Note on the importance of hydrocarbon fill for reservoir quality prediction in sandstones

Oil emplacement retarded the rate of quartz cementation in the Brae Formation deep-water sandstone reservoirs of the Miller and Kingfisher fields (United Kingdom North Sea), thus preserving porosity despite the rocks' being buried to depths of 4 km and 120degreesC. Quartz precipitation rates were reduced by at least two orders of magnitude in the oil legs relative to the water legs. Important contrasts in quartz cement abundances and porosities have emerged between the oil and water legs where reservoirs have filled with hydrocarbons gradually over a prolonged period of time (greater than 15 m.y.). The earlier the hydrocarbon fill, the greater is the degree of porosity preservation. Failure to consider this phenomenon during field development could lead to overestimation of porosity and permeability in the water leg, potentially leading in turn to poor decisions about the need for and placement of downflank water injectors. During exploration, the retarding effect of oil on quartz cementation could lead to the presence of viable reservoirs situated deeper than the perceived regional economic basement

Can CCS and NET enable the continued use of fossil carbon fuels after CoP21?

Carbon capture and storage (CCS) does not generate energy. CCS applied to fossil and modern bio-carbon fuels and feedstocks removes environmentally damaging CO2 emissions. CoP21 stipulated a maximum 2°C–1.5°C global warming from 2050 in perpetuity. Both CCS and negative emission technology (NET) are now required to manage the carbon stock in earth’s atmosphere and oceans. All components of CCS are operationally proven secure at the industrial scale. Fifteen CCS projects operate globally; seven are under construction. CCS systems increase electricity prices, to about £100/MWhr. CCS on industry is cheaper and storage costs minimal (£5–20/tonne). CCS reduces whole economy costs of carbon transition by 2.5 times. Policies of capex subsidy, oversupplied emissions certificates, weak carbon pricing, and weak emissions standards have all failed to develop large cost CCS mega-projects. New carbon certificates could link the extraction of carbon to an obligation to store a percentage of emissions. Certificates connect CCS and NET pathways to secure carbon storage for the public good

Independent Expert Scientific Panel – Report on Unconventional Oil and Gas

No abstract available

CO2 dissolution in formation water as dominant sink in natural gas fields

A primary concern facing Carbon Capture and Storage (CCS) technology is the proven ability to safely store and monitor injected CO2 in geological formations on a long-term basis. However, it is extremely challenging to assess the long-term consequences of CO2 injection into the subsurface from decadal observations of existing CO2 disposal sites.Noble gases are conservative tracers within the subsurface, and combined with carbon stable isotopes, have proved to be extremely useful in determining both the origin of CO2 and how the CO2 is stored within natural CO2 reservoirs from around the world [1,2]. This presentation will identify and quantify the principal mechanism of CO2 phase removal in nine natural gas fields in North America, China and Europe. These natural gas fields are dominated by a CO2 phase and provide a natural analogue for assessing the geological storage of CO2 over millennial timescales. Our study highlights that in seven gas fields with siliciclastic or carbonate-dominated reservoir lithologies, dissolution in formation water at a pH of 5–5.8 is the major sink for CO2 [2]. This pH range is obtained by modelling the carbon isotope fractionation that results from dissolution of CO2(g) to varying proportions of H2CO3(aq) and HCO3-(aq). This is a major breakthrough as accurate subsurface pH measurements are notoriously difficult to obtain. In two fields with siliciclastic reservoir lithologies, some CO2 loss through precipitation as carbonate minerals cannot be ruled out, but this is minor compared to the amount of CO2 lost to dissolution in the formation water within the same fields.Our findings imply mineral fixation is a minor CO2 trapping mechanism within natural reservoirs and hence suggests long-term models of geological CO2 storage should consider the potential mobility of CO2 dissolved in water.[1] Gilfillan et al., (2008) GCA 72, 1174-1198.[2] Gilfillan et al., (2009) Nature, doi:10.1038/nature07852<br/

