ABSTRACTPorosity, is a measure of the storage capacity of a
reservoir. It is defined as a ratio of the pore volume to the bulk
volume and it can be expressed as either in percent or fraction.
There are two types of porosity that can be determined which are
total or absolute porosity, and effective porosity,. Effective
porosity is the ratio of interconnected void spaces to the bulk
volume. Thus, the only pores that will contain fluids are the
effective porosity that formed from wells. The aim of this
experiment is to determine the effective porosity of a porous plug
by using liquid brine. Dry core rock sample is heat until fully
dried. Then, it is weighed using electronic balance before immersed
in a saturated liquid brine for 30 minutes. After that, the
saturated sample weight is measured. Base on the recorded data the
bulk volume and pore volume can be calculated. This calculation is
crucial in determine the effective porosity of the used samples.
Result from this experiment shows that the sample core rock 1 has
higher percentage of porosity when compared to the core sample 2
which are 0.4510 % and 0.1570 % respectively. This different is due
to the arrangement of the grains molecules in both samples together
with occurrence of the interconnected void space of the samples. As
for conclusion is best to say that core 1 is sandstone and core 2
is limestone as sandstone usually had higher porosity when compared
to limestone in the form of sedimentary type rocks.7
1.0 INTRODUCTION(Abu-Khamsin, n.d)measure of storage capacity of
a reservoir. It is defined as a ratio of pore volume (VP) to bulk
volume (VB) of a reservoir rock and may be expressed in either a
percent or a fraction (Abu-Khamsin, n.d). This calculation can be
expressed in a form of equation such:
By using this formulae both total or absolute and effective
porosity can be calculated. Total porosity is the ratio of all the
pores spaces in a rock to the bulk volume of the rock. Effective
porosity is the ratio of interconnected void spaces to the bulk
volume. Total porosity includes all existing pores regardless of
whether they are connected or not. While, effective porosity only
includes the interconnected pores. Thus, only the effective
porosity contains fluid that can be produced from wells (Chapter 4
: Porosity, n.d). For granular substance like sandstone, the
effective porosity may approach total porosity, yet shale and for
other highly cemented or vugular rocks like limestone, large
variation or pattern may occur between effective and total
porosity.2.0 OBJECTIVE
The objective of the experiment is to determine the effective
liquid porosity of porousplug by using liquid saturating method in
core laboratory.
3.0 THEORY
Due to its origin porosity is classified as either primary or
secondary. Primary porosity is referred to porosity in a rock
resulting from the sedimentation process. Secondary porosity is
defined as a porosity in a rock which happen after sedimentation
process like fracturing and recrystallization (Mohd Fauzi Hamid,
n.d). These changes in the original pore spaces may be created by
ground stresses, water propagation or various types of geological
activities after the original sediments were deposited. The
original porosity of the rock often increased due to the fracturing
or formation of solution cavities.
Porosity is independent of the size of the grains for such a
uniform rock grain size. The theoretical maximum porosity for cubic
packed rock of uniform size of spherical grains is 48%. These
maximum porosity of other packing arrangement can be shown as
follows (Glover, n.d).
Figure 2.1 : Ordered packing arrengements.
Table 3.1 : Maximum porosity of different packing
arrangementPackingMaximum Porosity (fractional)
Cubic0.476
Hexagonal0.395
Orthorhombic0.395
Rhombohedral0.260
Tetragonal0.302
Triclinic0.260
3.1 Effect of compaction on porosityCompaction is defined as a
process of volume reduction resulting from an external applied
pressure. For extreme compaction pressure, all materials show some
irreversible change in porosity. Generally grain compressibility in
most petroleum reservoirs is considered to be negligible. For
porous rock, the compressibility is depends on the
porosity.Porosity can be estimated through volumetric measurements
of core samples or from geological logs (which measure a
characteristic of the rock and infer porosity) or from Petro
graphic Image Analysis (PIA), which is pore level evaluation of a
small sample size. This section is directed towards the measurement
of porosity from rock samples or core because it provides the basic
concepts for understanding (Porosity, n.d).3.2 Grin sizePorosity is
not effected by grains size. Well rounded sediments that are packed
into the same arrangement generally have porosities from 26% to 48%
depending on the packing (Mohd Fauzi Hamid, n.d).
