Probabilistic evaluation of shallow groundwater resources at a hypothetical carbon sequestration site

Changes in Geochemical Composition of Groundwater Due to CO2 Leakage in Various Geological Media

This study evaluated the effects of CO2 leakage on the geochemical composition of groundwater in various geological media through long-term column experiments. Four columns were set up with soil representing a silicate aquifer; clean sand; a sand and limestone mixture; and alluvium soil, respectively. The experiments were conducted under the same experimental conditions for approximately one year. As the CO2-saturated synthetic groundwater was introduced into the columns, a decrease in pH and increases in electrical conductivity (EC), alkalinity, and concentrations of cations and trace elements were observed in all geological media. However, different patterns of changes were also observed depending on the mineralogical and physico-chemical characteristics of each material. As the column operation continued, while the pH decreased and low alkalinity values were more evident in the silicate soil and clean sand columns, the carbonate column continued to show high alkalinity and EC values in addition to high concentrations of most cations. The alluvium soil showed distinctive cation-exchange behaviors during the initial introduction of CO2. The results indicate that changes in the geochemical composition of groundwater will depend on the characteristic of the geological medium such as pH buffering capacity and cation exchange capacity. This study can be useful for monitoring and managing the impacts of CO2 leakage in various aquifer environments.

Sensitivity thresholds of groundwater parameters for detecting CO2 leakage at a geologic carbon sequestration site

Geologic carbon sequestration (GCS) projects in the USA are required to monitor groundwater quality for geochemical changes above the injection area that may be a result of CO₂ or brine leakage from the storage reservoir. Should CO₂ migrate into the groundwater around the compliance wells monitoring the shallower hydrologic units, each compliance parameter could react differently depending on its sensitivity to CO₂. Statistically determined limits (SDLs) for detection of CO₂ leakage into groundwater were calculated using background water quality data from the Illinois Basin Decatur Project (IBDP) sequestration site and prediction and tolerance intervals for specific compliance parameters. If the parameter concentrations varied outside of these ranges during the injection and post injection periods of a GCS project, then additional actions would be required to determine the reason for the changes in groundwater concentrations. Geochemical modeling can simulate the amount of CO₂ needed to alter water quality parameters a statistically significant amount. This information can then inform GCS operators and regulators as to which compliance parameters are relevant (sensitive) to CO₂ leakage for a given setting. For the system studied in here, Fe, Ca, K, Mg, CO₂, and pH were sensitive to CO₂ addition while Al, Cl, Na, and Si were not.

Unstable, Super Critical CO2-Water Displacement in Fine Grained Porous Media under Geologic Carbon Sequestration Conditions

In this study we investigated fluid displacement water with supercritical (sc) CO₂ in chalk under conditions close to those used for geologic CO₂ sequestration (GCS), to answer two main questions: How much volume is available for scCO₂ injection? And what is the main mechanism of displacement over a range of temperatures? Characterization of immiscible scCO₂ displacement, at the pore scale in the complex microstructure in chalk reservoirs, offers a pathway to better understand the macroscopic processes at the continuum scale. Fluid behavior was simulated by solving the Navier-Stokes equations, using finite-volume methods within a pore network. The pore network was extracted from a high resolution 3D image of chalk, obtained using X-ray nanotomography. Viscous fingering dominates scCO₂ infiltration and pores remain only partially saturated. The unstable front, developed with high capillary number, causes filling of pores aligned with the flow direction, reaching a maximum of 70% scCO₂ saturation. The saturation rate increases with temperature but the final saturation state is the same for all investigated temperatures. The higher the saturation rate, the higher the dynamic capillary pressure coefficient. A higher dynamic capillary pressure coefficient indicates that scCO₂ needs more time to reach capillary equilibrium in the porous medium.

