Contents 1-Abstract 2 2-Introduction 3 3-Theory 4 3.1-Total and effective porosity: 4 3.2Primary and Secondary porosity: 5 4- Experimental Set-up and Procedure 5 4.1-Materials and equipment 5 4.2- Experimental set-up and procedure 5 5-Results and calculations 7 6-Discussion and conclusion 8 6.1- Discussion of the results 8 6.2- Conclusion 8 7- References 9

1-Abstract

The aim of this report is to be familiar with the basic procedures of preparing rock samples and to measure the density of and porosity of rock samples.
Firstly the rock size and mass was measured to determine the density and bulk volume using an electronic ruler and mass balance. Then using the helium porosimeter the grain volume of each core plug sample was measured. Each of these measurements were repeated three times and then averaged for precision sake. The porosity was then calculated by subtracting the grain volume from the bulk volume, to determine the pore volume, and then dividing by the bulk volume. The porosity results for all the samples were reasonable except for sample 1 and 6 for which the porosity is calculated to be in negative % and make no logic. These possible reasons for these inaccurate results are that the group made high errors in reading the measurements or did the experiment in a wrong method.

2-Introduction
For the storage of oil and gas in any reservoir rock presence of porosity is vital. However, the degree of porosity in any rock depends upon various factors. But quality of reservoir rock, either good or poor, rests largely on porosity.

Porosity is the small voids and space between the grains of the rocks and often denotes with percent. Mathematically it is volume of the voids divided by total volume of the rock. Knowledge of porosity is of extreme importance for understanding the reservoir characteristics and behaviour. (Schlumberger, 2012)
Conventional laboratory experiments can be used to determine the porosity of the reservoir by injecting fluids, usually Helium gas, into their core plug samples. These experiments only measure the effective porosity, which is of the main interest to petroleum engineers.
This report documents a laboratory experiment for measuring porosity and density of 6 different core plugs using a helium porosimeter, a mass balance and an electronic ruler. The report outlines theoretical concepts of porosity underlying the experiment and the experimental setups and procedures. This report also documents the results and calculations furthermore it goes through a brief discussion and conclusion of those results.

3-Theory

Porosity is defined as the ratio of void volume of the rock to the bulk volume. Pore spaces in the rock are filled with fluids which could be oil, gas or water. Porosity is essentially needed to estimate the potential volume of hydrocarbons in the reservoir rocks. Well sorted (grains of approximately all one size) materials have higher porosity than similarly sized poorly sorted materials (where smaller particles fill the gaps between larger particles). Figure 1 illustrates how some smaller grains can effectively fill the pores, drastically reducing porosity, while only being a small fraction of the total volume of the material. Porosity data is obtained from direct measurements on the core samples and/or indirectly from well logging. Mostly, porosity data from core samples is used to calibrate the data from well logs.

Figure [ 1 ] comparison of well and poorly sorted grains
The porosity is usually denoted by the symbol Ø and is calculated by the following equation
Porosity=Pore VolumeBulk volume
Expressed in terms of symbols, this equation is represented as:
Ø = Vp/Vb
In the equation above Vp represents pore volume of the rock and Vb represents bulk volume. Pore volume is the total volume of void spaces where as bulk volume is the total volume of the rock.
3.1-Total and effective porosity:
Porosity is classified as total and effective porosity. Essentially total porosity is the ratio of total void spaces to bulk volume of the rock and includes isolated pores and the space occupied by clay-bound water. Whilst effective porosity is the only the ratio of interconnected pores to the bulk volume and is the main interest to petroleum engineers.
3.2Primary and Secondary porosity:
Porosity for both sandstones and limestone’s can be greatly modified by post-depositional events or digenesis. Cementation, chemical action and fracturing are events which can modify the original porosity of rocks. The original porosity of a formation is usually referred to as primary porosity. Porosity modified by post-depositional processes is referred to as secondary porosity. (UNSW, 2012)

Direct measurements of porosity from core samples in laboratory are determined from bulk and grain volume of the core samples. For irregular shaped core samples, volume is determined by gravimetric methods. In gravimetric methods, the apparent loss in weight of the sample when immersed completely in liquid of known density is measured. With volumetric methods the volume of the liquid displaced by the core sample, when fully immersed, is equal to the bulk volume of the sample. With these methods special equipment is required so that the liquid is not absorbed by the sample. For regular shaped samples bulk volume can be determined by measuring its dimensions. For instance, to measure the bulk volume of a cylindrical sample its diameter and length is required to be measured and then substitute the values in the following formula:
V = πr2h
In the equation above r is radius = (diameter/2) and h represents the length of the core plug.
4- Experimental Set-up and Procedure
4.1-Materials and equipment
The following materials and equipments were used in this experiment: * Cylindrical samples of rock to be tested * Electronic mass balance – to weigh the sample * Digital callipers – to measure diameter and height of sample * Helium porosimeter – to measure the grain volume of the sample
4.2- Experimental set-up and procedure
The Experiment was setup in the following way:
Grain volume of the core plug in our experiment and in typical laboratory experiments is determined by using Helium porositymeter. The underlying theory and formulae for calculations is as follows: 1. Two cells of known volume V1 and V2 are connected and at atmospheric pressure, pa. The clean and dry sample is placed in the second cell. 2. Valve B is closes and cell 1 is pressured with gas G through valve A to an absolute pressure (p1 +pa). This pressure is recorded and valve A is closed. 3. Valve B is opened and the two cells are allowed to equilibrate. The equilibrium absolute pressure, (p2 + pa), is recorded.
Note: A schematic for the above setup is shown in Figure 2
After this the following calculations can be run to find grain volume Vs:

