Logging

Actually geophysical methods of prospecting and exploration of mineral deposits were created with the aim not to produce expensive drilling of wells, but to be limited only to ground-based measurements. Nevertheless, it is often impossible to do well without drilling wells in the studied area, since the well is a source of core samples of rock extracted with a drilling tool to the surface. At the same time, coring and preservation in its original form upon delivery to the laboratory, is itself an expensive process. For this reason, the need for a coreless well study arose. An additional incentive for this was the fact that many wells are drilled on brittle and loose rocks, where the core cannot be raised to the surface.

When researching in wells, all analogues of types of ground-based geophysics can be used, primarily electrical exploration . Due to the presence of the well’s influence on instrument readings, as well as other factors, for one round-trip operation, measurements are taken at once by several different methods, the results of which are interpreted together.

In Russian, the term “logging” came from French, where the word core sample was called in the jargon of drillers with the word la carotte (“carrot”). The coring experts were jokingly called "carrots." The same word, in French, has a second meaning - petty fraud. The first logging methods did not differ in accuracy and often did not justify themselves, therefore logging specialists were called scammers not only for fun. Thanks to this, the term “logging” first took root in the French language, and then spread to the rest. Over time, in German the term was replaced by bohrlochmessung, in English by well logging, and in the French language the term des diagraphies is now used, but in Russia the old term remains.

Moreover, geophysical surveys of wells themselves have become strongly associated with this term, although they, in general, also include well operations and well geophysics.

For the first time, the term and the methodology itself were introduced by the brothers Conrad and Marcel Schlumberger (founders of the famous oilfield services company Schlumberger ). In their modification, electric logging was used, and the main area was the search for coal seams. Over time, logging methods began to be used in ore deposits, but then they were mainly used in oil and gas. To date, in this industry, the cost of logging does not exceed 4% of the cost of drilling operations, while providing most of the information received.

To carry out logging operations, a geophysical probe containing all the necessary equipment is lowered into the well. Part of the information received is immediately transmitted to the surface via a geophysical cable, which serves as a load-bearing power element, as well as a data transmission channel, and as an electrical conductor . In this case, some of the information can still be recorded in the memory of the probe itself and obtained after removing the probe to the surface. For technical reasons, any logging in the well is carried out from the bottom up, first lowering the probe to the required depth, and only then, slowly raising it, record the signals.

This way it is better to maintain a constant speed of the probe, while during the descent the probe can get stuck in the well (grab). However, this does not interfere with insignificant measurements sometimes carried out during the descent of the probe (densitometer, thermometry). If the probe travels too fast along the well, the equipment may simply not have time to measure even large anomalies. At the same time, too low a probe raising speed leads to an increase in the time of logging operations, and therefore to an increase in the cost of work in general.

Due to the fact that a real well, in contrast to an ideal one, is never straight and also has a variable radius, technical difficulties arise with the accurate determination of the current probe depth. For this reason, the current depth is measured in several ways at once:

• the so-called locator of couplings ( LM ), with the help of which the couplings are connected, which connect the pipes lowered into the well
• a counter of magnetic marks on a geophysical cable, each of which is located at a distance of 10 meters from the previous one (a double mark is placed every 100 meters for verification)

In real situations, some magnetic marks may be missed by the counter, and the locator may not notice one of the couplings, however, their joint use allows you to level out these errors and relate the position of the probe to the correct depth with sufficient accuracy.

One of the drawbacks of the technique is that the well itself affects the probe readings:

• metal walls of the well affect the magnetic measurements of the probe, are a good conductor, making electrical measurements difficult, and also interfere with the direct investigation of the annulus, preventing the use of many other methods
• the drilling fluid with which the well is filled contains water in large quantities, which makes it difficult to use methods that record the hydrogen content of oil, since the drilling fluid water also contains hydrogen atoms, as well as chlorine salts with a large neutron capture cross section and clay particles with natural gamma radiation
• in practice, there is also inconsistency in the actions of the drillers who created the well with the geophysicists who make measurements in it. In the course of drilling operations, it is more profitable and reliable for crews to work with heavier drilling mud containing an increased proportion of clay particles. However, these same particles settle on the walls of the well, creating a thick clay crust through which the measured geophysical fields will not be able to break through in the future.