4.0 APPARATUS AND MATERIALSApparatus Oven Desiccators with dry
silica Electronic balance 100 ml beaker 1000 ml beaker Glass rods
Damp cloth Graduated cylinderMaterials Core samples Sodium chloride
(NaCl) powder Distilled water5.0 PROCEDURE1. Core sample is dried
in an oven at 1000C and then placed in a desiccator with dry silica
gel to cool off.2. Weight, Wdry, diameter D, and length L, is
measured using electronic balance and vernier caliper
respectively.3. Completed dry sample is placed in a container or
beaker on a base of glass rods and slowly covered with brine until
the sample is totally immersed with about 2cm of brine above it.
The sample is saturated with 36 g/l NaCl brine, brine = 1.02
g/cm3.4. The sample is taken out from the container after 30
minutes and blot quickly with a damp cloth to remove surface
brine.5. Saturated sample is weighted, Wsat.6. Saturated sample is
put in a graduated cylinder filled with brine and the increase in
volume on the cylinder is measured. Bulk volume, Vb (cm3),
corresponds to the observed increase in the volume of brine
measured on the graduated cylinder.7. The test is repeated for the
other different sample.
6.0 RESULTCore No: 1 (sandstone)Diameter (cm): 5.45 cmLength /
thickness (cm) : 1.4 cmWdry (g)Wsat (g)Wbrine (g)Vp (cm3)Vb (cm3)ee
(%)
57.333057.42500.09200.0902200.004510.4510
Core No: 2 (limestone)Diameter (cm): 2.5 cmLength / diameter
(cm) : 2.5 cmWdry (g)Wsat (g)Wbrine (g)Vp (cm3)Vb (cm3)ee (%)
60.226560.26650.04000.0392250.00150.1570
7.0 SAMPLE CALCULATIONCore 1 (sandstone)Wbrine= Wsat
-WdryWbrine= 57.4250(g) 57.3330(g)= 0.0920(g)Vp = Wbrine / brine =
0.0920(g) / 1.02 (g/cm3)= 0.0902(cm3)Vb = 20mL
(1L/1000mL)(0.001m3/1L) = 20cm3
= Vp / Vb = 0.0902(cm3) / 20(cm3)= 0.00451 X 100% = 0.451%Core 2
(limestone)Wbrine = Wsat -Wdry Wbrine = 60.2665 (g) 60.2265 (g)=
0.0400 (g)Vp = Wbrine / brine = 0.0400(g) / 1.02(g/cm3)= 0.0392
(cm3)Vb= 25 mL (1L/1000mL)(0.001m3/1L)= 25cm3 = Vp / Vb = 0.0392
(cm3)/25(cm3)= 0.001570 X 100%= 0.1570%
8.0 DISCUSSIONAny porous materials like rocks are made of solid
grains and void space that contains the fluids, water, oil or gas.
In this experiment, the observation shows that not all pores of
rock sample are filled with brine. This due to the pores properties
which do not totally in interconnecting to each other that cause
void in some part of the rocks. At certain point, pores contain
water or brine that is saturated with mineral in the rock
structure.In this experiment, porosity of sample is differ to each
other where it is recorded higher in sandstone, 0.451% and low in
limestone, 0.1570%. This is due to its different in the arrangement
of grains of both samples. The arrangement in sandstone gives more
space and it is more interconnected while, limestone give a more
compacted arrangement of the grain which resulting less in space
and less interconnection. Porosity is one of the most crucial
properties of sedimentary rock that is the rock should be porous in
order to extract oil and gas. This is due to the only porous media
or rock with porosity have the ability to store hydrocarbon within
it. Thus, porosity is the main properties of a reservoir rock and
without it, rocks have no tendency in storing hydrocarbon which
resulting of no occurrence of oil and gas and formation of
wells.The principle to employ Boyles Law is that the gas molecules
is invade the pore space and not the solid (Boyle's and Charle's
Law, 2013). The different in using brine and helium are that the
molecules of helium gas is smaller compared to brine molecule. It
is easier for the helium gas to penetrate small pore and it is
inert does not inert does not absorb on rock surface as air may do.