Figure [ 2 ] Schematic for Helium porosimeter test procedure
Then after finding the bulk volume of the core plug with the aid of electronic ruler and the method described above, porosity can be calculated from the following equation:

Note: the porosity determined by this method is effective porosity.

Figure 2- UNSW Reservoir engineering notes, page 56
5-Results and calculations
Our group was provided with sample 3 and measured the following ridings: Reading | Diameter (cm) | Length (cm) | Mass(g) | Empty Volume (cm3 ) | Volume with sample (cm3) | 1 | 2.534 | 3.815 | 4.358 | 31.60 | 15.05 | 2 | 2.524 | 3.808 | 4.358 | 31.70 | 15.05 | 3 | 2.527 | 3.810 | 4.358 | 31.60 | 15.05 | Average | 2.53 | 3.81 | 4.358 | 31.64 | 15.05 |

The measurements of experiment for all the groups are listed in the following table: Sample | Diameter (cm) | Length (cm) | Mass (g) | Empty Volume (cm3 ) | Volume with sample (cm3) | 1 | 2.53 | 3.11 | 35.7 | 31.6 | 15.9 | 2 | 2.53 | 4.24 | 43.5 | 31.6 | 14.8 | 3 | 2.53 | 3.81 | 43.6 | 31.6 | 15.1 | 4 | 2.52 | 2.84 | 31.9 | 31.7 | 19.7 | 5 | - | - | - | - | - | 6 | 2.22 | 4.19 | 30.1 | 79.0 | 55.3 | 7 | 2.51 | 3.78 | 42.2 | 31.9 | 15.6 |

The results for the above measurements to calculate density and porosity are: Sample | Vb | Density | Vs | Vp | Porosity (%) | 1 | 15.63 | 2.28 | 15.7 | -0.07 | -0.42 | 2 | 21.32 | 2.04 | 16.8 | 4.52 | 21.18 | 3 | 19.15 | 2.28 | 16.5 | 2.65 | 13.86 | 4 | 14.16 | 2.25 | 12.0 | 2.16 | 15.3 | 5 | | | | | | 6 | 16.22 | 1.86 | 23.7 | -7.48 | -46.13 | 7 | 18.70 | 2.26 | 16.3 | 2.40 | 12.85 |

The symbols Vb, Vs and Vp in the table above represents bulk volume, grain volume and pore volume respectively.
General methodology for the above calculations is given below yet detailed calculations and their explanations for sample 3 are attached in the overleaf in appendices. The same protocol can be used to calculate the porosity and density for the other samples. I) Bulk volume (Vb)= (π/4)D2l II) Density of sample = M/Vb III) Vs=empty volume-sample volume IV) Vp=Vb-Vs V) Porosity % = (Vp/Vb)*100
6-Discussion and conclusion
6.1- Discussion of the results
Calculations for all samples seem reasonable as they are between 0-100% except for sample 1 and 6 which had porosity in negative percentage. For sample 1 the porosity is -0.42% and if errors in measurements and taking readings are considered then the porosity for this sample could be estimated to be 0% or slightly above it. This means sample 1 has no or legible porosity and this type of rocks does not contain enough hydrocarbons for commercial recovery and are no interest to petroleum engineers. However for sample 6 the results are very incredible and suggests that the group who carried out experiment for this sample have not only made extremely high errors while reading the measurements but perhaps have approached the whole experiment in a wrong manner.
6.2- Conclusion
In conclusion accurate porosity measurement, whether with cores in the lab or from logs, are of extreme significance to the petroleum industry as they are used to determine the original hydrocarbons in the reservoir. Laboratory measurement of porosity could be improved by using latest or digital porosimeter as they avoid human reading error factors. In our lab experiment all the samples porosity seems to be reasonable except for sample 1 and 6 because of high errors in measurements or possibly groups wrong approach to the experiment.

7- References
Bibliography
Schlumberger. (2012). Oil field glossary. Retrieved 08 06, 2012, from Schlumbereger: http://www.glossary.oilfield.slb.com/Display.cfm?Term=porosity
UNSW. (2012). Reservoir enginnering notes. UNSW.

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