To increase the information content of one hoisting operation, several instruments can be placed in one geophysical probe at once. There are cases when there are more devices than can fit in one probe, or these devices may be incompatible with each other and cannot be placed in one probe. Then, not one probe, but a bunch of several placed one after another can be lowered into the well. In addition, the so-called “braid” can be attached to the probe. Outwardly, it is a relatively short cable, on which sensors are placed like a garland, but at the same time, the information taken from them is sent to the main equipment located in the probe body.

Depending on the logging method, it may be necessary to center the probe along the axis of the well (in this case, the probe should not touch the walls of the well), or vice versa - tightly press the probe against the wall. In both cases, the result is achieved using springs located outside the housing. For centering, four springs are mounted on the casing, placed crosswise in plan view; to press against the well wall, one spring located at the side is sufficient.

Electric logging is a processing of field electrical exploration work, in relation to cramped conditions in the well. In general terms, the work is reduced to passing a current through two or more electrodes with the subsequent measurement of any electrical parameters: current strength, potential difference, frequency, dielectric constant, etc. It is the difference in the measured value that determines the variety of electric logging methods. Also, these differences are due, for example, to the configuration of the electrodes lowered into the well, that is, their relative position with respect to each other.

Apparent Resistance Method Group

• proper KS - apparent resistance with unfocused probes. The most common method of this group, which is a borehole analogue of the electric profiling method in electrical exploration
• resistivimetry . Using this method, the electrical resistivity of the fluid currently filling the well is measured. A fluid can be represented as a drilling fluid (its resistance is known in advance), and reservoir fluids ( oil , fresh or saline water), as well as their mixture
• BKZ - lateral logging sounding. This method is a downhole analogue of the method of vertical electrical sounding in electrical exploration.
• micro - logging is a type of CS with probes of very small size, closely pressed against the walls of the well. Using this method, mainly only reservoirs are searched for in the well
• BK - lateral logging. The difference from the classical CS lies in the focusing of the current by the probe
• MBK - micro-lateral logging. The difference between this method and micro-logging is in focusing the current with a probe

Logging Methods

In the current logging group, it is possible to create a wide variety of concepts and their modifications, however, in practice, only MSK (sliding contact method) is used to study wells in ore deposits and BTK (lateral current logging) to study coal wells.

Electromagnetic Methods Group

The main advantage of this group of methods is that they can be used in dry wells not filled with conductive drilling fluid. In addition, it can also be used in wells filled with oil-based drilling mud, which also do not conduct direct electric current. The following varieties are found:

• IR - induction logging. When conducting use relatively low frequencies - up to 200 kHz
• induction logging at high frequencies, the results of which depend on both the conductivity of the rocks and their dielectric constant:
  • VMP - wave conductivity method with a frequency of 1-5 MHz
  • VDK - wave dielectric logging with a frequency of up to 60 MHz
• VEMKZ - high-frequency electromagnetic well logging
• VIKIZ - high-frequency induction logging isoparametric sounding. The method is an analog of BKZ, but instead of direct current, an alternating current is used.

Group of methods of electrochemical activity

• PS is a method of spontaneous polarization, also known as the method of potentials of spontaneous polarization. It is a borehole analogue of the natural field method in electrical exploration
• EC - electrolytic logging. Borehole analogue of the method of induced polarization in electrical exploration
• MEP is an electrode potential method. This method exists exclusively in the downhole version and has no analogues in field electrical exploration.

Methods of radioactive logging operate with the presence of natural radioactivity in the rocks, which is measured during the logging operations. In the event that the rock initially has an extremely low background or is not radioactive at all, its preliminary irradiation is applied with the subsequent measurement of the formed background. According to the measured readings, it becomes possible to determine a number of physical properties of the rock: hydrogen content, clay content, density, etc.

