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Leakage from damaged caprock will depend on fracture properties in which we expect a difference between brittle and ductile deformations modes. We conducted experiments using integrated x-ray tomography and triaxial coreflood methods to characterize fracture-permeability behavior of shale as a function of the confining pressure at which fractures are created as a means of exploring the brittle-ductile transition. The experiments were conducted using a triaxial direct-shear coreflood system that creates through-going fractures for permeability measurements. Fracture characteristics were examined using 25-μm resolution x-ray computed tomography in both ex situ and in situ modes. At low confining pressure (3.5 MPa), permeability of fractured Utica shale was strongly dependent on fracture interactions with bedding planes and varied from 10s to 100s of mD (calculated with respect to the total sample area). Fracture systems propagating perpendicular to bedding planes resulted in tortuous paths with lower permeability. Fracture systems propagating parallel to bedding had substantially elevated permeability with some ex situ apertures greater than 500 μm. This behavior contrasted with fractures developed at high confining pressure (22 MPa). In situ x-ray tomography showed that localized deformation occurred on sub-resolvable fractures (less than 25-μm aperture) much smaller than apertures formed at low confining pressure. Permeability of the fractures formed at high confining pressure was less than 0.1 mD and about 3 orders of magnitude lower than the low confining pressure case.

Assessment of potential CO 2 and brine leakage from wellbores is central to any consideration of the viability of geological CO 2 sequestration. Depleted oil and gas reservoirs are some of the potential candidates for consideration as sequestration sites. The sequestration sites are expected to cover laterally extensive areas to be of practical interest. Hence there is a high likelihood that such sites will contain many pre-existing abandoned wells. Most existing work on wellbore integrity has focused on field and laboratory studies of chemical reactivity. Very little work has been done on the impacts of mechanical stresses on wellbore performance. This study focuses on the potential enhancement of fluid flow pathways in the near-wellbore environment due to modifications in the geomechanical stress field resulting from the CO 2 injection operations. The majority of the operational scenarios for CO 2 sequestration lead to significant rise in the formation pore pressure. This is expected to lead to an expansion of the reservoir rock and build-up of shear stresses near wellbores where the existence of cement and casing are expected to constrain the expansion. If the stress buildup is large enough, this can lead to failure with attendant permeability enhancement that can potentially provide leakage pathways to shallower aquifers and the surface. In this study, we use a numerical model to simulate key features of a wellbore (casing, annulus and cement) embedded in a system that includes the upper aquifer, caprock, and storage aquifer. We present the sensitivity of damage initiation and propagation to various operational and formation parameters. We consider Mohr-Coulomb shear-failure models; tensile failure is also likely to occur but will require higher stress changes and will be preceded by shear failure. The modeling is performed using the numerical simulator FEHM developed at LANL that models coupled THM processes during multi-phase fluid flow and deformation in fractured porous media. FEHM has been developed extensively under projects on conventional/unconventional energy extraction

As part of the United States Department of Energy (DOE) National Energy Technology Laboratory (NETL) sponsored project, The Quantification of Wellbore Leakage Risk Using Non-destructive Borehole Logging Techniques, the construction and integrity of a 68 year old well was studied. This study builds upon previous work examining the integrity of existing wells through shale formations. The objective of this study was to measure well integrity through potential caprocks and aid in understanding the potential leakage risk posed by old wells that intersect CO2 injection projects. The well was originally completed as a production well in the Gulf Coast region in 1945 and then plugged and abandoned in 1969. The well was re-entered and re-completed as an observation well for a CO2 injection project in 2008. It was replugged and abandoned after completing its observation role in 2013. For this study the well was logged using cement bond log and ultrasonic mapping tools, tested and sampled using a dynamic tester, and cored using a sidewall coring tool. The age of this well makes it the oldest well to be studied in this manner and this is the first well to be studied this way in the region. The results of the study indicate that much of the material behind the casing is unconsolidated cement. Logging results in many places in the well show poor isolation potential and indicate a microannulus. The logs also indicate removal of material during hydraulic testing which was confirmed by laboratory analysis. Of the six cores collected, four consisted of unconsolidated, soft, cement or rock, one consisted of heavily altered cement, and one consisted of slightly altered cement. The results of study are an interesting contrast to earlier field studies because they show an old well that lacked integrity at most of the test and sample points. However, the logging and a core sample in the squeezed zone indicate that there was zonal isolation between the injection and monitoring zones. The logging and mapping data along with the analysis of the “core” material collected provide insight into the potential leakage pathways within the well.

Poor wellbore integrity is a risk in CO2 storage that must be evaluated at any geologic sequestration site. The conditions of five wells from two fields in Wyoming were studied to better understand pre-injection leakage potential in existing wells. Ultrasonic and sonic logging tools mapped the condition of the casing and cement in each well. Permeability testing outside the casing was conducted using two different dynamic testing tools making point and vertical interference test measurements Permeability was also measured through laboratory testing of cased-hole sidewall cores. The results of laboratory measurements were generally in the microdarcy-to-nanodarcy range and indicate that the well cements have not degraded from exposure to the formation brines. The results of vertical interference tests when compared to lab measurements imply interfaces between casing and cement or cement and formation are more significant with respect to leakage than the quality of the cement at the tested location.

We present a preliminary model of geomechanical failure in wellbore systems induced by elevated pore- pressure following injection of CO 2 . The model is a coupled flow and geomechanics and operates on 2-D and 3- D detailed representations of the wellbore environment. We find that failure occurs at over-pressures of 6-7 MPa and that subsequent flow of water and/or CO 2 relieves pressure in the injection reservoir.

