RU2417387C2 - Geo-chemical procedure for prospecting oil deposits on sea bottom - Google Patents

Abstract

FIELD: oil and gas production.

SUBSTANCE: sea water is sampled. Collected material is analysed. There is determined concentrations of metals typical for oil deposits, notably vanadium and nickel. A region of oil deposit existence is established on base of observed change in contents of analysed elements in collected material. Also, not less, than 3-fold excess of background contents of the said elements in sea water indicates occurrence of an oil deposit.

EFFECT: increased reliability of oil deposit prospecting on sea shelf.

4 cl, 4 dwg

Description

The method relates to geochemical research methods and can be used to identify oil fields on the sea shelf.

Currently, the problem of creating a highly effective method for the search for oil deposits on the sea shelf is becoming increasingly acute all over the world. This is due to the limited proven reserves of oil on land, which, according to experts, should be enough only for the next 20-30 years. At the same time, it is believed that oil deposits under the seabed have much greater capacities. This is confirmed by the growing share of oil produced offshore in the total world production (approximately 1/3 at this time). Currently, more than 1700 oil fields are discovered on the shelf of the World Ocean (mainly abroad), whose reserves are estimated at 55 billion tons. Russia also has large potential oil reserves on the shelf of the northern seas, the development issues of which are closely related to the accuracy of the geochemical exploration of their fields. The overall economic efficiency of its production largely depends on the quality and efficiency of the search for oil deposits on the sea shelf, since today the cost of drilling only one test well on the seabed is 1-5 billion rubles. In this regard, the question of developing a fast and high-precision method for detecting oil deposits near the bottom of the sea shelf is becoming highly relevant.

It is known that the presence of oil deposits on the shelf is usually characterized by the infiltration of petroleum hydrocarbons and gases into the bottom layers of the water. However, the presence of gas evolution is also characteristic of gas deposits, and oil hydrocarbons, due to the low diffusion rate in sea water, almost complete lack of solubility in it and rapid degradation due to the processes of chemical and biochemical interaction with the components of sea water, are strongly localized near oil leakage sites to the surface of the seabed.

There are various direct and indirect geochemical methods for identifying oil and gas fields (Geochemical methods for searching for oil and gas fields. O. Baratashevich and others - M .: Nedra, 1980), the main of which are 5 main methods:

1) gas geochemical;

2) hydrogeochemical;

3) biogeochemical (geomicrobiological and phytogeochemical);

4) lithogeochemical;

5) bituminological.

The hydrogeochemical method includes pH and Eh - metry, the study of salt composition, etc.

During geochemical sensing at the regional and forecast-reconnaissance stages of oil and gas prospecting in the hydrosphere, the following types of surveys are carried out:

a) surface water surveys;

b) studies (surveys) gas-geochemical, biogeochemical, hydro-geochemical, hydro-gas-biochemical, pH and Eh - metry, elementometry;

c) groundwater surveys - wells for various purposes;

d) surveys of snow cover, etc.

Gas biochemical surveys of surface (water sources) and groundwater discovered in wells are widely used (Methods of processing and interpreting the results of hydrogeological studies for oil and gas prospecting. Saturday MI and others - M .: Nedra, 1980). Surface hydro-gas-biochemical surveys facilitated (using available local water sources - springs, wells, etc.) the discovery of a number of oil and gas fields, in particular, in the territory of Northern Ustyug, Southern Mangyshlak and the Buzachi Peninsula.

The closest set of essential features to the proposed method is a geochemical method for detecting heterogeneities according to the patent of the Russian Federation No. 2226282. This method is selected as a prototype. The known method relates to geochemical research methods and can be used to identify mineral deposits, as well as tectonic, anthropogenic and other structural-material heterogeneities of the geosphere. From the surface layer of the atmosphere, air is collected by pumping free air through a sampler, inhomogeneities contained in it, migrating in the direction of the day surface, and moving particles entering the air, the presence of which is associated with this heterogeneity or due to its existence. For analysis, particles no larger than 0.05 microns are selected. It is most convenient to collect particles on a graphite cuvette, which can be placed in the electrothermal atomizer of a multi-element atomic absorption spectrophotometer. The contents of the elements accompanying the desired heterogeneity are determined, and the presence, nature and parameters of the heterogeneity are determined by the detected change in the contents of the elements.