In the names of these methods abbreviations are used and a unified system of letter designations is adopted. Names usually consist of three letters:

• the first letter denotes the type of radiation that affects the studied rock: G - gamma radiation, H - neutron radiation
• the second letter indicates the type of measured response radiation (same notation)
• the third letter characterizes the scope, usually it is K - “logging”, but there may be others: O - “testing”, A - “analysis”, M - the method in general (rarely set)

The fourth letter can also be used, which in this case carries additional information - a variation or modification of the method. However, the names of some methods do not correspond to this classification and bear historically established names.

Gamma Method Group

• GK - gamma ray logging. A very simple and common method, measuring only natural gamma radiation from the rocks surrounding the well. There is a slightly more complicated version - spectrometric gamma-ray logging (SGK or GK-S), which makes it possible to distinguish gamma-quanta trapped in the detector of a geophysical probe by their energy. By this parameter, one can more accurately judge the nature of the rocks composing the thickness.
• GGC - gamma-gamma-ray logging. A geophysical probe irradiates the rock with gamma radiation, as a result of which the rock becomes radioactive and also emits gamma rays in response. It is these quanta that are recorded by the probe. There are two main varieties of the method:
  • density - GGK-P (sometimes the designation PGHK is found)
  • selective - GGK-S (can be designated as Z-GGK, S-GGK, etc.)
• RRK - X-ray logging. Its name formally does not correspond to the generally accepted system, so the name GRK (gamma-ray x-ray logging) is sometimes found, but RRK is commonly used.

Stationary Neutron Logging Method Group

In this group of methods, each geophysical probe is equipped with its own neutron source. The energy of the emitted neutrons can be different, but the neutron flux is kept constant during the logging. As sources of neutrons can be both spontaneously decaying elements, and nuclear reactions of two or more elements (for example, beryllium with an alpha particle).

• NGK - neutron gamma ray logging. The rock is irradiated with a neutron source, as a result the rock becomes radioactive and, in response, gamma rays emitted by it are recorded.
• NOC - neutron-neutron logging. The rock, as in NGK, is irradiated with neutrons, but in response, neutrons are also recorded, which may differ in their energy. Because of this, NOCs are divided as follows:
  • NNK-T - neutron-neutron thermal neutron logging
  • NNK-NT - neutron-neutron logging for epithermal neutrons
• MNC - multi-probe neutron logging using thermal or epithermal neutrons

Pulse Neutron Logging Method Group

В данной группе методов зонд снабжён источником нейтронов, но, в отличие от методов стационарного нейтронного каротажа, данный источник работает не непрерывно, а импульсами. Продукты взаимодействия импульсных вспышек нейтронов со средой многообразны, потому имеется большое число доступных методов:

• ИННК-Т
• ИННК-НТ
• ИНГК
• ИНГК-Спектрометрический ( ИНГК-С )

Каждый из этих импульсных методов может иметь модификации, имеющие разную степень распространённости (в том числе и вообще не применяемые на практике). Например, ИНГК-С существует во множестве вариаций, самой распространённой из которых является C/O-каротаж (углеродно-кислородный каротаж).

Неклассифицируемые методы

На практике иногда находит применение ГНК — гамма-нейтронный каротаж, в основе которого лежит ядерный фотоэффект. Из-за того, что теоретически его можно отнести как к гамма-методам, так и к нейтронным, то обычно его рассматривают отдельно.

Также разработан метод НАК — нейтронно-активационный каротаж. Суть метода заключается в том, что в горных породах, под действием искусственного нейтронного облучения, создают значительное количество искусственных радионуклидов, которые имеют собственную гамма-активность. Именно её измеряют при проведении НАК. В этом отношении метод отдалённо напоминает НГК.

В данном разделе помещены методы, изучающие менее характерные для геофизики физические поля.

