Anders Nermoen

Porosity and permeability development in compacting chalks during flooding of nonequilibrium brines: Insights from long‐term experiment

We report the complete chemical alteration of a Liege outcrop chalk core resulting from a 1072 flow-through experiment performed during mechanical compaction at 130°C. Chemical rock-fluid interactions alter the volumetric strain, porosity, and permeability in a nontrivial way. The porosity reduced only from 41.32% to 40.14%, even though the plug compacted more than 25%. We present a novel analysis of the experimental data, which demonstrates that the geochemical alteration does not conserve the volume of the solids, and therefore, the strain is partitioned additively into a pore volume and solid volume component. At stresses beyond yield, the observed deformation can be explained by grain reorganization reducing the pore space between grains and solid volume changes from the rock-fluid interactions. The mechanical and chemical effects are discussed in relation to the observed permeability development.

Evaluation of the compositional changes during flooding of reactive fluids using scanning electron microscopy, nano-secondary ion mass spectrometry, x-ray diffraction, and whole-rock geochemistry

Outcrop chalk of late Campanian age (Gulpen Formation) from Liege (Belgium) was flooded with in a triaxial cell for 516 days under reservoir conditions to understand how the non-equilibrium nature of the fluids altered the chalks. The study is motivated by enhanced oil recovery (EOR) processes because dissolution and precipitation change the way in which oils are trapped in chalk reservoirs. Relative to initial composition, the first centimeter of the flooded chalk sample shows an increase in MgO by approximately 100, from a weight percent of 0.33% to 33.03% and a corresponding depletion of CaO by more than 70% from 52.22 to 14.43 wt.%. Except for Sr, other major or trace elements do not show a significant change in concentration. Magnesite was identified as the major newly grown mineral phase. At the same time, porosity was reduced by approximately 20%. The amount of in the effluent brine remained unchanged, whereas was depleted and enriched. The loss of and gain in are attributed to precipitation of new minerals and leaching the tested core by approximately 20%, respectively. Dramatic mineralogical and geochemical changes are observed with scanning electron microscopy–energy-dispersive x-ray spectroscopy, nano secondary ion mass spectrometry, x-ray diffraction, and whole-rock geochemistry techniques. The understanding of how fluids interact with rocks is important to, for example, EOR, because textural changes in the pore space affect how water will imbibe and expel oil from the rock. The mechanisms of dissolution and mineralization of fine-grained chalk can be described and quantified and, when understood, offer numerous possibilities in the engineering of carbonate reservoirs.

How Stress and Temperature Conditions Affect Rock-Fluid Chemistry and Mechanical Deformation

We report the results from a series of chalk flow-through-compaction experiments performed at three effective stresses (0.5 MPa, 3.5 MPa and 12.3 MPa) and two temperatures (92℃ and and 130℃). The results show that both stress and temperature are important to both chemical alteration and mechanical deformation. The experiments were conducted on cores drilled from the same block of outcrop chalks from the Obourg quarry within the Saint Vast formation (Mons, Belgium). The pore pressure was kept at 0.7 MPa for all experiments with a continuous flow of 0.219 M MgCl2 brine at a constant flow rate; 1 original pore volume (PV) per day. The experiments have been performed in tri-axial cells with independent control of the external stress (hydraulic pressure in the confining oil), pore pressure, temperature, and the injected flow rate. Each experiment consists of two phases; a loading phase where stress-strain dependencies are investigated (approx. 2 days), and a creep phase that lasts for more than 150-160 days. During creep, the axial deformation was logged, and the effluent samples were collected for ion chromatography analyses. Any difference between the injected and produced water chemistry gives insight into the rock-fluid interactions that occur during flow through of the core. The observed effluent concentration shows a reduction in Mg2+, while the Ca2+ concentration is increased. This, together with SEM-EDS analysis, indicates that magnesium-bearing mineral phases are precipitated leading to dissolution of calcite, an observation . This is in-line with other flow-through experiments reported earlier. The observed dissolution and precipitation are sensitive to the effective stress and test temperature. Typically. H, higher stress and temperature lead to increased concentration differences of Mg2+ and Ca2+ concentration changes.. The observed strain can be partitioned additively into a mechanical and chemical driven component.

