Well logging is an interesting method that deals with the subsurface layers providing every necessary detail required about them. The process basically involves specialized tools which are sent down through the well bore to gather the information about the subsurface properties. This data collected is represented over a series of measurements covered over the depth range of the well bore and this assimilation is collectively known as well logs.
The first well log was made in 1927 at Pechelbronn Field in Alsace, France. Invented by the Schlumberger brothers, Marcel and Conrad, the tool measured the electrical resistance of the earth’s subsurface. They recorded the data at each meter as they retrieved the sonde, a specialized tool suspended from the cable, from the borehole. This data log of the corresponding resistivity change was used to identify the location of oil.
The well logs are useful in defining a number of parameters that include physical rock characteristics, lithology, mineralogy, pore geometry, porosity and permeability. By interpretation of the well log data we can also estimate the productive zones with their thickness and depth by defining the basic parameters like fluid composition and relative saturation. The other prominent methods that involve the well data is relating them to seismic to obtain a proper correlation pattern that will help in confirming the hydrocarbon presence.
For oil and gas prospecting the well data helps in the rock type identiﬁcation of geological environment, reservoir ﬂuid contact location and fracture detection. One can also estimate the total hydrocarbon in place along with the recoverable hydrocarbon. Determination of water salinity, reservoir pressure, porosity or pore size distribution is also done with its help. The other properties that can be interpreted from the data are water ﬂood feasibility, reservoir quality mapping, interzone ﬂuid communication probability and reservoir ﬂuid movement monitoring.
Applications of well logging data differ according to the specified specializations.
For the Geologist:
1.Depths of formation tops.
2. Geological environment and hydrocarbon accumulation.
3. Presence of hydrocarbons and its quantity.4.What are the reserves?
5. Conditions for an offset well.
For the Geophysicist:
1.Predicted formation tops.
2. Potential zone analysis for properties such as porosity as assumed from seismic data.
3. Analysis of synthetic seismic sections.
For the Drilling Engineer:
1.Hole volume for cementing.
2. Presence of any Key-Seats or severe Dog-legs.
3. Location for a good packer seat for testing.
4. Location to set a Whipstock.
For the Reservoir Engineer:
1.Thickness of pay zone.
2. Homogeneity of the section.
3. Volume of hydrocarbon per cubic meter.
4. Duration of well pay-out.
For the Production Engineer:
1.Determination of well completion zones.
2. Expected production rate.
3. Chances for water production.
4. Suitable well completion methods.
5. Determination of hydraulically isolated potential pay zone.
2. Methods of Logging
Performed on a well before the wellbore has been cased and cemented.
Logging is done through the bare rock sides of the formation.
Common type of logging method because the measurements are not obstructed.
Done during or after the well has been drilled.
Retrieve logging measurements through the well casing, or the metal piping that is inserted into the well during completion operations.
Cased-hole logging is performed more rarely but it provides valuable information about the well.