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Porosity and Permeability

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Porosity and Permeability

The vast majority of geological materials can hold water to a greater
or lesser extent, the amount of water if affected by the materials
porosity and permeability. Porosity is a parameter that describes the
amount of open space in geologic material, it can be stated as either
a fractional value or percentage of the material that is open space.
This open space is not however a void like the interior of a balloon
it is more like a sponge with small air bubbles and interconnecting
pores. These open pore spaces occur between sediment grains, in cracks
or fractures and also on a larger scale in cavernous openings formed
by dissolution of rock. The porosity values of rock typically range
from 0-50%, this space is often filled with water or air or mixture of
both.

Permeability is closely linked to porosity as permeability is a
parameter that describes the ability of geological material to
transmit water. Permeability is measured in Millidarcy (MD) or Darcies
(D). A Millidarcy is 1/1000 of a Darcy. A Darcy is the permeability
that will allow a flow of 1 cubic centimetre per second of a fluid
with 1 centipoise viscosity through a distance of 1 centimetre through
an area of 1 square centimetre under a differential pressure of 1
atmosphere. The greater the permeability of a rock, the easier it is
for fluids to flow through it. Typical permeability values may range
from 1 to 10 MD (poor permeability), 10 to 100 (good), and 100 to 1000
(excellent). However permeability is about the connected pores in
rocks, If for example a rock had a high porosity it could still have a
low permeability if the pores were not connected so water couldn't
enter them. Aquifers are geological material which transmit large
quantities of water, this meaning they have a high permeability. The
opposite to an aquifer is an aquatard these have a low permeability.

The porosity and permeability of rocks and other geologic materials is
hugely important in several fields such as water supply, damming,
mining, oil reservoirs and civil engineering. I intend to study the
effects of porosity and permeability on oil traps and reservoirs
before investigating myself the way it effects a range of rock types.

Oil reservoirs and traps

Oil is classed as a geological liquid this in essence means that it is
a liquid found in rock. Oil is found within the pores of sedimentary
rocks, mainly sandstone and course grained limestone's, It stays
trapped in these rocks for millions of years until it is extracted.
Oil is made up of primarily dead microscopic sea organisms, these
organisms died and sank to the bottom of the sea where they settled
along with sedimentary deposits, over time the amount of dead
organisms on the sea bottom increased and as they decomposed they
formed an 'ooze' like substance as they were mixed up with sand and
sedimentary deposits. This ooze was then covered by further layers of
shale and other depositants over millions of years which meant that
ooze came under a high pressure and also due to the earths internal
temperature a high temperature, this mix of high temperature and
pressure leads to the decaying organisms being denatured and over a
long time period turning into hydrocarbons; oil and natural gas. These
hydrocarbons started of as small globules between particles of shale
as shown in diagram;

Grey = shale

Blue= water and/or air

Black = hydrocarbons

[IMAGE]

But as pressure is increased by the weight of overlying sediment the
hydrocarbons were forced sideways for miles in some cases , the
majority of this hydrocarbon were dispersed either as gases to the
surface or remain in very low quantities spread over a large area.
However some of the hydrocarbons filtered into rock formations like
sandstone and limestone where they became trapped thus forming a
reservoir of oil.

To form such a reservoir the oil must be trapped, to prevent further
movement etc. the two main types of trap are structural traps where
the rocks have been deformed or faulted in some way and straigraphic
traps where the oil is trapped due to porosity of different rock
types.



Structural traps
================

Anticlines - If a permeable rock like a sandstone or limestone is
sandwiched between impermeable rock layers like shales or mudstones,
and the rocks are folded into an anticline, hydrocarbon can migrate
upward in the more permeable reservoir rocks, and will occur in the
hinge region of the anticline. Since anticlines in the subsurface can
often be found by looking at the orientation of rocks on the surface,
anticlinal traps were among the first to be exploited by petroleum
geologists.

