H14C-01
Direct Numerical Simulation of Fluid Driven Geomechanical Events during Carbon Sequestration
Geologic storage of supercritical CO2 introduces significant stress perturbations into the target formation and overburden. In some cases, this could activate existing fractures and faults, or drive new fractures through the caprock. We will present results of a recent investigation to identify conditions that will activate existing fractures/faults or make new fractures within the caprock using a range of computational tools. Understanding the geomechanical sources of risk to successful CO2 containment involves a wide range of scales. For example, at the largest scale, bounding fault stability must be considered. Many proposed sequestration targets are bounded by impermeable fault zones that are presumed to become flow paths if they slip. Such geologic features are activated at the field scale by pore pressure elevations. In contrast, fluid driven fracturing events that may introduce new flow paths across a caprock occur at smaller scales. Determining whether the creation of new fractures in the caprock leads to CO2 leakage, however, requires knowledge of how the new fracture intersects and interacts with prior networks of fractures within the caprock. A range of computational tools have been developed at LLNL to consider these scenarios at the most appropriate scales and including the most appropriate physical models. For example, the Livermore Distinct Element Code (LDEC) has been used to simulate the mechanical deformation of fracture networks extending up to 100m. Frac-HMC was developed to evaluate network permeability evolution at the scale of many tens of meters incorporating both mechanical and chemical effects. Massively parallel, non-linear continuum codes that use a smeared fault representation have also been developed to predict activation of multiple faults at the field scale. Most recently, LDEC has been extended to simulate hydraulic driven fracture events at the scale of individual fractures. We will present results spanning the range of small scale fluid driven fracturing to field scale fault activation. We will discuss our emerging framework for integrating these results across multiple scales to build understanding of how CO2 storage integrity is potentially controlled by small scale processes. For example, fluid driven fracturing can lead to percolation across the caprock, but evaluating the probability of such an event involves understanding the relationship between the new fracture and the prior fracture network. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.
H14C-02
Coupled Reservoir-Geomechanical Analysis of CO2 Injection Performance at In Salah, Algeria
In Salah Gas Project in Algeria has been injecting nearly one million tonnes CO2 per year over the past three years into a water-filled strata at a depth of about 1,800 m. Unlike most CO2 storage sites, the permeability of the storage formation is relatively low and comparable thin with a thickness of about 20 m. To mitigate high injection pressures, the In Salah Gas Project decided to use long-reach (about 1 to 1.5 km) horizontal injection wells. In this study we are using field data and coupled reservoir-geomechanical numerical modeling of CO2 injection to analyze geomechanical responses and to assess the effectiveness of this approach for CO2 storage in relatively low permeability formations. We use satellited-based measurements of surface uplift to constrain our coupled reservoir-geomechanical model. We conduct sensitivity studies to investigate whether the observed uplift can be explained by poro-elastic expansion within the 20-m thick reservoir or if there is a significant contribution from pressure changes within the adjacent caprocks. We investigate whether surface deformations from InSAR can be useful for detection of a permeable leakage paths through the overlying caprock layers.
H14C-03
Leaking And Non-leaking Systems: Study Of Natural CO2 Accumulations For Geological Sequestration
The potential risks of geological CO2 storage must be understood and geologists are required to predict how CO2 may behave once stored underground. As natural geological accumulations of carbon dioxide occur in many basins in Italy and volcanic and seismically active areas allow CO2 rich fluids to migrate to the near surface, many of these areas have been investigated in order to study long-term geochemical processes that may occur following geological storage of anthropogenic CO2. A study representing an example of "leaking" system is the Solfatara crater (Campi Flegrei, Southern Italy) characterised by the presence of both CO2 rich-waters and fumarole. Soil gas flux measurements show that the entire area discharges between 1200 and 1500 tons of CO2 a day. Most part of analysed waters is the effect of a mixing between a shallow meteoric water and a deep thermal Na-Cl end-member and/or seawater, resulting in sodium-chloride waters. A high dissolved CO2 content (max value 566.28 cc/l) is also present. Furthermore, the Campi Flegrei frequently undergo bradyseism related to the elastic response of the shallow crust to increasing pressure within a shallow magma chamber. The study of this phenomenon could be useful to detect ground deformation linked to geomechanical changes in a geological CO2 reservoir. In contrast, an example of "non-leaking" system is the Pisticci oil and gas Field (Southern Italy) where a great variety of hydrocarbons traps are formed by horst and tilted blocks in the Mesozoic carbonate substratum covered by an almost continuous sequence of Lower Pliocene marls and Middle Pliocene-Pleistocene marly blue clays. Soil gas surveys were performed after a MD 4.5 earthquake and two years later to test the permanence of the gas distribution pattern. CO2 distribution in soil gas seems not to be affected by changes in stress, as suggested by the average values of both surveys. The principal aim of our research has been to evaluate and mitigate risks for local populations as the studied areas are densely populated. To date, the obtained results suggest that gas uprising is generally well localised around restricted areas, often controlled by local tectonics (faults and/or fractures). This implies that, in the frame of geological CO2 sequestration, it is necessary to carefully assess the presence of pathways (fault and/or fractures) that might allow the migration of CO2 out of the reservoir.
