Geologic Carbon Sequestration Sites Research Articles

Mobilization and transport of organic compounds from reservoir rock and caprock in geological carbon sequestration sites

Supercritical CO2 (scCO2) is a good solvent for organic compounds such as benzene, toluene, ethyl-benzene, and xylene (BTEX), phenols, and polycyclic aromatic hydrocarbons (PAHs). Monitoring results from geological carbon sequestration (GCS) field tests have shown that organic compounds are mobilized following CO2 injection. Such results have raised concerns regarding the potential for groundwater contamination by toxic organic compounds mobilized during GCS. Knowledge of the mobilization mechanism of organic compounds and their transport and fate in the subsurface is essential for assessing risks associated with GCS. Extraction tests using scCO2 and methylene chloride (CH2Cl2) were conducted to study the mobilization of volatile organic compounds (VOCs, including BTEX), the PAH naphthalene, and n-alkanes by scCO2 from representative reservoir rock and caprock obtained from depleted oil reservoirs and coal from an enhanced coal-bed methane recovery site. Results showed that the extent of mobilization for the organic compounds was a function of the source rock. In fate and transport sand column experiments, moisture content was found to have an important influence on the transport of the organic compounds. In dry sand columns the majority of the compounds were retained in the column except benzene and toluene. In wet sand columns the mobility of the BTEX was much higher than that of naphthalene. Based upon the results determined for the reservoir rock, caprock, and coal samples studied here, the risk to aquifers from contamination by organic compounds appears to be relatively low; however, further work is necessary to fully evaluate the risks.

Optimal design of pressure-based, leakage detection monitoring networks for geologic carbon sequestration repositories

Monitoring of leakage at geologic carbon sequestration (GCS) sites requires the capability to intercept and resolve the onset, location, and volume of leakage in a timely manner. Pressure-anomaly monitoring represents one of the few monitoring technologies that possess such capabilities. To fully leverage the strength of pressure monitoring while meeting cost constraints, optimization of network design is necessary. This study presents an optimization method for designing cost-effective GCS monitoring networks under model and parameter uncertainty. A binary integer programming problem (BIPP) is formulated to minimize both the total volume of leakage and the number of uncovered potentially leaky locations. The BIPP is demonstrated for selecting optimal monitoring locations in both homogeneous and heterogeneous formations. The sensitivity of monitoring design to a number of model and design parameters is investigated, while model structure and parameter uncertainties are incorporated through user-specified scenarios. Results suggest that the BIPP is a viable approach for identifying optimal sensing locations even when the number of design variables is relatively large (∼105). The BIPP is general and can be readily used to facilitate the design of performance-based GCS monitoring networks.

Rheological characterization of olivine slurries, sheared under CO2 pressure

The injection of reactive mineral suspensions is a possible process to either reinforce or seal wells in geologic carbon sequestration sites. Among others, olivine slurries could be used as CO2‐triggered cements, as they carbonate under the thermodynamic conditions of deep storage. However, injection of such slurries requires to control their initial fluidity and its evolution during the displacement. For this purpose, we have characterized the rheological evolution of olivine suspensions, of particle sizes 1−10 μm, sheared at 100 s−1, in the ranges of 10−40% for the volume fractions, 20−50 bars for the CO2 pressure, and 60–110°C for the temperature. The apparent viscosity, measured at 100 s−1, increases exponentially, with a characteristic time ranging from less than 1 h to more than 10 h, and decreasing with olivine concentration, CO2 pressure, and temperature. Moreover, a careful characterization of the slurry rheology carried out in the initial stages of slow evolution, shows that the slurry obeys a Bingham plastic law, with a yield stress ΣB, and a Bingham viscosity µB increasing from 1 to 20 Pa and from 4 to 110 mPa s, respectively, when the olivine volume fraction φ is increased from 10 to 30%. Such variations can be accounted by classical models (Wildemuth and Williams [1984]: Rheologica Acta, 23,627–635, Krieger [1972]: Advances in Colloid and Interface Science, 3, 111–136). © 2013 American Institute of Chemical Engineers Environ Prog, 33: 572–580, 2014