[IMAGE]




Faults - If faulting can move permeable and impermeable rocks so that
the permeable rocks always have impermeable rocks above them, then an
oil trap can form. Since faults are often exposed at the Earth's
surface, the locations of such traps can often be found from surface
exploration.

[IMAGE]

Salt Domes - During the Jurassic Period, the Gulf of Mexico was a
'sealed' sea. This resulted in high evaporation rates & deposition of
a thick layer of salt on the bottom of the basin. The salt was
eventually covered with normal sediments. But salt has a lower density
than most sediments and is more ductile than most sedimentary rocks.
Because of its low density, the salt moved upward through the
sedimentary rocks as salt the sedimentary rocks as salt domes. The
intrusion of the salt deforms the sedimentary strata along its
margins, folding it upward to create oil traps. Because some salt
domes get close to the surface, surface sediments overlying the salt
dome are often domed upward, [IMAGE]making the locations of the
subsurface salt and possible oil traps easy to locate.



Stratigraphic Traps
===================

Unconformities - An angular unconformity might form a suitable oil
trap if the layers above the unconformity are impermeable rocks and
permeable rocks layer are sandwiched between impermeable layers in the
inclined strata below the unconformity. This type of trap is more
difficult to locate because the unconformity may not be exposed at the
Earth's surface. Locating possible unconformity traps like this
usually requires subsurface exploration techniques, like drilling
exploratory wells or using seismic waves to see what the structure
looks like.

[IMAGE]




Lenses - Layers of sand often form lens like bodies that pinch out. If
the rocks surrounding these lenses of sand are impermeable and if
deformation has produced inclined strata, oil and natural gas can
migrate into the sand bodies and will be trapped by the impermeable
rocks. This kind of trap is also difficult to locate from the surface,
and requires subsurface exploration techniques.

[IMAGE]

Reservoir Rock

A reservoir rock is one which has a high permeability and therefore
usually a moderately high porosity. It is in rocks with these
properties that oil can remain in the pores if the reservoir rock is
in a oil trap situation. Common reservoir rocks are sandstone and
limestone, these are both sedimentary rocks, these are good reservoir
rocks as they have a high permeability due to having a high porosity
as there are lots of gaps between grains.

Igneous rocks, like granite, do not have as higher porosity or
permeability this is because they are made of interlocking mineral
crystals. (see diagrams)

Igneous rock

Sedimentary rock



[IMAGE]




It is clear to see from diagrams as basic as these that there are
considerable more pores(shown in yellow) in a sedimentary rock than an
igneous rock. However to form a rock sediment grains need to be
cemented together, this cement can effect the porosity and
permeability and therefore the sediments effectiveness as a
sedimentary rock.

Impermeable rocks like the igneous rock shown are very important in
the creation of oil traps as without them the hydrocarbons would
disperse or evaporate leaving them in very low concentrations in the
reservoir rocks.



Impermeable rock
================

Impermeable rocks have either a very low or no permeability and a low
porosity, this is generally due to there mineral structure with
impermeable rocks often being igneous with interlocking crystals or
being extremely well cemented sedimentary rocks, where the cement
'clogs' up the pores between grains.



Factors affecting rocks porosity and permeability
=================================================

After looking at the structures of various rock types I have concluded
that there are 3 main factors effecting porosity and more importantly
to petroleum geologists permeability;

* Grain size- the larger the grains are, it is logical to conclude
that the larger the pores between them will be. Also the sorting
of grains as if they are poorly sorted smaller particles could
fill up pores between larger grains.

* Rock type- the rock type will effect the type of bonds between
minerals etc (e.g. interlocking crystals) and the type/ amount of
bonds would effect the porosity and permeability.

* Amount of cementation- this effects sedimentary rocks, but as this
is the most common type of reservoir rock this is very important
to geologists, if a rock has a higher percentage of cementation
then I have concluded that it will have a lower porosity as the
cement would be filling the pores.