Effectiveness and mechanism of natural attenuation at a petroleum-hydrocarbon contaminated site

This study applied an integrated method for evaluating the effectiveness and mechanism of natural attenuation (NA) of petroleum-hydrocarbon contaminated groundwater. Site groundwater and soil samples were analysed to characterize spatial and temporal variations in petroleum hydrocarbons, geochemical indicators, microbial diversity and isotopes. The results showed that the area of petroleum hydrocarbon contamination plume decreased almost 60% in four years, indicating the presence of natural attenuation. The ¹⁴C content and sequence analysis indicate that there are more relatively 'old' HCO₃^- that have been produced from petroleum hydrocarbons in the upgradient portion of the contaminated plume, confirming that intrinsic biodegradation was the major factor limiting spread of the contaminated plume. The main degradation mechanisms were identified as sulfate reduction and methanogenesis based on the following: (1) more SO₄^2- have been consumed in the contamination source than downgradient, and the δ³⁴S values in the resident SO₄^2- were also more enriched in the contamination source, (2) production of more CH₄ in the contamination source with the δ¹³C values for CH₄ was much lower than that of CO₂, and the fractionation factor was 1.030-1.046. The results of this study provide significant insight for applying natural attenuation and enhanced bioremediation as alternative options for remediation of petroleum-hydrocarbon contaminated sites.

Impact of naturally leaking carbon dioxide on soil properties and ecosystems in the Qinghai-Tibet plateau

One of the major concerns for CO2 capture and storage (CCS) is the potential risk of CO2 leakage from storage reservoirs on the shallow soil property and vegetation. This study utilizes a naturally occurring CO2 leaking site in the Qinghai-Tibet Plateau to analog a "leaking CCS site". Our observations from this site indicates that long-term CO2 invasion in the vadose zone results in variations of soil properties, such as pH fluctuation, slight drop of total organic carbon, reduction of nitrogen and phosphorus, and concentration changes of soluble ions. Simultaneously, XRD patterns of the soil suggest that crystallization of soil is enhanced and mineral contents of calcite and anorthite in soil are increased substantially. Parts of the whole ecosystem such as natural wild plants, soil dwelling animals and microorganisms in shallow soil are affected as well. Under a moderate CO2 concentration (less than 110000 ppm), wild plant growth and development are improved, while an intensive CO2 flux over 112000 ppm causes adverse effects on the plant growth, physiological and biochemical system of plants, and crop quality of wheat. Results of this study provide valuable insight for understanding the possible environmental impacts associated with potential CO2 leakage into shallow sediments at carbon sequestration sites.

Arsenic mobilization in shallow aquifers due to CO2 and brine intrusion from storage reservoirs

We developed an integrated framework of combined batch experiments and reactive transport simulations to quantify water-rock-CO₂ interactions and arsenic (As) mobilization responses to CO₂ and/or saline water leakage into USDWs. Experimental and simulation results suggest that when CO₂ is introduced, pH drops immediately that initiates release of As from clay minerals. Calcite dissolution can increase pH slightly and cause As re-adsorption. Thus, the mineralogy of the USDW is ultimately a determining factor of arsenic fate and transport. Salient results suggest that: (1) As desorption/adsorption from/onto clay minerals is the major reaction controlling its mobilization, and clay minerals could mitigate As mobilization with surface complexation reactions; (2) dissolution of available calcite plays a critical role in buffering pH; (3) high salinity in general hinders As release from minerals; and (4) the magnitude and quantitative uncertainty of As mobilization are predicated on the values of reaction rates and surface area of calcite, adsorption surface areas and equilibrium constants of clay minerals, and cation exchange capacity. Results of this study are intended to improve ability to quantify risks associated with potential leakage of reservoir fluids into shallow aquifers, in particular the possible environmental impacts of As mobilization at carbon sequestration sites.

Divergence of stable isotopes in tap water across China

Stable isotopes in water (e.g., δ²H and δ¹⁸O) are important indicators of hydrological and ecological patterns and processes. Tap water can reflect integrated features of regional hydrological processes and human activities. China is a large country with significant meteorological and geographical variations. This report presents the first national-scale survey of Stable Isotopes in Tap Water (SITW) across China. 780 tap water samples have been collected from 95 cities across China from December 2014 to December 2015. (1) Results yielded the Tap Water Line in China is δ²H = 7.72 δ¹⁸O + 6.57 (r² = 0.95). (2) SITW spatial distribution presents typical “continental effect”. (3) SITW seasonal variations indicate clearly regional patterns but no trends at the national level. (4) SITW can be correlated in some parts with geographic or meteorological factors. This work presents the first SITW map in China, which sets up a benchmark for further stable isotopes research across China. This is a critical step toward monitoring and investigating water resources in climate-sensitive regions, so the human-hydrological system. These findings could be used in the future to establish water management strategies at a national or regional scale.