• АК — акустический каротаж . В этом методе измеряют скорость распространения и затухание различных типов волн по стенкам скважины.
• КМВ — каротаж магнитной восприимчивости . Применение данного метода обосновано в случае необсаженных трубами скважин. Исключением являются скважины, обсаженные неметаллическими обсадными трубами.
• ЯМК — ядерно-магнитный каротаж. Данный метод, в силу его физического принципа, не включают ни в методы ядерного каротажа, ни в электрические. По своей сути он занимает промежуточное между ними положение и известен в двух модификациях:
  • на основе свободной прецессии ядер в магнитном поле Земли
  • на основе спинового эха в поле сильных постоянных магнитов
• ТК — термокаротаж. В ходе проведения данного каротажа могут измерять температуру и/или тепловое сопротивление стенок скважины.
• механический каротаж — фактически это метод из области бурения скважин, строго не относящийся к геофизике. Численно вычисляется как продолжительность проходки каждого погонного метра скважины при её бурении
• газовый каротаж — аналогично механическому, производится в процессе бурения. В данном случае измеряется содержание газов-углеводородов в отработанном буровом растворе, вышедшем на поверхность.

В силу узкой направленности каждого метода, а также из-за измерений в неблагоприятных условиях скважины, ни один метод каротажа не может давать объективной и достоверной информации. Данное обстоятельство было основным препятствием в начале XX века для развития данного раздела геофизических методов поисков полезных ископаемых (как уже отмечалось, поначалу каротажников считали мошенниками, откуда и произошло их название). Однако благодаря массовому появлению самых непохожих друг на друга методов, а также глубиной теоретической проработке каждого из них, при совмещении разных методов всё-таки удаётся получить почти всю необходимую информацию по разрезу скважины.

Метод ПС, почти сразу после его появления, стал применяться совместно с методом КС и этот комплекс получил название стандартного электрического каротажа. Комбинируя полученную с помощью разных методов информацию, можно более достоверно «расшифровать» содержимое недр.

На приведённом разрезе возникает сложная геологическая задача — нахождение глубины залегания угольных пластов. Метод КС не позволил без привлечения дополнительных изысканий отличить на данном разрезе каменный уголь от известняка (у обоих примерно одинаковые сопротивления при прочих равных условиях). Однако привлечение плотностного ГГК позволяет тут же выявить в разрезе известняк. Простой ГК также позволяет утвердиться в данной точке зрения, так как он хорошо реагирует на глинистость: в угольных пластах и в известняке нет глины, поэтому напротив них показания ГК проваливаются. Для сравнения также приведена диаграмма кавернометрии ( КМ ). В методе КМ измеряют диаметр скважины, который меняется по её глубине. Напротив хрупкого каменного угля стенки скважины при бурении разрушаются, поэтому диаметр скважины становится больше, а плотный известняк не поддался такому же разрушению, поэтому КМ его разрушений и не зафиксировала.

В данном разрезе обнаружен пласт бокситов , так как их естественная радиоактивность выше, чем у вмещающих пород, поэтому по ГК пласт выделяется максимумом. Метод КС прекрасно отбивает пласт пониженным сопротивлением, особенно его кровлю. Метод ПС также выделяет поляризуемый пласт бокситов, а провал показаний НГК свидетельствует о высоком содержании водорода (в бокситах много гидроксидов алюминия).

Комплексирование методов позволяет существенно расширить функциональность любого, даже самого простого метода. Особенно возрастает роль недорогого гамма-метода для выявления коллекторов, когда скважина заполнена буровым раствором . Удельное электрическое сопротивление этого раствора сопоставимо с сопротивлением пластовых вод. Метод ПС в этих условиях их плохо различает и данные ГК становятся основными для выделения коллектора .

• Геофизические исследования скважин
• Геолого-технологические исследования

• Сковородников И. Г. Геофизические исследования скважин. - Ed. 3-е, перераб. and add. — Екатеринбург: Институт испытаний, 2009. — 471 с. - 500 copies.

• Электрический каротаж
• Техническая инструкция по проведению геофизических исследований и работ приборами на кабеле в нефтяных и газовых скважинах