Acid treatment as a way to reduce shale rock mechanical strength and to create a material prone to the formation of permanent well barrier

Utilization of natural shale formations for the creation of annular barriers in oil and gas wells is currently discussed as a mean of simplifying cumbersome plugging and abandonment procedures. Shales that are likely to form annular barriers are shales with high content of swelling clays and relatively low content of cementation material (e.g., quartz, carbonates). Shales with large content of quartz and low content of swelling clays will be rather brittle and not easily deformable. In this paper we ask the question whether and to what extent it is possible to modify the mechanical properties of relatively brittle shales by chemically removing some cementation material. To answer this question, we have leached out carbonates from Pierre I shale matrix using hydrochloric acid and we have compared mechanical properties of shale before and after leaching. We have also followed leaching dynamics using X-ray tomography. The results show that removal of around 4–5 wt% of cementation material results in 43% reduction in Pierre I shale shear strength compared to the non-etched shale exposed to sodium chloride solution for the same time. The etching rate was shown to be strongly affected by the volume of fluid staying in direct contact with the shale sample.publishedVersio

Microstructure and Properties of Wire Arc Additive Manufacturing of Inconel 625

In the present investigation, wire arc additive manufacturing of Inconel 625 was carried out with the cold metal transfer variant of the metal inert gas process. The heat input varied between 0.46 and 0.63 kJ/mm, which is a rather low heat input with low deposition rate. The built walls were subjected to Charpy V and crack tip opening displacement (CTOD) fracture toughness testing, in addition to microstructure examination with light microscope and scanning and transmission electron microscope. The results obtained show that hardness increases from the base metal level of 210, via the heat-affected zone (in the building plate) with HV of 220, to the weld metal, with a hardness of around 240–250. All individual Charpy V values fall within the range from 160 to 200 J, while the CTOD fracture toughness is within the range from 0.49 to 1.05 mm. The microstructure examination revealed the microsegregation of certain elements to the interdendritic regions, causing three different particle types to form. Particles with a spherical morphology were identified as spinel (MgAl2O4). Some of the spinel particles were surrounded by disc-shaped precipitates, which were identified as (NbTi)(CN), having the same orientation as the spinel.publishedVersio

Electrical and thermal stability of Al-Cu welds: Performance benchmarking of the hybrid metal extrusion and bonding process

Advances in joining processes for aluminum and copper are sought after to facilitate the greater adoption of aluminum in electrical applications. Aluminum's chemical affinity to copper causes the joining and lifetime of Al-Cu welds to be vulnerable to the formation of various intermetallic compounds. Intermetallic compounds and the resulting weld structure are known to reduce the structural integrity and increase the electrical resistance of Al-Cu welds. In this study we evaluate the novel joining process, Hybrid Metal Extrusion and Bonding, for butt welding aluminum and copper. The weld structure was examined using scanning and transmission electron microscopy, and the weld resistance was measured using four-point measurements forecast to the weld interface. Energy dispersive spectroscopy and electron diffraction zone axis patterns were analysed to identify intermetallic compounds. Weld samples were examined pre and post heat treatment at 200 °C, 250 °C and 350 °C for total durations of over 1000 h. The results are compared to existing Al-Cu joining processes, and a new metric, weld interface resistivity, is proposed to compare the electrical properties of bimetallic welds. The Hybrid Metal Extrusion and Bonding process was found to form a thin, consistent and straight intermetallic layer with negligible impact on electrical resistance in the as-welded condition. Artificial ageing of samples by heat treatment established the overall growth rate of intermetallic compounds. The growth rate was used to evaluate the weld's operational lifetime versus temperature. The intermetallic growth rate of Hybrid Metal Extrusion and Bonding was quantified at 200 °C and compared to alternative processes. The Hybrid Metal Extrusion and Bonding process showed a significant performance advantage requiring the longest time to reach 2 μm thickness. Furthermore, the growth of intermetallic compounds did not increase the electrical resistance of the weld interface. The negligible impact on electrical resistance and slow intermetallic growth are promising results of the potential functional performance. This study is the first characterisation of the Hybrid Metal Extrusion and Bonding process for electrical applications showcasing its exciting potential for the joining of aluminum and copper.publishedVersio