Aerial detection of a simulated CO2 leak from a geologic sequestration site using hyperspectral imagery

Airborne remote sensing has been proposed as a potential large-area monitoring tool for geologic carbon sequestration (GCS) sites. Elevated soil CO2 levels from an underground CO2 leak could induce a plant stress response that is spectrally discernible from the air. A controlled subsurface CO2 release experiment was conducted during the growing season at the outdoor Zero Emissions Research and Technology (ZERT) center to simulate a CO2 leak scenario. Simultaneously, aerial imagery was collected to obtain a time series used to identify and characterize the simulated CO2 leak prior to, during, and after the three week CO2 release. A theoretical framework was developed for analytical strategies that could be implemented to detect a CO2 leak using aerial hyperspectral imagery with minimal a priori knowledge of when and where a surface leak is going to occur. Areas of inferred CO2 stressed vegetation were identified using an unsupervised clustering algorithm. The spectral signatures of this vegetation informed the development of a red edge index (REI) used to quantify the CO2 stress signal and chart vegetation health trajectories over the course of the CO2 release experiment. REI was found to be significantly lower (Welch's p-value<0.001) in CO2 stressed vegetation as compared to healthy vegetation. Furthermore, maximum differences in REI (REIhealthy vegetation−REICO2 stressed vegetation) were observed at the height of the CO2 release followed by a subsequent decrease in differences once CO2 injection ceased. These results suggest that there was a cumulative vegetation stress response followed by a possible vegetation recovery and that aerial hyperspectral imaging may be a plausible method for detecting CO2 leaks from GCS sites.

A Numerical Study on CO2 Seepage from Offshore Geologic Storage Site

The objective of the present study is to gain a fundamental understanding on CO2 behavior during the seepage from the geologic carbon sequestration site. To elucidate the seepage behavior in the marine sediment, CO2 bubble dynamics in partially saturated porous media was studied. Force balance on a CO2 bubble in porous media was calculated by considering buoyancy force, surface tension effect and drag force. To consider the effect of porosity, permeability and wetting fluid saturation on ascent of CO2 in porous media, a new surface tension force model was studied. Based on the calculation results, the bubble rising velocity showed strong dependency on porosity and grain diameter. But there was negligible permeability effect. The developed model showed different calculation results with correction factor of force balance. There is no agreement in previous researches whether the escaped CO2 bubble from the marine sediment can be released to the atmosphere or not. To understand the CO2 bubble behavior in the water column, we calculated the dissolution of CO2 bubble in sea water. A molecular diffusion and a convective mass transfer of CO2 bubble were calculated by considering offshore environmental conditions during the dissolution. Based on the calculation results, the bubble dissolution rate showed strong dependency on the initial bubble diameter and bubble rising velocity.

Assessing leakage detectability at geologic CO2 sequestration sites using the probabilistic collocation method

We present an efficient methodology for assessing leakage detectability at geologic carbon sequestration sites under parameter uncertainty. Uncertainty quantification (UQ) and risk assessment are integral and, in many countries, mandatory components of geologic carbon sequestration projects. A primary goal of risk assessment is to evaluate leakage potential from anthropogenic and natural features, which constitute one of the greatest threats to the integrity of carbon sequestration repositories. The backbone of our detectability assessment framework is the probability collocation method (PCM), an efficient, nonintrusive, uncertainty-quantification technique that can enable large-scale stochastic simulations that are based on results from only a small number of forward-model runs. The metric for detectability is expressed through an extended signal-to-noise ratio (SNR), which incorporates epistemic uncertainty associated with both reservoir and aquifer parameters. The spatially heterogeneous aquifer hydraulic conductivity is parameterized using Karhunen–Loève (KL) expansion. Our methodology is demonstrated numerically for generating probability maps of pressure anomalies and for calculating SNRs. Results indicate that the likelihood of detecting anomalies depends on the level of uncertainty and location of monitoring wells. A monitoring well located close to leaky locations may not always yield the strongest signal of leakage when the level of uncertainty is high. Therefore, our results highlight the need for closed-loop site characterization, monitoring network design, and leakage source detection.