I intend to perform three different basic experiments to see to what
extent my hypotheses are correct.



Experiment 1; How does rock type affect porosity?
=================================================

In this experiment I intend to study how different rock types affect
porosity. I want to see how much water can enter a rock through its
surface as this is important to geologists when studying reservoir
rocks in oil traps. I intend to use a very basic method, which is as
follows;

Equipment and justification for usage;

* Deep tray- so that the rock samples are completely submerged in
water so that the porosity is for the whole of the sample and not
just for the partially submerged section.

* Water( 5 litres per repeat)- this is a the most common geological
liquid, it is easily available and is non toxic so it does not
present a health hazard

* Scientific balance- to give an accurate measurement to 0.1 g, this
is important as the changes in mass may not be major

* Rock samples from the 3 rock groups(sedimentary, igneous and
metamorphic)- so I can study the porosity of the different rock
groups as they are of differing structures and compositions, also
using multiple samples from the different groups will make it
easier to see trends in the rock groups porosity.

* Stopwatch- to ensure accuracy in the timing and therefore
encourage fair testing.

* Paper towels- cheap, disposable method of removing surface water
without affecting the water contained in the inner pores.

Method;

1. The tray should be filled with enough water to cover the sample

2. The rock sample is weighed on the balance and the mass, in grams,
is recorded

3. The sample is then submerged in the water for 5 minutes

4. The sample is removed surface water is dried off with a paper
towel the mass is then measured again. This will be repeated 3
times for each rock sample.



Safety
======

This experiment has few risks and hazards associated with it, I will
still however make sure that all standard lab safety rules are
followed. As some of the samples I am using are of a reasonably high
mass I will be careful not to drop any as this may result in injury.



Fair testing
============

To achieve a fair test and therefore limit the chances of anomalous
results I will do the following;

* Use a stopwatch so that all the samples are submerged for the same
time period (5 min) I will start the stopwatch when the rock is
fully submerged and remove the rock at exactly 5 minutes.

* I will repeat the test 3 times for each rock sample so that I can
get an average % porosity, this will mean that if any of my
results are slightly anomalous the average will give a truer
porosity for the sample as the multiple repeats will diminish the
significance of anomalies from individual results.

* I will use the same quantity of water for each repeat (5 litres)
this will mean that all the testate's will have the same water
pressure on them making the test fairer than if the amounts of
water was different for each repeat.

* When drying the samples of surface water I will dry them all the
same amount by wrapping the sample in a paper towel and then
removing it as quickly as possible.

* I will use fresh water for each repeat as some substances may
dissolve into or be suspended in the water during the test, if I
reused this water it may effect

Simple as this method of measuring porosity is I feel it will be
sufficient to give an indication of the differing porosities of
different rock types.

I think that sedimentary rocks will have the highest porosity and
igneous rocks the lowest and metamorphic rocks in between them. I have
concluded this by looking at the rocks differing structures; a
sedimentary rock is made of cemented grains this means there are gaps
between grains and cement. Igneous rocks are crystalline and made of
interlocking mineral crystals therefore I think there porosity will be
low as, these crystals are bonded very tightly to each other, the gaps
present will not be as big or as numerous as between the grains in
sedimentary rock.



Results
=======

I have carried out my experiment as stated previously in the tables
below are my results;

Name

Mass before (g)

Mass after (g)

% Mass increase

% Average porosity

Observations

Conglomerate (well sorted)

350.3

414.4

18.30

18.3

Conglomerate (poorly sorted)

386

449.7

16.50

13.84

Pieces fell into water

Conglomerate (poorly sorted)