Conductive epoxy/carbon nanofiber coatings for scale control

Calcium carbonate (CaCO3) is one of the most widespread scaling minerals and has been a long-standing problem within many industrial sectors. Scaling of calcium carbonate on conductive surfaces can be prevented electrochemically by anodic polarization. Anodic polarization, however, cannot be applied directly to metal surfaces like e.g., steel that will suffer from corrosion when polarized anodically in an aqueous environment. Thus, in this paper it is proposed to apply a conductive coating to a metal surface to allow anodic polarization and inhibit surface scaling, without corrosion of the underlying metal surface taking place. To this end an epoxy/carbon nanofiber conductive coating was developed and deposited at steel surfaces. The coating showed good adhesion to the surface and the bulk and surface resistivities were in the order of 52.80 kΩcm and 31.87 kΩ/cm2, respectively. The anti-scaling performance of the coating without- and under anodic polarization was tested upon exposure to 1.5 wt % CaCl2 solution being in contact with CO2. The coating has been tested at several different potentials to find optimal conditions for scale inhibition. Potentials above +3 VOCP caused a degradation of the coating due to oxygen evolution at the anode, as well as evolution of chlorine gas. At +1.5 and +2 VOCP the coating remained intact and the precipitation of CaCO3 was limited. On the other hand, cathodic polarization of the coating surface enhanced scaling and no coating degradation was observed at cathodic polarization even at potentials as high as -5 VOCP. The coating has thus proven a good solution to control surface scale deposition. Both anodic scale inhibition and cathodic scale acceleration have been achieved at the coating surfaces.publishedVersio

Local mechanical properties and precipitation inhomogeneity in large-grained Al–Mg–Si alloy

Al–Mg–Si (6xxx series) alloys show excellent mechanical properties due to the precipitates formed during heat treatment. However, heat treatment of these alloys results in a soft precipitation free zone (PFZ) close to grain boundaries that weakens them and promotes fracture, and thereby reduces the ductility of the material. This study provides quantitative insights into the mechanical properties and underlying plasticity behavior of Al–Mg–Si (6xxx series) alloys through combined nanoindentation hardness measurements and in-depth characterization of the microstructure adjacent to the PFZ region and in the grain interior. Experimental nanoindentation, transmission microscopy (TEM) and electron channeling contrast imaging results confirm the weakening effect from PFZ by means of a reduced hardness close to grain boundaries. The nanoindentation hardness mapping also revealed an increase in hardness a few micrometers from the grain boundary with respect to the grain interior. Precipitate quantification from TEM images confirms that the hardness increase is caused by a locally higher density of precipitates. To the authors’ best knowledge, this harder zone has not been recognized nor discussed in previously reported findings. The phenomenon has important implications for the mechanical properties of large-grained ( µm) aluminium alloys.publishedVersio

Scanning transmission electron microscopy studies of precipitation in Al-Mg-Ge alloys