The effects of CO2‐brine rheology on leakage processes in geologic carbon sequestration

Leakage from geologic carbon sequestration (GCS) sites is inherently challenging to study because CO2, driven by buoyant forces, travels over long distances, undergoing phase changes and encountering numerous connate brine and formation chemistries as it rises to the surface. This work explores the effect that CO2has on the rheological properties of brine solutions over a range of GCS‐relevant temperature, pressure, ionic strength, and shear conditions. Under the fluid‐liquid equilibrium conditions that prevail in the deep subsurface, viscosity of CO2‐brine mixtures was found to be a function of temperature and pressure alone. Once leakage conditions ensue, discrete CO2bubbles form in brine, resulting in the vapor‐liquid equilibrium (VLE), and these mixtures exhibit complex linear viscoelastic, time dependent, and thixotropic behavior. The presence of CO2(g) bubbles on the flow of the bulk fluid could have important impacts on impeding (via shear drag force) leakage depending on the geometrical, geochemical and geophysical characteristics of a storage site. Under VLE conditions, the effective viscosity of CO2‐brine mixtures was found to be up to five times higher than brine alone but the microstructure was easily destroyed, and not readily regained, under high shear conditions. At higher temperatures and higher ionic strength, the effect is less pronounced. These results were considered in the context of flow through porous media, and the effect on buoyancy‐driven flow is significant. Understanding this effect is important for developing an accurate constitutive relationship for leaking CO2, which will lead to better capacity to select and monitor GCS sites.

Inversion of pressure anomaly data for detecting leakage at geologic carbon sequestration sites

Leakage from abandoned wells and geologic faults represents one of the greatest risks to the integrity of geologic CO2 sequestration sites. The ability to detect leakage in a timely manner is, therefore, crucial for mitigating the potential adverse impacts of leakage to the public and environment. We present an inversion approach for recovering both leakage locations and rates by using observed pressure anomaly data. The approach is based on formulation of a linear system of equations using the unit-step response method, which is applicable to both analytical and numerical models. Because the resulting system is often ill conditioned, we investigate the efficacy of regularization methods for stabilizing the solutions. Further, when prior information is insufficient to restrict the number of search locations, a global optimization algorithm is used to solve the challenging problem of joint location and leakage history inversion. The performance of several linear inversion solvers is compared while considering effects such as measurement error and spatial heterogeneity. The results are promising and suggest that our pressure-anomaly-based leakage detection algorithm can be used to identify leaky wells in practice. It can be deployed as an integrated component of CO2 risk management frameworks.

Transient CO2 leakage and injection in wellbore‐reservoir systems for geologic carbon sequestration

AbstractAt its most basic level, the injection of CO2 into deep reservoirs for geologic carbon sequestration (GCS) involves a system comprising the wellbore and the target reservoir, the wellbore being the only conduit available to emplace the CO2. Wellbores in general have also been identified as the most likely conduit for CO2 and brine leakage from GCS sites, especially those in sedimentary basins with historical hydrocarbon production. We have developed a coupled wellbore and reservoir model for simulating the dynamics of CO2 injection and leakage through wellbores, and we have applied the model to situations relevant to geologic CO2 storage involving upward flow (e.g. leakage) and downward flow (injection). The new simulator integrates a wellbore‐reservoir system by assigning the wellbore and reservoir to two different sub‐domains in which flow is controlled by appropriate laws of physics. In the reservoir, we model flow using a standard multiphase Darcy flow approach. In the wellbores, we use the drift‐flux model and related conservation equations for describing transient two‐phase non‐isothermal wellbore flow of CO2‐water mixtures. Applications to leakage test problems reveal transient flows that develop into quasi‐steady states within a day if the reservoir can maintain constant conditions at the wellbore. Otherwise, the leakage dynamics could be much more complicated than the simple quasi‐steady‐state flow, especially when one of the phases flowing in from the reservoir is near its residual saturation. A test problem of injection into a depleted (low‐pressure) gas reservoir shows transient behavior out to several hundred days with sub‐critical conditions in the well disappearing after 240 days. © 2011 Society of Chemical Industry and John Wiley &amp; Sons, Ltd

