Anette Krogsbøll

Chalk porosity and sonic velocity versus burial depth: Influence of fluid pressure, hydrocarbons, and mineralogy

Seventy chalk samples from four formations in the overpressured Danish central North Sea have been analyzed to investigate how correlations of porosity and sonic velocity with burial depth are affected by varying mineralogy, fluid pressure, and early introduction of petroleum. The results show that porosity and sonic velocity follow the most consistent depth trends when fluid pressure and pore-volume compressibility are considered. Quartz content up to 10% has no marked effect, but more than 5% clay causes lower porosity and velocity. The mineralogical effect differs between P-wave and shear velocity so that smectite-bearing chalk has a high Poissons ratio in the water-saturated case, but a low value in the dry case. Oil-bearing chalk has up to 25 units higher porosity than water-saturated chalk at similar depth but similar velocity, probably because hydrocarbons prevent pore-filling cementation but not pore-structure stiffening cementation in this presumably water-wet chalk. These results should improve the modeling of chalk background velocity for seismic inversion analysis. When describing the porosity-reducing process, pore-volume compressibility should probably be disregarded when correcting for fluid pressure because the cementing ions originate from stylolites, which are mechanically similar to fractures. We find that cementation occurs over a relatively short depth interval.

Permeability, compressibility and porosity of Jurassic shale from the Norwegian–Danish Basin

The Fjerritslev Formation in the Norwegian–Danish Basin forms the main seal to Upper Triassic–Lower Jurassic sandstone reservoirs. In order to estimate the sealing potential and rock properties, samples from the deep wells Vedsted-1 in Jylland, and Stenlille-2 and Stenlille-5 on Sjælland, were studied and compared to samples from Skjold Flank-1in the Central North Sea. Mineralogical analyses based on X-ray diffractometry (XRD) show that onshore shales from the Norwegian–Danish Basin are siltier than offshore shales from the Central Graben. Illite and kaolinite dominate the clay fraction. Porosity measurements obtained using helium porosimetry–mercury immersion (HPMI), mercury injection capillary pressure (MICP) and nuclear magnetic resonance (NMR) techniques on the shale samples show that MICP porosity is 6–10% lower than HPMI or NMR porosity. Compressibility, from uniaxial loading, and elastic wave velocities were measured simultaneously on saturated samples under drained conditions at room temperature. Uniaxial loading tests indicate that shale is significantly stiffer in situ than is normally assumed in geotechnical modelling. Permeability can be predicted from elastic moduli, and from combined MICP and NMR data. The permeability predicted from Brunauer–Emmett–Teller (BET)-specific surface-area measurements using Kozeny’s formulation for these shales, being rich in silt and kaolinite, falls in the same order of magnitude as permeability measured from constant rate of strain (CRS) experiments but is two–three orders of magnitude higher than the permeability predicted from the 1998 model of Yang & Aplin, which is based on clay fraction and average pore radius. When interpreting CRS data, Biot’s coefficient has a significant and systematic influence on the resulting permeability of deeply buried shale.

Constitutive model with time-dependent deformations

Abstract In many geological and engineering problems it is necessary to transform information from one scale to another. Data collected at laboratory scale are often used to evaluate field problems on a much larger scale. This is certainly true for geological problems where extreme scale differences are common in time as well as size. This problem is addressed by means of a new constitutive model for soils. It is able to describe the behavior of soils at different deformation rates. The model defines time-dependent and stress-related deformations separately. They are related to each other and they occur simultaneously. The model is based on concepts from elasticity and viscoplasticity theories. In addition to Hookes law for the elastic behavior, the framework for the viscoplastic behavior consists, in the general case (two-dimensional or three-dimensional), of a yield surface, an associated flow rule and a hardening law. The model is formulated in incremental terms and is therefore suitable for computational modeling and it has been implemented in a computer program used for analyzing the depositional history of an oil field in the Danish part of the North Sea. An important part of the problem in this case was the difference in time scale between the geological process of deposition (millions of years) and the laboratory measurements of mechanical properties (minutes or hours). In addition, the time scale relevant to the production history of the oil field was interesting (days or years).