Competition on the Rocks: Community Growth and Tessellation

Crustose lichen communities on rocks exhibit fascinating spatial mosaics resembling political maps of nations or municipalities. Although the establishment and development of biological populations are important themes in ecology, our understanding of the formation of such patterns on the rocks is still in its infancy. Here, we present a novel model of the concurrent growth, establishment and interaction of lichens. We introduce an inverse technique based on Monte Carlo simulations to test our model on field samples of lichen communities. We derive an expression for the time needed for a community to cover a surface and predict the historical spatial dynamics of field samples. Lichens are frequently used for dating the time of exposure of rocks in glacial deposits, lake retreats or rock falls. We suggest our method as a way to improve the dating.

Evaluation of porosity change during chemo-mechanical compaction in flooding experiments on Liège outcrop chalk

Abstract The mechanical strength, porosity and permeability of chalk are affected by chemical and mineralogical changes induced by fluids that are chemically out of equilibrium with the host rock. Here, two high-porosity Upper Cretaceous chalk cores from Liège were tested at effective stresses beyond yield at 130°C during flooding with MgCl2 and NaCl brines. Core L1 (flooded by MgCl2 brine) deformed more than L2 (flooded with NaCl brine), with volumetric strains of 9.4% and 5.1%, respectively. The porosity losses estimated from strain measurements alone are 5.82% for L1 and 3.01% for L2. However, this approach does not account for dissolution and precipitation reactions. Porosity calculations that are based on strain measurements in combination with (i) the weight difference between saturated and dry cores and (ii) the solid density measurement before and after flooding show an average porosity reduction of 3.69% between the two methods for L1. This discrepancy was not observed for core L2 (with the NaCl brine). The rock and effluent chemistry show that Ca2+ dissolved and Mg2+ is retained within the core for the L1 experiment. Therefore, accurate porosity calculations in chalk cores that are flooded by non-equilibrium brines (e.g. MgCl2) require both the volumetric strain and chemical alteration to be considered.

Towards causally cohesive genotype–phenotype modelling for characterization of the soft-tissue mechanics of the heart in normal and pathological geometries

A scientific understanding of individual variation is key to personalized medicine, integrating genotypic and phenotypic information via computational physiology. Genetic effects are often context-dependent, differing between genetic backgrounds or physiological states such as disease. Here, we analyse in silico genotype–phenotype maps (GP map) for a soft-tissue mechanics model of the passive inflation phase of the heartbeat, contrasting the effects of microstructural and other low-level parameters assumed to be genetically influenced, under normal, concentrically hypertrophic and eccentrically hypertrophic geometries. For a large number of parameter scenarios, representing mock genetic variation in low-level parameters, we computed phenotypes describing the deformation of the heart during inflation. The GP map was characterized by variance decompositions for each phenotype with respect to each parameter. As hypothesized, the concentric geometry allowed more low-level parameters to contribute to variation in shape phenotypes. In addition, the relative importance of overall stiffness and fibre stiffness differed between geometries. Otherwise, the GP map was largely similar for the different heart geometries, with little genetic interaction between the parameters included in this study. We argue that personalized medicine can benefit from a combination of causally cohesive genotype–phenotype modelling, and strategic phenotyping that captures effect modifiers not explicitly included in the mechanistic model.

The Dynamic Stability of Chalks During Flooding of Non-equilibrium Brines and CO2

SUMMARY We have shown how the dynamic evolution of the creep curve depends upon pore fluid composition and CO2 content. The presented experiment were performed on Kansas outcrop chalk which serves as model to understand chemical and physical procesess in chalk reservoirs. The test was performed at in-situ stress, pressure and temperature conditions. The stresses imposed to the chalk was in the plastic regime such that the plug deformed under pore collapse. The creep curve, i.e. deformation at constant stresses, is recognized by a monotonous increasing function with varying rate, dependent upon pore fluid composition. A statistical model is fitted to the different creep periods. Special emphasis is made on how the changes from the inert NaCl-brine to sea water brine (SSW) after 46 days; and from SSW to SSW+ CO2 at 116 days and 130 days altere the creep curve dynamics. Injection of SSW induces a period of accelerating creep whilst the addition of CO2 seems to reduce the observed creep rate. More data is required to support these observations.

