On September 13th 2014, AAPG Student Chapter UI conducted a short course which entitled “Introduction to Petrophysics” with speaker Al-Hafeez from Petrochina International. In this short course we were given explanation about petrophysics, the difference with the other sciences, how a petrophysicist can work in an oil company, and what parameters needed by a petrophysicist to observe a reservoir.
Petrophysics derived from the word petro means rock, and physics means nature. Therefore petrophysics means a natural study of rocks. Petrophysics very related with Archie law. When we drilled, we’d get data log and from this we obtain porosity value and saturation value. A petrophysicist has different responsibility with operation geologist in which operation geologist looking for layer that contains oil and water. But a petrophysicist also can know the reservation using porosity and saturation.
These are several points of view about petrophysics:
- Petrophysics is considered to be a difficult science
- A petrophysicist is marked as a scientist in a company
- Documentation and data arrangement by a petrophysicist isn’t as good as a geologist or geophysicist
Petrophysics considered to be a difficult science because very few fundamental theories that can be applied, this science examine more about material physics or theoretical physics. In petrophysics, the truth is difficult to be determined and only can be directly observed. Beside, there are many methods or formation that developed in laboratory that yet be patented. Furthermore there’s no specific major for petrophysics in universities.
Petrophysicist in an oil company can work together with geologist and geophysicist. When a geologists do the interpretation, but the gamma ray data hasn’t been corrected, it could be any mistake in interpretation. Petrophysics is like an entry door for well interpret and well coordinated data. Petrophysicist can handle all the data in an oil company.
Characterized sedimentation and quantification formation property, here we can learn about how fluids character on reservoir. Log analysis is different with petrophysics because petrophysics design the work so that time and cost can be expensed effectively. Petrophysics evaluated the reservoir formation, interpretation, and integrate the data by combining data using reservoir model. Petrophysics isn’t science, but include disciplines.
Phylosophy and methodology on petrophysics:
- Used for information or parameters for every data
- Team work is the most important thing
Total hydrocarbons in place
Porosity is fraction from Bulk Volume that isn’t contained by solid material.
Porosity is controlled by :
- Grain packing : for carbonate reservoir that has porosity 60-65% but we should see the character of the rocks itself.
- Grain shape : Divided four which are sphere, cube, cylinder, and disk. The thinner the grain the smaller the porosity will be.
- Grain size distribution
Methods to determine porosity:
1. Formation Density Log Methods
Using this instrument
We can get bulk density from rocks formation with formula
Below is the example of density log.
2. Neutron Log
Using this instrument below.
Neutron log formula.
Where Ih is indeks from hydrogen meanwhile Vi is volume of minerals or fluid within formation.
On neutron log can be found high gamma ray value, has porosity from shale because it might contain water so the neutron log is read to have high porosity.
3. Sonic Log
The principal of rocks propagation similar with neutron.
Using this instrument
Using formula we obtain
In which Rw is resisitivity of fluid on formation, m is cementation, and Sw is saturation of fluid. To define the value of m and n using this
- Picked plot
- FF-Pc_Ri Aparatus
Permeability is very important to reservoir simulation. Permeability is controlled by:
- Grain size
- Stress condition
- Pore geometry
- Grain size
- Grain shape
- Grain orientation
- Packing arrangement
- Degree and type of cementation
- Clay content
Basic principal of permeability is te most important thing in characterized a reservoir. There are some methods to count permeability:
- Mud log
- Pressure log
- Wireline log