Geophysical research: types, methods and technologies

2024 Author: Howard Calhoun | [email protected]. Last modified: 2023-12-17 10:16

Geophysical research is used to study rocks in near-wellbore and inter-well space. They are carried out by measuring and interpreting natural or artificial physical indicators of various types. Currently, there are more than 50 geophysical methods.

General characteristics

Geophysical research (GIS, production geophysics or logging) is a set of applied geophysics methods used to study geological profiles, obtain information about the technical condition of wells and identify minerals in the subsoil.

GIS is based on various physical properties of rocks:

• electric;
• radioactive;
• magnetic;
• thermal and others.

Production geophysical surveys of wells are the main type of geological documentation of wells. The purpose of their implementation is to solve a number of technical problems (comparison of sections foridentification of strata of the same age, determination of productive strata, marker horizons, lithological composition, the main characteristics of the formation that affect the development, development and operation of wells). The principle of any well logging method is to measure the values that characterize the properties of rocks and interpret them.

Electrical Methods

When conducting electrical geophysical surveys of oil wells, the following characteristics are measured:

1. Electrical resistivity (conductor minerals, semiconductors, dielectrics).
2. Electrical and magnetic permeability.
3. Electrochemical activity of rocks - natural (self-polarization potential method) or artificially induced (induced polarization potential method).

The first characteristic is associated with such a feature as increased resistivity of oil and gas saturated rocks, which is an identification feature of oil and gas deposits (they do not conduct electricity). Measurements are evaluated using the resistance increase factor, which allows you to determine the most important characteristics of the reservoir - the coefficient of porosity, water and oil and gas saturation. The most common techniques of this technology are described below.

Apparent resistance method

A probe with three grounding electrodes (one supply and 2 measuring electrodes) is lowered into the well, and the fourth (supply) is installed at the wellhead. When the probe moves vertically along the wellbore, the potential difference changes. Specific electricalresistance is called apparent because it is calculated for a homogeneous medium, but in fact it is inhomogeneous. Based on the data obtained, curves are built that can be used to determine the boundaries of the reservoir.

Side electrical sounding

Gradient probes of great length (a multiple of 2-30 well diameters) are used in measurements, which allows taking into account the influence of the drilling fluid and the depth of its penetration into the rocks, to determine the true formation resistivity.

Shielded grounding method with seven or three electrode probes

In a seven-electrode probe, the current strength is regulated so that the equality of potentials is ensured at the central and extreme points along the borehole axis. This is done to direct a focused beam of electric charge into the rock. The result is also apparent resistance.

Induction method

A probe with emitting and receiving coils, an alternator and a rectifier is lowered into the well. When creating the induced EMF, the apparent electrical conductivity of the formation is determined.

Dielectric method

Similar to the previous one, but the frequency of the electromagnetic field in the coil is an order of magnitude higher. This method is used to determine the nature of reservoir saturation with low water salinity.

There is also a method of microprobes (their size does not exceed 5 cm) to measure the electrical resistance of the rock,directly adjacent to the borehole wall.

Radiometry

Radiometric geophysical research methods are based on the registration of nuclear radiation (most often neutrons and gamma rays). The most common methods are:

• natural rock radiation (ɣ-method);
• scattered ɣ radiation;
• neutron-neutron (registration of neutrons scattered by the nuclei of atoms of rock);
• pulse neutron;
• neutron activation (ɣ-radiation of artificial radioactive isotopes arising from the absorption of neutrons);
• nuclear magnetic resonance;
• neutron ɣ-method (ɣ-radiative neutron capture radiation).

The techniques are based on the law of attenuation of the gamma radiation flux density, the effect of scattering and absorption of neutrons in the rock. Based on this, the density of rocks, their mineral composition, clay content, fracturing are determined, and radioactive contamination of downhole drilling equipment is monitored.

Seismoacoustic methods

Acoustic methods are based on the measurement of natural or artificial sound vibrations. In the first case, geological and geophysical studies of noises that occur when gas or oil enters the wellbore are carried out, as well as the vibration spectrum of the drilling tool is measured during rock penetration.

Methods for studying artificial oscillations of the sound or ultrasonic spectrum are based on measuring the time of wave propagation ordamping of the oscillation amplitude. The speed of sound propagation depends on several parameters:

• mineral composition of rocks;
• the degree of their gas-oil saturation;
• lithological features;
• clayness;
• stress distribution in rocks;
• cementation and others.

The probe lowered into the borehole consists of a transmitter and a receiver separated by acoustic insulators. Three- or four-element probes are usually used to reduce the effect of borehole geometry on measurement results. The downhole tool is connected to the surface equipment with a cable. The signal from the receiver is digitized and displayed on the screen.

With the help of this method, studies of lithological dissection of the reservoir section, large underground cavities are carried out, reservoir properties are determined and water cut is controlled.

Thermal logging

The basis of thermal logging in field geophysical surveys is the study of the temperature gradient along the wellbore, which is associated with different thermal properties of rocks (methods of natural and artificial thermal field). The thermal conductivity of the main rock-forming minerals ranges from 1.3-8 W / (m∙K), and at high gas saturation it drops several times.