Probabilistic Assessment of Above Zone Pressure Predictions at a Geologic Carbon Storage Site

Carbon dioxide (CO₂) storage into geological formations is regarded as an important mitigation strategy for anthropogenic CO₂ emissions to the atmosphere. This study first simulates the leakage of CO₂ and brine from a storage reservoir through the caprock. Then, we estimate the resulting pressure changes at the zone overlying the caprock also known as Above Zone Monitoring Interval (AZMI). A data-driven approach of arbitrary Polynomial Chaos (aPC) Expansion is then used to quantify the uncertainty in the above zone pressure prediction based on the uncertainties in different geologic parameters. Finally, a global sensitivity analysis is performed with Sobol indices based on the aPC technique to determine the relative importance of different parameters on pressure prediction. The results indicate that there can be uncertainty in pressure prediction locally around the leakage zones. The degree of such uncertainty in prediction depends on the quality of site specific information available for analysis. The scientific results from this study provide substantial insight that there is a need for site-specific data for efficient predictions of risks associated with storage activities. The presented approach can provide a basis of optimized pressure based monitoring network design at carbon storage sites.

CO2 Accounting and Risk Analysis for CO2 Sequestration at Enhanced Oil Recovery Sites

Using CO2 in enhanced oil recovery (CO2-EOR) is a promising technology for emissions management because CO2-EOR can dramatically reduce sequestration costs in the absence of emissions policies that include incentives for carbon capture and storage. This study develops a multiscale statistical framework to perform CO2 accounting and risk analysis in an EOR environment at the Farnsworth Unit (FWU), Texas. A set of geostatistical-based Monte Carlo simulations of CO2-oil/gas-water flow and transport in the Morrow formation are conducted for global sensitivity and statistical analysis of the major risk metrics: CO2/water injection/production rates, cumulative net CO2 storage, cumulative oil/gas productions, and CO2 breakthrough time. The median and confidence intervals are estimated for quantifying uncertainty ranges of the risk metrics. A response-surface-based economic model has been derived to calculate the CO2-EOR profitability for the FWU site with a current oil price, which suggests that approximately 31% of the 1000 realizations can be profitable. If government carbon-tax credits are available, or the oil price goes up or CO2 capture and operating expenses reduce, more realizations would be profitable. The results from this study provide valuable insights for understanding CO2 storage potential and the corresponding environmental and economic risks of commercial-scale CO2-sequestration in depleted reservoirs.

Reactive Transport Modeling of the Enhancement of Density-Driven CO2 Convective Mixing in Carbonate Aquifers and its Potential Implication on Geological Carbon Sequestration

We study the convection and mixing of CO2 in a brine aquifer, where the spread of dissolved CO2 is enhanced because of geochemical reactions with the host formations (calcite and dolomite), in addition to the extensively studied, buoyancy-driven mixing. The nonlinear convection is investigated under the assumptions of instantaneous chemical equilibrium, and that the dissipation of carbonate rocks solely depends on flow and transport and chemical speciation depends only on the equilibrium thermodynamics of the chemical system. The extent of convection is quantified in term of the CO2 saturation volume of the storage formation. Our results suggest that the density increase of resident species causes significant enhancement in CO2 dissolution, although no significant porosity and permeability alterations are observed. Early saturation of the reservoir can have negative impact on CO2 sequestration.

Improved estimation of hydraulic conductivity by combining stochastically simulated hydrofacies with geophysical data

Hydraulic conductivity is a major parameter affecting the output accuracy of groundwater flow and transport models. The most commonly used semi-empirical formula for estimating conductivity is Kozeny-Carman equation. However, this method alone does not work well with heterogeneous strata. Two important parameters, grain size and porosity, often show spatial variations at different scales. This study proposes a method for estimating conductivity distributions by combining a stochastic hydrofacies model with geophysical methods. The Markov chain model with transition probability matrix was adopted to re-construct structures of hydrofacies for deriving spatial deposit information. The geophysical and hydro-chemical data were used to estimate the porosity distribution through the Archie’s law. Results show that the stochastic simulated hydrofacies model reflects the sedimentary features with an average model accuracy of 78% in comparison with borehole log data in the Chaobai alluvial fan. The estimated conductivity is reasonable and of the same order of magnitude of the outcomes of the pumping tests. The conductivity distribution is consistent with the sedimentary distributions. This study provides more reliable spatial distributions of the hydraulic parameters for further numerical modeling.

