The Possibility of Extending the Detection Function of an Analytical System to Lower Concentrations

It is shown that the only way to increase the detection capacity of an analytical system is based on the use of the multitude values of the single analytical signal. This can be achieved in two modalities: 1) fixed multitude values (as mean or as sum), and 2) sequential multitude values (as signal sum and as frequencies sum). By using these procedures, the analytical detection can be applied under the classical detection limit at as low concentrations as desired, provided that the multitude of the individual values of the signal be sufficiently high

The Geomechanics of CO2 Storage in Deep Sedimentary Formations

This paper provides a review of the geomechanics and modeling of geomechanics associated with geologic carbon storage (GCS), focusing on storage in deep sedimentary formations, in particular saline aquifers. The paper first introduces the concept of storage in deep sedimentary formations, the geomechanical processes and issues related with such an operation, and the relevant geomechanical modeling tools. This is followed by a more detailed review of geomechanical aspects, including reservoir stress-strain and microseismicity, well integrity, caprock sealing performance, and the potential for fault reactivation and notable (felt) seismic events. Geomechanical observations at current GCS field deployments, mainly at the In Salah CO2 storage project in Algeria, are also integrated into the review. The In Salah project, with its injection into a relatively thin, low-permeability sandstone is an excellent analogue to the saline aquifers that might be used for large scale GCS in parts of Northwest Europe, the U.S. Midwest, and China. Some of the lessons learned at In Salah related to geomechanics are discussed, including how monitoring of geomechanical responses is used for detecting subsurface geomechanical changes and tracking fluid movements, and how such monitoring and geomechanical analyses have led to preventative changes in the injection parameters. Recently, the importance of geomechanics has become more widely recognized among GCS stakeholders, especially with respect to the potential for triggering notable (felt) seismic events and how such events could impact the long-term integrity of a CO{sub 2} repository (as well as how it could impact the public perception of GCS). As described in the paper, to date, no notable seismic event has been reported from any of the current CO{sub 2} storage projects, although some unfelt microseismic activities have been detected by geophones. However, potential future commercial GCS operations from large power plants will require injection at a much larger scale. For such largescale injections, a staged, learn-as-you-go approach is recommended, involving a gradual increase of injection rates combined with continuous monitoring of geomechanical changes, as well as siting beneath a multiple layered overburden for multiple flow barrier protection, should an unexpected deep fault reactivation occur

Subsurface impact of CO2: Response of carbonate rocks and wellbore cement to supercritical CO2 injection and long-term storage. Geologica Ultraiectina (310)

Capture of CO2 at fossil fuel power station coupled with geological storage in empty oil and gas reservoirs is widely viewed as the most promising option for reducing CO2 emissions to the atmosphere, i.e. for climate change mitigation. Injection of CO2 into such reservoirs will change their chemical and mechanical state, for example through acidification of the pore fluid or changes in the state of stress. Similar effects may influence the integrity of overlying caprocks and of old, plugged wells. To fully assess storage system integrity, these effects must be understood. The research reported in this thesis addresses the mechanical and chemical response of carbonate rich reservoir rocks, and of wellbore cements to CO2 injection and storage. State of the-art experimental techniques were employed. These included compaction experiments on simulated porous carbonates in order to investigate time-dependent compaction phenomena, compression tests to determine the mechanical failure behaviour of chalks and wellbore cements, and reaction experiments to explore the long-term chemical effects of CO2 on cement. The results show that CO2 can accelerate long-term compaction. However, under the saline conditions of typical limestone reservoirs, the effect will be negligible if the reservoir was stable before CO2 injection. Highly porous chalks are well known to be unstable to water injection. However CO2 has little further effect. Results obtained for wellbore cement (Class A Portland cement) similar to the cements used in depleted Dutch gas fields, such as the De Lier field, showed that the stress changes accompanying injection will not lead to mechanical damage. In addition, experiments on chemical interaction have shown that reaction of CO2 with the cement precipitates calcium carbonate into voids and cracks, thus improving the sealing properties. Provided such wells are plugged properly upon abandonment, mechanical and chemical integrity will be preserved

Fracture healing and transport properties of wellbore cement in the presence of supercritical CO2

