Well Logging Procedure 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 4 

Well logging is an important job in Oil and Gas industry.  It uses sophisticated tools, equipments, devices and software, The interpretation will be done by professional earth scientists with precision as it is very important in formation evaluation. That is why there are many trainings, seminars and handbooks of well logging, which are reasonably high in price, to make sure people contributing in well logging jobs are capable in using the software and basic procedure to get the representative results of the formation. The slide ppt (ppsx) of basic interpretation in well log results has been shown in this blog, but to elaborate it, we should know the tools, procedures, and simple facts in the field that is important to get the best results.

Why we need Well Logging?

in nontechnical language, Well logging is the method to see the condition around the borehole so we can have the data for some feet into the formation from the well. Formation means a rock unit, so the well log will see the condition of the rock around the well, and from these conditions we may conclude whether the rock will contain petroleum or not. Thus, still in nontechnical term, to measure the condition of the rock around the well, logging tools should “emit something” and measure the respond of the rock OR the logging tools may “measure emitted things” from the rock to get the properties of the rock. Whether the tools “emit something” or simply “measure emitted things” will differentiate the types of logging tools as active OR passive logs a. And from these measurement we may answer many questions from engineers in the field and the office. It is very important to the geologists as it will answer their prediction whether there is hydrocarbons, whether the potential zone porous, whether the top of formation as predicted, and what kind of hydrocarbon in the formation. It is a method for the reservoir engineer to know the pay zone thickness, homogeneity of zone, and porosity and permeability of the zone. It will help the decision of production engineer on which zone to complete, the production rates, whether there will be water production in following years, and even what stimulation should be used in the future.

Logging seems very powerful!

Indeed. It provides many information of the formation near the well. It will allow us to see changes in formation properties along the well. But it also has its shortcomings too. The biggest short coming is that the measurement is representative only to the rocks some feet around the well. That means we will have no clue if in 20 ft near the wellbore, the rock and fluid properties have changed. That is why the result is quite unrepresentative in the non-homogenous zone.

So, what are the tools? 

There are many logging tools that are used based on whether they are active or passive, and based on the properties they measure. The tools should be chosen based on the condition of the well, and the most important one, economical consideration. It’s all about money after all. The more equipment we use will cost more money. The longer we use the tools, the more we pay the rig (exploration rig, offshore rig and bigger rig will cost higher money. Exploration rig may cost $8000-15000 per day!).  The type of well will impact on the economical consideration too. Horizontal well will cost higher since the tool can’t be lower by wireline (should use pipe-conveyed logging), deep well will cost higher because the tools should be modified to resist the high pressure, and hot well (e.g. geothermal well) will cost higher because the tools should have high temperature design.

The active well log will emit electromagnetic, nuclear, or acoustic to the formation, then measure the respond to get the formation properties. For example, in Acoustic log, the logging tool will emanate acoustic and see the respond from the formation. The sound might travel through rock depends on the matrix’s physical structure. So, when the respond come to the acoustic receiver, the amplitude, speed and phase are function of all of the combined matrix densities, interconnections, cementation, fracturing, and porosities within the matrix.

The passive well log will respond to the conditions of the rock as the sources to measure the properties. They may respond to Natural gamma ray emitted by the formation (gamma ray log), spontaneous potential difference of formation and reference (SP log), and changes in borehole size (caliper log).

How do they work?

The well logging companies will operates the logging trucks with computer programs and software to gather the measurement data from the logging tool in real time. The wireline/cable is made of conductor material with outer armor to protect the cable from borehole conditions. As the measurement continue, the cable winch on the truck will be rolled and the measuring tool will move upward, creating the measurement along the wellbore depth. The data either recorded in the truck or transmitted directly from the wellsite to the office. The accuracy of the result also depend on the speed of the logging tools, as the faster upward movement of the logging tools, the result will less accurate.

Those are some overview of well logging job. It needs special persons who understand the fundamental of oil logging to give the correct interpretation, as the logging interpretation quite subjective, depend on experience, proficiency, and care of observer in interpreting the data. The well logging result analysis of every companies will also varies based on their own criteria, programs and experiences. But over all. the safety and economic should be the real consideration since the logging tools have radiation safety issue and need special training, also the cost of data misinterpretation will be greater than the cost of hiring major experienced logging service company.

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