In recent years, several studies have indicated the impact of nanoparticles (NPs) on various properties (such as viscosity and fluid loss) of conventional drilling fluids. Our previous study with commercial iron oxide NPs indicated the potential of using NPs to improve the properties of a laboratory bentonite-based drilling fluid without barite. In the present work, iron oxide NPs have been synthesized using the co-precipitation method. The effect of these hydrophilic NPs has been evaluated in bentonite and KCl-based drilling fluids. Rheological properties at different temperatures, viscoelastic properties, lubricity, and filtrate loss were measured to study the effect of NPs on the base fluid. Also, elemental analysis of the filtrate and microscale analysis of the filter cake was performed. Results for bentonite-based fluid showed that 0.019 wt% (0.1 g) of NPs reduced the coefficient of friction by 47%, and 0.0095 wt% (0.05 g) of NPs reduced the fluid loss by 20%. Moreover, for KCl-based fluids, 0.019 wt% (0.1 g) of additive reduced the coefficient of friction by 45%, while higher concentration of 0.038 wt% (0.2 g) of NPs shows 14% reduction in the filtrate loss. Microscale analysis shows that presence of NPs in the cake structure produces a more compact and less porous structure. This study indicates that very small concentration of NPs can provide better performance for the drilling fluids. Additionally, results from this work indicate the ability of NPs to fine-tune the properties of drilling fluids.

Understanding the chalk-fluid interactions and the associated mineralogical and mechanical alterations on a sub-micron scale are major goals in Enhanced Oil Recovery. Mechanical strength, porosity, and permeability of chalk are linked to mineral dissolution that occurs during brine injections, and affect the reservoir potential. This paper presents a novel “single grain” methodology to recognize the varieties of carbonates in rocks and loose sediments: Raman spectroscopy is a non-destructive, quick, and user-friendly technique representing a powerful tool to identify minerals down to 1 µm. An innovative working technique for oil exploration is proposed, as the mineralogy of micron-sized crystals grown in two flooded chalk samples (Liége, Belgium) was successfully investigated by Raman spectroscopy. The drilled chalk cores were flooded with MgCl₂ for ca. 1.5 (Long Term Test) and 3 years (Ultra Long Term Test) under North Sea reservoir conditions (Long Term Test: 130 °C, 1 PV/day, 9.3 MPa effective stress; Ultra Long Term Test: 130 °C, varying between 1–3 PV/day, 10.4 MPa effective stress). Raman spectroscopy was able to identify the presence of recrystallized magnesite along the core of the Long Term Test up to 4 cm from the injection surface, down to the crystal size of 1–2 µm. In the Ultra Long Term Test core, the growth of MgCO₃ affected nearly the entire core (7 cm). In both samples, no dolomite or high-magnesium calcite secondary growth could be detected when analysing 557 and 90 Raman spectra on the Long and Ultra Long Term Test, respectively. This study can offer Raman spectroscopy as a breakthrough tool in petroleum exploration of unconventional reservoirs, due to its quickness, spatial resolution, and non-destructive acquisition of data. These characteristics would encourage its use coupled with electron microscopes and energy dispersive systems or even electron microprobe studies.

Water weakening is a phenomenon that is observed in high porosity chalk formations. The rock interacts with ions in injected water and additional deformation occurs. This important effect needs to be taken into account when modelling the water flooding of these reservoirs. The models used on field scale are simple and only model the effect as a change in water saturation. In this paper, we argue that the water weakening effect can to a large extend be understood as a combination of changes in water activity, surface charge and chemical dissolution. We apply the de Waal model to analyse compaction experiments, and to extract the additional deformation induced by the chemical interaction between the injected water and the rock. The chemical changes are studied on a field scale using potential flow models. On a field scale, we show that the dissolution/precipitation mechanisms studied in the lab will propagate at a much lower speed and mainly affect compaction near the well region and close to the temperature front. Changes in surface charge travel much faster in the reservoir and might be an important contributor to the observed water weakening effect. We also discuss how mineralogical variations impacts compaction.