Precipitation in three Al-Mg-Ge(-Si-Cu) alloys has been investigated using transmission electron microscopy. The alloy compositions were chosen to be similar to previously studied Al-Mg-Si(-Cu) alloys to facilitate direct comparison. These alloys are strengthened by the precipitation of nanometresized, needle-shaped particles during heat treatment. A deeper understanding of precipitation at the atomic level is required in order to achieve greater control over alloy properties. The precipitation in the investigated Al-Mg- Ge(-Si-Cu) alloys was found to share similarities with that in Al-Mg-Si(-Cu) alloys, but there were also significant differences. The high atomic number of Ge relative to Al, Mg, and Si made Al-Mg-Ge alloys highly suited for study by the atomic-number sensitive technique high-angle annular dark-field scanning transmission electron microscopy (HAADF STEM). This was the most important technique employed in this thesis. The use of a state-of-the-art aberration-corrected microscope also made it possible to resolve details previously inaccessible. A near-hexagonal network of Ge columns when viewed along the needle direction was a unifying feature of all the precipitates in these alloys, as is the case in the metastable precipitates of the Al-Mg-Si(-Cu) alloy system. However, the β__ phase, the most important hardening phase in Al-Mg-Si alloys, was not observed. Instead, hardnesses similar to that of comparable Al-Mg-Si(-Cu) alloys were achieved through other precipitate phases. Two Al-Mg-Ge alloys were the main objects of study in this thesis: one Mg-rich and one Ge-rich, with an addition of Mg and Ge in the relation Mg2Ge and Mg5Ge6, respectively. Precipitate phases that form in overaged Al-Mg-Si alloys were observed around peak hardness in these Al-Mg-Ge alloys, as well as disordered precipitates. The precipitate phases known from Al-Mg-Si, U1 and β_, were finer and more coherent with the Al matrix in the Al-Mg-Ge alloys than their counterparts in Al-Mg-Si. These precipitates also displayed highly interesting interface structures, consisting of Ge atoms in columns not part of the bulk precipitate structure. The β_-like precipitate phase that was observed in the Mg-rich alloy was investigated by quantitative HAADF STEM. This method makes it possible to obtain quantitative compositional information from the specimen. It was found that the Ge-rich columns contained significantly less Ge than the Si columns of β_ in Al-Mg-Si alloys. A partial replacement of Ge by Al or vacancies might explain the smaller lattice parameter of the β_-like phase in Al-Mg-Ge compared with β_ in Al-Mg-Si alloys. Precipitation in an Al-Mg-Si-Ge-Cu alloy was also investigated with HAADF STEM. No repeating unit cell was observed in these precipitates near peak hardness. However, these precipitates contained a hexagonal network consisting of mixed Si and Ge columns with Mg, Al, and Cu columns occurring in between the network columns at specific sites. Structural units consisting of Al, Mg, Si, and Ge were often arranged in an ordered manner

Cement Self-Healing as a Result of CO2 Leakage

Avoiding CO2 leakages from storage reservoirs is crucial to ensure safe and cost-efficient Carbon Capture and Storage (CCS). This can only be done if effort is made to maintain well integrity throughout the entire life-cycle of a well. Cement integrity is especially important, since the interfaces between cement and rock or casing have been identified as weak links in today's well construction. The present paper focuses on the healing of fractures in well cement when the material is exposed to a CO2-brine water-alternating-gas (WAG) flooding scheme. Specimen characterization using computed tomography combined with electron microscopy documents the self-healing procedure in detail for a composite cement-rock specimen subjected to a WAG flooding scheme. The study revealed volumetric data on self-healing of cement cracks and chemical changes in the specimen as well as in aqueous chemistry upon CO2 exposure. The measured aqueous chemistry suggests CO2-cement interaction to be less pronounced with time thereby together with the observed cement self-healing suggesting that the risk of compromising the safety of a storage site by cement-CO2 chemical reactions is minimal.publishedVersio

Carbonation of silica cement at high-temperature well conditions

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Soft particles assisted grain refinement and strengthening of an Al-Bi-Zn alloy subjected to ECAP

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Multislice image simulations of sheared needle‐like precipitates in an Al‐Mg‐Si alloy

The image contrast of sheared needle‐like precipitates in the Al‐Mg‐Si alloy system is investigated with respect to shear‐plane positions, the number of shear‐planes, and the active matrix slip systems through multislice transmission electron microscopy image simulations and the frozen phonon approximation. It is found that annular dark field scanning transmission electron microscopy (ADF STEM) images are mostly affected by shear‐planes within a distance ∼6–18 unit cells from the specimen surface, whereas about 5–10 equidistant shear‐planes are required to produce clear differences in HRTEM images. The contrast of the images is affected by the Burgers vector of the slip, but not the slip plane. The simulation results are discussed and compared to experimental data.publishedVersio