Geophysical studies are used to study rocks in the near-well and inter-well spaces. 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 exploration (GIS, field 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 bowels.
GIS is based on various physical properties of rocks:
- electric;
- radioactive;
- magnetic;
- thermal and others.
Well logging is the main type of geological documentation for wells. The purpose of their implementation is to solve a number of technical problems (comparing sections to identify strata of the same age, determining productive formations, marking horizons, lithological composition, the main characteristics of the formation that affect the development, development and operation of wells). The principle of any GIS method is to measure the quantities characterizing the properties of the rocks, and their interpretation.
Electrical methods
When conducting electrical geophysical studies of oil wells, the following characteristics are measured:
- Electrical resistivity (minerals-conductors, semiconductors, dielectrics).
- Electric and magnetic permeability.
- The electrochemical activity of the rocks is natural (the method of potentials of its own polarization) or artificially induced (the method of potentials of induced polarization).
The first characteristic is associated with such a feature as increased resistivity of oil and gas-saturated rocks, which is an identification sign of oil and gas deposits (they do not conduct electric current). Measurements are estimated using the coefficient of increase in resistance, 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 methods of this technology are described below.
Apparent resistance method
A probe with three ground electrodes (one supply and 2 measuring) is lowered into the well, and the fourth (supply) is installed at the wellhead. With vertical movement of the probe along the wellbore, the potential difference changes. The electrical resistivity is called apparent because it is calculated for a homogeneous medium, but in fact it is heterogeneous. Based on the data obtained, curves are constructed from which the boundaries of the formation can be determined.
Lateral electrical sounding
In measurements, long-length gradient probes (a multiple of 2-30 borehole diameters) are used, which allows you to take into account the influence of the drilling fluid and the depth of its penetration into the rocks, to determine the true resistivity of the formation.
Shielded grounding method with a seven- or three-electrode probe
In a seven-electrode probe, the current is regulated so that equal potentials are provided at the central and extreme points along the axis of the well. This is done to direct a focused beam of electric charge into the rock. As a result, apparent resistance is also obtained.
Induction method
A probe with emitting and receiving coils, an alternator and a rectifier is lowered into the well. When creating induced EMF, the apparent conductivity of the formation is determined.
Dielectric constant 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 formation saturation with a small salinity of water.
There is also a microprobe method (their size does not exceed 5 cm) for measuring the electrical resistance of a rock immediately adjacent to a well 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 emissions (ษฃ-method);
- scattered ษฃ-radiation;
- neutron-neutron (registration of neutrons scattered by nuclei of rock atoms);
- pulsed neutron;
- activation neutron (ษฃ-radiation of artificial radioactive isotopes arising from neutron absorption);
- nuclear magnetic resonance;
- neutron method (ษฃ radiation of radiation capture of neutrons).
The methods are based on the law of attenuation of the gamma radiation flux density, the effect of neutron scattering and absorption in the rock. Based on this, determine the density of the rocks, their mineral composition, clay, fracture, control the radioactive contamination of downhole drilling equipment.
Seismic Acoustic Methods
Acoustic methods are based on the measurement of natural or artificial sound vibrations. In the first case, geological and geophysical studies of noise occurring when gas or oil enters the wellbore, and also measure the vibration spectrum of the drilling tool during the passage of rocks.
The methods for studying artificial vibrations of the sound or ultrasound spectrum are based on measuring the propagation time of the wave or the damping of the oscillation amplitude. The speed of sound propagation depends on several parameters:
- mineral composition of rocks;
- the degree of gas and oil saturation;
- lithological features;
- clay content;
- stress distribution in rocks;
- cementation and others.
The probe lowered into the well consists of an emitter and a vibration receiver, separated by acoustic insulators. To reduce the influence of well geometry on the measurement results, three- or four-element probes are usually used. The downhole projectile is connected to the ground equipment by cable. The signal from the receiver is digitized and visualized on the screen.
Using this method, studies of lithological partitioning of a section of a formation, large underground cavities are carried out, collector properties are determined and water cut is controlled.
Thermal logging
The basis of thermal logging in field geophysical studies is the study of the temperature gradient along the wellbore, which is associated with various thermal properties of rocks (methods of natural and artificial thermal fields). The thermal conductivity of the main rock-forming minerals ranges from 1.3-8 W / (m โ K), and at high gas saturation it drops several times.
