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What is Permeability?

The permeability of a rock is the measure of how effectively a fluid will flow through the rock.  The permeability is related to the porosity and the shape, distribution and connectedness of the pores.  That permeability may differ for different fluids / fluid phases – Relative Permeability (ie: the permeability of that rock to a heavy brine will differ to the permeability to a dry gas). 

Permeability can best be resolved by Darcy’s Law:  

q = – k p / u

Where q is the instantaneous flux, k is the permeability of the medium, u is the viscosity of the fluid, and p is the total pressure drop across the distance tested.

There are three main types of permeability to understand. 

1. Absolute permeability – The ability of a single fluid to flow through a rock, when only that one fluid is present.
2. Effective permeability – the effective permeability of a rock is the ability of a fluid phase to flow through the rock in the presence of other fluids.
3. Relative permeability – When considering multiple fluid phases in a rock, relative permeability is a ratio of the effective permeability of that phase to the absolute permeability of the single phase in that rock.

What is Brittleness and Ductility?

All solids can be categorized as either ductile or brittle, as associated to the material’s plastic deformation when it undergoes loading. Ductility is the ability of the solid material to deform plastically under loading while Brittleness is when the material has the tendency to not deform plastically under tensile loading, but instead to fracture / break.

Brittle solids can present ductile behaviours, and ductile solids can present ductile behaviours… dependent on temperature and pressure.  

Examples

• Calcite rocks such as limestone and dolostone – At shallow depths and lower temperatures may exhibit more brittle behaviours, but under higher temperatures and high confining pressures can exhibit more ductile behaviours
• Steel – while cold it is brittle, while warm it is more ductile

As modified from https://www.tulane.edu/~sanelson/eens1110/deform.htm

Brittle materials

• Rocks with high quartz / silica content
• Cast iron, glass, concrete

Ductile materials

• Rocks with high clay content
• Mild steel, tin, aluminum

One should also consider the difference between plastic deformation and elastic deformation of ductile materials.  Plastic deformation will not remain in the deformed state (there may be some elastic rebound), while elastic deformation will go back to / close to the original state. Rocks such as marble and salt formations will behave elastically.  

Also, in the case of fracturing of formations for oil and gas production, the ability of the formation to compress over and close in spite of the presence of proppant (proppant imbedment) should be considered.

What is Stress and Strain?

There are four types of stresses that can act on rocks

1. Confining stress – weight of all the rocks above
2. Compression – forces that act towards each other make up compressional stress… such as at convergent / collisional boundaries of tectonic plates (Juan de Fuca Plate moving under North America, Indian Plate and Eurasian Plate forming the Himalayas…)
3. Tension – forces that are acting to pull the rock apart make up tensional stress, causing rocks to lengthen and / or break… such as at divergent plate boundaries (East African Rift Valley, Red Sea Rift…)
4. Shear – forces that are parallel but in opposite directions make up shear stress… such as at transform plate boundaries (San Andreas Fault in California, Alpine Fault on South Island of New Zealand…)

From the four types of forces above, one can imagine that rocks at different depths in different regions of the world are experiencing different stresses. Much stress data from reliable sources is compiled into the World Stress Map – http://www.world-stress-map.org/

When the stresses act upon a material / rock, they cause that material to strain / deform.  This deformation can be broken down into two categories

1. Ductile deformation
2. Brittle deformation

Look for a follow-up post describing these deformations.

What is Pore Pressure?

Pore pressure is the pressure of fluid in the pore space of rock, and when it exceeds the hydrostatic pressure, an overpressure situation occurs.

Pore pressure definition and different pore pressure regimes (Normal, under and over pressure)

Pore pressure in the sediments can be over pressure, under pressure or normally pressured. Normal pore pressure scenario happens when the pore pressure in the sediment is equivalent to the hydrostatic column of the water at that particular depth. Under pressure can be the outcome of production from that particular formation and depletion. Overpressure scenario can happen by several phenomena and in general can be categorized in two: Type 1 and Type 2 over pressure regimes.

• Type 1 over pressure are related to porosity of the formation
• Type 2 Over pressure are not related to porosity of the formation

Depending on the type of the overpressure mechanism, several methods have been established to handle the estimation of pore pressure such as The ratio method, Eaton’s method, Effective stress methods and Bowers method.Type 1: Sediments are getting less compacted than expected due to several factors:

• Rapid Sedimentation
• Thick shale intervals
• Low permeability
• Highly compressible rock

Type 2: There is no relation between overpressure mechanics evolution and porosity and over pressure could be due to:

• Shale uplift
• Source of fluid
• There is no proper channel that fluid pressure escapes

What is Friction Angle?

For a planar, clean (no infilling) fracture, the cohesion is zero and the shear strength is defined solely by the friction angle. The friction angle of the rock material is related to the size and shape of the grains exposed on the fracture surface. Thus, a fine-grained rock and rock with a high mica content tend to have a low friction angle, whereas coarse-grained rock has a high friction angle.

Angle of Internal Friction

What is Unconfined Compressive Strength?

The unconfined compressive strength (UCS) of sedimentary rocks is a key parameter needed to address a range of geomechanical problems ranging from limiting wellbore instabilities during drilling, to assessing sanding potential and quantitatively constraining stress magnitudes using observations of wellbore failure.

Various applications can be used for the development of rock strength profiles along the wellbore. With testing on core samples, rock strength can be found using rock mechanical laboratory tests. Laboratory tests to find UCS can be done through uniaxial or Triaxial tests. Triaxial tests have to be carried out under different confining stresses to determine the failure criterion of a specific core depth.