492

547

11.18

Chalk

53.4

66.75

25.00

22.84

Rock went structurally weak

Chalk

72.6

87.4

20.39

Water went cloudy

Chalk

106.7

131.4

23.15

Shale

176.5

231.2

30.99

30.99

Rock fell apart

Sandstone

109.5

125.9

14.98

15.57

Pieces fell into water

Sandstone

127.6

149.9

17.48

Sandstone

81.4

93

14.25

Well cemented Quartzite

59.3

61.8

4.22

2.88

Well cemented Quartzite

82.4

83.9

1.82

Well cemented Quartzite

107.3

110.1

2.61

Slate

89.9

90.89

1.10

1.29

Slate

127.3

129.2

1.49

Slate

77.8

78.8

1.29

Schist

227.9

229.9

0.88

-0.02

Schist

280.2

287.2

2.50

Schist

154.6

149.3

-3.43

Gneiss

226.3

229.9

1.59

1.64

Gneiss

59.6

61.2

2.68

Gneiss

126.5

127.3

0.63

Gabbro

289

293.6

1.59

2.65

Gabbro

154

162.2

5.32

Gabbro

327

330.4

1.04

Weathered Gabbro

517

612

18.38

9.91

Rock went very structurally weak

Weathered Gabbro

376

398.9

6.09

Weathered Gabbro

296

311.6

5.27

Dolerite

106.5

108.3

1.69

1.73

Dolerite

225

227.8

1.24

Dolerite

216.5

221.4

2.26

Basalt

157.2

160

1.78

1.54

Basalt

222.3

224.9

1.17

Basalt

89.7

91.2

1.67

Obsidian

58.2

58.2

0.00

-3.19

Obsidian

50.5

50.2

-0.59

Obsidian

88.1

80.2

-8.97

The results highlighted in yellow are those that I think are clearly
anomalous, these anomalies are most likely due to errors in either
reading or the copying down of results.

Evaluation of experiment 1

I feel that my results do support my original hypothesis broadly
speaking they are not however totally conclusive, below I have listed
the problems which I either encountered during the experiment or
realised after;

* When the rocks are removed from the water they are losing water
and therefore mass as it runs out of the pores, it also loses mass
as its dried on the paper towel. This could reduce the % porosity

* The permeability being measured is the effective permeability
instead of the actual permeability, however generally in petroleum
geology effective permeability is more important.

* The rocks have surface water present, this is particularly
noticeable on the rougher rocks, this surface water would
unnaturally increase the mass and therefore make my results show
wrong % porosity.

* Several of my observations comment on cloudy water or the rock
breaking down, this is due to the gases already in the pores
creating a resistant pressure to the water as the gases are unable
to escape they could widen micro fractures around the pores
increasing permeability and possibly resulting in the rock
disintegrating. This would not only make it very hard to measure
the mass but would also mean that the rocks porosity has been
affected.

* There may have been structural changes to the rock due to its
complete emersion under low pressure, Calcium Carbonate (chalk)
for example went very weak and parts became suspended in the
water, yet in situ below the surface chalk beds do not
disintegrate with jointed chalk acting as an aquifer in some
cases, this is possible as the rock is under pressure due to
overburden etc, therefore my experiment is in fact testing
porosity out of situ (e.g. under different pressure) therefore
rocks properties may be different due to the environment its
tested in.

* The samples differing sizes and shapes may effect my results even
if they are in average % porosity as the size and shape of the
sample could mean that the rocks structure is different to that of
the rock in situ also this may be unfair if all the rocks of one
type for example are significantly smaller than samples of another
type

If I were to repeat this experiment there are several things that I
would change about the way that I carried it out these are listed
below;

* Performing the experiment in a vacuum as this would solve the
problem of air traps/pressure in the rock, practically however it
would be very hard to set up and perform the experiment in vacuum
without large industrial laboratory equipment.

* The use of water as the testing medium has caused several
problems, these are primarily to do with waters surface tension
which can form air traps in pores, one method of solving this
problem is by using mercury, which has very little surface tension
so it can fill all the pores giving a truly accurate effective
porosity, this effective porosity would however only be relevant
to mercury which isn't a common geological liquid.

* To be a totally fair test the rock samples should be of the same
shape and size, this could be achieved by using central sections
from a core bored from the rock.

There are numerous other potential solutions to the major problems
which are shown in this experiment, I intend to detail these in my
over all evaluation of the investigation.

Experiment 2; How does grain size affect the permeability of sediment?

In this investigation I intend to investigate if grain size affects
the permeability of sediment, I predict that the larger the grain
size, the larger the pores will be and therefore the higher the
permeability, this is shown in the diagrams below. Despite the
stylisation of these diagrams I feel they do still show that the
larger the grains the larger the pores would be meaning a higher
permeability (although not necessarily a higher % porosity).

[IMAGE]




To prove this hypothesis correct I intend to perform the following
simple experiment;

Equipment and justification for usage;

· 4 litre plastic drinks bottle- allows a large quantity of sediment
and also has space at the top for a reservoir of water.

· 30 cm3 of 5 different sizes of sediment ranging from 0.2-1cm- this
range of grain sizes is quite common in sedimentary rocks, it is also
coarse enough for the measurement of grains to be accurate, which will
encourage a fair test.

· Plastic funnel -to ensure water is not lost during pouring and to
act as an upper reservoir

· Clamp stand with boss and clamp - to hold the apparatus steady
during testing so that kinetic energy is not altering the results.

· Stopwatch- to measure time accurately

· Measuring cylinder- to make sure that the amount of water is the
same for each repeat

· Water (2 litres per repeat)- this is a the most common geological
liquid, it is easily available and is non toxic so it does not present
a health hazard

· Tights- to act as a filter preventing sediment leaving the bottle
but allowing water to pass through it, they are cheap, commonly
available and easy to fix to the bottle.

· Sellotape- to keep a tight seal so that no sediment can leave the
bottle during testing

[IMAGE]Method;

· The equipment should be set up as shown, with one grain size of
sediment in the bottle

· Water should then be poured into the funnel, timing should begin
when the water makes contact with the top of the sediment

· The water should be poured in as quickly as possible

· When there is no longer any surface water on the top of the sediment
and when the flow at the bottom subsides to drips timing should stop
and the result in seconds should be recorded.

· This should be repeated 4 times for each grain size

· Then the experiment should be repeated using a different grain size

Safety

This experiment has few risks and hazards associated with it, I will
still however make sure that all standard lab safety rules are
followed. I will make sure that the clamp stand is secured firmly so
that the apparatus will not fall over when the centre of mass is
changed by the addition of water. I will also position the experiment
over a sink so that the water that has passed through the sediment
does not form a slip/fall hazard on the floor.

Fair testing

To achieve a fair test and therefore limit the chances of anomalous
results I will do the following;

· I will repeat the test 4 times for each grain size so that I can get
an average time taken, (which will be an indication of the grain sizes
permeability) this will mean that if any of my results are slightly
anomalous the average will give a truer time for water flow through
the sample as the multiple repeats will diminish the significance of
anomalies from individual results.

· I will pour the same quantity of water (2 litres) into the sediment
each time this will mean that this is consistent for each grain size
reducing the chance of anomalous results.

· The same quantity of sediment will be used for each test; this means
that the volume of sediment will not affect my results as it is
constant for each experiment.

· The sediments I am using are brought from a reputable commercial
supplier; this means that the sizes should be consistent and accurate.

· The large container I am using as a sediment barrel and the use of a
funnel should create 2 reservoirs of water(1 in the funnel and 1 on
top of the sediment in the bottle) this should give a consistent flow
of water, this is vital for the experiment to be fair as the timing
starts when the water makes contact with the sediment not when it has
all been poured out of the measuring cylinder.



Results;
========

I carried out the experiment as I stated above below are the results;

Time taken (seconds)

Name of rock

Grain size(mm)

Repeat 1

Repeat 2

Repeat 3

Repeat 4

Average

White sand

0.2

840.6

917.4

903.5

898.3

889.95

cactii gravel

2

101.37

34.61

32.68

35.42

51.02

Aggregate

5

38.2

36.16

32.18

31.5

34.51

Aggregate

7

31.01

32.28

33.66

31.85

32.20

Aggregate

10

33.26

33.61

38.35

38.83

36.01



Experiment 2 evaluation
=======================

After completing my experiment I have decided that there are several
issues which have arisen from my simple method which may effect my
results I have detailed these below;

* The volume of water passing through the sediment is unknown as it
is not collected this means that it is impossible to work out
accurately the Darcy of the sediment as there may be water
remaining in the pores after the experiment

* Before the water is added there will be air trapped in the
sediments pores as water starts to flow through the sediment it
may not be able to move through all of the pores as air traps
could form due to the surface tension of water

Water,trapped air

*

* Water could potentially move down the sides of the sediment barrel
as it is not very wide so water could flow down the sides where
gaps would be present due to the difference in texture of the
smooth plastic and the coarse sediment, this means the
permeability given is not a true permeability of the sediment size
as it is effected by the container.

* The materials used for each grain size were different this could
have an effect on permeability as the sediment grains are not only
of differing materials so they would have different properties in
there relation to there attraction or repulsion of water they also
would be of different shapes with for example a particle from a
beach is likely to be rounded by its transportation in water,
whereas a particle from a scree slope is more likely to be angular
as it is transported by gravity and forms due to brittle
deformation.

* As the same sediment is used for each of the repeats of the
experiment the sediment noticeably reduces in volume as it
compacts, this means the pores are getting smaller as the water
flow/mass effects the packing of the grains. This would obviously
effect the permeability.

* The waters movement is effected by five main factors; the funnel,
sediment, filter, bottles shape and the water flow into the
bottle. Of these only one should be a variable the sediment. The
funnel and the top of the bottle act as reservoirs giving a
regular flow of water through out the experiment. The water flow
into the water is effected by the person pouring the water in,
this is significant as if the flow into the funnel is not constant
then the water in the "reservoirs" will be of differing masses,
the pressure from this would be different which would effect the
rate of water flow through the sediment. The bottles shape is also
effecting the permeability of the sediment, as it effects the rate
of water flow, where the water leaves the bottle acts as a bottle
neck, this means that although all the results are equally
effected by this it means that any calculations performed using
the experiment results are not going to be correct. The filter may
also effect the rate of water flow as it may be smaller than the
pores in the sediment, especially if the sediment is coarse in
grain size, this would mean that the filter would slow the rate of
flow effecting my results.

* As I measured the sediment by volume there will be more grains for
a smaller grain size in 30cm3 than for a larger grain size, this
could affect my results as there are more grains for the water to
move through in a smaller grain size than a larger one, this means
that potentially its actually the number of grains which is the
controlled and therefore my method may be generally flawed as my
aim was to investigate grain size.

* My experiment is only investigating the permeability of the
sediment in relation to water, this is not the most economically
important geological liquid compared to oil or gas. This
significantly reduces the value of experiment 2, as it is not
necessarily relevant to my original aim, which was to research
porosity and permeability in relation to petroleum geologists.

If I were to repeat this experiment I would do the following to either
eradicate or reduce the errors and problems listed above;

* To remove air from pores I would wet the sediment before the
experiment so the pores would be filled with water and not air. To
continue making the experiment a fair test I would add the 2
litres of water and then finish timing when 2 litres of water was
collected in a measuring cylinder at the bottom, this would mean
that I would have an accurate rate of flow for 2 litres through a
set quantity of sediment. From this information it would be
possible to work out the permeability of the sediment in Darcy's.

* To resolve the problem caused by packing during repeated
experiments on the same sediment a fresh container of the same
type and size of sediment could be used for each repeat as this
would mean that the average results would be more accurate.

