K. Bateman

Natural occurrences as analogues for the geological disposal of carbon dioxide

It is now generally accepted that anthropogenic CO2 emissions are contributing to the global rise in atmospheric CO2 concentrations. One possibility for reducing carbon dioxide emissions is to remove it from the flue gases of coal-fired power stations and dispose of it in underground geological reservoirs, possibly offshore in the North Sea. The feasibility of this option has been studied in detail by a consortium of European partners. As part of this study, natural occurrences of carbon dioxide were identified and preliminary information from these was obtained. The best characterised are found in the United States where the carbon dioxide reserves are exploited for use in tertiary enhanced oil recovery (EOR) programs in the Texas oilfields. The carbon dioxide reserves occur in geological structures and lithologies which are similar to those present in the North Sea. As such, these fields offer an ideal natural analogue for the disposal of carbon dioxide, since the interactions with groundwaters and reservoir lithologies have occurred on both spatial and temporal scales relevant to geological processes. Those carbon dioxide fields currently being exploited have already been studied to a limited extent by the oil companies involved. However, further study is required to provide information on the potential effects that disposing of large quantities of carbon dioxide might have on groundwaters and reservoir quality. In addition, more detailed information will be obtained on the interactions which occur during EOR using carbon dioxide. This paper presents data on some of the natural carbon dioxide fields, and compares the effects of these natural fluid-rock interactions with those observed in laboratory experiments performed to establish what reactions occur during the geological disposal of carbon dioxide.

Rate and mechanism of the reaction of silicates with cement pore fluids

Abstract The reaction mechanisms and rates of reaction of a number of the common rock-forming silicates with synthetic cement pore fluids have been evaluated in a series of laboratory experiments at 70°C. Mass transfer is dominated by the dissolution of the primary silicate and the precipitation of a range of Na-K-Al substituted calcium silicate hydrates, and a possible zeolite. Calcium was lost from, and silicon gained by, the fluid phase as a result of the reactions. Secondary solids formed thick layers on primary silicates, but dissolution of the silicates was not diffusion-limited. The rate of dissolution of the silicates was determined to be 2–3 orders of magnitude greater at pH 12–13 than at neutral pH, and confirm measurements by other authors. The rate of growth of calcium silicate hydrates was limited by the rate of supply of silicon from the primary silicates. Although the results of the laboratory experiments were dominated by the loss of calcium from the fluid and the precipitation of calcium silicate hydrates, thermodynamic modelling suggests that these may be replaced by zeolites and/or feldspars when groundwater residence times are considered.

Can CO2 hydrate assist in the underground storage of carbon dioxide

Abstract The sequestration of CO2 in the deep geosphere is one potential method for reducing anthropogenic emissions to the atmosphere without necessarily incurring a significant change in our energy-producing technologies. Containment of CO2 as a liquid and an associated hydrate phase, under cool conditions, offers an alternative underground storage approach compared with conventional supercritical CO2 storage at higher temperatures. We briefly describe conventional approaches to underground storage, review possible approaches for using CO2 hydrate in CO2 storage generally, and comment on the important role CO2 hydrate could play in underground storage. Cool underground storage appears to offer certain advantages in terms of physical, chemical and mineralogical processes, which may usefully enhance trapping of the stored CO2. This approach also appears to be potentially applicable to large areas of sub-seabed sediments offshore Western Europe.

Analcime reactions at 25–90°C in hyperalkaline fluids

Abstract Extensive use of cement and concrete is envisaged in the construction of geological disposal facilities for radioactive wastes. The hyperalkaline porefluids typical of groundwaters that have reacted with these materials have the potential to react chemically with other engineered barrier components such as bentonite, potentially degrading their performance. Analcime, NaAlSi2O6‧H2O, has been identified from previous modelling and experimental studies as a potential alteration product of bentonite. Laboratory experiments to investigate the stability of analcime under hyperalkaline porefluid conditions have been performed. Experiments used both batch and fluidized bed equipment at 25, 50, 70 and 90°C in K-based pH buffer solutions, both under- and over-saturated with respect to analcime. Results from dissolution experiments demonstrate that release of Na was greatly enhanced (by up to a factor of thirty) over that for Si and Al, particularly at pH 10 and 11. However, enhanced release of both Na and Al occurred in the batch experiments at pH 12-13. Near stoichiometric dissolution was observed in fluidized bed experiments under steady-state conditions at 70°C. Sodium was removed from the analcime structure by ion exchange for K, without involving dissolution and re-precipitation of the analcime framework. Scanning electron microscopy of reacted analcime grains showed that some grains had pronounced cracks parallel to original cleavage traces. These cracks were a result of volume decrease due to the substitution of K for Na ions and water molecules in the analcime structure to form leucite, KAlSi2O6. Synthesis of the dissolution data shows that the rate of dissolution increased with increasing temperature in the range 25-70°C and with pH at each temperature. Absolute rates of dissolution ranged from 10-10 mol m-2 s-1 at pH 9.5 at 25°C to 10-7 mol m-2 s-1 at pH 12 at 70 and 90°C. The rate of dissolution at any temperature was pH-dependent, such that the rate could be described by k (aH⁺)n, where k is the rate constant and n is -0.3 at 25°C, -0.4 at 50°C, -0.6 at 70°C and -0.7 at 90°C. Attempts to measure the growth rate of analcime in supersaturated solutions at 70 and 90°C were unsuccessful, although a limiting rate at 70°C, pH 10 was calculated to be 4 × 10-11 mol m-2 s-1, roughly 100× less than the rate of dissolution under the same conditions. These results imply that any trace amounts of analcime in bentonite will be converted to leucite by reaction with cement fluids with a high K/Na ratio. In some instances, leucite may thus incorporate K+ in preference to other phases (e.g. illite, K-feldspar) during alteration of bentonite by cement porefluids.

Granite-water interactions in a flow-through experimental system with applications to the Hot Dry Rock geothermal system at Rosemanowes, Cornwall, U.K.

Abstract A suite of laboratory experiments reacting granite with fluids typical of those circulated in a Hot Dry Rock (HDR) geothermal system at Rosemanowes, Cornwall has been conducted in the temperature range 60–100°C under flow-through conditions. Experiments of three types were performed: variable flow rate, using a streamwater-type fluid; variable temperature, using pH-buffer solutions; and variable fluid composition, using synthetic analogues of circulation fluids from the HDR site at Rosemanowes. The results of the variable flow rate experiments revealed gains in Si, Na, Ca, Li, B, F and Ba in the output fluids, the magnitudes of which generally decreased with increasing flow rate, whereas Mg, Sr, Fe and Mn were either lost from fluids or showed gains that were invariant with changes in flow rate. The principal reactant minerals were calcite, plagioclase, biotite and quartz, and a smectite clay was the “sink” for chemical components lost from the fluids. Output fluids from the variable temperature, pH-buffered experiments had chemical compositions dominated by dissolution, with few effects attributable to precipitation processes. Although increased temperature increased the net gains of most chemical components in output fluids, this was particularly so for Si, suggesting that the rate of quartz dissolution was increased relative to that of other minerals. Variations in the chemical composition of the fluids from the variable fluid composition experiments showed that only Mg was removed from typical Rosemanowes “injection” fluid, whereas Mg, Ca, Sr, Fe, Mn and, in some instances, F were removed from “production” fluids. Potassium was removed from production fluids with elevated initial concentrations of this component. Smectitic clays, zeolites, calcite and fluorite are inferred products of these reactions. A comparison of processes revealed by the laboratory data with the chemical exchanges observed during HDR circulation tests indicated that the latter involved sorption/precipitation processes to a greater extent, particularly for components such as Si and Li. The determination of the bulk rate of dissolution of the granite by chemical analysis of the fluids from the laboratory experiments varied between 10 −12 and 10 −10 kg/m 2 /s at any temperature. These measurements emphasise that rock dissolution is a heterogeneous process, involving the selective removal of the most reactive mineral phases. Rate constants of plagioclase dissolution estimated from the release of Na from the granite (∼10 −11 mol/m 2 /s at 80°C) are consistent with previous determinations on isolated pure minerals available in the literature. The rate of tourmaline dissolution has been estimated in a similar manner, using data for B, as ∼10 −11 mol/m 2 /s at 80°C. The use of a bulk granite dissolution rate derived from the laboratory experiments to predict the output of Na and Si from the reservoir at Rosemanowes would have under-estimated Na and over-estimated Si as a result of the absence or small contribution of processes such as fluid mixing and the precipitation of secondary solids in the laboratory experiments.

The role of biofilms in subsurface transport processes

Abstract Landfill and radioactive waste disposal risk assessments focus on contaminant transport and are principally concerned with understanding the movement of gas, water and solutes through engineered barriers and natural groundwater systems. However, microbiological activity can affect transport processes, changing the chemical and physical characteristics of the subsurface environment. Such effects are generally caused by biofilms attached to rock surfaces. Currently most existing transport models have to introduce additional assumptions about the relationships between the microbial growth and changes to the porosity and permeability. These relationships are particularly poorly understood. This paper reviews recent experimental work directed at the development of biofilms and their influence on subsurface flow and the transport of contaminants in intergranular and fracture porosity flow systems. The results are then discussed in terms of a more complex conceptual model.

