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Date: 2015-10-07; view: 551.

Unit 4. Well logging

C. Discussion

Topics:

1. Imaging techniques purposes;

2. The advantages of each of three imaging techniques (2-D, 3-D, 4-D);

3. Debate the proposition “One of the greatest innovations in the history of oil and natural gas exploration is the use of computers”.

Logging refers to performing tests during or after the drilling process to allow geologists and drill operators to monitor the progress of the well drilling and to gain a clearer picture of subsurface formations. There are many different types of logging, in fact; over 100 different logging tests can be performed, but essentially they consist of a variety of tests that illuminate the true composition and characteristics of the different layers of rock that the well passes through. Logging is also essential during the drilling process. Monitoring logs can ensure that the correct drilling equipment is used and that drilling is not continued if unfavorable conditions develop.

Various types of tests include standard, electric, acoustic, radioactivity, density, magnetic, temperature, mechanical, induction, caliper, directional and nuclear logging, to name but a few. The most prolific and often performed tests include standard logging (mud logging, coring) and wireline logging (electric logging, radioactive logging and acoustic (sonic) logging).

Standard logging consists of examining and recording the physical aspects of a well. For example, the drill cuttings (rock that is displaced by the drilling of the well) are all examined and recorded, allowing geologists to physically examine the subsurface rock. Also, core samples are taken, which consists of lifting a sample of underground rock intact to the surface, allowing the various layers of rock, and their thickness, to be examined. These cuttings and cores are often examined using powerful microscopes, which can magnify the rock up to 2000 times. This allows the geologist to examine the porosity and fluid content of the subsurface rock, and to gain a better understanding of the earth in which the well is being drilled.

Electric logging consists of lowering a device used to measure the electric resistance of the rock layers in the 'down hole' portion of the well. This is done by running an electric current through the rock formation and measuring the resistance that it encounters along its way. This gives geologists an idea of the fluid content and characteristics. A newer version of electric logging, called induction electric logging, provides much the same types of readings but is more easily performed and provides data that is more easily interpreted.

Acoustic (sonic) logs use a sound generator and microphone to measure the velocity of sound in the formation from one end of the sonde to the other. For a given type of rock, acoustic velocity varies indirectly with porosity.

Radioactive logging includes gamma-ray logging, neutron logging and density logging. The gamma-ray logging is simply a measurement of naturally occurring gamma radiation from the walls of the borehole. The basic idea is that sandstones and limestones are largely non-radioactive. 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.

Neutron logs irradiate the borehole walls with neutrons. They tend to pass through most minerals, but to be captured by hydrogen atoms. When that happens, a hydrogen atom emits a gamma ray. The number of these is proportional to the number of hydrogen atoms present in the formation. In most rocks, hydrogen occurs only in oil or water in the pores.

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 reemitted gamma rays detected by the sonde is inversely proportional to the density 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.

An example of the data obtained through various forms of logging is shown below. In this representation, the different columns indicate the results of different types of tests. The data is interpreted by an experienced geologist, geophysicist, or petroleum engineer, who is able to learn from what appear as 'squiggly' lines on the well data readout.

The drilling of an exploratory or developing well is the first contact that a geologist or petroleum engineer has with the actual contents of the subsurface geology. Logging, in its many forms, consists of using this opportunity to gain a fuller understanding of what actually lies beneath the surface. In addition to providing information specific to that particular well, vast archives of historical logs exist for geologists interested in the geologic features of a given, or similar, area.