The disadvantages of this method:

1. Lack of accuracy, since when analyzing air samples, it is necessary to take into account the wind rose.

2. The ambiguity in determining the source of deposits, because, for example, the presence of methane or ethane may indicate deposits of both oil and gas.

3. The inability to determine the sources of deposits at large distances from the epicenter.

The aim of this invention is to improve the accuracy and reliability of the method of searching for oil fields on the sea shelf.

To achieve this goal, it is proposed in the geochemical method of detecting oil deposits on the sea shelf, including sampling, analyzing the accumulated material and determining the area of oil deposits from the detected change in the contents of the determined elements in the accumulated material, to conduct sampling of sea water, not air. The concentration of metal impurities characteristic of oil fields, namely vanadium and nickel, should be used as the determined elements in the accumulated material, and at least a 3-fold excess of the background contents of vanadium and nickel in sea water should be considered a sign of the manifestation of an oil field.

Additional differences of the proposed method is that:

- when analyzing the accumulated material, a synchronous determination of the concentration of vanadium and nickel in sea water is carried out,

- determination of the concentration of vanadium and nickel in sea water is carried out according to the method of atomic emission spectroscopy with inductively coupled plasma,

- determination of the concentration of vanadium and nickel in seawater is carried out on analytical lines with wavelengths of 292,401 nm and 231,604 nm, respectively.

In all known large deposits, both in our country and in other countries (Gavar, Safania-Khafji and Manifa in Saudi Arabia, Burgan in Kuwait, Chikontenek in Mexico, etc.) (Champman E.L. Geology and water L .: Nedra, 1983. - 380 p.), Impurities of a large amount of metals were found in the composition of oils, among which are not typical of sea water, such as vanadium and nickel in amounts: V - from 0.00001 to 0.01 -0.1 wt.%, Ni - from 0.0001 to 0.001-0.01 wt.%.

For comparison, we note that the average content of these elements in the deep, clear waters of the World Ocean is for V - only 2.5 μg / L or 0.00000025%, for Ni - 1.7 μg / L or 0.00000017% (Gerlach C .A. Pollution of the seas: Diagnosis and therapy. 1985. - 263 p.).

As can be seen from these data, despite some unconditional dilution of impurities during the transition from oil to sea water, the vanadium content in the places where oil deposits appear on the shelf can exceed the average ocean background for this element by 400-40000 times, and for nickel, respectively, by 600-6000 times. This circumstance can be used to search for oil deposits on the Russian shelf of the northern seas, especially in relatively still clean seas - the Chukchi, Bering and Laptev Sea, where the background concentrations of V and Ni are about an order of magnitude lower than the average values for the World Ocean (V. Porta , O. Abollino, E. Mentasti and C. Sarzanini. Determination of ultra trace levels of metal ions in sea-water with on-line preconcentration and ETAAS. Journal of Analytical Atomic Spectrometry, 1991. V. 6. P. 119), that is, the conditions for searching by the proposed method are even more favorable.

It is known that large oil deposits are usually located near the seabed and, as a rule, are characterized by the infiltration of oil hydrocarbons and gas into the bottom layers of seawater (therefore, in principle, the closer to the surface of the seabed water samples are taken for analysis, the more precisely the location occurrence can be determined deposit). It is important to justify, however, such an analytical criterion for the analysis of the bottom water of the sea shelf, which, on the one hand, would make it possible to accurately determine the location of the oil deposit, that is, would be highly sensitive, and on the other hand, would guarantee a reliable difference between oil deposits and gas deposits (if, for example, the content of light hydrocarbon gases - methane, ethane, etc. in the water is chosen as such a criterion, then it is easy to confuse oil and gas deposits, since these accompanying impurities are inherent in both oil and gas and gas.

