WO2012006871A1

WO2012006871A1 - Method of prospecting directly for free gas in stratum

Info

Publication number: WO2012006871A1
Application number: PCT/CN2011/001166
Authority: WO
Inventors: 陈信平
Original assignee: 陈再迁
Priority date: 2010-07-16
Filing date: 2011-07-14
Publication date: 2012-01-19
Also published as: CN102906600A; CN102906600B; CN102337885A

Abstract

A method for prospecting directly for free gas in a stratum. In the method, well logging data are used to compute a Lamé constant and a shear modulus for an underground rock stratum. The Lamé constant, shear modulus, and density are used as test factors, and threshold values are determined for each. The test factors and threshold values thereof are used to prospect directly in the stratum for gases with economic value such as free natural gas and carbon dioxide. The present invention has the advantage of being low in cost.

Description

Method for directly detecting free gas in formation

The present invention relates to a method for directly detecting free gas in a formation. Specifically, the present invention relates to an exploration well and a development well for obtaining log data, and using the log data to calculate a Lame constant and shear of a subterranean formation Modulus, using the Lame constant, shear modulus, and density as hydrocarbon detection factors to determine their thresholds. Using these hydrocarbon detection factors and their thresholds, directly detect the free natural gas, carbon dioxide, etc. in the formation. The method of gas. Background technique

Natural gas, carbon dioxide and other gases in the formation are important natural resources. Natural gas is an important energy source, and carbon dioxide is an important chemical raw material. Book

The gas in the formation may be free or non-free. The free gas refers to a gas that is free to flow between pores and cracks in which the rocks communicate with each other. The flow of free gas follows Darcy Law and is driven by the pressure gradient of the fluid in the rock. Natural gas contained in sandstone, limestone and other rock formations is a typical free gas. Non-free gas refers to a gas that cannot move freely. There are various reasons for not being able to move freely. For example, natural gas in natural gas compound (also known as "combustible ice") is in a solid crystalline state and thus cannot move freely; The gas such as methane is in an adsorbed state and thus cannot move freely. The non-free gas in the adsorbed state is called an adsorbed gas. In fact, when coalbed methane exists in the adsorbed state on the pore surface of the rock, it adheres to the pore surface with a liquid film under the action of Van der Waals forces. The migration of adsorbed gases in rocks is a complex process that follows Fick's Law and is driven by the concentration gradient of the adsorbed gases.

Conventional natural gas reservoirs are natural gas contained in sandstones, limestones and other rock formations that have been well known and exploited, and are terms relative to unconventional natural gas reservoirs. Conventional natural gas reservoirs are always free. For more than a decade, people have gradually realized that mudstones, coal seams, etc. can also be natural gas reservoirs, and the natural gas they contain is called mudstone gas, coalbed methane, etc., and they are collectively referred to as unconventional gas reservoirs. Except for combustible ice, most unconventional natural gas reservoirs are in an adsorbed state.

The subterranean formation can be divided into potential reservoirs and non-reservoirs according to the ability of the underground rock to store free gas. Potential reservoirs refer to rock formations that may become free gas reservoirs, such as sandstone, limestone, volcanic rock, and so on. The main difference between the potential reservoir and the non-reservoir is: the former has a large porosity, the pore connectivity is good, and has a large enough permeability; the latter has a small porosity and a small permeability, which is close to zero. In a exploration area, there may be only one potential reservoir, and there may be multiple potential reservoirs. Floor. The non-reservoir with low permeability is used as the cap layer of the free gas reservoir, and together with the reservoir constitutes a trap; the trap is the space where the free gas exists. Typical rock formations that can be used as caprocks are mudstone, gypsum, salt rock, and the like.

Of the many exploration and development wells drilled for the exploration and development of free (ie conventional) natural gas, only a few wells are self-propelled, many wells require logging to obtain logging data and interpret well logging data to determine natural gas. The location and reserves of natural gas reservoirs can be finalized after multiple steps of reservoir location, well logging, fracturing and well testing. Even if it is a self-injection well, if it is predicted that there are multiple natural gas reservoirs, the self-spraying may be only one or two of the reservoirs, and it is still necessary to perform logging, interpretation, well logging, and pressure after pressing and treating the self-injection. Multiple steps such as cracking and well testing can ultimately determine the location and reserves of all natural gas reservoirs. The cost of fracturing and well testing is expensive, several times to ten times the cost of land drilling, and is several hundred times to nearly a hundred times the cost of logging. Therefore, the accuracy of logging data interpretation is crucial. It is impossible to miss the natural gas reservoir, otherwise the valuable natural gas reserves will be lost. Nor can it be misreported to the natural gas reservoir, otherwise the cost of fracturing and well testing will be huge. waste. At present, logging data interpretation uses a variety of resistivity logging, density logging, porosity logging, and lithology logging; comprehensive interpretation of multiple data, and the results of various data interpretations are mutually validated, with a view to improving the accuracy of interpretation. degree. However, the accuracy of logging interpretation is still unsatisfactory. The rock formations interpreted as natural gas reservoirs, the results of fracturing and well testing may only produce water without gas; and the rock formations interpreted as water layers, after some time, because the adjacent wells were confirmed to be natural gas reservoirs by the well test. Layers and the like have provoked people to reinterpret logging data, fracturing and well testing, and found that the original interpretation of the water layer is actually the natural gas layer; all these types of misinterpretations occur from time to time. Conventional natural gas exploration requires new technical methods to improve the accuracy of logging data interpretation.

Unconventional natural gas reservoirs and conventional natural gas reservoirs can coexist in symbiosis. For example, coalbed methane is one of the important unconventional natural gas resources. In the sandstone near the coal seam, free natural gas often coexists with it. In the long geological history, the methane produced in the coal seam passes through various channels such as faults and fissures, and migrates to the sandstone and other reservoirs near the coal seam, and is freely present in the pores of rocks such as sandstone to form free natural gas. Tibetan, known as the "shallow free gas reservoir." "Shallow" means that the buried depth of such free-standing natural gas reservoirs is often smaller than that of coal-bed methane reservoirs that are natural gas source rocks, much smaller than conventional gas reservoirs that were previously known. How to explore and develop shallow free natural gas reservoirs while exploring and developing coalbed methane has become one of the current research hotspots.

However, conventional natural gas reservoirs that coexist with unconventional natural gas reservoirs have their own particularities. The reservoirs of such natural gas reservoirs are usually tight sandstones with low porosity and low gas saturation, making existing techniques for exploring conventional gas reservoirs often unsuitable for exploration of such natural gas reservoirs. For example, because such natural gas reservoirs are densely sanded and have low porosity, even if the natural gas is completely filled with sandstone pores, there is little difference between the elastic characteristics of the gas-saturated tight sandstone and the elastic characteristics of the tight sandstone without gas. Not enough to cause significant changes in seismic wave reflection characteristics; and because of shallow burial and severe surface noise interference, existing seismic exploration techniques (including special seismic techniques for exploration of conventional natural gas reservoirs) are used to explore shallow free gas reservoirs. The effect is very poor. For example, due to the compactness of sandstones in such natural gas reservoirs, low porosity, low gas saturation, coupled with the plugging effect of mud mud formed on the borehole wall, conventional logging data explains the natural gas reservoirs. The effect is even worse. Although countries with abundant CBM resources (for example, the United States, Canada, Australia, etc.) have been committed to the exploration and development of shallow free natural gas reservoirs, little progress has been made so far because no detection technology suitable for such natural gas reservoirs has been found. In order to explore and develop natural gas reservoirs such as shallow free natural gas reservoirs, new technological methods need to be developed. Summary of the invention

The object of the present invention is to provide a method for directly detecting free gas in a formation. Specifically, the object of the present invention is to provide an exploration well and a development well to obtain logging data, and to calculate a Lame constant of a subterranean formation based on the logging data. And shear modulus, using the Lame constant, shear modulus, and density as hydrocarbon detection factors to determine their thresholds, using these hydrocarbon detection factors and their thresholds to directly detect free natural gas in the formation, A method of gas such as carbon dioxide.

The invention is implemented by the following technical means:

A method for directly detecting free gas in a formation, which utilizes an exploration well and a development well, first performs logging in an exploration well and a development well, obtains logging data, and calculates a Lame constant and shear of the underground rock formation based on the logging data. Cutting modulus, using the Lame constant, shear modulus, and density as hydrocarbon detection factors to determine their thresholds, using these hydrocarbon detection factors and their thresholds to directly detect free natural gas, carbon dioxide, etc. in the formation. Wait for the gas.

The method consists of the following steps:

(1) Perform the following operations for each exploration and development well in the exploration and development area:

(a) Logging in the well to collect logging data such as shear wave velocity, longitudinal wave velocity, density, natural potential, natural gamma, porosity, resistivity, etc. of the subterranean formation;

(b) Calculating the Lame constant and shear modulus of the underground rock according to the shear wave velocity, longitudinal wave velocity and density obtained by the logging;

(c) Calculating the lithologic composition of the subterranean formation based on the relevant logging data, and obtaining the lithology log;

(2) For all exploration and development wells in the exploration and development area, the Lame constant, shear modulus, and density are used as hydrocarbon detection factors to determine the threshold values of these hydrocarbon detection factors, including the following sub-steps:

(d) distinguish between potential reservoirs and non-reservoirs;

(e) Statistical analysis to obtain the relationship between the Lame constant, shear modulus, density and lithology type and lithology composition of various rock layers. The method is: Using scatter plots to determine the Lame constant and shear mode of different types of rock The difference in quantity and density and its variation with lithological composition; when the amount of data is large enough, the regression method is used to determine the correlation between the tensile constant, shear modulus, density and lithological composition of various rock layers. (f) Determine the best possible threshold for the Lame constant, shear modulus, and density of the free gas reservoir by: (i) statistically analyzing the Lame constant and shear mode of the known free gas reservoir in the exploration area. Quantity, density, threshold value; (ii) The Lame constant, shear modulus, and density of the reservoir rock sample measured by the laboratory in water saturation and gas saturation, and the threshold value is determined.

(g) Determine the relationship between the Lame constant, shear modulus, density and lithological composition, porosity of the free gas reservoir; (3) Determine each potential reservoir of each exploration and development well in the exploration area Whether the layer is a reservoir of free gas includes the following substeps:

(h) Apply the results of the foregoing step (f) and the aforementioned step (g) to each well to determine the threshold value of the hydrocarbon detection factor applicable to each of the exploration and development wells.

(i) the gates of the hydrocarbon detection factors applicable to each of the exploration and development wells obtained from the well density curve and the aforementioned step (b), the calculated lambian constant and shear modulus, and the aforementioned step (h) Limits, as well as lithology logs, porosity log curves, etc., determine whether each potential reservoir of each exploration and development well in the exploration area is a reservoir of free gas.

(j) Study the position of each well on the geological structure, and finally determine whether the reservoir of the free gas of each well obtained in the above step (i) is a reservoir worthy of gas test, by: making a geological structure map, determining the structural circle Closed, the well located in the favorable part of the structural trap is most likely to encounter the reservoir with economic value worthy of test; study the gradual change of lithology in the horizontal and vertical directions in the exploration area, determine whether there is lithologic trap and lithology The location and extent of the trap, the well located in the favorable part of the lithologic trap may also encounter a reservoir with economic value worthy of test; a well that is neither within the structural trap nor within the lithologic trap Drilling into a reservoir worthy of trials with economic value.

Compared with the prior art, the present invention has the following obvious advantages and beneficial effects:

The invention uses the logging data observed in the well to detect the shallow free gas reservoir, and the observation environment noise is small and the data is reliable. Therefore, the present invention is particularly easy to implement.

The present invention generally does not require an increase in field engineering costs; even if it is necessary to increase the cost of field engineering, the increase is at most the acquisition cost of the transverse wave velocity. Therefore, the implementation of the present invention has the advantage of being low in cost. DRAWINGS

Figure 1 shows the log curves observed in the Z exploration well of a coalbed methane exploration area. From left to right, the longitudinal wave velocity, the shear wave velocity, and the density;

2 is a Lame constant and a shear modulus calculated according to the log curve in FIG. 1;

Figure 3 is a partial potential reservoir of well Z in a coalbed methane exploration area determined according to the method described in the foregoing step (d); Figure 4 is a relationship between sandstone reservoir porosity and gas-saturated reservoir density in a coalbed methane exploration area. relationship; Figure 5 is the relationship between the porosity of sandstone reservoirs in a coalbed methane exploration area and the shear modulus of gas-saturated reservoirs. Figure 6 is the relationship between the porosity of sandstone reservoirs in a coalbed methane exploration area and the Lame constant of gas-saturated reservoirs. Relationship;

Figure 7 shows two shallow free natural gas reservoirs drilled in the Z well in a coalbed methane exploration area. detailed description

The present invention will be further described below in conjunction with specific embodiments.

The invention utilizes exploration wells and development wells in resource exploration and development to obtain logging data, and calculates the Lame constant and shear modulus of the underground rock layer according to the logging data, and takes the Lame constant, the shear modulus and the density as Hydrocarbon detection factors, determine their thresholds, use these hydrocarbon detection factors and their thresholds to directly detect free-form natural gas, carbon dioxide, and other economically valuable gases in the formation.

In order to demonstrate the ability of the hydrocarbon detection factors provided by the present invention to predict free gas in the formation, it is necessary to utilize the Gassmann square in petrophysics (see Gassmann, F., 1951, "Elastic waves through a packing of spheres", Geophysics. Vol. 16, p. 673-685). The expression of the Gassmann equation chosen for use in the present invention is:

(3)

Where μ is the shear modulus of the water-saturated rock,

Is the shear modulus of the rock skeleton, ie the shear modulus of the gas-saturated rock,

λ is the Lame constant of a water-saturated rock.

λ, which is the Lame constant of the rock skeleton, that is, the Lame constant of the gas-saturated rock.

K _b is the volumetric compressive modulus of the rock skeleton, ie the volumetric compressive modulus of the gas-saturated rock.

K _S is the volumetric compressive modulus of the minerals that make up the rock skeleton.

K _f is the volumetric compressive modulus of the fluid filled in the pores of the rock.

φ is the porosity of the rock.

Equation (1) shows that: When the pore fluid properties of the rock change, the shear modulus μ of the rock does not change. In fact, laboratory measurements of rock samples indicate that the shear modulus of the rock increases slightly as the water in the pores of the rock is replaced by natural gas. This variation, as well as the difference in shear modulus between different types of rocks, is used by the present invention to detect reservoirs of free gas in the formation. Equation (2) shows that: The Lame constant λ is a parameter closely related to the pore fluid properties of the rock, and when the pore fluid properties of the rock change, the Lame constant λ decreases with the volumetric compressive modulus of the rock pore fluid. It is monotonously reduced. In this regard, it can be proved as follows: Since the numerator of the second term at the right end of equation (2) is the square of the real number, the numerator is always a positive number. 5, and the incompressibility of the rock increases with the porosity of the rock, as the rock porosity φ is always less than 0.5, and the measurement results of various rock-forming minerals and various rocks are always less than 0.5. And the decrease, that is, the same kind of rock, φ increases, then decreases, therefore, + is always less than ¹ ; and since the rock porosity φ is always positive, the pore fluid ^ is always positive, therefore, the equation (2 The denominator of the second term on the right is always positive. Since the numerator and denominator of the second term at the right end of equation (2) are both positive, the second term at the right end of equation (2) is always a positive number. It is thus inferred that for the same rock sample, when ^ decreases, /^) increases, the second term at the right end of equation (2) monotonically decreases, and the Lame constant λ also monotonically decreases. For a particular rock sample, the change in the Lame constant λ is purely due to changes in pore fluid properties, and, theoretically, changes in pore fluid properties only result in a change in the Lame constant λ. The volumetric compressive modulus of water is about 2. 3 Gpa, and the gas volume compressive modulus under standard conditions is always less than 1 Mpa, which differs by three orders of magnitude. Therefore, the gas can be considered as a gas relative to the volumetric compressive modulus of water. The volumetric compressive modulus is close to zero. In equation (2), when /^ approaches zero, ) approaches infinity, and the second term of equation (2) approaches zero. The difference in the Lame constant between gas-saturated rocks and water-saturated rocks is the second term of equation (2). The invention utilizes the phenomenon that the Lame constant of the gas-saturated rock is much lower than the Lame constant of the water-saturated rock, and uses the Lame constant as the most important hydrocarbon detecting factor to detect the occurrence of free gas in the underground rock formation. ("GPa" is the abbreviation for the pressure unit "megapascal". By definition, the unit of pressure is used to indicate the value of the elastic modulus.)

The third hydrocarbon detection factor used in the present invention is density. The density of the gas is much smaller than the density of water (3⁄4. When the water in the pores of the rock is replaced by a gas, the density of the rock will decrease, and the degree of reduction is related to the porosity and gas saturation of the rock. Assuming the free gas reservoir The porosity is χ (expressed as a percentage), then the density of the gas-saturated rock will decrease A _Pl compared to the density of the water-saturated rock _:

If the rock is not a fully gas-saturated rock but a partially gas-saturated rock, and the gas saturation expressed as a percentage is ξ, then the density difference Δ ρ ₂ between the water-saturated rock and the partially gas-saturated rock is

△ ρ ₂ =(ρ -Ρ ξχ (4) In general, it is difficult to judge whether or not the rock contains gas and whether it has economic value based on the density change. However, the present invention does not use density as a hydrocarbon detecting factor in isolation, but Using density as one of the hydrocarbon detection factors, it is comprehensively judged whether the potential reservoir is a gas-bearing reservoir and whether it has economic value. The present invention provides specific methods and steps for effecting detection after providing a theoretical basis for detecting free gas in a subterranean formation.

The method for detecting free gas in a subterranean formation provided by the present invention consists of the following steps:

(a) Logging in the well to collect logging data such as shear wave velocity, longitudinal wave velocity, density, natural potential, natural gamma, porosity, and resistivity of the subterranean formation.

(b) Obtain the shear wave velocity Vs, the longitudinal wave velocity Vp, and the density p according to the logging, and calculate the shear modulus and the Lame constant of the subterranean formation according to the following formula:

μ = ?V _S ² ( 5 )

= pV _p ² - 2pV^ ( 6 )

(c) Calculate the lithologic composition of the subterranean formation according to the relevant logging data, and obtain the lithology logging curve; this step is a processing step already existing in the logging material processing flow, and the present invention will not be described again.

(d) Distinguish between potential reservoirs and non-reservoirs. The basis for the distinction is: geological logging records and logging histograms of each well obtained during drilling, as well as porosity logging curves and log lithology obtained by logging data processing. Composition, lithology log. The method of distinguishing is: roughly classify the potential reservoirs and non-reservoirs according to the types of rock and their lithological characteristics at different depths recorded by the logging records and the log column icons, and determine the types of potential reservoirs in the exploration area, and determine Candidate potential reservoirs; determine the final potential reservoir based on porosity log curves, lithology logs, lithology components, and considering the caprocks of candidate potential reservoirs.

(e) Statistical analysis to obtain the relationship between the Lame constant, shear modulus, density and lithology type and lithology composition of various rock layers. The method is: Using scatter plots to determine the Lame constant and shear mode of different types of rock The difference in quantity and density and its variation with lithological composition; when the amount of data is large enough, the regression method is used to determine the correlation between the tensile constant, shear modulus, density and lithological composition of various rock layers.

(f) Determine the best possible threshold for the Lame constant, shear modulus, and density of the free gas reservoir by: (i) statistically analyzing the Lame constant and shear mode of the known free gas reservoir in the exploration area. Quantity, density, threshold value; (ii) The Lame constant, shear modulus, and density of the reservoir rock sample measured by the laboratory in water saturation and gas saturation, and the threshold value is determined. These two methods can be used at the same time, mutually confirming and correcting; in the early stage of exploration, they can only rely on the measurement results of the laboratory; as the exploration and development progresses, the data of the identified free gas reservoirs are gradually increased; In the later stages, it is possible to transition to data using only known free gas reservoirs. (g) Determine the relationship between the Lame constant, shear modulus, density and lithological composition, porosity of the free gas reservoir by: using a scatter plot to determine the Lame constant of the free gas reservoir, shear mode The trend of quantity and density with lithological composition and porosity; when the amount of data is large enough, the regression method is used to determine the Lame constant, shear modulus, density and lithology composition and porosity of the free gas reservoir. Relevant relationship.

(3) Determining whether each potential reservoir of each exploration and development well in the exploration area is a reservoir of free gas, including the following sub-steps:

(h) the best possible threshold value of the Lame constant, shear modulus, density obtained in the above step (f), and the Lame constant and shear modulus of the free gas reservoir obtained in the aforementioned step (g) The relationship between density and its lithological composition and porosity is applied to each well to determine the threshold value of the hydrocarbon detection factor applicable to each exploration and development well.

(i) the gate of the hydrocarbon detection factor applicable to each exploration well and development well obtained from the density log and the aforementioned step (b) and the shear modulus, the shear modulus, and the aforementioned step (h) Limits, as well as lithology logs, porosity log curves, etc., determine whether each potential reservoir of each exploration and development well in the exploration area is a reservoir of free gas.

(j) Study the position of each well on the geological structure, and finally determine whether the reservoir of the free gas of each well obtained in the above step (i) is a reservoir worthy of gas test, by: making a geological structure map, determining the structural circle Closed, the well located in the favorable part of the structural trap is most likely to encounter the reservoir with economic value worthy of test; study the gradual change of lithology in the horizontal and vertical directions in the exploration area, determine whether there is lithologic trap and lithology circle The position and extent of the closure, wells located in the favorable part of the lithologic trap may also encounter reservoirs of economic value that are worthy of test; wells that are neither located within the structural trap nor within the lithologic trap are unlikely to be drilled In the case of a reservoir worthy of trials with economic value.

Having described in detail each step of carrying out the invention, a shallow free gas reservoir coexisting with coalbed methane is explored by using the technical method of the present invention as an example for carrying out the invention.

Coalbed methane is the gas in the coal seam. Its main component is methane, which is the cause of coal mine gas outburst and explosion. However, before the coal is mined, the gas is extracted, which is natural gas, which is clean energy and is called coalbed methane. China's coalbed methane resources amount to 38 trillion cubic meters, ranking third in the world, equivalent to China's conventional natural gas resources. In recent years, China has carried out CBM exploration and development projects in major coal reserves, and has achieved economic, environmental protection and labor protection benefits. In the field of coalbed methane exploration and development, one of the current hot issues is to explore and develop coalbed methane while simultaneously exploring and developing shallow free gas reservoirs near the coal seam that coexist with coalbed methane.

Because coalbed methane is present in the coal seam in the adsorption state, it has the characteristics of local enrichment; and because the pore connectivity of the coal seam is poor, the permeability is mainly determined by the joint fissure of the coal seam, resulting in a low permeability of the coalbed methane reservoir; therefore, the coal seam Gas development wells are small, usually between 200m and 300m, to ensure oil recovery. This feature of coalbed methane exploration and development projects is particularly suitable for The technical methods provided by the present invention are used to explore shallow free gas reservoirs. In order to explore shallow free gas reservoirs, as long as the potential reservoirs are identified as natural gas reservoirs in CBM exploration wells and development wells, it is possible to ensure the identification of all shallow economic free gas reservoirs of economic value. For example, if the well spacing of the CBM development well network is 300 m, then the present invention can detect all shallow free natural gas reservoirs having an equivalent radius greater than 150 m in the plane, and the shallow free state natural gas reservoir having an equivalent radius of less than 150 m is generally not Economic Value. Therefore, the present invention is particularly suitable for detecting shallow free gas reservoirs coexisting with coalbed methane.

In a coalbed methane exploration area, the C coal seam of the main reservoir of coalbed methane has a depth of about 680m, a thickness of 2m~8m, and a gas content of 10~30m7 tons per ton of coal. Above and below the main reservoir, there are a plurality of thin coal seams, respectively. The main types of rocks in the exploration area are mudstone, sand mudstone, sandstone and limestone. Among them, limestone is mostly thin, with limited distribution, dense texture, few cracks or pores, and it is impossible to become a reservoir of free gas. Mudstone, sand-shale, sandstone are mostly thin interbeds, and the thickness of single-layer mudstones and sandstones generally does not exceed 10m; most sandstones are dense and have a porosity of less than 5%. In summary, the potential reservoirs in the exploration area are sandstones with large porosity, and the direct caprocks of these potential reservoir sandstones are thicker mudstones.

Figure 1 shows the log curves observed in an exploration well (hereinafter referred to as "Z well") in the exploration area. From left to right, the longitudinal wave velocity, the shear wave velocity, and the density are shown. The leftmost column in the figure is the number of the horizon of the exploration target. Now, in the figure, the numbers of all the coal seams encountered by the Z well are marked, where A, B, C, ... are the number of the coal seam, and the number in front of the last letter "m" is the thickness of the coal seam; B-coal - 1. 2m" means "B coal seam, thickness 1. 2m". "C_Coal - 3. 3m" is the number of the main reservoir of coalbed methane encountered in the well, and its thickness is 3. 3m. The longitudinal wave velocity and density of the logging curve in Figure 1 are both inevitable logging projects in the conventional logging series. The shear wave velocity is not a logging project in the conventional logging series. The current practice of resource exploration is to perform a special logging series that includes shear wave velocity logging only in some exploration wells. Implementation of the present invention requires observation of shear wave velocity throughout the exploration and development wells, unless some exploration wells and/or development wells have been excluded from the possibility of drilling free natural gas.

Figure 2 is the Lame constant and shear modulus calculated from the log curve in Figure 1, using equations (5) and (6) in step (b).

Figure 3 is a partial potential reservoir of the Z well determined in accordance with the method described in step (d). Please note that in order to display these potential reservoirs, the scale of the map is smaller than the scales of Figures 1, 2, and 4. However, due to the limitation of the size of the map, only the potential and the promising potential from the coal seam can be displayed. Reservoir. These potential reservoirs are labeled as "potential reservoir -4", "potential reservoir -5", ..., etc. See the "Target horizon number" column.

The statistical analysis and research on the logging and logging data of 5 exploration wells have found that the limestone in this exploration area has the largest Lame constant and shear modulus, which is expressed on the Lame constant and shear modulus curves. Sharp peaks are consistent with logging records where limestone is mostly thin; the Lame constant and shear modulus of thick mudstones are followed by a small and gentle change; the shear modulus of sandstone may be greater than Less than the shear modulus of mudstone, closely related to its shale content and porosity, If the amount is large, the shear modulus is small; if the porosity is small, the shear modulus is large; the Lame constant of sandstone is affected by the shale content and porosity, and is more related to the properties of sandstone pore fluid.

The exploration area is a new area in the evaluation stage. There is no known free natural gas reservoir for reference. Therefore, the relative gates of the constant, shear modulus and density can only be determined according to the results of laboratory rock sample determination. The limit value, that is, the threshold value is determined based on the difference between the Lame constant, the shear modulus, the density, and the relative change between the water-saturated sandstone and the gas-saturated sandstone. According to the results of the determination of 53 sandstone samples in water saturation state and gas saturation state, the Lame constant of gas-saturated sandstone is 2~5Gpa lower than that of water-saturated sandstone, and the relative change is -5%~-33%; The shear modulus of gas-saturated sandstone is mostly higher than the shear modulus of water-saturated sandstone, but it is also lower than the shear modulus of water-saturated sandstone, which is not completely consistent with the prediction of Gassmann equation, mainly because there is The sandstone samples have a large shale content, and the water infiltration softens the strength of the clay between the quartz mineral particles of the sandstone. However, since the absolute value of the relative change of shear modulus of most sandstone samples under gas saturation and water saturation is less than 10%, the Gassmann equation can be considered as suitable for the sandstone in this exploration area; gas-saturated sandstone density and water-saturated sandstone The difference in density substantially conforms to the formulas (3) and (4) cited in the present invention, and will not be described again. In view of the low degree of exploration in this exploration area, there is no known free natural gas reservoir for reference. Therefore, it is decided to determine the best possible hydrocarbon constants, shear modulus and density based on laboratory measurements. Threshold. The specific method is as follows: From the sandstone samples measured in the laboratory, the sandstone samples with porosity greater than 5% are selected, and the Lame constant, shear modulus and density under gas saturation are used to eliminate individual abnormalities and abnormalities. After the results, the average values were taken as the best possible threshold values for the hydrocarbon detection factor Lame constant, shear modulus, and density. The best possible threshold for determining the Lame constant is 10 GPa, the best possible threshold for shear modulus is 4 GPa, and the best possible threshold for density is ^2.3 g/cm ³ .

Similarly, since there are no proven free natural gas reservoirs available, the Lame constant, shear modulus, density and lithological composition of the free gas reservoir can only be analyzed based on laboratory measurements of sandstone samples. The relationship between porosity. The main components of these sandstones are quartz and clay minerals with a clay content ranging from 2.6% to 15.3%. The statistical analysis of laboratory results shows that the Lame constant of gas-saturated sandstone increases with the increase of clay content, the density increases slightly with the increase of clay content, and the shear modulus decreases slightly with the increase of clay content. Only when the clay content is greater than 10%, it is necessary to consider the influence of clay content on the Lame constant, shear modulus and density. Considering the high diagenesis and dense texture of the sandstone in this exploration area, the statistical analysis is reasonable. of. However, porosity has a significant effect on the Lame constant, shear modulus and density. Figure 4 shows the effect of porosity on the density of gas-saturated reservoirs. Because of the linear relationship between the density of rock and its porosity:

P=P skeleton - (P skeleton - p fluid) χ ( 7 ) Where P is the density of the rock skeleton, p _# is the density of the rock pore fluid; for gas-saturated sandstone, p _# is the density of natural gas under subsurface pressure and temperature conditions (3. Since _P > _{P #} , the density of the rock is always This decreases with increasing porosity. Figure 4 is a good reflection of this linear relationship, with a linear fit of R squared up to 0.96. Figure 5 shows the effect of porosity on the shear modulus of a gas-saturated reservoir. The modulus of cut obviously decreases with increasing porosity. Although there is no linear function relationship between the porosity and the shear modulus similar to (7), the linear fit has an R-squared value of 0.65. It is shown that the linear empirical relationship between porosity and shear modulus obtained by linear fitting is still reliable. Figure 6 shows the effect of porosity on the Lame constant of gas-saturated reservoir. The Lame constant also increases with porosity. Large and decreasing, this trend is more obvious and reliable, but the data is highly dispersive, and the R-squared value of the linear fit is 0.1, indicating that the linear empirical relationship obtained by linear fitting is not reliable.

The best possible threshold values for the hydrocarbon detection factor Lame constant, shear modulus, density, and the Lame constant, shear modulus, density of the free gas reservoir, its lithological composition, and porosity are obtained. The relationship, applied to the Z well, determines the threshold for the hydrocarbon detection factor applicable to the Z well. The application process is: According to the relationship between the Lame constant, shear modulus, density and its lithological composition and porosity of the free gas reservoir, the best possible threshold for adjusting the Lame constant, shear modulus and density Value, obtain the threshold value of the hydrocarbon detection factor applicable to the Z well. The final threshold value for the Z-well is 9 GPa, the shear modulus threshold is 3 GPa, and the density threshold is 2. 2 _g / cm ³ .

For the investigation of the occurrence of free natural gas, shear modulus is the most important hydrocarbon detection factor for evaluating reservoir quality. For the Z well, although the shale content and porosity of the sandstone reduce the shear modulus of the sandstone, the influence of the shale content and porosity on the reservoir quality is reversed. Large shale content may cause clay to fill pores and block pore throats, reducing the effective porosity of the reservoir and reducing the permeability of the reservoir; these all degrade the quality of the reservoir. The increase in porosity will inevitably lead to an increase in the reservoir's ability to contain natural gas and an increase in reservoir permeability, which will improve the quality of the reservoir. Therefore, it is necessary to carefully identify the factors that contribute to the reduction of sandstone shear modulus. The lithology log shows that for the potential reservoirs in Figure 3, the "potential reservoir-6" and "potential reservoir-7" clay content is less than 5%, and their log porosity is greater than 9%, according to This infers that their lower shear modulus values are due to increased porosity.

For the exploration of the occurrence of free natural gas, the Lame constant is the most important hydrocarbon detection factor for judging the pore fluid properties of sandstone. On the logging curve, the relatively small Lame constant corresponding to the sandstone layer indicates that the water in the pores of the sandstone may be replaced by natural gas. For the Z well, as mentioned above, an increase in the shale content in the sandstone will result in an increase in the Lame constant, and an increase in the shale content will result in a deterioration in the reservoir quality; an increase in the porosity of the sandstone will result in a decrease in the Lame constant. , and the increase in porosity will lead to better reservoir quality. Therefore, on the logging curve of the Z well, for the Lame constant, the smaller Lame constant is always conducive to interpreting the sandstone layer as a reservoir. For the potential reservoirs in Figure 3, the Lame constants for "potential reservoir-6" and "potential reservoir-7" are less than the threshold and can be interpreted as natural gas causing such a low Lame constant. As mentioned above, there is a direct, simple, and definite linear relationship between the density of sandstone and the density of its pore fluids, which can be used to evaluate the various potential reservoirs of each exploration and development well. For the potential reservoirs in Figure 3, the density of "potential reservoir-6" and "potential reservoir-7" is less than 2.15 g/cm ³ , which is less than the threshold of the hydrocarbon detection factor density of 2.2 g/cm ^{3 .} These two sandstone layers can be interpreted as natural gas reservoirs.

The other potential reservoirs in Figure 3 are interpreted as water layers because their Lame constant, shear modulus, and density do not meet the hydrocarbon detection factor threshold.

The study and mapping of the geological structure around the Z well found that the "potential reservoir-6" and "potential reservoir-7" of the Z well are located in the vicinity of the shaft portion of a gentle anticline structure, and the mudstone on it constitutes Anticline trap. This structurally advantageous part supports the interpretation of "potential reservoir-6" and "potential reservoir-7" as free natural gas reservoirs.

Figure 7 shows the final interpretation of the Z well. The "natural gas reservoir-1 - 2.5 m" and "natural gas reservoir -2 - 3. 5 m" marked in the column "Target horizon number" in the figure indicate the interpretation of the two-layer natural gas reservoir, of which "2.5m" and "3.5m" are the thicknesses of two free natural gas reservoirs, respectively. The results of the test gas prove that the interpretation shown in Figure 7 is correct.

The well logging data used in the implementation steps of the present invention, except for the shear wave velocity, are well logging data that must be collected during exploration and development; while the shear wave velocity may or may not be acquired. Therefore, the use of the present invention may not require an increase in field engineering costs at all; even if additional field engineering costs are required, the increase is at most the acquisition cost of the shear wave velocity. Therefore, the implementation of the present invention has the advantage of being low in cost.

It should be noted that the above embodiments are merely illustrative of the present invention and are not intended to limit the technical solutions described herein; therefore, although the present specification has been described in detail with reference to the various embodiments described above, It will be understood by those skilled in the art that the present invention may be modified or equivalently substituted without departing from the spirit and scope of the invention.

Claims

Claim

A method for directly detecting free gas in a formation, characterized in that: using an exploration well and a development well to obtain logging data, and using the logging data to calculate a Lame constant and a shear modulus of the subterranean formation, The plum constant, shear modulus, and density are used as hydrocarbon detection factors to determine their threshold values. Using these hydrocarbon detection factors and their thresholds, direct detection of free economical gases such as natural gas and carbon dioxide in the formation; Includes the following steps:

(a) Logging in the well to collect logging data of shear wave velocity, longitudinal wave velocity, density, natural potential, natural gamma, porosity, and resistivity of the subterranean formation;

(b) Calculate the Lame constant and shear modulus of the subterranean formation based on the shear wave velocity, longitudinal wave velocity, and density obtained from the logging data;

(d) distinguish between potential reservoirs and non-reservoirs;

(e) Statistical analysis of the relationship between the Lame constant, shear modulus, density and lithology type, and lithology composition of various rock layers;

(f) determining the best possible threshold for the Lame constant, shear modulus, and density of the free gas reservoir;

(g) determining the relationship between the Lame constant, the shear modulus, the density of the free gas reservoir and its lithological composition, and porosity;

(h) applying the results of the aforementioned step (f) and the aforementioned step (g) to each well to determine the threshold value of the hydrocarbon detection factor applicable to each of the exploration and development wells;

(i) the gates of the hydrocarbon detection factors applicable to each of the exploration and development wells obtained from the well density curve and the aforementioned step (b), the calculated lambian constant and shear modulus, and the aforementioned step (h) Limits, as well as lithology logs, porosity log curves, to determine whether each potential reservoir of each exploration and development well in the exploration area is a reservoir of free gas;

(j) Study the position of each well on the geological structure, and finally determine whether the reservoir of the free gas of each well obtained in the above step (i) is a reservoir worthy of gas test.

2. A method of directly detecting free gas in a formation according to claim 1, characterized in that in said step (b), the Lame constant is calculated using =V _p ² - 2 V _s ² , using /= V _s ² calculates the shear modulus.

3. The method of directly detecting free gas in a formation according to claim 1, wherein: said step

(e) Using scatter plots to determine the difference of the Lame constant, shear modulus, and density of different types of rock and their variation with lithological composition; using regression method to determine the Lame constant and shear mode of various rock layers The correlation between quantity, density and its lithological composition.

4. The method of directly detecting free gas in a formation according to claim 1, wherein: said step

(f) statistically analyze the Lame constant, shear modulus, and density of known free gas reservoirs in the exploration area to determine these hydrocarbon detection factors The best possible threshold for the child; the Lame constant, shear modulus, and density of the reservoir rock sample measured by the statistical analysis laboratory under water saturation and gas saturation, and the best possible threshold for determining these hydrocarbon detection factors Limit.

5. The method of directly detecting free gas in a formation according to claim 1, wherein: said step

(g), using scatter plots to determine the trend of Lame constant, shear modulus, density as a function of lithology and porosity of free gas reservoirs, using regression method to determine the Lame constant and shear of free gas reservoirs The relationship between modulus, density and its lithological composition and porosity.

6. The method of directly detecting a free gas in a formation according to claim 1, wherein: said step

(h), the Lame constant obtained in step (f), the shear modulus, the best possible threshold value of the density, the Lame constant of the free gas reservoir obtained in the step (g), the shear modulus, and the density The relationship between its lithological composition and porosity is applied to each well to determine the threshold of the hydrocarbon detection factor applicable to each exploration and development well.

7. The method of directly detecting free gas in a formation according to claim 1, wherein: said step

(i), based on the density curve obtained from the logging and the calculation of the Lame constant and the shear modulus obtained in step (b), the hydrocarbon detection factor applicable to each exploration well and development well obtained in step (h) Threshold values, etc., as well as lithology log and porosity log, determine the reservoir of free gas for each exploration and development well in the exploration area.

8. The method of directly detecting a free gas in a formation according to claim 1, wherein: said step (j), constructing a trap and a lithologic trap, according to each well in a structural trap or lithology The relative position of the trap, and finally determine the reservoir that needs to test the gas.