Summary: Ppt Lec 2 - Hydraulic Head, Darcy’S Law, And Hydraulic Properties Of Various Rock Types

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  • 1 Part 1: Porosity and storage

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  • Describe the properties of total porosity (n [-]) and write down the formula for n [-]. Include in the description what ratio porosity describes.

    Porosity determines how much water a certain rock/soil can hold, i.e. the ratio of pore space volume (Vpores) to the total volume of the rock sample (Vtotal) . Porosity is usually designated n and is dimensionless: 

    Note that specific yield and specific retention are important qualities of total porosity.
  • There's a graph describing the definition of specific yield (Sy), specific retention (Sr) and effective porosity(n) by sediment blocks and water in their pore spaces. Describe in words what this graph displays and indicate which term belongs to which graph description.

    Effective porosity: when pore space is completely full of water. Then the volume of pores equals effective porosity. 

    Specific retention is when water is retained around the grains after the sediment has fully drained.   

    Specific yield is described by the bucket underneath the specific retention sediment block. It's the volume of water yielded after drainage of the sediment.  

    Or: Specific yield (Sy) is the that part of total porosity (n) directly
    released by gravity drainage.
    Specific retention (Sr) is the remainder.

  • When does specific retention (Sr) decrease in a well sorted aquifer? What does Sy do?

    Sr decreases when grain size increases, whereas Sy increases with increasing grain size.
  • What is the difference between consolidated and unconsolidated rock? What is the general effect on porosity of being con or unconsolidated?

    Compaction and cementation of sediments to the degree that they become coherent, relatively solid rock. Typical consequences of consolidation include an increase in density and acoustic velocity, and a decrease in porosity.
  • Name whether these rocks are (un)consolidated and whether they have high/low porosity and why: 

    See fig
  • 1.1 Porosity types of a range of rock types

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  • Primary porosity is developed at the time the rock was formed. Secondary porosity is the result of processes active after the rock formed. Give two examplex of primary and secondary porosity in rock types that you specify.

    For example: pore space in sedimentary rock,
    vesicles in volcanic rocks.

    Void space between grains in either siliclastic or
    carboniferous sedimentary rocks (unconsolidated or
    Crystalline rock such as igneous and metamorphic
    rocks have hardly any primary porosity (but nevertheless
    can hold considerable amounts of water in mineral assemblages, e.g. biotite)
    Volcanic rocks can have considerable primary
    porosity as a result of the formation of tunnels
    (large-scale) and/or vesicles (small-scale)
    For example: rock dissolution, fracturing, faulting.

    Secondary fracture porosity (tectonic) in sandstone and
    limestone leading to dual porosity/permeability media     

    Dissolution and caveformation in limestone areas
  • In unconsolidated sands and gravels, there's primary porosity of voids between sediment grains (e.g. 0.25-0.40). Draw how the highest/lowest porosity is achieved by specific packing and sorting.

  • Same packing but smaller grainsize doesnot affect total (primary) porosity. True or false? 

  • How does the storage of water in clays change when they get wet? What effect does this have on their porosity?

    Clay minerals have a plate-like shape. Wetted clay stores
    water in between these plates. The bond between water and
    clay minerals and water molecules is strong due to the
    di-pole nature of water molecules.

    Clays and to a lesser extent shale (claystone with a sandy
    fraction) can hold a relative large amount of water in this
    way and therefore are considered to have a high
    porosity (0.6-0.8).
  • What is the primary porosity of crystalline metamorphic and igneous rocks and can you give a reason why?

    Crystalline metamorphic and igneous rocks
    hardly have any primary porosity at all
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