Brine with larger molecule compared to helium yet it still able to
enter the pores as it is small enough. This liquid saturation
method help in yielding effective porosity, however complete
saturation is seldom obtained and therefore porosity is commonly
lower than that determined from Boyles Law method.Liquid saturation
method is an accurate method of determining the volume of the rock
sample that filled with brine when saturated in subsurface
(Tatomir, 2007). This accuracy achieve due the characteristic of
the reservoir rock subsurface, which already filled with brine.
Thus, the volume of the rock can be determined by using volume
displacement method. The most significance of applying this method
is that the determination of porosity convenient while preparing
samples for other tests, and samples can be used for further
testing.
9.0 CONCLUSIONGenerally, sedimentary rocks have the highest
percentage of porosity compared to the metamorphic rocks and
igneous rocks (Salman Bloch, 2002). In this experiment sandstone
and limestone used as sample are both from the types of sedimentary
rocks.Sandstone is usually more porous than limestone as it
arrangement of grains is based on the grains size while, limestone
normally undergo further compaction and dissolution of minerals.
This compaction and dissolution of minerals result in lowering the
porosity of the limestone. Compaction is the process of volume
reduction resulting from an externally applied pressure. All
materials show some irreversible change in porosity for the extreme
compaction pressure. This is because of the distortion and crushing
of the grain or matrix elements of the materials and in some
recrystallization cases (Yuyuan Zhao, 2004).In this experiment core
1 has higher percentage of porosity compared to core 2 which are
0.4510 % and 0.1570 % respectively. The different is calculated to
be 0.294 % and thus it can be say that core 1 is sandstone as it
gain higher in porosity while core 2 is limestone as it porosity is
lower.10.0 RECOMMENDATION1. The rock samples used in this
experiment had been repeatedly heated many time and also used by
other groups. This situation may actually help in giving some error
to the actual result for the carried experiment. The theoretical
value will be slightly differ from the actual result as the
structure of stratification of the rock may be damaged due to
repeat heating applied. Thus, it is recommended that in proceeding
this experiment, it would be wise to use a different rock sample
for every different experiment.2. When lifting the core sample from
brine solution it is advised to use some helping device like
clipper in order to avoid an error resulting from the polluted
solution as non-proper method used.3. When, weighed the dry core
and saturated core, used the electronic balance so that the value
observe and recorded will be more precise as it consist of long
decimal places. Using the covered electronic balance would also
help in getting more accurate reading as it get rid the effect of
the wind to weighing system.4. The eyes position must be
perpendicular to the scale of graduated cylinder so that parallax
or misreading error can be avoided. This will help in reducing
error in this experiment.REFFERENCES
Abu-Khamsin, D. S. (n.d). Basic Properties of Reservoir Rocks.
1-102.Boyle's and Charle's Law. (2013). Lear extra live,
1-3.Chapter 4 : Porosity. (n.d). 1-7.Glover, D. P. (n.d). Porosity.
Petrophysics MSc Course Notes, 1-11.Mohd Fauzi Hamid, W. R. (n.d).
Rock and Fluid Properties. Fundamentals Of Petroleum Engineering,
1-52.n.d. (n.d). Chapter 4: Porosity. 1-7.Porosity. (n.d). Chapter
2, 1-8.Salman Bloch, R. H. (2002). Origin and predictability.
Anomalously high porosity and permeability in deeply buried
sandstone reservoirs, 1-28.Tatomir, A.-B. (2007). Saturation
Determination for Multiphase Systems in Porous Medium Using Light
Transmission Method. 1-74.Yuyuan Zhao, F. H. (2004). Optimisation
of compaction and liquid-state sintering in sintering and
dissolution process for manufacturing Al foams. Materials Science
and Engineering,
117-125.http://petrowiki.org/Porosity_determination
APPENDICES
Figure 2 : Core Sample 2Figure 1 : Core Sample 1
Figure 3 : Immersed Core Sample 2 with Brine solutionFigure 3 :
Immersed Core Sample 1 with Brine solution