Experimental simulation of the alkaline disturbed zone around a cementitious radioactive waste repository: numerical modelling and column experiments

Abstract One approach to describe the migration of an alkaline plume from the cementitious engineered barriers of a geological disposal facility for radioactive wastes is to employ coupled chemistry and flow computer models. Although evidence from natural systems is useful to constrain reaction mechanisms and minerals to be incorporated into such models, time-dependent information is generally lacking. A series of laboratory column experiments has been conducted in order to test the capabilities of two of the currently available, coupled models to predict product solids and output fluid compositions with time. The coupled models PRECIP and CHEQMATE were used to provide predictive calculations based upon known experimental parameters and data available from the literature. The predictions did not replicate all the variations in mineralogy observed in the experiments, primarily due to restrictions in the availability of kinetic and thermdynamic data for the range of secondary phases of interest. However, the model predictions did reproduce the general variation of secondary phases with time and distance along the columns.

An experimental evaluation of the reaction of granite with streamwater, seawater and NaCl solutions at 200°C

Abstract Experiments reacting granite with streamwater, seawater, and NaCl solutions have been conducted at 200°C, 50 MPa under batch conditions using direct-sampling autoclaves for durations up to 80 days. Granite-streamwater experiments at 10:1 and 2:1 water/rock ratios produced fluids of low TDS ( The reaction of granite with seawater produced a fluid of low pH (≈ 3.5), and principal changes in the fluid chemistry as follows: a gain of SiO2; and losses of sulphate, magnesium, and calcium. Appreciable mobilisation of heavy metals into the fluid phase such as Fe (up to 15 mg/l) and Mn (up to 2 mg/l) were noted. Solid precipitates consisted of anhydrite, caminite (magnesium hydroxide sulphate hydrate) and a mixed-layer smectite-chlorite clay. The low pH of the evolved fluid is attributed to the precipitation of the hydroxyl-consuming mineral phases, caminite and the smectite-chlorite clay. The reaction of granite with NaCl solutions (0.008 and 0.028 M) produced fluids which were similar to those of the streamwater experiments. However, pH was lower in the NaCl experiments (=7.4–7.8), so that the steady-state concentrations of a number of the cationic constituents of the fluids were slightly greater in these experiments compared with the streamwater experiments. The dominant constituent of the product fluids (excepting NaCl) was SiO2, the concentration of which was buffered by quartz solubility. The sole secondary solid product identified was a sparsely-developed illitic clay. The injection of a Li-Rb-Cs-Na-Cl fluid into the experiments after 1000 hours demonstrated that the concentrations of the rare alkalis in the fluids were controlled by non-equilibrium processes. Other chemical components perturbed by dilution returned to the steady states developed prior to the injection process, demonstrating buffering by metastable reactions with the rock. The steady-state composition of the fluids was adequately predicted using an assumption of chemical equilibrium between the principal minerals of the granite and thermodynamic data for pure mineral compositions.

Microbiological influences on fracture surfaces of intact mudstone and the implications for geological disposal of radioactive waste

Abstract The significance of the potential impacts of microbial activity on the transport properties of host rocks for geological repositories is an area of active research. Most recent work has focused on granitic environments. This paper describes pilot studies investigating changes in transport properties that are produced by microbial activity in sedimentary rock environments in northern Japan. For the first time, these short experiments (39 days maximum) have shown that the denitrifying bacteria, Pseudomonas denitrificans, can survive and thrive when injected into flow-through column experiments containing fractured diatomaceous mudstone and synthetic groundwater under pressurized conditions. Although there were few significant changes in the fluid chemistry, changes in the permeability of the biotic column, which can be explained by the observed biofilm formation, were quantitatively monitored. These same methodologies could also be adapted to obtain information from cores originating from a variety of geological environments including oil reservoirs, aquifers and toxic waste disposal sites to provide an understanding of the impact of microbial activity on the transport of a range of solutes, such as groundwater contaminants and gases (e.g. injected carbon dioxide).

Influence of biofilms on transport of fluids in subsurface granitic environments – some mineralogical and petrographical observations of materials from column experiments

Abstract Landfill and radioactive waste disposal risk assessments focus on contaminant transport and are principally concerned with understanding the movement of gas, water and solutes through engineered barriers and natural groundwater systems. However, microbiological activity can impact on transport processes changing the chemical and physical characteristics of the subsurface environment. Such effects are generally caused by biofilms attached to rock surfaces. This paper will present some mineralogical and petrographical observations of materials extracted at the completion of an experimental column study which examined the influences of biofilm growth on groundwater flow through crushed diorite from the Äspö Hard Rock Underground Research Laboratory, Sweden.