3D geometry and topology of pore pathways in Opalinus clay: Implications for mass transport

Clay-rich sedimentary rocks are considered as seal lithologies for hosting radioactive waste or as caprocks for geological carbon sequestration sites. Evaluation of the rock's sealing capacity requires a comprehensive understanding of mass transport processes, which, in turn, demands knowledge of the 3D structure of pore space. Here, the use of focused ion beam nanotomography (FIB-nt) in building realistic pore space models is demonstrated along with a novel approach employed to analyze the topology of the pore space. The method was applied to three samples of the Opalinus clay of in northern Switzerland which is considered as a candidate host rock formation for the disposal of radioactive waste. Due to resolution limitations the lower limit of analyzed pore radii was about 10 nm. Pore radii > 10 nm were related to a physical porosity of about 2 vol.%. Comparing the pore size distribution determined by FIB-nt with the one obtained by N 2 adsorption analysis, FIB-nt revealed the structure of about 20% of the total pore space. The total external porosity determined by N 2 adsorption analysis was in the range of 10 to 12 vol.%. Our approach to analyze the complex 3D structure of the pore space was based on converting the voxel based 3D structure into a 3D graph of the pore skeleton. A 3D graph representation permitted determination of the spatial distribution of pore space geometrical properties such as pore path orientation, pore path tortuosity and pore path length. Pore-paths in Opalinus clay show a preferred orientation within the bedding plane in combination with a comparatively low pore path tortuosity. Pore path tortuosity perpendicular to the bedding plane is higher by a factor of as much as five. Anisotropy in pore space is caused by spatial density variations of pore path orientation (i.e. preferred orientations of pore paths) in combination with an elongated pore shape (i.e. low tortuosity).

Leakage risk assessment of the In Salah CO2 storage project: Applying the certification framework in a dynamic context

Abstract The Certification Framework (CF) is a simple risk assessment approach for evaluating CO2 and brine leakage risk at geologic carbon sequestration (GCS) sites. In the In Salah CO2 storage project assessed here, five wells at Krechba produce natural gas from the Carboniferous C10.2 reservoir with 1.7–2% CO2 that is delivered to the Krechba gas processing plant, which also receives high- CO2 natural gas (∼10% by mole fraction) from additional deeper gas reservoirs and fields to the south. The gas processing plant strips CO2 from the natural gas that is then injected through three long horizontal wells into the water leg of the Carboniferous gas reservoir at a depth of approximately 1,800 m. This injection process has been going on successfully since 2004. The stored CO2 has been monitored over the last five years by a Joint Industry Project (JIP)–a collaboration of BP, Sonatrach, and Statoil with co-funding from US DOE and EU DG Research. Over the years the JIP has carried out extensive analyses of the Krechba system including two risk assessment efforts, one before injection started, and one carried out by URS Corporation in September 2008. The long history of injection at Krechba, and the accompanying characterization, modeling, and performance data provide a unique opportunity to test and evaluate risk assessment approaches. We apply the CF to the In Salah CO2 storage project at two different stages in the state of knowledge of the project: (1) at the pre-injection stage, using data available just prior to injection around mid-2004; and (2) after four years of injection (September 2008) to be comparable to the other risk assessments. The main risk drivers for the project are CO2 leakage into potable groundwater and into the natural gas cap. Both well leakage and fault/fracture leakage are likely under some conditions, but overall the risk is low due to ongoing mitigation and monitoring activities. Results of the application of the CF during these different state-of-knowledge periods show that the assessment of likelihood of various leakage scenarios increased as more information became available, while assessment of impact stayed the same. Ongoing mitigation, modeling, and monitoring of the injection process is recommended.

