Electric Logs

After a well is drilled, "electric logging tools" are run into the hole.  Electric logging tools have several things in common.  Most of them are shaped like a long, heavy pipe, 20 to 90 feet long.  They are narrow enough to go into the hole, which is usually about 8-10 inches in diameter.  They are lowered into the hole on a "wireline", a thick, flexible sheathed cable that conducts electricity down to the "tools", and transmits the tool readings back up. The truck carries enough wireline on a large spool to lower the tool many thousands of feet.

The wireline is spooled out the back the  truck, over a pulley, and down into the hole.  There are many, many different kinds of electric logs, and the Petroleum Geologist will select the ones that will give him the the best data about the particular hole that is being logged.

Note that "electric logging tools" are are run from the back of the truck into  the hole.  An electric-logging truck is a very advanced portable scientific and measurement laboratory on wheels.

The electric log tools produce a long piece of paper called an "electric log".  "Electric log" is a general description for any of several kinds of logs.  PG's get much valuable information from electric logs, including rock type, porosity, presence of oil, water, or gas, and many other things.  The electric log is the most important tool of the PG. 

Common types of electric logs include gamma-ray, caliper, resistivity, density porosity, and neutron porosity.  There are dozens of other, more specialized, types.

If the electric logs indicate that the well contains oil or gas, the  PG will order that the well be completed.  

The logging truck is shown, backing up to "log a hole".

The Gamma-Ray and Resistivity Log

Take a look at the first picture of an electric log below.  Usually, several tools are run on each logging trip into the hole.  The first run will usually be a "resistivity run" using a combined tool "string" (several tools attached together) consisting of a gamma-ray tool, a caliper tool, and three different types of resistivity tools.  The log generated from this run is called a "Gamma-Ray & Resistivity Log".

The Gamma-Ray Portion

The gamma-ray tool reads natural gamma-ray radiation given off by rocks.   Shales release a lot of natural gamma-rays, so they read high on the gamma-ray log.   Sandstones and limestones do not release many gamma-rays, so they read low.   Look at the log to the right.  The gamma-ray log is on the far left side.  The scale, in gamma-ray API (American Petroleum Institute) units, is at the the top (0-125).  There are 10 divisions on the gamma-ray scale so each division=12.5 units.  If the line should run off the right side of the log, it will reappear on the left, then the "backup scale" (125-250) is used.

The thick black horizontal lines drawn on the log represent 10-foot sections.   The thinner black lines between them are 2 feet each.  I drew a thick blue line on the log at a depth of 14,600 feet.   This is the top of a sandstone.  Picking tops like this one is a very important part of reading electric logs.  

Look just below the blue line, and you will see that the gamma-ray log is reading about 15-20 units.  This low reading indicates a sandstone or limestone.   Now look above the thick line.  The gamma-ray

 is reading about 62 units.   This indicates a shale formation.  Sandstones and limestones are usually easy to tell from shales when you have a gamma-ray log, because their gamma-ray readings are usually less than about 50 units, and really clean, nice sandstones or limestones often read 20 units or less.

The Resistivity Portion

Now look at the right side of the above resistivity log.  There are three curves (wiggly lines) here.   Each curve measures the resistivity of the rock, including the fluid contained in the rock.   The reading is made in "ohms", a measurement of resistance of the rock formation to the flow of electricity.  Porous formations containing mostly salt water (the kind of water normally found deep underground) have low resistivity, often about  1-10 ohms.  Formations that contain oil and gas have very high resistivities...perhaps 50-500 ohms.  Perforating a salt-water zone is expensive and wasteful, and the PG wants to avoid that!  The vast majority of porous rocks hold only salt water!

The three curves each penetrate, or "look", a different depth into the rock..  The one we are interested in is marked "Deep Resistivity" and is drawn as a line with long dashes.  This tool "looks" sideways into the the rock to a depth of 6 feet or more.  At this distance from the borehole,  the natural oil or gas in the rock is usually undisturbed by the drilling process, and we can get a really good resistivity reading.  Within the sandstone shown here, the Deep Resistivity reads up to 300 ohms.  This very high resistivity indicates a reservoir that probably contains oil or gas.  But to be sure of this, we need to know something about the porosity of the rock.  That's because rocks with little or no porosity also have very high resistivities,  but will not produce any oil or gas!