Artificial thermal fields are created during drilling using flushing fluid or installation of electric heaters in the well. To measure the temperature gradient, borehole electrical resistance thermometers are most often used. As the main sensitive element, copper wire and semiconductor materials are used.
The temperature change is recorded indirectly - by the magnitude of the electrical resistance of this element. The measuring circuit also contains an electronic generator, the oscillation period of which varies depending on the resistance. Its frequency is measured with a special device, and the constant voltage generated in the frequency meter is transmitted to the visual observation equipment.
Conducting geophysical studies using this methodology allows obtaining information on the geological structure of the field, identifying oil, gas and water-producing strata, determining their flow rate, detecting anticlinal structures and salt domes, thermal anomalies associated with the influx of hydrocarbons. The application of this technology in areas with active volcanic activity is especially relevant.
GIS geochemical methods
Geochemical methods of research are based on a direct study of the gas saturation of the drilling fluid and sludge generated during flushing of the well. In the first case, the determination of the content of hydrocarbon gases can be carried out directly in the process of drilling or after it. The drilling fluid is degassed in a special installation, and then the hydrocarbon content is determined using a gas analyzer-chromatograph located in a logging station.
Sludge, or particles of drilled rock contained in the drilling fluid, are studied by a luminescent or bituminological method.
Magnetic Logging
Magnetic methods for conducting geophysical surveys of wells include several methods for differentiating rocks:
- by magnetization;
- magnetic susceptibility (creating an artificial electromagnetic field);
- by nuclear magnetic properties (this technology also applies to nuclear logging).
The magnetic field is due to the presence of magnetic ore bodies and formations that underlie and overlap them. Sensitive elements of downhole equipment are magnetomodulation sensors (flux probes). Modern instruments can measure all three components of the magnetic field vector, as well as magnetic susceptibility.
Nuclear magnetic logging consists in determining the characteristics of a magnetic field that is induced by hydrogen nuclei in a pore fluid. Water, gas and oil differ in the content of hydrogen nuclei. Due to this property, it is possible to study the reservoir and its permeability, identification of the type of fluid, differentiation of the types of constituent rocks.
Gravel
Gravity exploration is a method of geophysical field research based on the heterogeneous distribution of the gravity field along the length of the wellbore. According to the purpose, 2 types of such logging are distinguished - to determine the density of the rocks of the layers that cross the well, and to identify the location of geological objects causing an anomaly of gravity (changing its value).
A jump in the latter 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. These data allow 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 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 the generator, and the final parameter is the oscillation of its frequency.
Equipment
Geophysical research methods are carried out using field geophysical stations, the main elements of which are:
- downhole tools;
- a winch with a mechanical or electromechanical drive (from a power take-off, an electrical network or an autonomous current source);
- drive control unit;
- control system for the main indicators of the lifting procedures (depth of immersion, speed of descent into the well, tension force) - display unit, tension unit, depth sensor;
- downhole lubricator for sealing the wellhead during well logging (includes stop valves, gland, receiving chamber, manometers and other instrumentation);
- ground measuring equipment (on a car chassis).
For servicing deep wells, equipment can be located in the bodies of two cars. Laboratories for geophysical research of wells are mounted on the chassis of URAL, GAZ-2752 Sobol, KamAZ, GAZ-33081 and others. The car body usually includes 2 compartments - the worker, in which the equipment is located, and a "change house" for maintenance personnel.
The main equipment requirements are high accuracy and reliability of geophysical surveys. Work in wells is fraught with harsh conditions - great depth, significant temperature drops, vibrations, shaking. Equipment is supplied in accordance with the requirements of the customer, the method used and the objectives of the work. To conduct geophysical surveys in offshore wells, all equipment is transported in containers.
Interpretation of Results
The results of geophysical surveys go through phased processing from the values โโof measuring instruments to determining the geophysical parameters of the formation:
- Conversion of downhole equipment signals.
- Determination of the true physical properties of the studied rocks. At this stage, additional field geophysical work may be required.
- Determination of lithological and reservoir properties of the reservoir.
- Using the results to solve one of the tasks - identifying mineral deposits, their distribution throughout the region, determining the geological age of rocks, porosity, clay, gas and oil saturation, permeability; the selection of reservoirs, the study of the features of the geological section and others.
Interpretation of geophysical surveys is carried out by various methods depending on the technology used (electrical, radiometric, thermal and others) and measuring equipment. Modern geophysical organizations operate automated data collection and processing systems (Prime, Pangea, Inpres, PaleoScan, SeisWare, DUG Insight and others).