A U.S. NGO’s perspectives on monitoring of saline and EOR geologic carbon injection and sequestration sites

Abstract Geologic carbon sequestration offers both the potential to provide a permanent sink for industrial CO2 emissions, and added value to tertiary enhanced oil and gas recovery (EOR). Subsurface injection of carbon dioxide is a proven technology developed in the EOR industry over the past three decades. However, EOR efforts have not focused on monitoring, reporting and verification (MRV) of permanent geologic sequestration of CO2. The challenge, particularly for early deployment of MRV, will be the implementation of protective monitoring plans that do not become unnecessary barriers to commercialization of geologic carbon sequestration. For well-characterized low risk sites, a three-tiered MRV structure could serve to improve cost-effectiveness and encourage commercial-scale development of CCS and geologic sequestration technologies. For these low risk sites, early first-tier MRV would be comprised of narrowly focused subsurface and surface monitoring techniques designed to test CO2 behavior against the reservoir model. A finding of steady-state predictable injection, consistent with the objectives set out in the operating permit, would require minimal MRV expense and few, if any, airside tools. MRV at these steady-state sites would be periodically re-evaluated for operation cost-effectiveness, eliminating monitoring techniques that yield either redundant or minimally useful subsurface information. However, evidence of adverse migration of the CO2 plume pressure front, or mechanical failure of cap rock would “trigger” advanced MRV. This second pre-planned tier of MRV would be used to evaluate short and long-term risk to drinking water, air and other resources. This, in turn would either lead to modification of injection strategies, once re-initiated, to mitigate identified risk, or permanent injection shut down and third and final stage static post-injection MRV. Saline and EOR-based geologic carbon sequestration should be treated under the same overarching principles, however MRV may require different approaches.

Time-window-based filtering method for near-surface detection of leakage from geologic carbon sequestration sites

We use process-based modeling techniques to characterize the temporal features of natural biologically controlled surface CO2 fluxes and the relationships between the assimilation and respiration fluxes. Based on these analyses, we develop a signal-enhancing technique that combines a novel time-window splitting scheme, a simple median filtering, and an appropriate scaling method to detect potential signals of leakage of CO2 from geologic carbon sequestration sites from within datasets of net near-surface CO2 flux measurements. The technique can be directly applied to measured data and does not require subjective gap filling or data-smoothing preprocessing. Preliminary application of the new method to flux measurements from a CO2 shallow-release experiment appears promising for detecting a leakage signal relative to background variability. The leakage index of ±2 was found to span the range of biological variability for various ecosystems as determined by observing CO2 flux data at various control sites for a number of years.

Model Components of the Certification Framework for Geologic Carbon Sequestration Risk Assessment

Chapter 1 MODEL COMPONENTS OF THE CERTIFICATION FRAMEWORK FOR GEOLOGIC CARBON SEQUESTRATION RISK ASSESSMENT Curtis M. Oldenburg 1 , Steven L. Bryant 2 , Jean-Philippe Nicot 3 , Navanit Kumar 2,5 , Yingqi Zhang 1 , Preston Jordan 1 , Lehua Pan 1 , Patrick Granvold 4 , Fotini K. Chow 4 Earth Sciences Division, 90-1116, 1 Cyclotron Rd., Lawrence Berkeley National Laboratory, Berkeley CA 94720 Department of Petroleum and Geosystems Engineering, The University of Texas at Austin, 1 University Station C0300, Austin, TX 78712 Bureau of Economic Geology, Jackson School of Geosciences, The University of Texas at Austin, University Station, Box X, Austin, Texas 78713 Department of Civil and Environmental Engineering, University of California, Berkeley, Berkeley, CA 94720 Shell Oil and Production Company, 200 North Dairy Ashford, Houston, TX 77079 ABSTRACT: We have developed a framework for assessing the leakage risk of geologic carbon sequestration sites. This framework, known as the Certification Framework (CF), emphasizes wells and faults as the primary potential leakage conduits. Vulnerable resources are grouped into compartments, and impacts due to leakage are quantified by the leakage flux or concentrations that could potentially occur in compartments under various scenarios. The CF utilizes several model components to simulate leakage scenarios. One model component is a catalog of results of reservoir simulations that can be queried to estimate plume travel distances and times, rather than requiring CF users to run new reservoir simulations for each case. Other model components developed for the CF and described here include fault characterization using fault-population statistics; fault connection probability using fuzzy rules; well-flow modeling with a drift-flux model implemented in TOUGH2; and atmospheric dense-gas dispersion using a mesoscale weather prediction code. INTRODUCTION We have developed a novel and practical risk-based framework for certifying that the leakage risk of a potential geologic carbon sequestration (GCS) site is below agreed-upon thresholds [1, 2, 3]. The approach we developed, known as the Certification Framework (CF), proposes a standardized way for project proponents, regulators, and the public to analyze and understand risks and uncertainties of GCS in a simple and transparent way. The CF goes beyond the scope of regulations of deep underground injection permitted by the U.S. Environmental Protection Agency (EPA) which protect underground sources of drinking water (USDW) to consider risks to a broader set of resources and environmental assets as well as loss of emission-reduction credits due to CO 2 leakage. The CF uses physically grounded models for the movement of injected CO 2 and brine, and for assessing the likelihood that CO 2 or injection-related pressure perturbation will intersect wells and faults. The CF and an example case study have been fully described elsewhere [3]. This paper describes the essential model components that we have developed for carrying out a CF analysis. These model components include the catalog of simulation results, fault population statistics, fault connectivity analyses, well-bore flow, and dense gas dispersion.