* To prevent water travelling down the sides of the sediment barrel
a larger one could be used with the water poured down the centre
this would mean that the results were an accurate permeability of
the central sediment and not effected by the container.

· The problems caused by differing materials for different grain sizes
could be resolved by using one 'universal' sediment, which is
available in a range of different sizes with a similar grain shape.

· To solve the problem with the filter I could use different sized
meshes for the different grain sizes having the mesh as close to the
grain size as possible but still smaller than it. This would mean that
the mesh would not be effecting the water movement, as it would be
larger than the pores in the sediment.

· The problems caused by the flow of water into the funnel being at
different rates and for different times due to human error could be
solved by using a mechanical pouring device this would mean a steady
water flow could be provided for a set amount of time, this would make
my experiment fairer by reducing the opportunities for human error.

There are numerous other potential solutions to the major problems
which are shown in this experiment, I intend to detail these in my
over all evaluation of the investigation.

Experiment 3; how does the % of cementation affect the permeability of
sedimentary rocks?

In this experiment I intend to investigate if the % of cementation
effects the permeability of sedimentary rocks. As it is complex and
time consuming to work out the % cementation of 'natural' sedimentary
rocks, I intend to create my own with a closely controlled % of
cementation by using Portland cement, I have chosen to use Portland
cement as it is reasonably insoluble in a short period of time, this
is important as my results would be affected if the cement dissolved
in the water. I predict that as the Portland cement is viscous and
will therefore stick to the grains so as the % of cement increases the
permeability will decrease as the cement will take up more and more of
the pore space as it bonds the grains, this would obviously restrict
waters flow through the pores.

I intend to use the same method as for experiment 2 with a few
differences, which I have detailed below;

· Instead of changing the size of grains each time I intend to use the
same sized sediment for all of my experiment (7mm aggregate)

· I will add 0-5% of pre made cement (1 part Portland cement powder to
3 parts sand with water) to the sediment mix it in and leave it to set
overnight, I will then perform the experiment in the same way as for
experiment 2.

· I will then repeat this 3 times for each % of cement

Safety

As well as following normal lab safety procedures I will also have to
take additional precautions as Portland cement is potentially
dangerous. Cement powder is an alkaline, it can have a caustic effect
when in contact with bare skin for a period of time, it is also toxic.
Due to these factors I will wear gloves, safety goggles and a dust
mask when mixing the cement, I will also avoid touching the cement
with bare skin.

Fair testing

To achieve a fair test and therefore limit the chances of anomalous
results I will do the following;

· I will repeat the test 3 times for each % of cement so that I can
get an average time taken, (which will be an indication of the %
cementations effect on permeability) this will mean that if any of my
results are slightly anomalous the average will give a truer time for
water flow through the sample as the multiple repeats will diminish
the significance of anomalies from individual results.

· I will pour the same quantity of water (2 litres) into the sediment
each time this will mean that this is consistent for each % of
cementation reducing the chance of anomalous results.

· The same quantity of aggregate will be used for each test; this
means that the volume of sediment will not affect my results as it is
constant for each experiment and therefore the only variable is the %
of cementation.

· All of the sediment/cement mixes will be left to set for the same
period of time so that the cement is equally cured in them all, this
means that the state of the cement should be the same for all meaning
the results should only be affected by the %

· The grain size and material is constant for all so this should not
effect the quality of my results.

Results

I carried out the experiment as stated above below are my results;

Percent of cement

Repeat 1 (sec)

Repeat 2 (sec)

Repeat 3 (sec)

Average (sec.)

5%

152

132

160

148.00

4%

122

92

134

116.00

3%

79

68

74

73.67

2%

45

53

40

46.00

1%

38

27

33

32.67

0%

22

35

29

28.67

These results appear to support my hypothesis that the higher the % of
cementation the longer it would take for water to move through and
there for the lower the permeability. My results also appear to be
reasonably accurate with only one possible anomaly (high lighted in
yellow) this is probably cause by human error in the recording or
reading of the results.