As such a criterion, a comprehensive analytical criterion is proposed - synchronous in time (with continuous or periodic water sampling when the vessel is moving), a parallel excess of background concentrations of V and Ni in bottom seawater by 3-10 times or more.

Three or more excess of background concentrations ensures the reliability of the analysis. The fact that analysis is carried out on V and Ni, whose contents in sea water are very low, and in any oil the content of these accompanying impurities is quite high compared to the background, provides high accuracy in determining the place where oil hydrocarbons seeping onto the surface of the shelf and guarantees the detection of these places at very great distances from their epicenter, since vanadium and nickel impurities, unlike petroleum hydrocarbons, are highly soluble in sea water and have high coefficients iffuzii (of the order of 1-10 cm ² / sec according to the data (VP Kochergina and Timtchenko IE Monitoring hydro ocean fields L .: Gidrometeoizdat, 1987. -. 280)..

Consider the general probabilistic model of the arrival of metal impurities (metal ions), characteristic of oil, into sea water. This model can be used for practical calculations, with the aim of accelerating the location of oil seepage into bottom seawater when detecting 3 times or more excess concentrations of impurities of characteristic metals over their background concentrations.

It is clear that the distribution of metal impurities (V and Ni) characteristic of oil from their source located on the seabed will be determined by a sufficiently large set of factors:

1) the power and size of the source;

2) transport function: convective (turbulent or laminar) diffusion. This function is determined by the diffusion coefficients and the spatial velocity field of sea currents;

3) the loss function, which leads to equalization from the maximum excess concentration at the source point to the average (background) value at the impurity distribution boundary. This function is determined by various chemical, physical and biochemical processes that can occur during the propagation of an impurity in space from a point source (for example, the conversion of an impurity into an insoluble form, followed by sedimentation on the seabed).

In general terms, the solution of the spatial field problem with concentration C = C (X, Y, H) can be represented as follows:

where W is the power of the source, the mass of the metal impurity per unit time: W = m / t, erf (z) is the Gaussian error integral,

z = f (X, Y, H, D, K), D is the diffusion coefficient (laminar or turbulent), and K is the constant of a certain process of the loss function:

Determining the spatial field of concentrations, taking into account all the factors and parameters that determine them, is a very difficult and time-consuming task requiring the use of numerical methods of solution (development of an algorithm and a special computer program, theoretical or experimental determination of all necessary constants and parameters, etc.) .

The first two factors determine the entry of impurities into space (Figure 1). The loss function is necessary to comply with the stationary condition for the distribution of the impurity in space, i.e. determines the boundaries of the pollution area with a constant supply of impurities from the source. Figure 1 shows the curve of the formation of the field of concentration of the impurities of the characteristic metal in the marine environment.

In this regard, when modeling, you can use certain assumptions and simplifications:

1) there is a stationary source (conditionally point) of metal impurities in a semi-limited space of the environment (water area) with a constant input power of the characteristic component, the source of which is located on the seabed (the place of oil seepage), where there are no significant laminar or turbulent flows;

2) the transfer of this characteristic impurity is determined by diffusion;

3) the diffusion coefficient does not depend on the direction of the diffusion flow, i.e. impurity transfer is uniform in all directions;

4) the characteristic impurity is in a single form, the loss function is determined by a single process (as some chemical process).

