Formation evaluation

Radioactivity logs are of two main types: Density logs and Neutron logs. Density logs use a gamma ray source to irradiate the sides of the borehole. Electrons in the material of the formation absorb some of the gamma rays and reemit them at a characteristic frequency. The number of gamma rays reemitted is directly proportional to the electron density of the formation. That is very nearly the same as the actual density or specific gravity of the formation. If one knows the density of the minerals in the solid part of the formation, it is a simple algebra problem to determine the amount of water or oil filled porosity. Gas filled porosity reads a little high.

Neutron logs irradiate the borehole walls with neutrons. They tend to pass through most minerals, but to be captured by hydrogen nucleii. When that happens a gamma ray is emitted: a gamma ray of capture. The number of these are proportional to the number of hydrogen nucleii present in the formation. For most lithologies, hydrogen is restricted to the oil or water in the pores. Gas, opposite to its effect on density logs, makes the neutron log read low. The Neutron-Density combination logs take advantage of this by presenting both curves on one log. Where they overlay one another, the pore fluid is water or oil, where they separate, it is gas. Density logs and Neutron logs are usually run together on the same sonde. A very common modern logging configuration is the 'Triple Combo' an Induction log run on the same sonde with a Density-Neutron log. This ideally provides all the parameters to evaluate a suspected pay section with only one logging run.

Both sonic and density-neutron logs give porosity as their primary information. Sonic logs read farther away from the borehole so they are more useful where sections of the borehole are caved. Because they read deeper, they also tend to average more formation than the density-neutron logs do. Modern sonic configurations with pingers and microphones at both ends of the log, combined with computer analysis, minimize the averaging somewhat. Averaging is an advantage when the formation is being evaluated for seismic parameters, a different area of formation evaluation. A special log, the Long Spaced Sonic, is sometimes used for this purpose. Seismic signals average tens to hundreds of feet of formation, so an averaged sonic log is more directly comparable to a seismic waveform.

Density-neutron logs read the formation within about four inches of the borehole wall. This is an advantage in resolving thin beds. It is a disadvantage when the hole is badly caved. Corrections can be made automatically if the cave is no more than a few inches deep. A caliper arm on the sonde measures the profile of the borehole and a correction is calculated and incorporated in the porosity reading. However if the cave is much more that four inches deep, the density-neutron log is reading little more than drilling mud.

The SP log, known variously as a "Spontaneous Potential", "Self Potential" or "Shale Potential" log is simply a voltmeter measurement of the voltage or electrical potential difference between the mud in the hole at a particular depth and a copper ground stake driven into the surface of the earth a short distance from the borehole. A salinity difference between the drilling mud and the formation water acts as a sort of natural battery and will cause several voltage effects. This "battery" causes a movement of charged ions between the hole and the formation water where there is enough permeability in the rock. The most important voltage is set up as a permeable formation permits ion movement, reducing the voltage between the formation water and the mud. Sections of the borehole where this occurs then have a voltage difference with other nonpermeable sections where ion movement is restricted. Vertical ion movement in the mud column occurs much more slowly because the mud is stagnant, not being circulated while the drill pipe is out of the hole. The copper surface stake provides a reference point against which the SP voltage is measured for each part of the borehole. There can also be several other minor voltages, due for example to mud filtrate streaming into the formation under the effect of an overbalanced mud system. This flow carried ions and is a voltage generating current. These other voltages are secondary in importance to the voltage resulting from the salinity contrast between mud and formation water.

The nuances of the SP log are still being researched. It is a simple tool but unlike the other logging tools, nature designs the measurement and we can only make it. In theory, almost all porous rocks contain water. Some pores are completely filled with water. Others have a thin layer of water molecules wetting the surface of the rock, with gas or oil filling the rest of the pore. In sandstones and porous limestones there is a continuous layer of water throughout the formation. If there is even a little permeability to water, ions can move through the rock and decrease the voltage difference with the mud nearby. Shales do not allow water or ion movement. Although they may have a large water content, it is bound to the surface of the flat clay crystals comprising the shale. Thus mud opposite shale sections maintains its voltage difference with the surrounding rock. As the SP logging tool is drawn up the hole it measures the voltage difference between the reference stake and the mud opposite shale and sandstone or limestone sections. The resulting log curve reflects the permeabilty of the rocks and, indirectly, their lithology. SP curves degrade over time, as the ions diffuse up and down the mud column. It also can suffer from stray voltages caused by other logging tools that are run with it. Older, simpler logs often have better SP curves than more modern logs for this reason. With experience in an area, a good SP curve can even allow a skilled interpreter to infer sedimentary environments such as deltas, point bars or offshore tidal deposits.

The Gamma Ray log is simply a measurement of naturally occurring gamma radiation from the walls of the borehole. The basic idea is that sandstones are largely nonradioactive quartz and limestones are nonradioactive calcite. Shales however may have a high content of radioactive potassium isotopes. Thus the presence or absence of gamma rays in a section of borehole is an indication of the amount of shale or clay in the surrounding formation. The Gamma Ray log is useful in holes drilled with air or with oil based muds. These wells can have no SP voltage. Even in waterbased muds, the gamma ray and SP logs are often run together. They comprise a check on each other and can indicate some unusual shale sections which may either not be radioactive, or may have an abnormal ionic chemistry. The gamma ray log is also useful to detect coal beds (which tend to concentrate small amounts of uranium). Finally, the gamma ray log will work inside a steel casing making it essential when a cased well must be evaluated.