A method for determining a shale gas accumulation type

Abstract

The application provides a shale gas accumulation type determination method, which comprises the following steps: according to the throat radius of connected pores in the shale sample in the bedding direction, the connected pores are classified; according to the volume of each type of connected pores, the accumulation type of the shale gas is determined; wherein, the types of the connected pores include Darcy diffusion type connected pores, slip flow type connected pores, Fick diffusion type connected pores and Knudsen diffusion type connected pores. The determination method can more accurately determine the accumulation type of the shale gas.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of geological exploration, in particular to a method for determining shale gas accumulation type. BACKGROUND

[0002] As shale is a fine-grained sedimentary system, the porosity and permeability conditions are extremely poor, and the accumulation process of shale gas in shale formation is complex, and the flow form of shale gas under high temperature and high pressure environment in shale formation is variable, which easily leads to unclear understanding of shale gas accumulation type. The shale gas accumulation type is the key to shale gas geological evaluation, which determines the shale gas accumulation process and accumulation efficiency. Therefore, accurately judging the shale gas accumulation type can provide guidance for low-cost and efficient exploration and development of shale gas field and technical support for the discovery of shale gas field.

[0003] The prior art usually determines the shale gas accumulation type at the block scale of the gas field, considering the regional tectonic background, preservation conditions and sedimentary environment, combining with the shale gas content. However, in the accumulation process of shale gas, the gas flow form and connected pore distribution are the basis of shale gas accumulation, and determine the path and way of shale gas accumulation in the underground, so the method for determining the shale gas accumulation type in the prior art has insufficient accuracy and cannot meet the current requirements for shale gas reservoir distribution and productivity prediction.

[0004] Therefore, it is urgent to study a method for accurately determining the shale gas accumulation type. SUMMARY

[0005] The present application provides a method for determining shale gas accumulation type, which can more accurately determine the accumulation type of shale gas and meet the current requirements for shale gas reservoir distribution and productivity prediction.

[0006] The present application provides a method for determining shale gas accumulation type, which includes:

[0007] According to the throat radius of the connected pores in the shale sample in the bedding direction, the connected pores are classified;

[0008] According to the volume of each type of connected pores, the accumulation type of the shale gas is determined;

[0009] The categories of the connected pores include Darcy diffusion type connected pores, slip flow type connected pores, Fick diffusion type connected pores and Knudsen diffusion type connected pores.

[0010] The method for determining shale gas accumulation type as described above, wherein the determination of the accumulation type of the shale gas according to the volume of each type of connected pores includes:

[0011] If V1≥2(V2+V3+V4), the shale gas is a buoyancy accumulation type;

[0012] If 2(V2+V3+V4)>V1≥1 / 2(V2+V3+V4), the shale gas is a composite accumulation type of non-buoyancy accumulation and buoyancy accumulation;

[0013] If V1<1 / 2(V2+V3+V4), the shale gas is a non-buoyancy accumulation type;

[0014] V1 is the volume of Darcy diffusion type connected pores;

[0015] V2 is the volume of slip flow type connected pores;

[0016] V3 is the volume of Fick diffusion type connected pores;

[0017] V4 is the volume of Knudsen diffusion type connected pores.

[0018] The method for determining the shale gas accumulation type as described above, wherein the classifying the connected pores according to the throat radius of the connected pores in the shale sample in the bedding direction comprises:

[0019] If r≥500λ, the connected pore is a Darcy diffusion type connected pore;

[0020] If 500λ>r≥50λ, the connected pore is a slip flow type connected pore;

[0021] If 50λ>r≥0.5λ, the connected pore is a Fick diffusion type connected pore;

[0022] If r<0.5λ, the connected pore is a Knudsen diffusion type connected pore;

[0023] Wherein, r is the throat radius of the connected pore, nm;

[0024] λ is the average molecular free path of the shale gas, nm.

[0025] The method for determining the shale gas accumulation type as described above, wherein λ is determined by formula 1;

[0026]

[0027] In formula 1, λ is the average molecular free path of the shale gas, nm;

[0028] K is the Boltzmann constant, J / K;

[0029] T is the in-situ temperature of the shale gas, K;

[0030] d is the diameter of the shale gas molecule, m;

[0031] P is the in-situ pressure of shale gas, Pa.

[0032] The method for determining the shale gas accumulation type as described above, wherein the volume of each type of connected pore is obtained according to the volume distribution of the connected pores.

[0033] The method for determining the shale gas accumulation type as described above, wherein the volume distribution of the connected pores is the product of the total volume of the connected pores and the volume proportion of the connected pores.

[0034] The method for determining the shale gas accumulation type as described above, wherein the total volume of the connected pores is determined by a helium porosimeter.

[0035] The method for determining the shale gas accumulation type as described above, wherein the volume proportion of the connected pores is determined by a high-pressure mercury injection apparatus.

[0036] The method for determining the shale gas accumulation type as described above, wherein the shale sample is in a cylindrical shape.

[0037] The method for determining the shale gas accumulation type as described above, wherein the diameter of the shale sample is 2.5 cm, and the length is 3-7 cm.

[0038] The present application provides a method for determining the shale gas accumulation type, which determines the shale gas accumulation type by the volume of each type of connected pore in the shale sample in the bedding direction, has high accuracy, and is suitable for wide application. BRIEF DESCRIPTION OF DRAWINGS

[0039] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the related art, the drawings needed to be used in the description of the embodiments of the present application or the related art are briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present application, and for those skilled in the art, other drawings can also be obtained without creative labor on the basis of these drawings.

