Friday, September 11, 2015

Porosity Log Overview

This post is an elaboration of the "Basic Logging Interpretation" slide which I share in my previous post [CLICK HERE]. This explanation will fit the best if you want to present the "Basic Logging Interpretation" slide at page 7

It is very important to understand that the resistivity log which has high resistivity measurement doesn't always detect the hydrocarbon containing zone. There are two possibilities: first, the low resistivity zone might contain low resistivity fluid, which is hydrocarbon, and if that is the case, we have found our sweet pay zone. But second, the formation might be very small in porosity that the resistivity measurement becomes low.
Most of the rock is nonconductive to the electricity, so when the porosity of the rock is very small, there will only small quantity of fluid in the rock. Even when the fluid is very conductive, since the quantity is small, the log will show low resistivity measurement. That is why, the result from porosity log should be analyzed.

There are 3 types of porosity log: Sonic log, Density log, and Neutron log. Although the results will represent the porosity condition of the formation, the truth is none of them measure porosity directly. The density and neutron logs use nuclear to interpret the fluctuation as their porosity measurement. On the other hand, sonic log use acoustic measurements to be interpreted as porosity. Rather than choosing one of them, service companies may use combination of these logs to give better indications of lithology and accurate estimation of porosity, since lithology should be specified first if using a single porosity measurement for calculating the correct porosity.

Sonic Log

Sonic log is a log that measures interval transit time (Δt) of a compression sound wave travelling through the formation along the axis of the borehole. The Δt is the transit time of the wave and is the reciprocal of the velocity. The acoustic pulse from a transmitter s detected at two or more receivers. The time of the first detection of the transmitted pulse at each receiver is processed to produce Δt. The Interval transit time itself will depend on both lithology and porosity. Important thing to be noted is when we use the sonic log, porosity calculated in gas zones will be slightly higher than the actual values. It could be happened since the travel time in gas is higher than in water.

Density Log

The density log will count the returning gamma rays which represent electron density in formation. Gamma rays emitted from a chemical source by the logging tool, and then the gamma ray will interact with electrons of the elements in the formation. The detectors in the tool will count the number of returning gamma rays which represent formation electron density. There will be relation between  electron density to the formation density. This relation will be represented as a constant. Formation bulk density, which is a function of matrix density and formation fluid density will be used in an equation to find the porosity of formation. that is why it is very important to choose representative matrix density and fluid density to make accurate porosity measurement. Matrix density will vary  with lithology and it will make the porosity measurement become demanding. It should be noted that presence of gas in the pore space causes porosity from density log is higher than the actual porosity. We’ll see later that Density-Neutron combinations will create cross-over in the gas zone.

Neutron Log

Neutron logs will measure the hydrogen content in a formation by emitting neutron. Neutrons are emitted from a chemical source from the logging tool. Then, the emitted neutrons will collide with nuclei in the formation. The collision will make neutrons loses energy, neutron is absorbed and a gamma ray is emitted. The element which closely approximates the mass of a neutron is hydrogen. When collision between neutron and hydrogen happened, average energy transfers to the hydrogen nucleus is about ½ that of the original energy of neutron.  It may be concluded that materials with large hydrogen content like water or hydrocarbons will slow down neutrons. Since hydrogen in a porous formation is concentrated in the fluid-filled pores, energy loss can be related to the formation’s porosity.

Neutron-Density cross plot often to be used to detect gas zone. The characteristic of gas zone are Density porosity measurement is too high and Neutron porosity measurement is too low. It means the Neutron porosity will be lower than density porosity, thus creating Cross-over line between them in the gas zone.

Those are some basic informations about porosity log. We’ll discuss about them in detail in the future.


Do you love this post? Thank you! I feel appreciated. This post is under the category of Petroleum Engineering,  you may love to see other similar posts HERE

Irianto Petrus Binsardo

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