This paper investigates the process and rate of carbonation reaction of Class A wellbore cement exposed to CO2-saturated solution at confined conditions similar to those employed in geological storage of CO2. The main goal was to investigate whether reaction improves or degrades the sealing/healing capacity of fractured Type A cement plugs. Batch reaction experiments were performed for up to three months, on both intact and fractured Class A Portland cement cylinders, at a constant confining pressure of 30 MPa, a temperature of 80 °C and a CO2 pressure of 10 MPa. The experiments were carried out on water-saturated samples, exposing them to the supercritical CO2 at one end. All samples were jacketed in sleeves to seal them from the high pressure confining medium. The results indicate that cement carbonation front advanced in time, leading to a densification of the material. Extrapolation of the reaction rates to 1-year period indicates a carbonation depth of about 1.38 mm, and about 7.56 mm after 30 years of exposure to CO2-saturated solution. Thermogravimetric analyses, Scanning Electron Microscopy observations and permeability measurements indicate that carbonation of wellbore cement leads to a decrease of the porosity of the material on the reaction front and moreover, has the potential for healing pre-existent fractures and for improving the sealing properties of good-quality cement samples in time, at reservoir conditions

The Possibility of Extending the Detection Function of an Analytical System to Lower Concentrations

It is shown that the only way to increase the detection capacity of an analytical system is based on the use of the multitude values of the single analytical signal. This can be achieved in two modalities: 1) fixed multitude values (as mean or as sum), and 2) sequential multitude values (as signal sum and as frequencies sum). By using these procedures, the analytical detection can be applied under the classical detection limit at as low concentrations as desired, provided that the multitude of the individual values of the signal be sufficiently high

The influence of water and supercritical CO2 on the failure behavior of chalk

Reduction of compressive strength by injection of water into chalk is a well-known mechanism responsible for increased compaction in chalk reservoirs. This raises the question of whether such effects might be enhanced in the context of long-term storage of CO2 or of CO2 injection for enhanced oil and gas recovery (EOR/EGR) purposes. Therefore, data regarding the effect of supercritical CO2 on the mechanical behavior of chalk are needed. The effect of supercritical CO2 on the short-term failure behavior of wet chalk was accordingly investigated by means of conventional triaxial deformation experiments, performed on Maastrichtian chalk cores under dry conditions, in the presence of saturated chalk solution and using CO2-saturated solution at temperatures simulating reservoir conditions (20-80°C) and effective confining pressures up to 7MPa. Increasing temperature from 20 to 80°C did not show any significant effects on the strength of the dry samples. Addition of aqueous solution to the samples led to drastic weakening of the chalk, the effect being more pronounced at high effective confining pressures (Peff>3MPa). Addition of 10MPa supercritical CO2 to wet samples did not produce any significant additional effect in comparison with the wet samples. All samples showed a yield strength envelope characterized by shear failure at low effective mean stresses giving way to a compaction cap at high mean stresses. The weakening effect of aqueous solution was explained in terms of a reduction in frictional resistance of the material, due to water-enhanced grain-contact cracking, and perhaps pressure solution, with a possible contribution by disjoining pressure effects caused by water adsorption. While CO2 does not seem to reduce short-term failure strength of wet chalk, processes such as intergranular pressure solution have to be considered for assessing mechanical stability of chalk in the context of long-term CO2 storage or EOR/EGR operations

The effect of CO2 on creep of wet calcite aggregates

This paper reports uniaxial compaction creep experiments performed on porous calcite aggregates in the presence of CO2 at controlled conditions similar to those relevant for geological storage of CO2 in carbonate reservoirs. The experiments were conducted on pre-compacted calcite aggregates of various mean grain sizes in the range 1 to 250 μm, under dry and wet conditions, at temperatures of 28–100°C and applied effective stresses of 4–40 MPa. Carbon dioxide was added to wet samples at pressures up to 10 MPa. The results demonstrate that dry granular calcite shows virtually no creep, but that significant creep occurs when saturated aqueous solution is added. In wet samples, the strain rate increases with increasing grain size and applied stress. When CO2 is added from the outset, the strain rate decreases with increasing grain size up to 106 μm, and increases with grain size above 106 μm. Below 106 μm, the strain rate also increases with applied stress and strongly with CO2 (partial) pressure, but decreases with increasing temperature. The mechanical data together with microstructural evidence indicate that combined grain scale microcracking and diffusion controlled pressure solution best explain the behavior observed. Notably, in experiments where CO2 was added before loading, pressure solution dominated creep at fine grain size, giving way to subcritical cracking control at grain sizes above 106 μm. Our results point to pressure solution accelerating by up to 50 times at CO2 pressures increased from 6 to 10 MPa. Integrating our findings, we suggest that if a depleted carbonate reservoir exhibits measurable compaction creep due to diffusion-controlled pressure solution, then injection of CO2 has the potential to speed this up by amounts up to 50 times or more