Short-Range Atmospheric Dispersion of Carbon Dioxide

We present a numerical study aimed at quantifying the effects of concentration-dependent density on the spread of a seeping plume of CO2 into the atmosphere such as could arise from a leaking geologic carbon sequestration site. Results of numerical models can be used to supplement field monitoring estimates of CO2 seepage flux by modelling transport and dispersion between the source emission and concentration-measurement points. We focus on modelling CO2 seepage dispersion over relatively short distances where density effects are likely to be important. We model dense gas dispersion using the steady-state Reynolds-averaged Navier-Stokes equations with density dependence in the gravity term. Results for a two-dimensional system show that a density dependence emerges at higher fluxes than prior estimates. A universal scaling relation is derived that allows estimation of the flux from concentrations measured downwind and vice versa.

The consequences of failure should be considered in siting geologic carbon sequestration projects

Geologic carbon sequestration is the injection of anthropogenic CO 2 into deep geologic formations where the CO 2 is intended to remain indefinitely. If successfully implemented, geologic carbon sequestration will have little or no impact on terrestrial ecosystems aside from the mitigation of climate change. However, failure of a geologic carbon sequestration site, such as large-scale leakage of CO 2 into a potable groundwater aquifer, could cause impacts that would require costly remediation measures. Governments are attempting to develop regulations for permitting geologic carbon sequestration sites to ensure their safety and effectiveness. At present, these regulations focus largely on decreasing the probability of failure. In this paper we propose that regulations for the siting of early geologic carbon sequestration projects should emphasize limiting the consequences of failure because consequences are easier to quantify than failure probability.

Modeling Gas Transport in the Shallow Subsurface During the ZERT CO2 Release Test

We used the multiphase and multicomponent TOUGH2/EOS7CA model to carry out predictive simulations of CO2 injection into the shallow subsurface of an agricultural field in Bozeman, Montana. The purpose of the simulations was to inform the choice of CO2 injection rate and design of monitoring and detection activities for a CO2 release experiment. The release experiment configuration consists of a long horizontal well (70 m) installed at a depth of approximately 2.5 m into which CO2 is injected to mimic leakage from a geologic carbon sequestration site through a linear feature such as a fault. We estimated the permeability of the soil and cobble layers present at the site by manual inversion of measurements of soil CO2 flux from a vertical-well CO2 release. Based on these estimated permeability values, predictive simulations for the horizontal well showed that CO2 injection just below the water table creates an effective gas-flow pathway through the saturated zone up to the unsaturated zone. Once in the unsaturated zone, CO2 spreads out laterally within the cobble layer, where liquid saturation is relatively low. CO2 also migrates upward into the soil layer through the capillary barrier and seeps out at the ground surface. The simulations predicted a breakthrough time of approximately two days for the 100kg d−1 injection rate, which also produced a flux within the range desired for testing detection and monitoring approaches. The seepage area produced by the model was approximately five meters wide above the horizontal well, compatible with the detection and monitoring methods tested. For a given flow rate, gas-phase diffusion of CO2 tends to dominate over advection near the ground surface, where the CO2 concentration gradient is large, while advection dominates deeper in the system.

