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Loading history during drilling, completion and operations of the well can affect wellbore integrity. Debonding along well cement interfaces leads to the creation of micro-annuli and potential leakage pathways. We investigate the risk of debonding by modeling the integrity of an example well throughout its life-time using a finite element analysis tool. Well cement properties under downhole conditions are uncertain and therefore treated in a probabilistic manner. Simulation results for an example well show that a flexible annular cement reduces the risk of debonding and creation of micro-annuli along well cement interfaces. A quantitative measure of risk reduction is provided by probability density functions. Overall, probability functions of model output parameters from simulations with real well data can be used as input to a well integrity risk assessment framework that can incorporate diverse data and uncertain information.

Geological storage of captured CO2, which typically will contain certain amounts of impurities, in salineaquifers is of potential to reduce greenhouse gas emissions into the atmosphere. The co-injection of theimpurity SO2has an effect on the chemical reactivity of the fluid and solid phases as well as on the physicalbehaviour of the CO2plume. To address these influences we assessed the impact of SO2on the physic-ochemical behaviour of the CO2phase by pressure-volume-temperature calculations and geochemicalmodelling of fluid-rock interactions. As case study, the Ketzin pilot site in Germany was chosen. Theresults reveal that the presence of SO2causes a greater porosity decrease compared to pure CO2duringthe injection period. Certainly, any textural changes that occur in the vicinity of the injection well are ofimportance for selecting the right injection strategy and to reduce operational costs. We further assessedthe impact of SO2on the overall CO2storage capacity at different depth. Particularly, there is a maxi-mum reduction of the storage capacity at low depth, but the general impact of SO2concentrations

To ensure the safety of a CO 2 storage site and containment of CO 2 in the subsurface, the integrity of wellbore materials must be maintained. Field and laboratory studies have shown CO 2 -induced reactivity of wellbore cement, but these results have to be extrapolated to the extended time span of CO 2 storage. Geochemical modeling provides a tool for the prediction of cement alteration; however, large uncertainties in input parameters exist and signifi cant variation in subsurface conditionsis expected. This asks for a systematic investigation of the sensitivity of modeled cement alteration towards these factors. In this paper we report PHREEQC simulations of CO 2 diffusion into cement and subsequent chemical reactions. The sensitivity of cement alteration toward reaction rates, initial porosity, temperature/mineralogy and fl ow/no fl ow conditions were investigated. The base case model indicated that intact cement and tight interfaces between the reservoir and the cement would yield less than 1% porosity change after 300 days of diffusion. For porosity increase or degradation to occur at the cement interface, leaching/fl ow along the wellbore was required. The sensitivity scenarios yield CO 2 penetration depths between 0.3 cm and 1.4 cm after 300 days of diffusion. The maximum was reached for the high porosity (fast diffusion) scenario that facilitates CO 2 transport through the cement matrix. The minimum CO 2 penetration was for enhanced calcium silicate hydrate (C-S-H) decalcifi cation, which increases calcite precipitation, CO 2 consumption, and hence decelerates CO 2 penetration. This is related to high temperatures (and more crystalline C-S-H) or to higher kinetic rate constants used.

Risk assessment-led characterisation of a site for the geological storage of CO2 in the UK northern North Sea was performed for the EU SiteChar research project as one of a portfolio of sites. Implementation and testing of the SiteChar project site characterisation workflow has produced a ‘dry-run’ storage permit application that is compliant with regulatory requirements. A site suitable for commercial-scale storage was characterised, compatible with current and future industrial carbon dioxide (CO2) sources in the northern UK. Pre-characterisation of the site, based on existing information acquired during hydrocarbon exploration and production, has been achieved from publicly available data. The project concept is to store captured CO2 at a rate of 5 Mt per year for 20 years in the Blake Oil Field and surrounding Captain Sandstone saline aquifer. This commercial-scale storage of 100 Mt CO2 can be achieved through a storage scenario combining injection of CO2 into the oil field and concurrent water production down-dip of the field. There would be no encroachment of supercritical phase CO2 for more than two kilometres beyond the field boundary and no adverse influence on operating hydrocarbon fields provided there is pressure management. Components of a storage permit application for the site are presented, developed as far as possible within a research project. Characterisation and technical investigations were guided by an initial assessment of perceived risks to the prospective site and a need to provide the information required for the storage permit application. The emphasis throughout was to reduce risks and uncertainty on the subsurface containment of stored CO2, particularly with respect to site technical performance, monitoring and regulatory issues, and effects on other resources. The results of selected risk assessment-led site characterisation investigations and the subsequent risk reassessments are described together with their implications for the understanding of the site. Additional investigations are identified that could further reduce risks and uncertainties, and enable progress toward a full storage permit application. Permit performance conditions are presented as SiteChar-recommended useful tools for discussion between the competent authority and operator. © M. Akhurst et al.

Many uncertainties exist in geochemical modeling. Mineral reactive surface area is one of the uncertain parameters. QEMSCAN analyses are performed on sandstone samples from a Dutch CO2 natural analogue to determine reactive surface areas. Geochemical modeling is performed using QEMSCAN surface areas and surface areas which are conventionally used in modeling. The model predicts that the QEMSCAN reactive surface areas result in higher reaction rates and faster equilibration of the sandstones with CO2. The fact that reactions are predicted to occur in a sandstone which has been in contact with CO2 for geological times suggests that the reservoir mineralogy is not yet in equilibrium with CO2. This would indicate that, even if mineral reactive surface areas are determined in detail, geochemical models strongly overestimate reaction kinetics. Other uncertain parameters which can affect kinetics, like mineral nucleation and ion diffusion, should be evaluated in order to be able to better predict long-term CO2- mineral reactions and storage integrity.

Large scale implementation of CO2 storage can significantly reduce emission of greenhouse gasses into the atmosphere. However, safe and long-term containment of CO2 in storage reservoirs must be ensured. Wellbores in the subsurface present possible leakage pathways for CO2 to the surface and hence wellbore cement reactivity is of major concern. Previous experimental studies of cement reactivity often use high brine to cement ratios which may lead to overestimations of the rate of cement alteration. We aim to study cement reactivity under more realistic CO2 storage conditions. Limited brine is used to represent a wellbore environment with brine mainly present in pore space. The experimental results show a cease or significant reduction of reaction progression after 7 days due to saturation of the fluid. This inhibits further cement dissolution and re-dissolution of secondary calcite. The observed reaction zones are matched by geochemical modeling, showing from core to rim: unreacted cement (zone A), portlandite dissolution and increased porosity (zone B), major calcite and reduced porosity plus minor ferrihydrite precipitation (zone Ci) and minor calcite precipitation (zone Cii). The calibration of the geochemical model aids the development of an accurate reactive transport model for long-term cement alteration and integrity prediction.

The Werkendam (WED) and Barendrecht-Ziedewij (BRTZ) gas fields are CO2- and CH4-bearing stratigraphic equivalents in the Netherlands. A comparison in petrographic characteristics and burial histories of the two fields is performed to investigate long-term mineral reactions induced by the presence of CO2. The mineral relations in BRTZ are used as a CO2-free reference for WED. However, the results show that the differences in paragenetic sequence between the two fields are partially due to different temperature evolutions and fluid influxes. The mineral relations that can be linked to the presence of CO2 in WED are the partial dissolution of anhydrite and feldspar and the precipitation of siderite, quartz and potentially minor dolomite. The amount of CO2 sequestered in siderite (and potentially dolomite) is small. PHREEQC geochemical modelling was able to simulate the observations of the mineral reactions induced by carbonized brine. Yet, a sensitivity study on the type of illite used in the model, and the inclusion of minor minerals showed that significantly different reactions can be induced when input parameters are slightly varied. Furthermore the presence and type of Fe-minerals determines if and how much siderite forms. This shows that careful selection of initial mineralogy is required as model input. In one model run mineral reactions were predicted which are known from petrographic studies not to occur under the applied conditions. Hence, besides careful mineral selection, an expert opinion on diagenetic processes is necessary to guide the model towards the proper mineral reactions.