OR/14/048 Sampling and analyses

Rochelle, C A, Purser, G, and Milodowski, A E. 2014. Results of laboratory carbonation experiments on NRVB cement. British Geological Survey Internal Report, OR/14/048.

Sampling procedure and sample preservation

General

For each set of batch experiments, fluid samples from both CO₂-pressurised and N₂-pressurised experiments were treated in the same way. This involved degassing a sample of aqueous fluid (if present) straight into a sterile polythene syringe.

At the end of each experiment, as much as possible of the remaining fluid (if present) inside the vessel was removed whilst still at pressure. This was to minimise the potential for carbonate mineral precipitation as this could result from the solution degassing, and hence an artefact of sampling. However, previous experience has indicated that such degassing tends to result in carbonate precipitation only after a few hours. Consequently, for solution samples that were taken and preserved in a matter of a few tens of minutes, such precipitation is not thought to represent a significant problem.

Solid samples were extracted after slow depressurisation of the vessel by venting the CO₂ or N₂ to the atmosphere. Depressurisation was done slowly to maximise the ability of trapped CO₂ to exit the cement samples — not doing this increased the risk of hydrofracturing the sample.

Preservation of solid products

On opening a pressure vessel, the (slightly damp) sample of reacted cement was very gently dried with an absorbent paper towel, weighed, and its length and diameter measured. The cores were then immediately placed in flat-roll plastic tubing, which was flushed with Ar before being sealed by crimp-welding. Each sample was sealed and stored within a two layers of Ar-flushed crimp-welded flat-roll plastic tubing, to prevent reaction with atmospheric CO₂ prior to petrographical analysis.

The cores of cement were then vacuum-dried at 20°C using an Edwards Modulyo freeze drying unit. However, the samples were not freeze-dried, as the pre-freezing process would have potentially created damage to the fabric of the samples. This is because the cement plugs were too large to allow rapid freezing throughout the mass of the sample, and slow freezing of the porewater within the core of cement plugs would have led to the slow but disruptive growth of ice crystals. Instead, the moist cement plugs were placed directly in the freeze drier unit, which was then evacuated, allowing the samples to gently dry at approximately 20°C, free from any contact with atmospheric CO₂, over a period of 48 hours at which point a constant vacuum of approximately 1 x 10^-1 Torr was achieved, indicating no further loss of water from the samples.

The dried cement plugs were then transferred into a glass desiccator containing soda lime prior to further preparation for mineralogical analysis. The soda lime served to adsorb any atmospheric CO₂ in the vessel, thereby preventing any potential carbonation of the solids during storage prior to analysis. The unreacted cement reference sample, which had been stored in a sealed container in water saturated with Ca(OH)₂ for the duration of the experiments, was also opened and treated in the same way as the reacted cement samples.

Preservation of fluid samples

After sampling, each of the reacted fluids was split into several sub-samples: Approximately 1 ml was taken for immediate analysis of pH.

Approximately 2–4 ml was taken for immediate analysis of alkalinity/carbonate/bicarbonate.

Approximately 10–12 ml was filtered using a 0.2 µm ‘Acrodisc^®’ nylon syringe filter. A volume (in the order of 8 ml) of this sample was placed into a polystyrene tube and acidified with 2% v/v (i.e. in the order of 0.08–0.4 ml) of concentrated ‘ARISTAR^®’ nitric acid. This was analysed subsequently for major and trace cations by inductively coupled plasma — optical emission spectroscopy (ICP-OES).

A further aliquot of the filtered sample (in the order of 1–2 ml) was taken and placed in a polyethylene tube for analysis of anions by ion chromatography (IC). This sample was diluted (typically to 20–50% concentration) to minimise the potential for carbonate mineral precipitation prior to analysis.

Samples were stored in a fridge (at about 5°C) prior to analysis.

Analytical techniques

Petrographic characterisation of the solid components

General
The reacted cement plugs, together with reference starting cement plugs, were initially photographed with a digital camera. The morphology and nature of any alteration products on the surfaces of the plugs were then examined in by optical binocular stereo microscope, which was then followed by more detailed observation using scanning electron microscopy (SEM) with secondary electron imaging (SEI). Alteration profiles through the reacted cement plugs were examined in polished thin sections prepared as longitudinal slices through the plugs. These sections were examined initially by optical petrographic (polarizing) microscope in transmitted-light, and then in more detail by backscattered scanning electron microscopy (BSEM). Identification of phases observed during SEM (BSEM and SEI) observation was aided by energy-dispersive X-ray microanalysis (EDXA) to provide semi-quantitative microchemical analysis of reacting materials and alteration products. During SEM analysis, selected areas of some polished thin sections of the reacted cement samples were also imaged by X-ray microchemical mapping using EDXA, to examine the distribution of major chemical components and to provide further information on the movement of reaction fronts through the cement plugs.

