by Nikolai RYKHLINSKY, Cand. Sc. (Phys. & Math.), Institute of Innovation Geophysics Methods (Moscow),
Lev GOLUBCHIKOV, Cand. Sc. (Phys. & Math.), Coordination Center "Controlled Thermonuclear Synthesis-International Projects" under the RSC "Kurchatov Institute"
Although there is certain progress throughout the world in geological-and-geophysical survey for oil and gas and in prospecting drilling, the ratio of productive wells to their total number is, just as before, insignificant. The wild cat principle, oil-and-gas producers believe, has been operating in the past hundred years: no one knows where it will jump.
Diagram of electromagnetic probing with focusing of the current.
Today the average "luck ratio" in attempts to tap our planet's wealth with the help of seismic methods accounts for about 30 percent on the ground and 20 percent on the sea shelf (i.e., solely one of three and five wells proves productive, respectively). In order to raise these indicators we need direct independent geophysical information, and for gaining it, we have to solve a topical economic problem, especially for offshore drilling, where each well sinking claims millions of dollars.
Specialists in the field at home and abroad are now trying to solve this complicated problem by making use of the artificially activated electric field underground, including below the bottom on the shelf of seas and oceans. However, the widely known geological electric survey methods produce only summary information of the sum total of the elements of the surveyed plot structure, based, as they are, on measurement of its electric resistance alone. The fact is that they are in principle suitable solely for the search of objects whose indicator in this respect is radically different from the surrounding rock varieties (for instance ore bodies, deep-lying fragments of the destroyed building's foundation, permafrost borders). But they are not suitable in the case of oil and gas pools, that are hardly distinguishable from geological medium.
Electromagnetic probing ("feeling") of possible oil or gas fields with focusing of the current is a new method of addressing the problem, i.e., the geological electric survey, proposed by Nikolai Rykhlinsky, a coauthor of this article. His method, that has been recently improved and patented in Russia, is currently undergoing the patenting procedure in Europe, the USA and China. The method makes it possible to determine characteristics, most important for the search of hydrocarbons, that on the whole are typical solely of hydrocarbons: specific electric conductivity, polarization induced by the probing signal (i.e., relocation of charges) and the duration of its droop. Moreover, the method makes it possible to bring to the minimum the effect on the results of measurement of the geological objects located nearby where the probing signal's electric field penetrates and is diffused.
How can we obtain the indicated electrophysical parameters? According to the current rules, first, they conduct bidimensional (preferably tridimensional) seismic survey for revealing "traps" in the earth's crust-vacancies where hydrocarbons may be accumulated. Two generation lines are placed on the plot chosen according to the survey data (cables of the same length-several-hundred-meter long-activating the opposite electromagnetic fields) for the current to be focused at the place of reception of the "reply" (reflected) signals. Heteropolar rectangular pulses of 4 - 32 m per sec. are sent to subsoil, i.e., leaving more time for the electric field to spread there. The difference in the momentary values of electric potentials (characteristics showing the electric field voltage and other properties) is measured in the intervals between them in certain spots after definite periods of time. The reflected signals are also recorded. If an oil or gas pool is discovered, these signals acquire the respective characteristics within the limits of the pool outline, and this fact is clearly shown by up-to-date meters. The figures received as a result serve as source information for determination of the three above-mentioned parameters and of the layout of
Generated (blue curved) and reflected (red) signals. The distance from the focusing spot is indicated along X axis.
the pool distribution (by means of solution of the so-called inverse problem of electrophysics).
In the event of offshore survey the ship tows the generation and reception cables lengthwise and crosswise the outline of the supposed field. They are synchronized with the use of the space system of geostationary sputniks for determination of the probing zone, which makes it possible to carry out its topographic survey. And the possibility of measuring the reflected signal in the same physical spot in the direct and return navigation of the ship is among the obvious advantages of the method in question in this case.
Information, accumulated from a considerable quantity of probing signals (usually from 10 to 100 in practice), determined, just as the duration of the entire session, by the operator with respect to concrete conditions of field works, is used to ensure greater precision of the methods in question.
The physical basis of the offered method is as follows. The effective dielectric permeability (the force of interaction of the charges) of the rock, consisting of sediment deposits, is by six orders higher than that of vacuum. As a result of the effect of the electromagnetic field, the surveyed geological medium containing hydrocarbons accumulates energy, received from the pulse of the activated current. And as soon as the current is turned on, it will slowly lose it, just as a condenser (it should be noted that neither water nor clay, or other substances, occupying vacancies in the earth's crust, have this property). The period of time, required for it, directly depends on the depth of location and specific properties of the surveyed anomalous object (in our case-oil and gas pools). It is precisely this factor and focusing of the current that make it possible to implement the method in question.
The offered method has been presented in the form of a set of current feeding equipment, meters, mathe-
Presentation with the help of electromagnetic probing of electric resistance 1/σο coefficient of the set polarization η at the time of its droop τ for the profile of the perpendicular section of the surveyed plot in the Ob River mouth.
matical programs for field survey and for processing its results. The probing parameters are set, depending on the specific features of the surveyed region. The required standard instrument program allowing for flexible adaptation to the survey assignment operates in the meter recording probing signals, and it is coupled with the receiver of the system of geostationay sputniks. A personal computer that can operate under field conditions and has an active long-term memory, with filtered data obtained in the course of the session stored in it, controls the operation of the generation-and-reception complex. Subsequently the data are recorded on the compact disc and transmitted to the center for secondary processing and for recommendations to be issued on prospective drilling.
The new method was tested in Ob Inlet drinking water and in the shelf of the Caspian Sea at a depth up to 100 m. (If the depth is up to 1,000 m and the thick layer of salted water with good electric conductivity screens the reflected signals, there is a version of the offered method technology making it possible to suppress this effect that distorts the measurement results.) For instance, in the Ob Inlet probing showed the presence of a clearly outlined gas deposit at a depth of about 3,400 m in a sandstone layer about 15-meterthick, and subsequent drilling confirmed the fact. In other cases the tests of the new method on the ground in this country from Kaliningrad to Vladivostok produced "the luck ratio" close to 100 percent. Only one of over 160 drilled deep wells has produced doubtful results, for it is located on the border of permafrost and the water surface.
By the start of the 21st century practically all of the large oil fields have been discovered and are currently operated. Moreover, in 2006 the volume of produced "black gold" exceeded that of prospected oil on the ground and on the sea bottom. According to various forecasts, the end of the "hydrocarbon" civilization should be expected in the first half of the 21st century. And what comes next? No serious analysis has been offered so far. The new method of electric geological survey, especially in the shelf zones of the World Ocean, will allow us to put off the dangerous line and estimate objectively the potential of new oil and gas fields, not prospected earlier due to the high cost of works.
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