The newer resistivity tools take up to eight or more separate readings to look deep inside the rock formation.  Such tools are called "array" tools.

Another Log -- The Porosity Log

Now, let's look at the "porosity run" to the right.  On the left is a log you are familiar with, the gamma-ray log (solid line).  It is so useful it is usually run on every logging trip into the hole.  Near the gamma-ray log is a "caliper" log.  This simple log measures the width of the drilled hole, in inches.  The caliper scale is at the top; 6-16 inches, and the curve is shown by a thinner, dashed line.  Note that the hole is very large through the shale formations (almost 16 inches) and narrow through the sandstone (about 8 inches).  This is because the shales are relatively soft (as far as rocks go) and they have been "washed out" (the hole has been widened) by the circulating drilling mud that is used in all wells to cool the bit and bring rock cuttings to the surface.  The sandstone is harder, and does not wash out so easily, so the hole size shown on the caliper is closer to the actual size of the drill bit.  In the sandstone, the hole is a little more than 8 inches in diameter.

On the right side of the log are two types of porosity measurements -- DENSITY porosity and NEUTRON porosity.  These measurements are taken from two different tools.

The scale at the top of the log shows (negative) -10% porosity on the right, 0% about 1/4 of the way over, and 30% porosity on the left.  Sandstones have an absolute maximum of about 28% porosity.  Shale porosity is much higher, but because the grain size of shale is so small, oil or gas trapped in shale cannot usually be removed by drilling for it, so shale porosity is generally not important.

The Density Log tool consists of a highly radioactive gamma-ray source that is beamed into the formation.  A radiation counter is mounted higher on the tool.   Radiation passes through the rock and is recorded by the counter.  If the rock is heavy and dense, few gamma-rays reach the counter.  If the rock is light and porous, like many sandstones and porous limestones, a lot of gamma rays reach the counter.  The curve is mathematically adjusted until it reads approzimately "true" formation porosity, and this is recorded on the log.   In the sandstone above, the density porosity is about 18-24%.....in other words, 18-24% of the rock is made up of holes !  That's a very good thing!

The Neutron Log is a type of porosity log that measures the hydrogen present in the water atoms in a formation.  Since shales contain a lot of water, bound up tight in their tiny pores, the Neutron Log reads very high porosities in shale formations.   So, the PG must look at the Gamma-Ray log first to see if the rock is shaly.  If it is, the Neutron Porosity doesn't mean much!

Look above the sandstone at the neutron log and you will see that it is running "off-scale" to the left.  When gas is present, the neutron log reads low porosity (in this case, 8-12%), and there is a big spread between the density and neutron curves.  This spread (colored yellow on the log) is called "gas effect".  The presence of sandstone also causes the two curves to spread, but not nearly so much as gas effect.  Geologists like to see gas effect.

The resistivity and the neutron-density are just two of the many types of electric logs, but they are the most commonly run nowadays.  Other log types are shown below.

PG's use the measurements from both the density and neutron logs and  combine the two numbers   together with a chart or a math formula.  The result is the PG's best guess as to the actual porosity of the formation.  Most rocks require about 8% porosity, and more hydrocarbons than water in the pores, before they will produce enough oil or gas to pay for drilling the well.   The the PG combines the porosity measurements she has made with measurements from the resistivity run.  The result gives the PG an idea of the percentages of oil, gas, and water in the formation.

After the logs are analyzed, a decision is made to either complete the well or plug it.  Completing it means that heavy steel casing will be run into the hole and cemented on the  outside.  Holes are then shot through the casing from the inside at the depth where the PG thinks the oil or gas is located.  When the PG gets oil or gas from the hole, he officially has a "well", not a "hole".

If the hole contains no economic amounts of oil or gas, it will be "plugged."  This means that no expensive casing will be run.  At this time, the hole is not a well, The hole is plugged with cement, and the ground is returned to normal.  You would never know that a hole had been drilled there.   

But, of course, the dry hole is still important to the PG because it gives him more data about the formations, and the dry hole will be carefully noted on his maps of the area.  A failed or plugged hole is called a "dry hole", never a "dry well".