Neutron logging and its varieties are radiation methods of geophysical research. Depending on the type of radiation detected (neutrons or gamma-photons), several modifications of this technology are distinguished. Downhole equipment has a similar layout. Neutron logging allows you to determine one of the most important indicators of the oil and gas-bearing formation - the porosity coefficient, and also to divide the reservoirs according to the type of fluid contained in them.
Methods of geophysical research
In geophysics, several methods are used to study rocks, which can be conditionally divided into 2 large groups: electrical (electromagnetic) and non-electrical. The first group includes the following methods:
- Studies by unfocused probes: o apparent resistance method; o microprobe; o resistivity measurement; o current logging.
- Methods of research with focused probes: o lateral logging; o divergent logging.
- Electromagnetic techniques: o induction logging; o wave electromagnetic logging; o downhole radio wave method.
- Methods for measuring electrochemical activity: o method of potentials of spontaneous orientation; o method of electrode potentials; o method of evoked potentials.
The second group includes the following technologies:
- Seismic acoustic methods: o acoustic logging (including reflected wave method); o vertical downhole profiling; o interwell acoustic transmission; o seismic exploration.
- Nuclear physics methods.
- Thermal logging.
- Magnetic research methods: o downhole magnetic exploration; o magnetic susceptibility logging; o nuclear magnetic logging.
- Downhole gravity exploration.
- Gas and mechanical logging.
Radiometric methods
Nuclear physics research methods include a large group of technologies:
- gamma-ray logging (measurement of natural radioactivity);
- gamma gamma method;
- neutron methods;
- labeled atom technology;
- activation gamma method.
These methods are a powerful tool for studying geological formations crossed by a well. They are based on measuring the parameters of ionizing radiation emitted by the nuclei of atoms of substances contained in rock. Like acoustic logging, radiometric methods can be divided into methods that measure natural and artificial fields (radiation). As the radioactive particles are used those that have the greatest penetrating ability - neutrons (n) and gamma rays.
The essence of neutron technology
Neutron logging is one of the methods of geophysical research, which is based on exposure to a stream of fast neutrons. As a result, they slow down, disperse and absorb in the rock.
Downhole probes for neutron logging contain the following main nodes:
- radioactive radiation source;
- particle counter (n or gamma rays);
- filters that exclude direct radiation from the source to the detector.
Neutron characteristics of rocks
Once in rocks, fast neutrons slow down and lose energy due to interactions with atoms. In this state, they are scattered in the substance and captured by the nuclei of atoms of chemical elements in fractions of milliseconds.
The most intense moderator is hydrogen. The small path that a neutron travels to reaching its thermal state is characteristic of rocks with a high hydrogen content (oil and water saturated reservoirs, minerals that contain a lot of crystallization water).
The following neutron characteristics of the rocks are distinguished:
- The way to slow fast neutrons to a thermal state (in it, the particle energy approaches the value of the average kinetic energy of the thermal motion of rock molecules and atoms).
- Diffusion length (the path from the occurrence of a thermal neutron to its absorption).
- The lifetime of particles in a thermal state.
- The dispersion index in the rock.
- Particle migration length (total distance traveled during deceleration and diffusion).
In practice, these properties are evaluated using the conditional neutron porosity coefficient.
Varieties
Neutron logging includes several types of research, differing by 2 main criteria:
- The operation mode of the radiation source: o stationary methods; o pulsed methods (used mainly after casing wells).
- The nature of the recorded secondary radiation: o n-neutron logging (measure the amount of n scattered by the atomic nuclei of the rock substances); o neutron gamma method (ษฃ-radiation resulting from the capture of n); o neutron activation logging (ษฃ-radiation of artificial radionuclides released during the absorption of n).
The logging modification depends mainly on the type of detector (helium, scintillation, semiconductor counters) and the filters surrounding it. Stationary methods are included in the complex of compulsory research in the exploration drilling of wells.
Neutron neutron technique
This method of geophysical research is based on the first neutron characteristic of rocks and has 2 varieties: registration of thermal or epithermal neutrons. The energy of the latter is somewhat greater than the thermal energy of atoms.
Hydrogen among all elements is anomalous not only with respect to the scattering geometry, but also with respect to the loss of neutron energy in collision with it. For gas reservoirs, higher readings are characteristic than for water- and oil-saturated ones, since the specific hydrogen content in them is less.
The greater the porosity of the oil and gas bearing formation, the lower the indications of the epithermal method n. The data obtained during neutron-neutron logging, allows you to calculate the coefficient of porosity. Due to the reduced sensitivity of the epithermal particle counters, this method has less statistical accuracy.
Thermal neutrons are removed from the radioactive source for a longer path than the epithermal ones, and their average lifespan is determined by an inversely proportional relationship to the contents of chlorine, boron and rare earth elements. Chlorine is present in the reservoir waters of high salinity. Oil and gas bearing rocks are characterized by a longer duration of the existence of thermal particles. The principle of the neutron-neutron measurement technique for thermal n is based on this property.
Neutron gamma ray logging
In a neutron gamma study, the gamma radiation that is produced when thermal n is captured is measured. Aquifers are distinguished by greater readings, compared with oil, by 15-20% (with the same porosity). A significant difference from the previous methods is that the readings of this technology increase with increasing mineralization of the drilling fluid.
Since in case of neutron-gamma-ray logging a natural radioactive background is also recorded in the rocks, correction coefficients are introduced to interpret the obtained results. In oil and gas wells, this method is used for the same purposes as the neutron-neutron technique - the separation of rocks according to different hydrogen contents, the determination of the porosity coefficient, the detection of gas-liquid and water-oil contact in a cased well. There are also combined techniques that record n โโand gamma radiation, which improves the accuracy of measurements.
Pulse technology
Pulse logging is a type of neutron research methods based on neutron emission for short time intervals (100-200 microseconds). There are also 2 modifications of this technology:
- registration of thermal n;
- measurement of ษฃ-quanta of radiation capture.
By registering one of these parameters by 2 time values, the average lifetime of thermal neutrons in the formation rocks is obtained. This allows you to judge the presence of certain chemical elements. Aquifers have significantly lower readings for longer time delays than oil and gas bearing ones.