A quantitative analysis of hydraulic interaction processes in stream-aquifer systems

The hydraulic relationship between the stream and aquifer can be altered from hydraulic connection to disconnection when the pumping rate exceeds the maximum seepage flux of the streambed. This study proposes to quantitatively analyze the physical processes of stream-aquifer systems from connection to disconnection. A free water table equation is adopted to clarify under what conditions a stream starts to separate hydraulically from an aquifer. Both the theoretical analysis and laboratory tests have demonstrated that the hydraulic connectedness of the stream-aquifer system can reach a critical disconnection state when the horizontal hydraulic gradient at the free water surface is equal to zero and the vertical is equal to 1. A boundary-value problem for movement of the critical point of disconnection is established for an analytical solution of the inverted water table movement beneath the stream. The result indicates that the maximum distance or thickness of the inverted water table is equal to the water depth in the stream, and at a steady state of disconnection, the maximum hydraulic gradient at the streambed center is 2. This study helps us to understand the hydraulic phenomena of water flow near streams and accurately assess surface water and groundwater resources.

Coupled Geochemical Impacts of Leaking CO2 and Contaminants from Subsurface Storage Reservoirs on Groundwater Quality

The leakage of CO2 and the concomitant brine from deep storage reservoirs to overlying groundwater aquifers is considered one of the major potential risks associated with geologic CO2 sequestration (GCS). In this work both batch and column experiments were conducted to determine the fate of trace metals in groundwater in the scenarios of CO2 and metal-contaminated brine leakage. The sediments for this study were from an unconsolidated sand and gravel aquifer in Kansas, containing 0-4 wt % carbonates. Cd (114 μg/L) and As (40 μg/L) were spiked into the reaction system to represent potential contaminants from the reservoir brine. Through this research we demonstrated that Cd and As were adsorbed on the sediments, in spite of the lowered pH due to CO2 dissolution in the groundwater. Cd concentrations in the effluent were below the Cd MCL, even for sediments without detectable carbonate to buffer the pH. Arsenic concentrations in the effluent were also significantly lower than the influent concentration, suggesting that the sediments tested have the capacity to mitigate the coupled adverse effects of CO2 leakage and brine intrusion. The mitigation capacity of sediment is a function of its geochemical properties (e.g., the presence of carbonate minerals, adsorbed As, and phosphate).

Field demonstration of CO2 leakage detection in potable aquifers with a pulselike CO2-release test

This study presents two field pulselike CO2-release tests to demonstrate CO2 leakage detection in a shallow aquifer by monitoring groundwater pH, alkalinity, and dissolved inorganic carbon (DIC) using the periodic groundwater sampling method and a fiber-optic CO2 sensor for real-time in situ monitoring of dissolved CO2 in groundwater. Measurements of groundwater pH, alkalinity, DIC, and dissolved CO2 clearly deviated from their background values, showing responses to CO2 leakage. Dissolved CO2 observed in the tests was highly sensitive in comparison to groundwater pH, DIC, and alkalinity. Comparison of the pulselike CO2-release tests to other field tests suggests that pulselike CO2-release tests can provide reliable assessment of geochemical parameters indicative of CO2 leakage. Measurements by the fiber-optic CO2 sensor, showing obvious leakage signals, demonstrated the potential of real-time in situ monitoring of dissolved CO2 for leakage detection at a geologic carbon sequestration (GCS) site. Results of a two-dimensional reactive transport model reproduced the geochemical measurements and confirmed that the decrease in groundwater pH and the increases in DIC and dissolved CO2 observed in the pulselike CO2-release tests were caused by dissolution of CO2 whereas alkalinity was likely affected by carbonate dissolution.