CDIO Projects in Civil Engineering Study Program at DTU

In 2008 all Bachelor of engineering study programs at the Technical University of Denmark (DTU) have been adopted to the “Conceive – Design – Implement – Operate” approach. As part of the necessary changes it was decided that all seven study programs should have a cross disciplinary project or a design build project on each of the first four semesters. In this paper the four projects in the civil engineering study program are described along with a brief description of the entire study program. The aim is to provide additional information and documentation to accompany an exposition where students present their projects. Learning outcomes, training and assessment of personal, professional and social engineering skills are described from a project point of view. Progression of engineering skills is discussed from a study program perspective. The interrelation between the various elements to the final learning outcome is discussed with respect to the concept of the study program as it is today. Barriers for reaching the ultimate goal, that all students become “engineers who can engineer” at a high technical level, are identified and discussed. It is concluded that the study program has all the potential to prepare students to cope with the challenges in engineering practice, but it also shows that the degree of success depends on the amount of barriers along the way.

Quantifying Porosity, Compressibility and Permeability in Shale

The Fjerritslev Formation in the Norwegian-Danish Basin forms the main seal to Upper Triassic-Lower Jurassic sandstone reservoirs. In order to estimate rock properties Jurassic shale samples from deep onshore wells in Danish basin were studied. Mineralogical analysis based on X-ray diffractometry (XRD) of shale samples show about 50% silt and high content of kaolinite in the clay fraction when compared with offshore samples from the Central Graben. Porosity measurements from helium porosimetry-mercury immersion (HPMI), mercury injection capillary pressure (MICP) and nuclear magnetic resonance (NMR) show that, the MICP porosity is 9-10% points lower than HPMI and NMR porosity. Compressibility result shows that deep shale is stiffer in situ than normally assumed in geotechnical modelling and that static compressibility corresponds with dynamic one only at the begining of unloading stress strain data. We found that Kozeny’s modelled permeability fall in the same order of magnitude with measured permeability for shale rich in kaolinite but overestimates permeability by two to three orders of magnitudes for shale with high content of smectite. The empirical Yang and Aplin model gives good permeability estimate comparable to the measured one for shale rich in smectite. This is probably because Yang and Aplin model was calibrated in London clay which is rich in smectite.

Recovery of Porosity and Permeability for High Plasticity Clays

Stress history is normally evaluated based on an evaluation of geological history and if possible combined with oedometer tests where preconsolidation stress σp and compaction properties are measured. Often the constrained modulus M is used to define the stiffness, and the value will typically depend on the stress state of the sample. The stress state of a sample is typically defined by overconsolidation ratio OCR defined as the ratio between vertical preconsolidation stress and actual vertical stress (OCR = σp/σ0). Janbu (1963), Maine & Kulhawy (1982) and others have published typical relations that describe stress state and constrained modulus as functions of OCR, plasticity index and other quantities. They are all based on the assumption that the soil sample “remembers” previous load levels, and that stiffness of the sample is consequently increased due to the preloading. Definition and applicability of the reconsolidation stress is now questioned when high plasticity calys are considered. Even if it is known, that a clay layer has been exposed to a high stress level, for instance due to the weight of an ice shield, the actual stress level according to that preloading might not be the relevant parameter when estimating stiffness properties for use in basin modeling or other models.

Different Methods of Predicting Permeability in Shale

Permeability is often very difficult to measure or predict in shale lithology. In this work we are determining shale permeability from consolidation tests data using Wissa et al., (1971) approach and comparing the results with predicted permeability from Kozeny’s model. Core and cuttings materials were obtained from Fjerritslev shale Formation in Juassic interval of Stenlille and Vedsted on-shore wells of Danish basin. The calculated permeability from specific surface and porosity vary from 0.09 to 48.53 µD while that calculated from consolidation tests data vary from 1000 µD at a low vertical effective stress to 9 µD at high vertical effective stress of 100 MPa. The indirect permeability calculated from consolidation tests falls in the same magnitude at higher vertical effective stress, above 40 MPa, as that of the Kozeny model for shale samples with high non-clay content ≥ 70% but are higher by two to five orders of magnitudes at lower vertical effective stress below 40 MPa as the content of clay minerals increases causing heterogeneity in shale material. Indirect permeability from consolidation can give maximum and minimum values of shale permeability needed in simulating fluid flow in shale useful in assessing their integrity for CO2 storage, gas shale exploitation and other engineering applications.