Petrophysics is not an exact science. A lot of petrophysical computations are based on empirical equations derived from laboratory observations and measurements. Therefore, it is important to note that computation of petrophysical parameters has a lot of uncertainties due to uncertainties in measurements, tool errors, errors in assumptions and models used, etc. It is somewhat futile to try to be accurate up to the third decimal place in the computations, when the input data used has its intrinsic errors and uncertainties. There have been many cases of heated arguments between petrophysicists and the end users (who shall remain nameless), about 0.1PU (porosity unit) difference in porosity computed from the bulk density log. The density log measurement has a standard deviation of +0.015gm/cc which translates to +0.9PU uncertainty in porosity for a sandstone reservoir. Here is a presentation that I made several years ago, to illustrate the importance of petrophysical uncertainties in hydrocarbon resource assessment.  Monte Carlo simulations were run to determine the sensitivities of various input parameters in the computation of Volume of Shale, Porosity and Water Saturation as they have major impacts on Net Sand, Net Pay and average petrophysical input parameters used for Hydrocarbon Resource Assessment.  The purpose of this simple presentation is to provide some basic information, as an introduction to uncertainties in petrophysical evaluations. It is not meant to be a full blown coverage of the complex subject of uncertainty.

Further to the presentation on “Saturation Height Function” (prepared by Mr Leong) which I posted previously, I would like to stress that Leverett-J method of generating SHF is one of the many methods used in the industry. Some of the common methods used are: Cuddy Method, Leverett-J, Skelt-Harrison, Lambda Function, Thomeer, Brooks-Corey and Heseldin. Personally, I have only used Cuddy Method, Leverett-J and Skelt-Harrison. There are numerous technical papers on the subject of SHF. I have found the following three papers to be very helpful.

1) Skelt, C and Harrison R., 1995, An integrated approach to saturation height analysis: SPWLA 36th Annual Logging Symposium, paper NNN.

2) SPE 71326 Saturation Height Methods and Their Impact on Volumetric Hydrocarbon in Place Estimates by Harrison et al.

3) WHICH SATURATION-HEIGHT FUNCTION? by Stephen J Adams (SPWLA-2016 57th Annual Logging Symposium)

I and my colleague A.H Asari Ramli authored and presented a paper “WATER SATURATION FROM GENERIC CAPILLARY PRESSURE CURVES” at the 17th Japan Formation Evaluation Symposium in 2011. Happy reading!!!

Here is a presentation on “Capillary Pressure and Saturation Height Function” made by Mr. Leong Hoon Yoong, a good friend and former colleague of mine. He has kindly given his permssion to post this presentation on public forum. Mr. Leong is a very talented consultant petrophysicist with many years of experience working for major oil companies as company staff, as well as with several consultant companies. I believe this presentation will be very useful especially for the young professionals.

Capillary pressure data is used by petrophysicists to derive a saturation function (SHF), which relates water saturation as a function of height above the Free Water Level in a reservoir. This SHF is then used by geologists to populate water saturation values in a static model of the reservoir, which is then passed onto reservoir engineers for dynamic reservoir model simulation.

Petrophysical cutoffs are usually used in oil and gas companies to compute average petrophysical parameters, such as average porosity and average water saturation values, for input parameters in hydrocarbon volume calculation in an oil or gas reservoir of interest. This is often a topic of contention, especially between petrophysicists who provide petrophysical input parameters and geologists who compute hydrocarbon volumes. Some contend that petrophysicists are double-dipping when they impose all standard cutoff parameters, namely clay volume, porosity and water saturation cutoffs on a reservoir. Cutoffs are used to discriminate reservoir rock from non-reservoir rock, remove tight or low porosity zones and eliminate intervals with very high water saturation. Obviously, by doing so, the net thicknesses such as Net Sand and Net Pay are reduced. However, the average values of porosity and water saturation are improved. If the cutoffs are relaxed or even totally eliminated, the net thicknesses will certainly increase. However, poor quality rock will now be included in the computation of average reservoir parameters, which will downgrade the reservoir quality, resulting in lower values of average porosity and higher values of water saturation. Each operating company has its own policy and method of computing optimum cutoff parameters. Sometimes, confusion rises among partners in a joint venture, when different cutoff values are used, resulting in different net thicknesses and average petrophysical input parameters. Here is a presentation on petrophysical cutoffs that I made many years ago, during a workshop on standardization of petrophysical cutoff parameters. I hope that readers of my post will find it useful. Again, I want to stress that my presentation materials are pretty basic and meant for young professionals who are just starting their careers in petrophysics. There are several technical papers on petrophysical cutoffs, for those who want to know more about the subject.

Here is another presentation that I made some time ago. It was presented initially to the petrophysics community in my company and later on to a wider audience consisting of geologists, geophysicists, reservoir engineers as well as others. The objective of this presentation was to clarify the different types of porosity, namely total porosity and effective porosity computed by petrophysicists. There is a confusion or misunderstanding of the petrophysical definitions of porosity, which are somewhat different from those understood by geoscientists and reservoir engineers. Although we petrophysicists may be practitioners of the “Dark Art”, as mentioned by some readers on LinkedIn, we are not able to distinguish between connected and unconnected porosity, when we analyze standard logs. To make it more confusing, there are other definitions of porosity, like open porosity and closed porosity, as used in some countries. I hope that my fellow professionals, especially the younger ones, will find this presentation worth reading.

After briefly discussing “What is Petrophysics (formation evaluation)” in my previous post, “Formation Evaluation in a Nutshell”, let me now pose the question “Who Wants to be a Petrophysicist?” Several years after giving my presentation on Formation Evaluation, I prepared another one on what it takes to become a petrophysicist. This was meant to be a pep talk to young petrophysicists and other young professionals who may want to consider pursuing a career in petrophysics. It is a light-hearted, tongue-in-cheek presentation and not to be taken as an in-depth technical discussion. It does not contain any highly technical or confidential material. I prepared this presentation by incorporating reference materials from different open sources and have acknowledged them on the presentation slides. I do hope that the original authors of these reference materials would not mind my sharing them with others as my presentation is solely meant for educational and knowledge sharing purposes. During my classes on petrophysics, I always said that learning should be fun, especially for a subject like petrophysics. I hope that the readers of this post will find my presentation useful and fun. The presentation file can be downloaded from this site as follows.

Thank you for the overwhelming response to my post on LinkedIn, on the subject of petrophysics or formation evaluation. I have tried my best to respond and share my PDF file to those who requested. However, I may have missed some of the requests. You can download the PDF file “Formation_Evaluation_Nutshell” on this blog site.

Thanks for joining me!

Good company in a journey makes the way seem shorter. — Izaak Walton

Petrophysics is a kind of exotic sub-surface discipline, which even a lot of people in the oil and gas industry do not know about. During my long career as a petrophysicist, I was asked by many people about “what petrophysics is” and “what petrophysicists do”. I prepared a short presentation called “Formation Evaluation in a Nutshell” and presented it to several of my colleagues and young oil and gas professionals during Community of Practice (CoP) sessions and training courses. As I am retired now, I think it may be appropriate and beneficial to some of the young professionals in the oil and gas industry to have an idea about petrophysics, which has been my lifelong pet subject. I have attached my presentation materials in PDF format and posted it on LinkedIn for all to read and share. These materials are meant to be an introduction to the very wide subject of petrophysics and not to replace formal training courses in petrophysics.

Ko Ko Kyi on ResearchGate