Well log is a continuous record of measurement made in bore hole respond to variation in some physical properties of rocks through which the bore hole is drilled. Traditionally Logs are display on girded papers shown in figure1. Now a days the log may be taken as films, images, and in digital format.
HISTORY
- 1912 Conrad Schlumberger give the idea of using electrical measurements to map subsurface rock bodies.
- in 1919 Conrad Schlumberger and his brother Marcel begin work on well logs.
Logging Unit - The first electrical resistivity well log was taken in France, in 1927.
- The instrument which was used for this purpose is called SONDE, the sound was stopped at periodic intervals in bore hole and the and resistivity was plotted on graph paper.
- In 1929 the electrical resistivity logs are introduce on commercial scale in Venezuela, USA and Russia
- For correlation and identification of Hydrocarbon bearing strata.
- The photographic – film recorder was developed in 1936 the curves were SN,LN AND LAT
- The dip meter log were developed in 1930
- the Gamma ray and Neutron Log were began in 1941.
LOGGING UNITS
- logging cable
- winch to raise and lower the cable in the well
- self-contained 120-volt AC generator
- set of surface control panels
- set of downhole tools (sondes and cartridges)
- digital recording system
GR (gamma ray) logs measure radioactivity to determine what types of rocks are present in the well. Because shales contain radioactive elements, they emit lots of
gamma rays. On the other hand, clean sandstones emit very few gamma rays.
SP (spontaneous potential) logs indicate the permemabilities of rocks in the well by measuring the amount of electrical current generated between the drilling fluid and the formation water that is held in pore spaces of the reservoir rock. Porous sandstones with high permeabilities tend to generate more electricity than impermeable shales. Thus, SP logs are often used to tell sandstones from shales
Resistivity logs determine what types of fluids are present in the reservoir rocks by measuring how effective these rocks are at conducting electricity. Because fresh water and oil are poor conductors of electricity they have high resistivities. By contrast, most formation waters are salty enough that they conduct electricity with ease. Thus, formation waters generally have low resistivities. There are many different types of resistivity logs, which results in a confusing array of acronyms.
BHC (borehole compensated) logs, also called sonic logs, determine porosity by measuring how fast sound waves travel through rocks in the well. In general, sound waves travel faster through high-density shales than through lower-density sandstones.
FDC (formation density compensated) logs, also called density logs, determine porosity by measuring the density of the rocks. Because these logs overestimate the porosity of rocks that contain gas they result in “crossover” of the log curves when paired with Neutron logs (described under CNL logs below).
CNL (compensated neutron) logs, also called neutron logs, determine porosity by assuming that the reservoir pore spaces are filled with either water or oil and then measuring the amount of hydrogen atoms (neutrons) in the pores. Because these logs underestimate the porosity of rocks that contain gas they result in “crossover” of the log curves when paired with FDC logs (described above).
NMR (nuclear magnetic resonance) logs may be the well logs of the future. These logs measure the magnetic response of fluids present in the pore spaces of the reservoir rocks. In so doing, these logs measure both porosity and permeability, as well as the types of fluids present in the pore spaces.
Dipmeter logs determine the orientations of sandstone and shale beds in the well, as well as the orientations of faults and fractures in these rocks. The original dipmeters did this by measuring the resisitivity of rocks on at least four sides of the well hole. Modern dipmeters actually make a detailed image of the rocks on all sides of the well hole. Borehole scanners do this with sonic (sound) waves, whereas FMS (formation microscanner) and FMI (formation micro-imager) logs do this by measuring the resisitisvity. These modern, essentially 3D logs are known as image logs since they provide a 360°ree; image of the bore hole that can show bedding features, faults and fractures, and even sedimentary structures, in addition to providing basic dipmeter data on the orientations of bedding.
References:
1-Bassiouni, Z: Theory, Measurement, and Interpretation of Well Logs, SPE Textbook Series
2-Schlumberger, Log Interpretation Charts, Houston, TX (1995)
3-Western Atlas, Log Interpretation Charts, Houston, TX (1992