For a stationary, constant in time, cloud of contamination with metal impurities from a point source, the condition for the equality of diffusion transfer and the loss process must be satisfied, therefore, the density of the diffusion flux through a certain surface S is determined by the rate of the component loss process, ν:

where the speed of the process is defined as:

where: C _f - background content of metal impurities in sea water (R = ∞);

V - hemisphere volume:

S - hemisphere surface:

q - diffusion flow:

After substituting expressions (1.5) - (1.7) in equation (1.4) we get:

After separately integrating the right and left sides of equation (1.8) for the spatial distribution function of the concentration of the characteristic component from a point source, we can write the following expression:

where: C _max - the maximum concentration of metal impurities near the source of pollution (R = 0), which is determined by the power of the source.

Figure 2. Dependence of the relative concentration of the characteristic metal impurity on the distance from a point source is shown.

Expression (1.9) describes the spatial function of concentrations as a certain distribution law (see Figure 2), which corresponds to the equation for determining the concentration field, presented in the general form (1.1) (Monin A.S., Yaglom A.M. Statistical hydromechanics. Part 1, Moscow: Nauka, 1967. - 640 p.):

where K _P is the transfer coefficient of the characteristic impurity of the metal (turbulent diffusion coefficient).

In the case of uniform distribution, the cloud of characteristic impurity (V, Ni) will be a hemisphere centered at the source point. In this case, the concentration distribution of the characteristic metal impurity in the bottom layer will be characterized by a volumetric Gaussian bell (Figure 3).

Figure 3 shows the area of contamination (C = 3C _f ) of the marine environment by a characteristic admixture of metal from a point source with a uniform distribution.

In the case of an uneven distribution of the characteristic impurity, for example, in the presence of stable laminar flows near the seabed, other relations must be used instead of expression (1.10).

So, for example, to calculate the concentration of a metal impurity in the case of a point source located at a depth of H _about , constant power W, turbulent diffusion with a unidirectional laminar flow along the OX axis at a constant speed, the following expression can be used:

Expression (1.11) describes an impurity jet having the shape of an elliptical paraboloid elongated along the OX axis. The concentration distribution of the characteristic impurity at a certain constant value of H, for example, in the bottom layer, in this case is presented graphically in FIG. 4.

Expression (1.11) shows that in the direction of the laminar flow, the concentration of the impurity of the characteristic metal decreases proportionally to X ^-1 , and exponentially to the axis of the flow. An analysis of the order of magnitude in expression (1.11) allows us to draw conclusions about the boundaries of the pollution cloud of the characteristic impurity and thereby determine the most appropriate boundaries of the search system (the width and depth of the search zone) when detecting significant excess concentrations of impurities V and Ni over the background.

Figure 4 shows the distribution of the relative concentration of the characteristic impurity of the metal in the bottom layer of sea water in the case of a constant laminar flow directed along the OX axis.

An analysis of the orders of magnitude in the above expression allows us to draw conclusions about the boundaries of the pollution cloud with a characteristic impurity of metal. So, for example, with a 1000-fold excess of the impurity concentration over the background at the source point in the bottom layer, the boundary in the direction of flow corresponds to a distance from the source of ~ 10 km to 50 km, and in the direction perpendicular to the flow, ~ from 1 km to 5 km. The exact values depend on the power and size of the source, flow rate, transfer coefficients and their dependence on the coordinate (distance from the source), etc. In practice, in the study of bottom shelf waters, these parameters can be refined experimentally.

It is important to note that a simultaneous increase in the contents of both V and Ni in seawater can only be associated with the presence of an oil deposit and is a guarantee that there will be no error due to gas deposits. Background concentrations of V and Ni in pure seawater, as mentioned above, are extremely low.

The excess content of V and Ni impurities in seawater can be determined with maximum sensitivity and accuracy using the developed ICP-AES technique. A simultaneous increase in the content of nickel and vanadium impurities in seawater can be unequivocal evidence of the infiltration of petroleum hydrocarbons from the bottom into seawater and serve as the basis for the start of trial drilling in this area.