Induced Shear Failure by Temperature Reduction at Uni-axial Strain Conditions

This study improvises uniaxial strain condition during cooling by keeping constant overburden, and adjusting radial stress at cooler temperatures in order to re-establish the same radial dimensions prior to cooling. The amount of radial stress reduction by thermal contraction could be sufficient to trigger shear failure. Experiments are performed on Mons chalk and Kansas chalk so the role of induration can be assessed. Calcite thermal expansion is highly anisotropic. Weakening caused by temperature fluctuation could give insight to what gives chalk its strength, cementation, or repulsive electrostatic forces. For each chalk type, shear failure line is determined. The samples are heated to 90oC and loaded to 70% of the axial stress required to induce shear failure. Then the temperature is reduced by 60°C. The change in confining pressure necessary to restore zero radial strain is estimated. The two chalks show different behaviour. Mons demonstrates this cooling would induce shear failure, but has no significant effect on its strength. Kansas, is able to restore uniaxial strain conditions without shear failure. The strength of the Kansas sample was unaffected, however the change in confining pressure needed to restore the uniaxial strain condition decreased with each additional cycle, indicating changing elastic properties.

Elastic and Plastic Behavior of Chalks at Deviatoric Stress Condition: Experiments Performed with Four Different Brines

This paper deals with exploring elastic (bulk modulus and Young’s modulus) and plastic parameters (yield stress, creep and rebound) during deviatoric loading and time-dependent deformation. A series of experiments were carried out at Ekofisk reservoir temperature (130°C) to study the effect of four different fluids, viz., distilled water (DW), NaCl-brine, MgCl2-brine and seawater (SSW), on Mons outcrop chalk. The cores were deviatorically loaded and left to creep at a constant value of 69-73% of the axial yield stress obtained from reference tests with the same brine. Variations in the bulk modulus and Young’s modulus were observed as function of saturation fluid, although the significance of these observations require more data. SSW had the lowest yield stress followed by NaCl and MgCl2, and highest for DW, which conforms the results from earlier studies. The final creep strain was highest for SSW and was 1.3-1.5 times higher than for other brines. The core initially saturated by SSW showed the highest plastic component of the total strain inferring that the ions in SSW does play an important role in inducing permanent damage.

Integrated Approach to CO2 EOR and Storage Potential Evaluation in an Abandoned Oil Field in Czech Republic

The paper presents the results of the experimental and simulation activities of the Czech-Norwegian CO2 Pilot Preparation project (REPP-CO2) carried out under Norway Grants. A relatively small hydrocarbon field located in Vienna basin was selected as a candidate for the CO2-EOR and storage (CCUS) pilot. The field produced in 1950-1970’s, the available reservoir data is somewhat limited and uncertain as typical for old abandoned fields. Nevertheless, based on available geological knowledge, core material and fluid samples (sometimes from the neighboring analog fields) a geological model was build and an integrated approach to evaluation of CO2-EOR and storage (CCUS) potential was suggested. As a first approximation to the CCUS potential, a material balance model was established to evaluate aquifer size and connectivity, as well as potential CO2 storage capacity. The material balance study was based on available production history. Laboratory investigations of available core material and fluid samples allowed to identify and reduce the uncertainties related to fluid properties, geochemistry and geomechanics. An approach was suggested to link core scale geomechanical experiments to the field scale, while addressing the uncertainty in geomechanical parameters in a systematic way. Material balance studies, geological modelling and interpretation of experimental data enabled us to create a simulation model matched to available production and pressure data, therefore laying out a good basis for evaluation of CO2-EOR and storage (CCUS) potential. Simulations taking into account advantages in drilling, monitoring and reservoir technology over four decades since the field abandonment indicated a potential to recover approximately as much oil as was produced from the virgin reservoir. The CO2-EOR is also believed to create a business case suitable for paving the way for the storage project where estimated capacity is up to 1 million tons depending on technical and economic conditions.