PADAMOT : project overview report

Background and relevance to radioactive waste management International consensus confirms that placing radioactive wastes and spent nuclear fuel deep underground in a geological repository is the generally preferred option for their long-term management and disposal. This strategy provides a number of advantages compared to leaving it on or near the Earth’s surface. These advantages come about because, for a well chosen site, the geosphere can provide: • a physical barrier that can negate or buffer against the effects of surface dominated natural disruptive processes such as deep weathering, glaciation, river and marine erosion or flooding, asteroid/comet impact and earthquake shaking etc. • long and slow groundwater return pathways from the facility to the biosphere along which retardation, dilution and dispersion processes may operate to reduce radionuclide concentration in the groundwater. • a stable, and benign geochemical environment to maximise the longevity of the engineered barriers such as the waste containers and backfill in the facility. • a natural radiation shield around the wastes. • a mechanically stable environment in which the facility can be constructed and will afterwards be protected. • an environment which reduces the likelihood of the repository being disturbed by inadvertent human intrusion such as land use changes, construction projects, drilling, quarrying and mining etc. • protection against the effects of deliberate human activities such as vandalism, terrorism and war etc. However, safety considerations for storing and disposing of long-lived radioactive wastes must take into account various scenarios that might affect the ability of the geosphere to provide the functionality listed above. Therefore, in order to provide confidence in the ability of a repository to perform within the deep geological setting at a particular site, a demonstration of geosphere “stability” needs to be made. Stability is defined here to be the capacity of a geological and hydrogeological system to minimise the impact of external influences on the repository environment, or at least to account for them in a manner that would allow their impacts to be evaluated and accounted for in any safety assessments. A repository should be sited where the deep geosphere is a stable host in which the engineered containment can continue to perform according to design and in which the surrounding hydrogeological, geomechanical and geochemical environment will continue to operate as a natural barrier to radionuclide movement towards the biosphere. However, over the long periods of time during which long-lived radioactive wastes will pose a hazard, environmental change at the surface has the potential to disrupt the stability of the geosphere and therefore the causes of environmental change and their potential consequences need to be evaluated. As noted above, environmental change can include processes such as deep weathering, glaciation, river and marine erosion. It can also lead to changes in groundwater boundary conditions through alternating recharge/discharge relationships. One of the key drivers for environmental change is climate variability. The question then arises, how can geosphere stability be assessed with respect to changes in climate? Key issues raised in connection with this are: • What evidence is there that 'going underground' eliminates the extreme conditions that storage on the surface would be subjected to in the long term? • How can the additional stability and safety of the deep geosphere be demonstrated with evidence from the natural system? As a corollary to this, the capacity of repository sites deep underground in stable rock masses to mitigate potential impacts of future climate change on groundwater conditions therefore needs to be tested and demonstrated. To date, generic scenarios for groundwater evolution relating to climate change are currently weakly constrained by data and process understanding. Hence, the possibility of site-specific changes of groundwater conditions in the future can only be assessed and demonstrated by studying groundwater evolution in the past. Stability of groundwater conditions in the past is an indication of future stability, though both the climatic and geological contexts must be taken into account in making such an assertion

Fault seal controls on security of CO2 storage in aquifers

Structural traps for engineered storage of CO2 usually rely on a component of fault seal. In assessing the performance risk of storage sites, the conditions under which natural CO2 and CO2/hydrocarbon mixtures are retained by faults is poorly known. Mechanical failure can occur by flow along the fault plane due to extension, compression or shear. Geometric juxtaposition of aquifers or lack of low permeability fault gouge can enable flow across the fault plane. It is well established that faults which are close to being critically stressed have markedly different properties with respect to both their fluid flow and geomechanical characteristics. Here we examine three case studies. In the first two, the Rotliegend Sandstone reservoirs of the Oak and Fizzy Fields in the Southern North Sea, both of which are natural fault-bound gas fields with high CO2 content, we modify standard fault seal approaches to account for the different physical and chemical properties of CO2 to oil and CH4. In particular the impact of IFT and contact angle on threshold capillary pressure is investigated. Faults of both the Oak and Fizzy fields are analysed for fracture stability and slip tendency and are found to be stable (relative to present-day stresses) in all modelled scenarios and could withstand CO2 column heights in excess of trap height. However, under detailed assessment of fault seal potential for CO2-CH4 mixtures, both fields appear to be limited in column height by cross-fault leakage through carbonate layers of the overlying Zechstein Group. The third case study assessed the Captain Sandstone saline aquifer of the Inner Moray Firth. The in situ stress field was characterised using data available from hydrocarbon exploration wells. A range of potential stress fields were identified, and regional 3D geometric mapping of the major faults was then used to assess fault stability under the different potential stress regimes. Additionally, stereographic plots of fault dip angle and strike were used to deduce the pore pressure perturbation that could cause the mechanical reactivation of faults of any orientation. This accounted for unmapped faults that might truncate the storage reservoir and its overburden. In the stress scenario with the highest differential stress magnitudes low overpressures in the region of ~1.5 MPa could cause the reactivation of preferentially oriented faults, whereas higher induced pressures may be supported in lower differential stress regimes. Higher overpressure would also be required to cause the reactivation of the non-optimally oriented faults

Application of mineralogical, petrological and geochemical tools for evaluating the palaeohdrogeological evolution of the PADAMOT study sites