H14C-04
Numerical analysis of wellbore integrity: results from a field study of a natural CO2 reservoir production well
An important aspect of the risk associated with geological CO2 sequestration is the integrity of existing wellbores that penetrate geological layers targeted for CO2 injection. CO2 leakage may occur through multiple pathways along a wellbore, including through micro-fractures and micro-annuli within the "disturbed zone" surrounding the well casing. The effective permeability of this zone is a key parameter of wellbore integrity required for validation of numerical models. This parameter depends on a number of complex factors, including long-term attack by aggressive fluids, poor well completion and actions related to production of fluids through the wellbore. Recent studies have sought to replicate downhole conditions in the laboratory to identify the mechanisms and rates at which cement deterioration occurs. However, field tests are essential to understanding the in situ leakage properties of the millions of wells that exist in the mature sedimentary basins in North America. In this study, we present results from a field study of a 30-year-old production well from a natural CO2 reservoir. The wellbore was potentially exposed to a 96% CO2 fluid from the time of cement placement, and therefore cement degradation may be a significant factor leading to leakage pathways along this wellbore. A series of downhole tests was performed, including bond logs and extraction of sidewall cores. The cores were analyzed in the laboratory for mineralogical and hydrologic properties. A pressure test was conducted over an 11-ft section of well to determine the extent of hydraulic communication along the exterior of the well casing. Through analysis of this pressure test data, we are able estimate the effective permeability of the disturbed zone along the exterior of wellbore over this 11-ft section. We find the estimated range of effective permeability from the field test is consistent with laboratory analysis and bond log data. The cement interfaces with casing and/or formation are the most likely pathway by comparison of test data to cement core analysis. The results of this work demonstrate that field pressure tests can be an effective means to estimate effective permeabilities along existing wellbores, thus providing an important tool for managing the risk of geological CO2 sequestration.
H14C-05
Assessing the Leakage Potential of CO2 into Ground Water Resources at SACROC, West Texas
In this paper we apply CO2-PENS to characterize the long-term CO2 storage performance at SACROC, specifically with respect to potential impacts on ground water resources located above the CO2 injection depth. CO2-PENS is a hybrid process/system model composed of a library of process level modules linked together to allow simulations to span a range of spatial and temporal scales. SACROC is the oldest CO2-EOR operation in the Permian Basin and has been operational for over 35 years. We describe how CO2-PENS is used to couple CO2 migration at three different scales; from a reservoir module, through a borehole leakage module, and into an aquifer impact module. Both the reservoir module and wellbore leakage module are based on abstractions of the underlying physics of multiphase fluid flow, using reductions in complexity to allow fast Monte-Carlo simulation while capturing the behavior of these processes. Finally, we show how results from CO2-PENS can be used to delineate areas that are susceptible to ground water impacts from CO2 leakage and discuss how this information can be used in risk analysis.
H14C-06
Prediction of Groundwater Quality Changes in Response to CO2 Leakage from Deep Geological Storage
If carbon dioxide stored in deep saline aquifers were to leak into overlying aquifer containing potable groundwater, the intruding CO2 would lower groundwater pH and could enhance the solubility of hazardous inorganic constituents present in the aquifer minerals. As an effort to evaluate risks associated with geologic sequestration of CO2, this work assesses these potential effects using reactive transport modeling. A systematic geochemical evaluation of more than 38,000 groundwater quality analyses from aquifers throughout the United States provided the prerequisites for reactive transport modeling. For example, galena (under reducing conditions) and cerussite (under oxidizing conditions) control aqueous Pb (lead) whereas arsenopyrite component in pyrite controls aqueous As (arsenic) generally under reducing conditions. Reactive transport simulations are performed which focus on the chemical evolution of Pb and As in the groundwater after the intrusion of CO2. The simulations use representative mineralogies for shallow potable aquifers in the USA and two measured mineralogies for deep confined aquifers. The resulting concentrations of Pb and As in the groundwater are then compared to the EPA specified health- based limits for drinking water. A significant increase of aqueous Pb and As occurs, although in most situations they remain below health-based limits. Sensitivity studies are also conducted for variation in hydrological, geochemical and mineralogical conditions and several critical parameters. The results indicate that aquifers containing more carbonate (through pH buffer) and clay minerals (by adsorption) are less vulnerable to CO2 intrusion. Adsorption/desorption from minerals surface significantly impact the mobilization of Pb and As. Adsorption dampens the effect of galena and arsenopyrite dissolution by removing Pb and As from aqueous phase under reducing conditions. Under oxidizing condition desorption is primarily responsible for increasing the concentration of aqueous Pb while precipitation of cerussite downstream stabilize aqueous Pb concentration.