A possible option for the marine deployment of the search equipment complex may be a combination of an inductively coupled plasma atomic emission spectrometer with a FIAS-400 continuous-flow continuous injection sampling attachment (in conjunction with all other necessary instruments and sensors). Such a scheme of analytical control of the content of metal impurities in seawater will automatically and simultaneously determine nickel, vanadium and (if necessary) other metal impurities in seawater associated with oil fields (Method 200.7. Determination of Metals and Trace Elements in Water and Wastes by Inductively Coupled Plasma-Atomic Emission Spectrometry. Revision 4.4. EMMC Version. EPA. Methods for the Determination of Metals in Environmental Samples. Supplememt I. May 1994.

Method 200.15. Determination of Metals and Trace Elements in Water by Ultrasonic Nebulization Inductively Coupled Plasma-Atomic Emission Spectrometry. Revision 1.2. EMMC Version. EPA Methods for the Determination of Metals in Environmental Samples. Supplement I. May 1994).

Additionally, one or two channels of the spectrometer can be used to determine elements that are constantly present in seawater but are not associated with oil fields, for example, magnesium and (or) potassium to implement the principle of the internal standard, which increases the accuracy of determining the coordinates of the oil deposit.

After the discovery of oil deposits on the shelf according to the above criterion, to confirm positive results in places where nickel and vanadium contents are highest, discrete samples can be taken for routine analysis for the presence of petroleum hydrocarbons (infrared spectroscopy, gas-liquid chromatography, luminescence, etc.). )

Due to the fact that the information obtained during the oil search on the content of nickel and vanadium impurities in sea water will be 2 equidistant (equally spaced) parallel time series for their statistical computer processing (performed directly on board the vessel in real time), in order to identify anomalies in the content of the elements being determined, dispersion analysis methods, for example, sign criteria or the cumulative map method, should be used.

Based on the foregoing, the technical and economic effect of the proposed method is as follows:

1. Since the analysis is performed not of air but of water, therefore, the results are more accurate (there is no wind rose).

2. The analysis is carried out on V and Ni, which ensures the unambiguous determination of oil deposits (for example, methane, ethane is inherent in both oil and gas).

3. There is the possibility of determining deposits at large distances from the epicenter.

4. The search is conducted in the direction of increasing V and Ni (in the direction of the epicenter).

5. Threefold or more excess of V and Ni compared to the background ensures the reliability of the method and high accuracy in determining the place where leaking petroleum hydrocarbons reach the surface of the bottom of the shelf.

6. It is possible to ship the instruments and equipment of NPP-ISP with a flow-injector prefix of continuous sampling type FIAS-400.

7. In addition, 1 or 2 channels of the spectrometer can be used to determine elements that are constantly present in sea water but are not associated with oil fields (magnesium, potassium) to implement the principle of the internal standard, which increases the accuracy of determining the coordinates of the oil deposit.

Claims (4)

1. A geochemical method for the detection of oil deposits on the sea shelf, including sampling, analysis of accumulated material and determining the area of presence of oil deposits by the detected change in the contents of the determined elements in the accumulated material, characterized in that they take samples of sea water as the determined elements in the accumulated the material uses concentrations of metal impurities characteristic of oil fields, namely vanadium and nickel, and the sign of the manifestation of an oil field is considered not less than 3-fold excess of background vanadium and nickel content in seawater. 2. The geochemical method for detecting oil deposits on the sea shelf according to claim 1, characterized in that when analyzing the accumulated material, a simultaneous determination of the concentration of vanadium and nickel in sea water is carried out. 3. The geochemical method for detecting oil deposits on the sea shelf according to claim 2, characterized in that the determination of the concentration of vanadium and nickel in sea water is carried out by the method of atomic emission spectroscopy with inductively coupled plasma. 4. The geochemical method for detecting oil deposits on the sea shelf according to claim 3, characterized in that the concentration of vanadium and nickel in seawater is determined by analytical lines with wavelengths of 292.401 nm and 231.604 nm, respectively.

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