity-dependent strength variation in sedimentary rock is well documented in the literature (Lawrence et al., 2013; Rathnaweera et al., 2014) demonstrating that significant strength deteriorations occur with increasing salinity. Conversely, the effect of salinity on the tensile capacity of sedimentary rock (Rostom et al., 2013; Bergsaker et al., 2016) has not been studied in detail. The present study aims to address this gap by investigating the dependence of sedimentary rock strength properties on the saturation fluid of host rock. A series of UCS tests and Brazilian disk tensile strength (BTS) tests were performed on sandstone samples saturated in different concentrations of NaCl brine to determine the variation of strength characteristics of representative sedimentary ore deposits. The experimental results were used to calibrate and validate a numerical rock assembly in PFC3D 5.0 (Particle flow code 3D) to mimic the strength deterioration of sedimentary rock under brine saturation, which is an essential component for accurate rock fracture simulation using numerical models.

Section snippets

Sample preparation

A dense, coarse-grained, silicate sandstone from the Sydney basin was selected to carry out the tests, as it is a good representative of the sedimentary mineral deposits found in Australia. Table 1 shows the physical and mineralogical properties of the sandstone. Cylindrical sandstone specimens that are 38 mm in diameter and 76 mm in height were cored from a single block of sandstone for the UCS tests. Sampling was carefully carried out and all samples were cored perpendicular to bedding planes

UCS testing

From the series of UCS tests conducted, deterioration in rock strength was perceived under different saturation concentrations and compared with dry, unsaturated samples. Fig. 4 shows the stress-strain behaviour of both dry specimens and NaCl brine saturated reservoir rock specimens under unconfined loading conditions. The axial stress variation was plotted against both lateral and axial strains.

The sandstone specimens saturated with water showed a considerable drop in the compressive strength

Linear contact model and flat-jointed model

In recent years a number of attempts have been made to replicate the strength characteristics of rock-like material using discrete element methods (DEM), where fracture initiation, propagation, and coalescence can be captured in detail (Fan et al., 2015; Yang et al., 2016). However, the majority of research on the subject has only considered calibrating micro-parameters of the material model to match the compressive strength properties of the experimental results of the rock types tested.

Fracture evolution – experimental and numerical modelling results

Accurate characterization of the stress-strain behaviour of intact rock is important in predicting failure stresses as demonstrated in the previous section. In this section, the fracture initiation, propagation, and coalescence leading to unstable fracture propagation and the final fracture obtained from the numerical and experimental results are compared. The experimental data (consisting of acoustic emission (AE) measurements and surface strain distribution collected by ARAMIS technology) are

Conclusions

The influence of pore-fluid salinity on the strength degradation of sedimentary sandstone was investigated in this study. A series of experiments consisting of UCS and BTS tests were carried out on fully saturated sandstone specimens. The experimental results were used to calibrate and validate a numerical rock assembly, which mimics the strength deterioration of sandstone with increasing pore-fluid salinity. The numerical rock assembled using the flat-jointed model in PFC3D was extensively

Declarations of interest

None.

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View full text

Micro-mechanics based numerical simulation of NaCl brine induced mechanical strength deterioration of sedimentary host-rock formations

Highlights

Abstract

Introduction

Section snippets

Sample preparation

UCS testing

Linear contact model and flat-jointed model

Fracture evolution – experimental and numerical modelling results

Conclusions

Declarations of interest

Eng. Geol.

Hydrometall.

Eng. Geol.

Materials Characterization

Eng. Geol.

J. Colloid Interface Sci.

Chemosphere

Eng. Geol.

Geomorphology

Int. J. Rock Mech. Min. Sci. Geomech. Abstr.

Eng. Geol.

Sci. Total Environ.

J. Clean. Prod.

J. Pet. Sci. Eng.

International journal of rock mechanics and mining sciences

Fuel

Tectonophysics

J. Cult. Herit.

Hydrometall.

Int. J. Rock Mech. Min. Sci.

Ultrasonics