H14C-07
CO2 Leak Detection Using Hyperspectral Plant Signatures During the 2008 ZERT CO2 Sequestration Field Experiment in Bozeman, MT
Geologic sequestration of carbon dioxide is a potential method of mitigating increased carbon dioxide (CO2) emissions into the atmosphere. However, the impact CO2 sequestration would have on various physical, chemical, and biological systems needs to be explored in order to effectively utilize this technique long term. We used plants to detect the effects of releasing pure carbon dioxide gas through a 100-meter long horizontal well, varying between 1 and 2.5 meters below ground surface, at a flow rate of 0.3 tons per day for 29 days, beginning on July 9th, 2008. An influx of CO2 gas into the soil induces stress on vegetation, which can be observed in their visible to near infrared reflectance spectral signatures. We followed the health of a 20 meter by 30 meter patch of plants (including alfalfa, dandelion, Timothy grass, Tall fescue grass, and Orchard grass), located along and across the injection well. 68 sites were chosen within the plant patch where spectral signatures were measured almost daily with an Analytical Spectral Devices "FieldSpec Pro" spectrometer. These sites were located over the injection well and up to 10 meters off the well. This array gave us data both within and outside of the CO2 leak zone so we could normalize our observations for all other environmental factors affecting the plants. On August 5, 2008, airborne hyperspectral imagery was also acquired from a low flying aircraft using a hyperspectral camera developed by Resonon, Inc. of Bozeman, MT. In addition, other groups participating in the ZERT experiment measured various parameters of the field site, such as CO2 flux mapping, soil CO2 concentrations, atmospheric CO2 concentrations, plant isotopics, soil moisture, groundwater chemistry, and depth to groundwater. Four to five days after the start of the CO2 injection, stress was observed in the spectral signatures of plants within 1 meter of the well. After approximately ten days of CO2 injection, plants located up to 2.5 meters from the well exhibited at least moderate amounts of stress. Locations of plant stress corresponded to areas of high measured CO2 flux. In addition, correlating the spectral signatures to adjacent soil CO2 concentration probes gave a lower limit to the CO2 concentration needed to act as a stressor on the plants. During the injection, soil probes located over the well and at 2.5 meters north of the well measured CO2 concentrations that equilibrated at approximately 13.5% and 8% by volume, respectively. The spectra of the plants near these soil probes exhibited some degree of stress. The other soil probes farther from the well (at 5, 7.5 and 10 m) did not measure CO2 soil concentrations above 4% by volume, and none of the plants adjacent to those probes displayed discernable stress. We infer that the lower limit of CO2 needed to stress the plants over a period of 30 days is between 4% and 8% by volume.
H14C-08
Multispectral imaging of plant stress for detection of CO2 leaking from underground
Multispectral imaging of plant stress is a potentially useful method of detecting CO2 leaking from underground. During the summers of 2007 and 2008, we deployed a multispectral imager for vegetation sensing as part of an underground CO2 release experiment conducted at the Zero Emission Research and Technology (ZERT) field site near the Montana State University campus in Bozeman, Montana. The imager was mounted on a low tower and observed the vegetation in a region near an underground pipe during a multi-week CO2 release. The imager was calibrated to measure absolute reflectance, from which vegetation indices were calculated as a measure of vegetation health. The temporal evolution of these indices over the course of the experiment show that the vegetation nearest the pipe exhibited more stress than the vegetation located further from the pipe. The imager observed notably increased stress in vegetation at locations exhibiting particularly high flux of CO2 from the ground into the atmosphere. These data from the 2007 and 2008 experiments will be used to demonstrate the utility of a tower-mounted multispectral imaging system for detecting CO2 leakage from below ground with the ability to operate continuously during clear and cloudy conditions.