Certification framework based on effective trapping for geologic carbon sequestration

We have developed a certification framework (CF) for certifying the safety and effectiveness of geologic carbon sequestration (GCS) sites. Safety and effectiveness are achieved if CO 2 and displaced brine have no significant impact on humans, other living things, resources, or the environment. In the CF, we relate effective trapping to CO 2 leakage risk which takes into account both the impact and probability of leakage. We achieve simplicity in the CF by using (1) wells and faults as the potential leakage pathways, (2) compartments to represent environmental resources that may be impacted by leakage, (3) CO 2 fluxes and concentrations in the compartments as proxies for impact to vulnerable entities, (4) broad ranges of storage formation properties to generate a catalog of simulated plume movements, and (5) probabilities of intersection of the CO 2 plume with the conduits and compartments. We demonstrate the approach on a hypothetical GCS site in a Texas Gulf Coast saline formation. Through its generality and flexibility, the CF can contribute to the assessment of risk of CO 2 and brine leakage as part of the certification process for licensing and permitting of GCS sites around the world regardless of the specific regulations in place in any given country.

Percolation-theory and fuzzy rule-based probability estimation of fault leakage at geologic carbon sequestration sites

Leakage of CO2 and displaced brine from geologic carbon sequestration (GCS) sites into potable groundwater or to the near-surface environment is a primary concern for safety and effectiveness of GCS. The focus of this study is on the estimation of the probability of CO2 leakage along conduits such as faults and fractures. This probability is controlled by (1) the probability that the CO2 plume encounters a conductive fault that could serve as a conduit for CO2 to leak through the sealing formation, and (2) the probability that the conductive fault(s) intersected by the CO2 plume are connected to other conductive faults in such a way that a connected flow path is formed to allow CO2 to leak to environmental resources that may be impacted by leakage. This work is designed to fit into the certification framework for geological CO2 storage, which represents vulnerable resources such as potable groundwater, health and safety, and the near-surface environment as discrete “compartments.” The method we propose for calculating the probability of the network of conduits intersecting the CO2 plume and one or more compartments includes four steps: (1) assuming that a random network of conduits follows a power-law distribution, a critical conduit density is calculated based on percolation theory; for densities sufficiently smaller than this critical density, the leakage probability is zero; (2) for systems with a conduit density around or above the critical density, we perform a Monte Carlo simulation, generating realizations of conduit networks to determine the leakage probability of the CO2 plume (P leak) for different conduit length distributions, densities and CO2 plume sizes; (3) from the results of Step 2, we construct fuzzy rules to relate P leak to system characteristics such as system size, CO2 plume size, and parameters describing conduit length distribution and uncertainty; (4) finally, we determine the CO2 leakage probability for a given system using fuzzy rules. The method can be extended to apply to brine leakage risk by using the size of the pressure perturbation above some cut-off value as the effective plume size. The proposed method provides a quick way of estimating the probability of CO2 or brine leaking into a compartment for evaluation of GCS leakage risk. In addition, the proposed method incorporates the uncertainty in the system parameters and provides the uncertainty range of the estimated probability.

Case studies of the application of the Certification Framework to two geologic carbon sequestration sites

Abstract We have developed a certification framework (CF) for certifying that the risks of geologic carbon sequestration (GCS) sites are below agreed-upon thresholds. The CF is based on effective trapping of CO 2 , the proposed concept that takes into account both the probability and impact of CO 2 leakage. The CF uses probability estimates of the intersection of conductive faults and wells with the CO 2 plume along with modeled fluxes or concentrations of CO 2 as proxies for impacts to compartments (such as potable groundwater) to calculate CO 2 leakage risk. In order to test and refine the approach, we applied the CF to (1) a hypothetical large-scale GCS project in the Texas Gulf Coast, and (2) WESTCARB’s Phase III GCS pilot in the southern San Joaquin Valley, California.