Evaluation of experiment 3

All the points I have made in my evaluation of experiment 2 (with the
exception of the comments about differing materials for different
grain sizes) are valid and relevant with reference to experiment 3, as
are my possible solutions to the problems as well as this there are
however some problems which are specific to experiment 3 these I have
detailed below;

· The % of cement I am using is of a very low volume and as it is
mixed by hand for a short period of time the chances are that it is
not very well distributed through out the sediment, this means that
water could flow through pores and sections of the sediment which have
very little or no sediment present. This would make my experiment
unfair as the distribution of cement would be different for each %.

· The water which came out the bottom of the bottle was a murky cement
like colour the probable cause for this was that the cement had not
fully set so some of it dissolved in the water, this would reduce the
%of cement in the sediment barrel and therefore change my results.

· The cement has to be left to set overnight otherwise it would not be
acting as a bonding agent and would just be a pore fluid present in
the sediment. But as cement sets it contracts this causes fractures
and faults, during the testing it would be possible for water to
travel down these faults which would mean that the results are not an
accurate representation of permeability

· The % of cement is not natural when compared to sedimentary rocks
and also the human synthesised material is very different in it
properties to the matrix which bonds sediment grains in natural
sedimentary rocks (for example cement has a higher viscosity meaning
it bonds grains in a different way to that which the matrix does when
under pressure. This means that the experiment is only testing the
permeability of a man made sedimentary "rock" and would therefore not
necessarily have the same properties or the same permeability as a
normal sedimentary rock.

· As the cement is added as a % it means that the volume will be
different for each of the cementation percentages (e.g. for 0% there
will be 30cm3 of sediment and 30cm3 of material generally in the
sediment barrel but for 5% cementation there will be 30 cm3 of
sediment and 1.5 cm3 of cement meaning 31.5 cm3 of material) although
these are small amounts it is still significant especially if my
experiment was repeated on a larger scale.

This last point is a major fundamental flaw in my experiment, however
below I have detailed some possible methods of minimising these
problems;

· To make sure cement distribution is even I would use a mechanical
mixer which would mix the sediment-cement mix for a long period of
time to attempt to ensure even distribution.

· It would be possible using a light microscope and a thin section of
a sedimentary rock to work out the percentage of cementation by
measuring and counting this information could then be used to work out
accurate % of cement. A simpler method however would be to use cores
of real sedimentary rock with a known % of cementation.

· If the cement was left for a longer time period to cool, especially
in colder conditions, it would be fully set (minimising dissolution in
the water) and have less fractures as it would be setting for a longer
period of time so it would contract less.

Investigation evaluation and conclusion

After completing my investigation I feel that I have answered and
supported some of the original questions and ideas I had. However I
have also discovered that the methods I chose were far to basic for
accurate results, which could lead to calculations as to the Darcy
level of materials etc. this means that my experiment is somewhat
limited in its success as it has proved hard to back up my personal
conclusions with conclusive data.

It would be very hard for me to repeat the experiments with the
modifications I suggested due to limitations of equipment and time,
and even with these modifications I still feel that the experiments
would not be an accurate indication of porosity and permeability as
they are all taking place with small rocks samples that are out of
situ. This is the primary problem with my experiment and my
investigation as a whole as it is all being carried out in a
laboratory away from the rocks natural environment. If possible I
think the best method of assessing rocks porosity and permeability
would be to use radioactive tracers in geological liquids and chart
their movement through rocks in situ. This would be an accurate, if
potentially costly, method of assessing rocks properties as reservoir
and impermeable rocks.

Overall I feel that my results did support my hypothesis, particle in
experiment 3, but I feel my methods featured to many fundamental
problems for my investigation to be a complete success.

How to Cite this Page

MLA Citation:
"Porosity and Permeability." 123HelpMe.com. 18 Apr 2014
    <http://www.123HelpMe.com/view.asp?id=122524>.




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