Different types of wireline well log

There are many different types of log – here are some of the most common examples.

The first type of log, invented by the Schlumberger brothers in the 1920s, was an Electrical Log used to measure resistivity. It passed an electrical current into the formation and the resistivity to the current indicated the presence of hydrocarbons. Nowadays, resistivity is measured in other ways, but the electrical log is still used to measure the current generated by contact between mud and natural waters in the pores of a reservoir rock. This is known as Spontaneous or Self Potential (abbreviated to SP) and is recorded on Track 1 of the log. Different types of sedimentary rocks have different SP signatures, enabling identification by the petrophysicist.

Nowadays, resistivity (or its inverse, conductivity) is measured by an Induction Log (IL), which allows the electric current to be focused and induced in a very specific zone of the formation. The tool can measure shallow, medium or deep resistivity depending on how far into the formation the current is passed. Oil and gas have very high resistivity and their presence shows a kick on the log.

A third type of log is the Gamma Ray Log (GRL), which uses a scintillation counter to measure natural radioactivity in the rock. There are three main sedimentary rocks – shale, limestone and sandstone. Only shales are radioactive and they kick to the right on the log. Other sedimentary rocks kick to the left. Gamma ray logs are frequently used because they can be run in both openhole and cased-hole and are relatively cheap.

Then, there are two types of log which act as radioactive sources. The first is a Neutron Porosity Log (NL), which bombards the rock formation with high speed atomic particles (neutrons). When a neutron meets a hydrogen particle, a lot of its energy is absorbed and gamma rays are produced. On the log, either the slow- moving neutrons or the gamma rays are counted, and this shows the porosity of the rock because the greater the number of neutrons and gamma rays, the more porous the rock. The log shows porosity as a percentage.

The other is a Formation Density Log (FDL), also called a Gamma-Gamma Log. This measures the density of the subsurface rock by emitting gamma rays, which are absorbed when they encounter dense formation, but less so when they meet porous rock. So, in this case, a high measurement is an indicator of hydrocarbons.

The last log is the Caliper Log (CAL). This is used to measure the diameter of the hole. It has sensors which brush the side of the hole as it is run up and generate a record of the size. The hole size is important for making calculations of cement quantities for casing, and also for the calibration of compensated logs.

Task 3. Fill in the table below in as much detail as you can from the information provided in Task 2. See if you can complete the table by carrying out further research on the Internet:

Задание 3. Заполните таблицу, используя информацию из задания 2. Воспользуйтесь интернетом при необходимости: