RU2065615C1 - Method for locating hydrocarbon deposits - Google Patents

Abstract

FIELD: oil and gas geophysics; forecasting of hydrocarbon deposits. SUBSTANCE: region of interest is subjected to gravimetrical measurements, closed annular zones with abnormally high values of gravity are discriminated by abnormal-gravity readings, density of rock beyond zones, geothermal coefficient, compressibility, coefficient of volumetric thermal expansion are measured, two wells are drilled down to target horizon depth, one within annular zone and other within section around it; density of lithologically similar rock in these wells is determined, and hydrocarbon deposit is recognized by relation between densities of rock at target horizon depth beyond zone, within annular zone and within section around annular zone. EFFECT: facilitated procedure. 2 dwg

Description

 The invention relates to the field of oil and gas geophysics and can be used in predicting hydrocarbon deposits.

 A known method for predicting hydrocarbon deposits, including a detailed geological survey, seismic exploration, well drilling, core sampling and research.

 The disadvantage of this method is its low reliability due to the lack of the ability to accurately identify the boundaries of the deposits.

 There is also known a method for predicting hydrocarbon deposits [1] comprising carrying out gravimetric surveys on the investigated area and identifying a closed zone with abnormally high gravity gradients at the border.

 The disadvantages of the method are the low accuracy and the ability to identify deposits due to the inability to distinguish the gradients of gravity at the boundaries of the deposits from their gradients above the graben with a slight displacement, as well as heterogeneities in the upper part of the section. In addition, the method does not allow to accurately identify the boundaries of the deposits.

 The closest technical solution to the claimed one is a method for predicting hydrocarbons [2] including detailed geological surveying, high-precision gravity exploration, determination of anomalous gravity, identifying promising areas of gravitational anomalies, drilling.

 However, this method does not allow to determine the boundaries of the deposits, in addition, it does not precisely allow to identify gravitational anomalies due to the actual hydrocarbon deposits.

 The technical result of the invention is to improve the accuracy and reliability of identifying hydrocarbon deposits.

где

значение плотности вне зоны на глубине целевого горизонта;
σ₁ значение плотности в пределах кольцеобразной зоны;
σ₂ значение плотности в пределах ограниченного кольцеобразной зоной участка;
при этом значение плотности вне зоны на глубине целевого горизонта определяют из условия

где σ_n-1(P) значение плотности в (n-1)-ом интервале глубиной Н_n-1 как функции давления;
β_n-1 коэффициент сжимаемости породы в (n-1)-ом интервале;
g ускорение свободного падения;
H_n значение глубины n-го интервала;
H_n-1 значение глубины (n-1)-го интервала;
α_n коэффициент объемного теплового расширения в n-ом интервале глубин;
T₀/H₀ величина геотермического градиента в приповерхностном слое.The stated result is achieved by the fact that in the method for predicting hydrocarbon deposits, which consists in conducting high-precision gravity exploration, determining abnormal gravity values, identifying promising areas, drilling, identifying hydrocarbon deposits, promising areas are identified in the form of closed annular zones with abnormally high values of gravity, outside the zones, under normal conditions, rock density, compressibility coefficient, coefficient of volumetric thermal expansion, geote the static gradient, then, at different pressures, the compressibility coefficient, at different temperatures, the coefficient of volumetric thermal expansion, set the intervals for measuring density changes with depth, determine the density value outside each zone at the depth of the target horizon by determining the density in the upper interval and each following it based on the data density values of the previous interval. Then, at least two wells are drilled within the prospective section to the depth of the target horizon within the annular zone and within the limited area with the subsequent determination of the densities of lithologically similar rocks in these wells, and the presence of hydrocarbon deposits is judged from the condition

Where

density value outside the zone at the depth of the target horizon;
σ ₁ is the density value within the annular zone;
σ ₂ is the density value within the area bounded by the annular zone;
the density value outside the zone at the depth of the target horizon is determined from the condition

where σ _n-1 (P) is the density value in the (n-1) -th interval with a depth of H _n-1 as a function of pressure;
β _n-1 rock compressibility factor in the (n-1) th interval;
g acceleration of gravity;
H _n is the depth value of the n-th interval;
H _n-1 value of the depth of the (n-1) -th interval;
α _n coefficient of volumetric thermal expansion in the n-th interval of depths;
T ₀ / H ₀ value of the geothermal gradient in the surface layer.

 The sources of patent and scientific and technical information known to the author do not describe a method for predicting hydrocarbon deposits, in which, to increase reliability, promising areas are identified in the form of closed annular zones with abnormally high gravity and are drilled within the annular zone and within the limited area with subsequent determination of the presence of deposits from a certain ratio. Achievable technical result of the invention is based on the use of the properties of compaction of rocks in the lateral parts of the reservoir, exceeding the density of rocks in the rocks containing the reservoir and outside the reservoir. Thus, the presence of a closed local maximum of gravity at the boundaries of the reservoir is detected using high-precision gravity exploration, and drilling wells within the ring-shaped zone and the area limited by it allows determining density values both in the inner and outer regions of the zone. The density values are compared with the theoretical density value at the depth of the target horizon, which, in turn, is calculated by a certain ratio. Moreover, the estimation of density values at one hypsometric level is necessary to establish the phenomenon of rock compaction on the sides of the reservoir, as mentioned above.

 The method is illustrated by drawings, where in FIG. Figure 1 presents the identified promising areas in the form of closed annular zones and shows the location of the wells both inside the zone and outside.

 In FIG. Figure 2 shows the observed gravity field above the reservoir and the distribution of rock density in the reservoir and beyond for the Bahar oil and gas field (Caspian Sea).

 In FIG. 1: 1 structure contour, 2 oil and gas potential according to drilling data, 3 closed annular zone with abnormally high values of gravity, 4 wells within the annular zone and within the area bounded by it, 5th place of rock sampling outside the zone to determine the density value at depth of the target horizon.

 In FIG. 2: 6 position of the earth’s surface, 7 wells drilled in the field along the specified profile, 8 contour of the deposit, 9 density change curve at the depth of the deposit along the profile, 10 - regional background of gravity, 11 gravitational effect from the structure, 12 - curve of the observed field gravity, 13 local maximum gravity, 14 boundaries of the zone of gravity gradients, 15 minimum gravity above the reservoir, 16 approximate position of the well inside the zone contoured by the local maximum gravity, 17 approximate position of the well within local maximum gravity, 18 position of the well outside the zone contoured by the local maximum gravity.

 The method is implemented as follows. On the studied area according to a system of parallel profiles on the surface of the Earth 6 measure the value of gravity using a gravimeter. After that, corrections are introduced into the observed field for the regional background and the effect of the structure (subtracting them from the observed gravity field) and the residual gravity anomaly and the minimum gravity observed above the reservoir are isolated 15. Then, the areas of abnormal gravity gradients on the profiles between limiters 14. Next, a closed zone of gravity gradients located between the boundaries 14 observed on all profiles is isolated on the studied area. After that, within the selected closed zone of anomalous gravity gradients, an additional highly accurate gravimetric survey is carried out to isolate local gravity maxima in the gravity gradient zone 13. The existence of such local gravity maxima is indicated by the experimental data (at the Bahar and Bulla Sea oil and gas fields ) an increase in the density of rocks directly on the sides of the reservoir with a slight decrease in the marginal part (see curve 9 of the tight distribution in Fig. 2 within the reservoir with contours 8 and beyond its contour, observed at the Bahar field), which should cause local gravity maxima, which are observed in practice (see curve 12 in Fig. 2) on all profiles. In the presence of such a local maximum of gravity along the entire closed zone of anomalous gradients of gravity, a more confident conclusion is made about the presence of a deposit. Then, wells (at least two) are drilled so that at least one well 16 (see Fig. 2) is drilled inside the zone contoured by the local maximum gravity, at least one well 17 (see Fig. 2) is drilled within a local maximum gravity 13. Out of the zone contoured by a local maximum gravity 13, a well 18 is drilled for sampling rocks.

 Place 18 sampling of rocks is located outside the annular zone at a distance of the order of the average width of the annular zone from its outer border. The choice of this distance is based on a pattern established by actual data, according to which the given distance from the outer edge of the annular zone is the minimum at which normal compaction of the rocks can already take place.

где

значение плотности пород вне зоны на глубине целевого горизонта;
σ₂ значение плотности пород внутри зоны, т.е. в пределах ограниченного зоной участка;
σ₁ значение плотности пород в пределах кольцеобразной зоны.Throughout the section of wells 16, 17, 18, the density of coeval sediment is determined (for example, using gravity logging). After that, the measured densities of lithologically homogeneous uniformly aged sedimentary rocks are compared for these wells and the section interval is revealed for which a low density of rocks in the inner zone contoured by a local maximum of gravity is observed, the maximum density for rocks of the same interval under a local maximum of gravity and several a lower rock density for the same interval outside the zone contoured by a local gravity anomaly reveals a density ratio d by condition

Where

the density of rocks outside the zone at the depth of the target horizon;
σ ₂ is the density of rocks within the zone, i.e. within a limited area area;
σ ₁ is the rock density within the annular zone.

проводят для пород, находящихся на одной и той же глубине.When this ratio is fulfilled, they conclude that there is a deposit on the area under study. Moreover, the comparison of the densities

carried out for rocks located at the same depth.

The value of the density of rocks outside the zone is determined as follows. Outside the promising area, under normal conditions, the values of rock density σ ₀ in the near-surface layer Н ₀ , compressibility coefficient β ₀ , volumetric thermal expansion coefficient α ₀ , and geothermal gradient Т ₀ / Н ₀ are measured.

 Moreover, the size of the intervals should be no more than half the power of the target horizon.

The compressibility coefficient β _n is measured as a function of the change in pressure P _n , the coefficient of volumetric thermal expansion α _n as a function of the change in temperature T _n . Determine the pressure and temperature at a depth of H ₁ .

Then determine the density value in the upper interval at a depth of H ₁ . Based on the data obtained, the density value is determined in the second interval, etc. Those. the found density value in the previous interval, for example in the (n-2) th, allows you to determine the density value in the subsequent (for example, in the (n-1) th), as a result of which the density value is determined outside the zone at the depth of the target horizon.

(2)
где

значение плотности на глубине n-го интервала,
H глубина целевого горизонта,
n номер интервала, в котором изменение плотности пород можно считать постоянной,
σ_n(P) значение плотности как функции давления,
α_n коэффициент объемного теплового расширения.The mathematical expression for calculating the density of rocks outside the zone was obtained from the following considerations. A change in density depends on a change in temperature with depth. The change process is described by the equation

(2)
Where

density value at the depth of the n-th interval,
H is the depth of the target horizon,
n is the number of the interval in which the change in rock density can be considered constant,
σ _n (P) value of density as a function of pressure,
α _n coefficient of volumetric thermal expansion.

(3)
Известно, что изменение температуры T_n с глубиной может быть определено через среднее значение геотермического градиента и плотности разреза:
T_n (T₀/H₀) H_n-1, (4)
где T₀/H₀ величина геотермического градиента в приповерхностном слое в интервале глубин от 0 до Н₁.The solution of equation (2) takes the form

(3)
It is known that the temperature change T _n with depth can be determined through the average value of the geothermal gradient and the density of the section:
T _n (T ₀ / H ₀ ) H _n-1 , (4)
where T ₀ / H _{0 the} value of the geothermal gradient in the surface layer in the depth interval from 0 to H ₁ .

(5)
где Н_n-1 глубина (n-1)-го интервала, в котором плотность пород можно считать постоянной.Taking into account relation (4), expression (3) takes the form

(5)
where H _{n-1 is the} depth of the (n-1) -th interval in which the density of the rocks can be considered constant.

The value of the density increment as a function of pressure in the first interval with a depth of H ₁ is determined by the expression
Δσ ₁ (P) = σ ₀ β ₀ g (H ₁ -H ₀ ), (6) (6)
where σ _{0 is the} initial value of the density in the surface layer,
β ₀ compressibility coefficient in the surface layer,
g acceleration of gravity,
H _{1 the} depth of the first interval,
H _{0 the} depth of the surface layer.

Given that Δσ ₁ = σ ₁ -σ _0. , The density value in the first interval of depth H ₁ has the form
σ ₁ (P) = σ ₀ β ₀ g (H ₁ -H ₀ ) + σ ₀ , (7) (7)
By analogy, the density value in the nth layer with a depth of H _n takes the form
σ _n (P) = σ _n-1 (P) β _n-1 g (H _n -H _n-1 ) + σ _n-1 (P), (8) (8)
where σ _n-1 (P) is the density value as a function of pressure in the (n-1) th interval with a depth of H _n-1> ,
β _n-1 compressibility factor of the (n-1) -th interval.

(9)
Предложенный способ позволяет с большой степенью точности и надежности выявлять залежь на исследуемой площади, как ее пространственное положение, так и положение по глубине, позволяет исключить возможность ошибки при выявлении залежи и отличие ее от грабена и плотностных неоднородностей в верхней части разреза.Given relation (5), the expression for determining the density in the n-th interval at a depth of H _n takes the form

(9)
The proposed method allows with a high degree of accuracy and reliability to identify the deposit on the studied area, both its spatial position and the position in depth, eliminates the possibility of errors in identifying deposits and its difference from graben and density inhomogeneities in the upper part of the section.

Claims (1)

A method of searching for hydrocarbon deposits, which consists in conducting high-precision gravity exploration, determining abnormal gravity values, identifying promising areas, drilling, identifying hydrocarbon deposits, characterized in that the prospecting areas are detected in the form of closed annular zones with abnormally high gravity values, outside the zones under normal conditions, the density of rocks, geothermal gradient, compressibility coefficient, coefficient of volumetric thermal expansion, then at different at different pressures, the compressibility coefficient, at different temperatures, the coefficient of volumetric thermal expansion, sets the intervals for measuring density changes with depth, determines the density value outside the zones at the depth of the target horizon by determining the density in the upper interval and each following it based on the density values of the previous interval, then conduct drilling within a prospective area of at least two wells to the depth of the target horizon within each annular zone and in In the course of the limited area with the subsequent determination of the densities of lithologically similar rocks in these wells, the presence of hydrocarbon deposits is judged from the condition

Where

density value outside the zone at the depth of the target horizon;
σ ₁ is the density value within the annular zone;
σ ₂ is the density value within the area bounded by the annular zone,
the value of the intervals of measurements of changes in density with depth is chosen not more than half the power of the target horizon, and the density value outside the zone at a depth of the target horizon is determined from the condition

where σ _n-1 is the density value in the (n 1) th interval with depth [MH _n-1 , kg / m ³ ;
β _n-1 - rock compressibility coefficient in the (n 1) th interval at various pressure values, MPa ^-1 ;
g acceleration of gravity, m / s ² ;
H _{n the} value of the depth of the n-th interval, m;
H _n-1 value of the depth of the (n-1) th interval, m;
α _n - coefficient of volumetric thermal expansion in the n-th interval, C ^-1 ;

the value of the geothermal gradient in the surface layer, ^o C / m,
moreover, sampling of rocks to determine these parameters is carried out outside the annular zone at a distance from its outer boundary equal to the average width of the annular zone.

Images