Artificial thermal fields are created during drilling with the help of flushing fluid or installation of electric heaters in the well. To measure the temperature gradient most oftendownhole electric resistance thermometers are used. Copper wire and semiconductor materials are used as the main sensing element.

Change in temperature is recorded indirectly - by the magnitude of the electrical resistance of this element. The measuring circuit also contains an electronic oscillator whose oscillation period varies with resistance. Its frequency is measured by a special device, and the constant voltage generated in the frequency meter is transmitted to the visual observation equipment.

Carrying out geophysical research using this technique allows obtaining information about the geological structure of the field, identifying oil, gas and water-bearing formations, determining their flow rate, detecting anticlinal structures and s alt domes, thermal anomalies associated with the influx of hydrocarbons. The use of this technology is especially relevant in areas with active volcanic activity.

Geochemical GIS methods

Geochemical research methods are based on a direct study of the gas saturation of the drilling fluid and cuttings formed during well flushing. In the first case, the determination of the content of hydrocarbon gases can be carried out directly during drilling or after it. The drilling fluid is degassed in a special unit, and then the hydrocarbon content is determined using a gas analyzer-chromatograph located in the logging station.

Slurry, or particles of drilled rock,contained in the drilling fluid are studied by luminescent or bituminological methods.

Magnetic logging

Magnetic methods for conducting well logging include several ways to differentiate rocks:

• by magnetization;
• on magnetic susceptibility (creation of an artificial electromagnetic field);
• for nuclear magnetic properties (this technology is also referred to as nuclear logging).

The strength of the magnetic field is due to the presence of magnetic ore bodies and layers that underlie and overlap them. Magnetic modulation sensors (flurosondes) serve as sensitive elements of downhole equipment. Modern instruments can measure all three components of the magnetic field vector, as well as magnetic susceptibility.

Nuclear magnetic logging is to determine the characteristics of the magnetic field, which is induced by hydrogen nuclei in the pore fluid. Water, gas and oil differ in the content of hydrogen nuclei. Thanks to this property, it is possible to study the reservoir and its permeability, identify the type of fluid, and differentiate the types of constituent rocks.

gravity exploration

Gravity exploration is a method of geophysical exploration of deposits based on a non-uniform distribution of the gravity field along the length of the wellbore. By purpose, 2 types of such logging are distinguished - to determine the density of rocks of the layers that cross the well, and to identify the location of geological objects that cause an anomaly in gravity (change in its value).

The jump of the last indicator occurs when moving from a reservoir with a lower density to denser rocks. The essence of the method is to measure the vertical gravity and determine the thickness of the reservoir. This data allows you to find out the density of rocks.

String and quartz gravimeters are used as the main downhole equipment. The first type of devices is the most widely used. Such gravimeters are an electromechanical vibrator in which an alternating voltage is applied to a vertically fixed string with a suspended load. The vibrator is connected to a generator, and its frequency fluctuations serve as the final parameter.

Equipment

Geophysical research methods are carried out with the help of field geophysical stations, the main elements of which are:

• downhole tools;
• winch with a mechanical or electromechanical drive (from the power take-off, electrical network or independent power source);
• drive control unit;
• control system for the main indicators of tripping procedures (depth of immersion, speed of descent into the well, tension force) - display unit, tension unit, depth sensor;
• borehole lubricator for sealing the wellhead during well logging (includes shutoff valves, stuffing box, receiving chamber, pressure gauges and other instrumentation);
• ground measuring equipment (on the chassis of a car).

Deep well maintenance equipmentcan be located in the bodies of two cars. Laboratories for geophysical exploration of wells are mounted on the chassis of URAL, GAZ-2752 Sobol, KamAZ, GAZ-33081 and others. The body of the car usually includes 2 compartments - a worker, in which the equipment is located, and a "change house" for service personnel.

The main requirements for equipment are high accuracy and reliability of geophysical surveys. Work in wells is associated with difficult conditions - great depth, significant temperature drops, vibrations, shaking. The equipment is completed according to the requirements of the customer, the method used and the goals of the work. For geophysical research in offshore wells, all equipment is transported in containers.

Interpretation of results

The results of geophysical surveys are processed step by step from the values of measuring instruments to the determination of the geophysical parameters of the reservoir:

1. Conversion of downhole equipment signals.
2. Determination of the true physical properties of the studied rocks. Additional field geophysical work may be required at this stage.
3. Determination of lithological and reservoir properties of the formation.
4. Using the results obtained to solve one of the tasks set - identifying mineral deposits, their distribution throughout the region, determining the geological age of rocks, coefficients of porosity, clay content, gas and oil saturation, permeability; identification of reservoirs, study of featuresgeological section and others.

Interpretation of geophysical surveys is carried out by various methods depending on the technology used (electrical, radiometric, thermal, etc.) and measuring equipment. Modern geophysical organizations operate automated data collection and processing systems (Prime, Pangea, Inpres, PaleoScan, SeisWare, DUG Insight and others).