The role of Work Package (WP) 2 of the PADAMOT project – ‘Palaeohydrogeological Data Measurements’ - has been to study late-stage fracture mineral and water samples from groundwater systems in Spain, Sweden, United Kingdom and the Czech Republic, with the aim of understanding the recent palaeohydrogeological evolution of these groundwater systems. In particular, the project sought to develop and evaluate methods for obtaining information about past groundwater evolution during the Quaternary (about the last 2 million years) by examining how the late-stage mineralization might record mineralogical, petrographical and geochemical evidence of how the groundwater system may have responded to past geological and climatological changes. Fracture-flow groundwater systems at six European sites were studied: • Melechov Hill, in the Bohemian Massif of the Czech Republic: a shallow (0-100 m) dilute groundwater flow system within the near-surface weathering zone in fractured granitic rocks; • Cloud Hill, in the English Midlands: a (~100 m) shallow dilute groundwater flow system in fractured and dolomitized Carboniferous limestone; • Los Ratones, in southwest Spain: an intermediate depth (0-500 m) dilute groundwater flow system in fractured granitic rocks; • Laxemar, in southeast Sweden: a deep (0-1000 m) groundwater flow system in fractured granitic rocks. This is a complex groundwater system with potential recharge and flushing by glacial, marine, lacustrine and freshwater during the Quaternary; • Sellafield, northwest England: a deep (0-2000 m) groundwater flow system in fractured Ordovician low-grade metamorphosed volcaniclastic rocks and discontinuous Carboniferous Limestone, overlain by a Permo-Triassic sedimentary sequence with fracture and matrix porosity. This is a complex coastal groundwater system with deep hypersaline sedimentary basinal brines, and deep saline groundwaters in crystalline basement rocks, overlain by a shallow freshwater aquifer system. The site was glaciated several times during the Quaternary and may have been affected by recharge from glacial meltwater; • Dounreay, northeast Scotland: a deep (0-1400 m) groundwater flow system in fractured Precambrian crystalline basement overlain by fractured Devonian sedimentary rocks. This is within the coastal discharge area of a complex groundwater system, comprising deep saline groundwater hosted in crystalline basement, overlain by a fracture-controlled freshwater sedimentary aquifer system. Like Sellafield, this area experienced glaciation and may potentially record the impact of glacial meltwater recharge. In addition, a study has been made of two Quaternary sedimentary sequences in Andalusia in southeastern Spain to provide a basis of estimating the palaeoclimatic history of the region that could be used in any reconstruction of the palaeoclimatic history at the Los Ratones site: • The Cúllar-Baza lacustrine sequence records information about precipitation and palaeotemperature regimes, derived largely from the analysis of the stable isotope (δ18O and δ13C) signatures from biogenic calcite (ostracod shells). • The Padul Peat Bog sequence provided information on past vegetation cover and palaeogroundwater inputs based on the study of fossil pollen and biomarkers as proxies for past climate change. Following on from the earlier EC 4th Framework EQUIP project, the focus of the PADAMOT studies has been on calcite mineralization. Calcite has been identified as a late stage mineral, closely associated with hydraulically-conductive fractures in the present-day groundwater systems at the Äspö-Laxemar, Sellafield, Dounreay and Cloud Hill sites. At Los Ratones and Melechov sites late-stage mineralization is either absent or extremely scarce, and both the quantity and fine crystal size of any late-stage fracture mineralization relevant to Quaternary palaeohydrogeological investigations is difficult to work with. The results from the material investigated during the PADAMOT studies indicate that the fracture fillings at these sites are related to hydrothermal activity, and so do not have direct relevance as Quaternary indicators. Neoformed calcite has not been found at these two sites at the present depth of the investigations. Furthermore, the HCO3 - concentration in all the Los Ratones groundwaters is mainly controlled by complex carbonate dissolution. The carbonate mineral saturation indices do not indicate precipitation conditions, and this is consistent with the fact that neoformed calcite, ankerite or dolomite have not been observed petrographically

Natural CO2 sites in Italy show importance of overburden geopressure, fractures and faults for CO2 storage performance and risk management

The study of natural analogues can inform the long-term performance security of engineered CO2 storage. There are natural CO2 reservoirs and CO2 seeps in Italy. Here, we study nine reservoirs and establish which are sealed or are leaking CO2 to surface. Their characteristics are compared to elucidate which conditions control CO2 leakage. All of the case studies would fail current CO2 storage site selection criteria, though only two leak CO2 to surface. The factors found to systematically affect seal performance are overburden geopressure and proximity to modern extensional faults. Amongst our case studies, the sealing reservoirs show elevated overburden geopressure whereas the leaking reservoirs don’t. Since the leaking reservoirs are located within <10 km of modern extensional faults, pressure equilibration within the overburden may be facilitated by enhanced crustal permeability related to faulting. Modelling of the properties that could enable the observed CO2 leakage rates finds high permeability pathways (such as transmissive faults or fractures) become increasingly necessary to sustain leak rates as CO2 density decreases during ascent to surface, regardless of the leakage mechanism into the overburden. This work illustrates the value of characterising the overburden geology during CO2 storage site selection to inform screening criterion, risk assessment and monitoring strategy