We present experimental results based on computed x-ray microtomography (CMT) for quantifying capillary trapping mechanisms as a function of fluid properties using several pairs of analog fluids to span a range of potential supercritical CO 2 -brine conditions. Our experiments areconducted in a core-flood apparatus using synthetic porous media and we investigate capillary trapping by measuring trapped non-wetting phase area as a function of varying interfacial tension, viscosity, and fluid flow rate. Experiments are repeated for a single sintered glass bead core using three different non-wetting phase fluids, and varying concentrations of surfactants, to explore and separate the effects of interfacial tension, viscosity, and fluid flow rate. Analysis of the data demonstrates distinct and consistent differences in the amount of initial (i.e. following CO 2 injection) and residual (i.e. following flood or WAG scheme) non-wetting phase occupancy as a function of fluid properties and flow rate. Further experimentation and analysis is needed, but these preliminary results indicate trends that can guide design of injection scenarios such that both initial and residual trapped gas occupancy is optimized.

Wellbore integrity is one of the key performance criteria in the geological storage of CO2. This is significant in any proposed storage site but may be critical to the suitability of depleted oil and gas reservoirs that may have 10’s to 1000’s of abandoned wells. Much previous work has focused on Portland cement which is the primary material used to seal (create zonal isolation) wellbore systems. This work has emphasized the reactivity of Portland cement to form calcium carbonate. However, an increasing number of field studies [e.g., 1], experimental studies [e.g., 2], and theoretical considerations indicate that the most significant leakage mechanism is likely to be flow of CO2 along the casing-cement microannulus, cement-cement fractures, or the cement-caprock interface. The magnitude of flows along these interfaces is a complex function of the pressure gradient, geomechanical properties that support the interface and dissolution/precipitation reactions that lead to widening or closure of the interface. In this study, we investigate the casing-cement microannulus through core-flood experiments. The experiments were conducted on a 5-cm diameter sample of cement that was cured with an embedded rectangular length of steel casing. Prior to the experiment, the casing was loosened creating a poorly bonded interface. However, we discovered that under confining pressure this interface was non-transmissive, suggesting that in the wellbore environment an open casing-cement microannulus requires a relatively low differential between pore and confining pressure. For the experiments, we created an artificially transmissive interface by scoring grooves in the steel casing (0.2-0.8 mm in depth). The core-flood experiments were conducted at 40 ∘C, 14 MPa pore pressure, and 28 MPa confining pressure for a period of 400 hours. During the experiment, 6.2 L of a 50:50 mixture of supercritical CO2 and a 30,000 ppm NaCl-rich brine flowed through 10-cm of limestone before flowing through the 6-cm length cement-casing composite. Approximately 41,000 pore-volumes of fluid moved through the casing-cement grooves. Scanning electron microscopy revealed that the CO2-brine mixture impacted both the casing and the cement. The Portland cement was carbonated to depths of 50–150 μm by a diffusion-dominated process. There was no evidence of mass loss or erosion of the Portland cement. By contrast, the steel casing reacted to form abundant precipitates of iron carbonate that lined the channels and in one case almost completed filled a channel. These results are compared to field studies to constrain the magnitude of possible CO2 migration in real wellbore systems.

The geomechanical behavior of caprock and wellbore systems determines the robustness of the CO2 storage system to disturbances from stress, pressure and temperature. In this study, we conduct triaxial coreflood and x-ray tomography experiments to directly measure permeability of water and supercritical CO2 in caprock (shale and anhydrite), cement and synthetic wellbores. The observed plastic behavior and large deformation that occurred prior to distinct sample failure demonstrates substantial stress accommodation in these systems. Fracture development resulted in total sample permeability ranging from 10-1000 mD depending on sample properties and the applied stress configuration.

A vital aspect to public and regulatory acceptance of carbon sequestration is assurance that groundwater resources will be protected. Theoretical and laboratory studies can, to some extent, be used to predict the consequences of leakage. However, direct observations of CO 2 flowing through shallow drinking water aquifers are invaluable for informing credible risk assessments. To this end, we have sampled shallow wells in a natural analog site in New Mexico, USA, where CO 2 from natural sources is upwelling from depth. We collected major ion, trace element, and isotopic ( 3 H, 18 O, and Sr) data and, coupled with laboratory experiments and reactive transport modeling, have concluded that the major control on groundwater quality at this site is not chemical reaction of CO 2 with the aquifer but intrusion of saline waters upwelling with the CO 2 . Using reactive transport modeling based on field data, we show the difference in reactivity of the CO 2 and CO 2 /saline water source terms, particularly with respect to carbonate mineralogy. Sr isotopes were used to investigate whether aquifer waters were affected by carbonate mineral reaction with CO 2 or by saline water intrusion. Preliminary data suggest that Sr isotopes can successfully be used to discriminate between the two types of source terms at Chimayo; this technique shows promise for monitoring CCS sites. In developing predictive capabilities for future sites, it is critical to identify the solid phases and specific reactions controlling dissolved trace metal concentrations in both the presence and absence of CO 2 . We have conducted laboratory experiments to identify these phases and have found that some elements (e.g., U, Ca) are largely controlled by ion exchange and/or carbonate minerals. In the experiments, the concentration of some metals increases after exposure to CO 2 (although concentrations remain below the U.S. EPA primary drinking water standards); we are currently extending these experiments to determine if the reactions causing the increase are reversible and, if so, on what time scales. Metal scavenging by secondary mineral precipitation, as observed at other natural analog sites, may be important at certain temporal scales. We are using the information gained from this field and laboratory study to develop predictive models for application to risk assessment at future CCS sites. The models will be particularly useful in identifying the temporal and spatial scales of water quality changes and in developing possible mitigation strategies in the case of leaks at engineered CCS sites.