The depositional geometry and facies distribution of an Early Miocene (Burdigalian) carbonate system in the Perfugas Basin (NW Sardinia) comprise a well-exposed example of a transition from a ramp to ...a steep-flanked platform. The carbonate succession (Sedini Limestone Unit) is composed of two depositional sequences separated by a major erosional unconformity. The lower (sequence 1) records a ramp dominated by heterozoan producers and the upper (sequence 2) is dominated by photozoan producers and displays a gradual steepening of the depositional profile into a steep-flanked platform. This paper shows the process of creating a digital outcrop model including a facies model. This process consists of combining field data sets, including 17 sedimentary logs, and a spatial dataset consisting of differential global positioning system data points measured along key stratigraphic surfaces and sedimentary logs, with the goal of locking traditional field observations into a 3D spatial model. Establishing a precise geometrical framework and visualizing the overall change in the platform geometry and the related vertical and lateral facies variations of the Sedini carbonate platform, allows us to better understand the sedimentary processes leading to the geometrical turn-over of the platform. Furthermore, a detailed facies modeling helps us to gain insight into the detailed depositional dynamics. The final model reproduces faithfully the depositional geometries observed in the outcrops and helps in understanding the relationships between facies and architectural framework at the basin scale. Moreover, it provides the basis to characterize semiquantitatively regional sedimentological features and to make further reservoir and subsurface analogue studies.
The past two decades of research on Carbon Capture and Storage (CCS) seem to have finally become fruitful as global leaders and energy-intensive industries are cooperating to materialize CCS projects ...and reach the promised reduction in CO2 emissions. Traditionally, CCS projects targeted mostly high permeability sandstone formations, despite the numerous carbonate fields undergoing CO2 injection for Enhanced Oil Recovery (EOR) in the United States or Canada. Because of the reactivity between calcite minerals and CO2 saturated water, chalk formations, characterized by high porosity and low permeability, have been previously portrayed as infeasible CO2 storage sites. Although previous laboratory investigations were carried out to assess the performance of CO2-EOR in North Sea chalk fields, these studies did not result in any field-scale demonstration projects; this may soon change since a positive movement towards CO2 storage in depleted oil fields has been recently initiated. In this work, we reviewed existing studies on CO2 injection in chalk to address the suitability of this type of formation for CCS. Although the evidence on the thermo-hydro-mechanical-chemical behaviour of chalk in the presence of CO2-saturated aqueous solutions is mixed, the majority of flooding tests performed on reservoir core samples do not support further weakening relative to water injection conditions nor significant changes in the petrophysical properties. Along with the weakening effect and using the Danish North Sea chalk fields as a case study, we addressed events that impact the storage site safety such as fault reactivation, and caprock and well integrity. Furthermore, monitoring techniques relevant to offshore locations are also discussed. Based on studies on other types of carbonates, and considering the characteristics of chalk (e.g., permeability, wettability, and reactivity) we analysed the relevance of different trapping mechanisms (i.e., solution, capillary, and mineral) but also several effects (i.e., chemical, biological, mechanical) that can lead to loss of injectivity. The main observations and conclusions in this work can be easily extrapolated to other chalk formations worldwide.
•A growing number of countries are contemplating CO2 storage to reduce CO2 emissions•Existing research largely leans towards CO2 storage in highly permeable formations•Global deployment of CO2 storage can benefit by diversifying the lithology options•Porous chalk formations can represent reliable CO2 storage sites.
The locus and shape of the yield surface of chalk in the stress space represent key input parameters in geomechanical models, as it dictates the onset and amount of plastic deformation. The study ...aims at reconstructing for the first time the initial end cap and shear failure line of intact reservoir chalk in plots of mean‐deviatoric stress versus porosity. One hundred and thirty‐seven stress‐strain measurements describe the effects of initial porosity between 30% and 45% and initial water saturation on the yield surface in order to cover a wide spectrum of reservoir rocks. The physico‐chemical processes taking place at pore collapse under oil‐ and water‐saturated conditions are discussed. The experimental data collected from previous works quantify the hardening and softening effects observed with decreasing porosity and increasing water saturation. The results also indicate that the stress interval characterizing the elastic to elasto‐plastic transition zone and the curvature of the end cap are dependent on porosity and quartz content. These observed trends suggest changes in the sequence of local failures acting in the rock matrix at the onset of compaction. Comparisons with previous studies suggest that the water weakening effect differs between outcrop and reservoir chalk likely due to a change in mineralogy. A yield function is proposed to build the yield surface and to capture the porosity‐dependent change in the shape of end cap and the water weakening effect. The outcomes provide key input data describing the mechanical behavior and properties of chalk under laboratory conditions required for compaction simulation studies.
Key Points
Yield surface of 30%–45% porosity chalk at 0% and 100% water saturation is reconstructed in plots of mean‐deviatoric stress versus porosity
Yield functions considering the water weakening effect are proposed to model a wide spectrum of compacting clean chalks
Key mechanical properties of the North Sea chalk for compaction studies
This paper examines the nature of interpersonal risk, its propagation and how risk aversion can act as a barrier to learning and knowledge translation for innovation. A study of the social relations ...between a top management team and director-level staff in a high-reliability organisation frames the argument that the team's risk mitigation processes concerning new proposals are a factor in producing and maintaining a risk-averse corporate culture. This condition impedes organisational learning and knowledge processes when staff adjust their presentations to reduce risk exposure associated with new ideas, uncertainty and untested sense-making. A related risk to the firm arises when employees find presenting innovative proposals too risky, and cease making presentations on key corporate initiatives. The 'precautionary principle' is suggested as one factor that contributes towards risk-aversion in the firm's culture.
This contribution shows how unsupervised Markovian segmentation techniques can be accelerated when implemented on graphics hardware equipped with a Graphics Processing Unit (GPU). Our strategy ...exploits the intrinsic properties of local interactions between sites of a Markov Random Field model with the parallel computation ability of a GPU. This paper explains how classical iterative site-wise-update algorithms commonly used to optimize global Markovian cost functions can be efficiently implemented in parallel by fragment shaders driven by a fragment processor. This parallel programming strategy significantly accelerates optimization algorithms such as ICM and simulated annealing. Good acceleration are also achieved for parameter estimation procedures such as K-means and ICE. The experiments reported in this paper have been obtained with a mid-end, affordable graphics card available on the market.
This paper shows how Markovian segmentation algorithms used to solve well known computer vision problems such as motion estimation, motion detection and stereovision can be significantly accelerated ...when implemented on programmable graphics hardware. More precisely, this contribution exposes how the parallel abilities of a standard Graphics Processing Unit (usually devoted to image synthesis) can be used to infer the labels of a label field. The computer vision problems addressed in this paper are solved in the maximum a posteriori (MAP) sense with an optimization algorithm such as ICM or simulated annealing. To do so, the fragment processor is used to update in parallel every labels of the segmentation map while rendering passes and graphics textures are used to simulate optimization iterations. Results show that impressive acceleration factors can be reached, especially when the size of the scene, the number of labels or the number of iterations is large. Hardware results have been obtained with programs running on a mid-end affordable graphics card.
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