Chalk is a very fine-grained carbonate and can accommodate high porosity which is a key characteristic for high-quality hydrocarbon reservoirs. A standard procedure within Improved Oil Recovery (IOR) is seawater-injection which repressurizes the reservoir pore pressure. Long-term seawater-injection will influence mineralogical processes as dissolution and precipitation of secondary minerals. These secondary minerals (<1 micrometer) precipitate during flooding experiments mimicking reservoir conditions. Due to their small sizes, analysis from traditional scanning electron microscopy combined with energy dispersive X-ray spectroscopy is not conclusive because of insufficient spatial resolution and detection limit. Therefore, chalk was analyzed with high-resolution imaging by helium ion microscopy (HIM) combined with secondary ion mass spectrometry (SIMS) for the first time. Our aim was to identify mineral phases at sub-micrometer scale and identify locations of brine–rock interactions. In addition, we wanted to test if current understanding of these alteration processes can be improved with the combination of complementary imaging techniques and give new insights to IOR. The HIM-SIMS imaging revealed well-defined crystal boundaries and provided images of excellent lateral resolution, allowing for identification of specific mineral phases. Using this new methodology, we developed chemical identification of clay minerals and could define their exact location on micron-sized coccolith grains. This shows that it is essential to study mineralogical processes at nanometer scale in general, specifically in the research field of applied petroleum geology within IOR.

This experiment was designed to study the water-weakening effect of artificially fractured chalk caused by the injection of different compositions of brines under reservoir conditions replicating giant hydrocarbon reservoirs at the Norwegian Continental Shelf (NCS). NaCl, synthetic seawater (SSW), and MgCl_2, with same ionic strength, were used to flood triaxial cell tests for approximately two months. The chalk cores used in this experiment originate from the Mons basin, close to Obourg, Belgium (Saint Vast Formation, Upper Cretaceous). Three artificially fractured chalk cores had a drilled central hole parallel to the flooding direction to imitate fractured chalk with an aperture of 2.25 (±0.05) mm. Two additional unfractured cores from the same sample set were tested for comparison. The unfractured samples exposed a more rapid onset of the water-weakening effect than the artificially fractured samples, when surface active ions such as Ca²⁺, Mg²⁺ and SO₄²⁻ were introduced. This instant increase was more prominent for SSW-flooded samples compared to MgCl₂-flooded samples. The unfractured samples experienced axial strains of 1.12% and 1.49% caused by MgCl₂ and SSW, respectively. The artificially fractured cores injected by MgCl₂ and SSW exhibited a strain of 1.35% and 1.50%, while NaCl showed the least compaction, at 0.27%, as expected. Extrapolation of the creep curves suggested, however, that artificially fractured cores may show a weaker mechanical resilience than unfractured cores over time. The fracture aperture diameters were reduced by 84%, 76%, and 44% for the SSW, MgCl₂, and NaCl tests, respectively. Permeable fractures are important for an effective oil production; however, constant modification through compaction, dissolution, and precipitation will complicate reservoir simulation models. An increased understanding of these processes can contribute to the smarter planning of fluid injection, which is a key factor for successful improved oil recovery. This is an approach to deciphering dynamic fracture behaviours.

Variation of electron-induced x-ray emission with the crystal direction of an incoming electron beam (channeling) is used to study the crystal site preference of different elements in a promising thermoelectric half-Heusler material. This material has Sb, Co and Ti as the main constituting elements, with lower contents of Hf, Zr and Sn. It is confirmed experimentally, in accordance with previous assumptions, that Sb and Sn occupy one of the octahedral sites, 4a (0, 0, 0), Ti, Zr and Hf the other octahedral site, 4b (½, ½, ½), and Co atoms occupy the tetrahedral site, 4c (¼, ¼, ¼) in the cubic spacegroup F$\overline{4}$ 3m with lattice constant a = 0.598 ± 0.001 nm. Furthermore, a strong asymmetry along the polar directions is observed, which is as it should be in a half-Heusler material, when the Co atoms have a strong preference for one of the two sets of tetrahedral sites. Such asymmetries can not be observed by kinematical diffraction according to Friedel’s law, thus demonstrating that the observation of element characteristic signals under channeling conditions is a unique technique for determining the sense of polar directions and distinguishing crystal symmetry alternatives in small crystal grains and nanocrystals.