Polished thin section preparation
Petrographic polished thin sections were prepared from each of the vacuum-dried reacted cement plugs and a reference unreacted cement plug. The plugs were longitudinally-sliced through the median line by dry-cutting with a diamond rock saw. One half of the cement plug was then vacuum-impregnated with epoxy-resin in order to stabilise the material for thin section preparation. A blue dye was added to the epoxy-resin so that the porosity in the cement could be readily identified and discriminated from artefacts of section preparation (e.g. grain-plucking), during examination of the polished thin sections in transmitted light under the optical microscope. Thin slices were cut from the resin-impregnated blocks and mounted on glass microscope slides with colourless epoxy-resin, and ground and polished to a thickness of 30 µm. The sections were finished to a high-quality polish with 0.45 µm diamond paste. All section cutting and polishing was conducted in the absence of water, using alcohol as a lubricant, to prevent further cement hydration reactions occurring during section preparation.

The cement plugs used in these experiments (25 mm diameter, 50 mm long) were too large to fit in their entirety onto the standard-size (28 mm x 48 mm slices) polished thin sections prepared from the cement plugs. However, the thin sections were cut so that they presented a profile that allowed the plug to be examined from at least the centre of plug to outer surface of the cement plug, to include the both sides and one end of each plugs.

Optical petrography
Prior to detailed petrographical observations by BSEM-EDXA, the polished thin sections were examined in transmitted light using a Zeiss Axioplan 2 optical petrographic (polarising) microscope. Low magnification images of whole thin sections were also be recorded by digitally scanning of the thin section using an Epsom Perfection 1240U flatbed scanner equipped with a transmitted light (transparency) scanning attachment.

Scanning electron microscopy
Scanning electron microscopy (SEM) analyses was carried out using either:

1. A LEO 435VP variable pressure digital scanning electron microscope (VPSEM) fitted with a KE Developments solid-state 4-element (diode) backscattered electron (BSEM) detector. The SEM instrument was also equipped with an Oxford Instruments INCA Energy 450 energy-dispersive X-ray microanalysis (EDXA) system with a thin window Si-Li X-ray detector capable of detecting elements from boron to uranium.
2. A FEI QUANTA 600 environmental scanning electron microscope (ESEM), equipped with both a conventional Evert-Thornley secondary electron detector and an environmental (large field) secondary electron detector, and a solid-state 2-element (diode) BSEM detector. The ESEM instrument was also equipped with an Oxford Instruments INCA Energy 450 EDXA system with an Oxford Instrunment X-MAX large area (50 mm²) silicon drift detector (SDD) capable of detecting elements from boron to uranium.

SEI observations of the reacted core surfaces were made by direct observation of the whole (intact) cement core samples in the FEI QUANTA 600 ESEM instrument, under low-vacuum (variable pressure) mode, using a water-vapour atmosphere at a pressure of 0.95 Torr, and with an electron beam accelerating voltage of 15 kV and beam current of 7.5 pA. The polished thin sections were examined under BSEM using both the FEI QUANTA 600 ESEM and the LEO 435VP variable pressure SEM instruments. Uncoated polished thin sections were examined in the variable pressure (low vacuum) mode: (a) on the FEI QUANTA 600 ESEM using a water-vapour atmosphere at a pressure of 0.95 Torr, and with an electron beam accelerating voltages of 15–20 kV and beam current of 40–100 pA; or (b) on the LEO 435VP VPSEM using a nitrogen atmosphere of 0.4 Torr, and with an electron beam accelerating voltages of 15–20 kV and beam current of 500–800 pA. Higher-resolution BSEM-EDXA imaging of the polished thin sections was also undertaken using the SEM instruments in high-vacuum mode, after coating the sections with a thin layer of carbon (approximately 25 nm thick). The BSEM image brightness is proportional to the average atomic number of the material, thus allowing the differentiation of phases on the basis of their chemistry (Goldstein et al., 1981). Phase identification was also be aided by microchemical information obtained from observation of semi-quantitative EDXA spectra recorded from features of interest. EDXA data were acquired and processed using the Oxford Instruments Microanalysis Suite (version: Issue 18d+SP3) software package.

Chemical analysis of experimental fluids

Appropriate fluid samples were taken for chemical analysis of cations using inductively coupled plasma — optical emission spectroscopy (ICP-OES), and for anions using ion chromatography (IC). Bicarbonate/carbonate analysis was by titration against a known volume of sulphuric acid.

pH measurements were made on cooled and depressurised samples using either an Orion^® ‘900A’ pH meter or an Orion^® ‘3 Star’ pH meter. Both were calibrated using NBS traceable buffers chosen from pH 4, 7 10 and 12.46.

Details of major elements/species that will be analysed, typical detection limits and associated analytical errors are given in Table 3. The errors are based on long-term internal quality control standards.