[0040] Figure 1 The proportion curve of the connected pores in the embodiments of the present application;

[0041] Figure 2 The volume distribution curve of the connected pores in the embodiments of the present application. DETAILED DESCRIPTION

[0042] The technical solutions in the embodiments of the present application will be clearly and completely described below, obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work belong to the protection scope of the present application.

[0043] The present application provides a shale gas accumulation type determination method, comprising:

[0044] According to the throat radius of the connected pores in the shale sample in the bedding direction, the connected pores are classified;

[0045] According to the volume of each type of connected pores, the accumulation type of the shale gas is determined;

[0046] The connected pores include Darcy diffusion type connected pores, slippage flow type connected pores, Fick diffusion type connected pores and Knudsen diffusion type connected pores.

[0047] The shale is formed by layer-by-layer deposition, and the bedding direction in the present application refers to the direction perpendicular to the deposition direction.

[0048] The shale sample is taken from the shale, so the bedding direction of the shale sample is the bedding direction of the shale.

[0049] The shale sample has connected pores and non-connected pores in the bedding direction, wherein the non-connected pores are closed pores through which fluid cannot pass; the connected pores are connected through throats, and fluid can flow in the connected pores through the throats. The shale gas in the present application flows in the connected pores.

[0050] In the present application, the volume of the connected pores includes the volume of the throats and the volume of the connected pores connected by the throats.

[0051] The present application does not particularly limit the number of throats and the throat radius, for example, the connected pores can be connected by N throats, N is greater than or equal to 1, and the radii of the N throats can be different or the same.

[0052] In the present application, the connected pores can be divided into Darcy diffusion type connected pores, slippage flow type connected pores, Fick diffusion type connected pores and Knudsen diffusion type connected pores.

[0053] The Darcy diffusion type connected pores refer to the shale gas flowing in the connected pores in the form of Darcy diffusion;

[0054] The slippage flow type connected pores refer to the shale gas flowing in the connected pores in the form of slippage flow;

[0055] The Fick diffusion type connected pores refer to the shale gas flowing in the connected pores in the form of Fick diffusion;

[0056] Knudsen diffusion type connected pore refers to shale gas flowing in the connected pore in the form of Knudsen diffusion.

[0057] According to the throat radius of the connected pore of the shale sample in the bedding direction, the connected pore is divided into the Darcy diffusion type connected pore, the slippage flow type connected pore, the Fick diffusion type connected pore and the Knudsen diffusion type connected pore in the present application; and the accumulation type of the shale gas is determined according to the volume of each type of connected pore.

[0058] The method for determining the accumulation type of the shale gas in the present application can determine the accumulation type of the shale gas through the parameters of the microscale, has high accuracy, provides more direct technical support for determining the formation process of the shale gas reservoir and predicting the production capacity change of the shale gas in the geological evaluation of the shale gas, and is suitable for being widely applied.

[0059] In some embodiments of the present application, determining the accumulation type of the shale gas according to the volume of each type of connected pore comprises:

[0060] If V1 is greater than or equal to 2(V2+V3+V4), the shale gas is of the buoyancy accumulation type;

[0061] If 2(V2+V3+V4) is greater than V1 and V1 is greater than or equal to 1 / 2(V2+V3+V4), the shale gas is of the composite accumulation type of the non-buoyancy accumulation and the buoyancy accumulation;

[0062] If V1 is less than 1 / 2(V2+V3+V4), the shale gas is of the non-buoyancy accumulation type;

[0063] V1 is the volume of the Darcy diffusion type connected pore;

[0064] V2 is the volume of the slippage flow type connected pore;

[0065] V3 is the volume of the Fick diffusion type connected pore;

[0066] V4 is the volume of the Knudsen diffusion type connected pore.

[0067] The method of the present application can better predict the distribution of the shale gas reservoir, for example,

[0068] The shale gas reservoir of the buoyancy accumulation type is distributed at the high part of the structure;

[0069] The shale gas reservoir of the non-buoyancy accumulation type is distributed at the low part of the structure and the slope area, and has a wider distribution range.

[0070] And the method can better predict the production capacity of the shale gas and formulate relevant development measures, for example,

[0071] If the shale gas is of the buoyancy accumulation type, the production speed of the shale gas is faster, the recovery rate is higher, and the benefit is better.

[0072] If the shale gas is a non-buoyancy accumulation type, the shale gas is produced at a slow speed and a low yield, and measures for increasing production need to be taken during the mining process to generate benefits.

[0073] In some embodiments of the present application, the classification of the connected pores according to the throat radius of the connected pores in the bedding direction of the shale sample comprises:

[0074] If r is greater than or equal to 500 lambda, the connected pore is a Darcy diffusion type connected pore;

[0075] If 500 lambda is greater than r and r is greater than or equal to 50 lambda, the connected pore is a slip flow type connected pore;

[0076] If 50 lambda is greater than r and r is greater than or equal to 0.5 lambda, the connected pore is a Fick diffusion type connected pore;

[0077] If r is less than 0.5 lambda, the connected pore is a Knudsen diffusion type connected pore;

[0078] Wherein, r is the throat radius of the connected pore, nm;

[0079] Lambda is the average molecular free path of the shale gas, nm.

[0080] In the present application, the average molecular free path of the shale gas and the throat radius of the connected pore have the following relationship:

[0081]

[0082] Wherein, Kn is the Knudsen number, decimal;

[0083] R is the throat radius, m.

[0084] And when Kn is less than or equal to 0.001, the shale gas is in a Darcy diffusion form;

[0085] When 0.001 is less than or equal to Kn and Kn is less than or equal to 0.01, the shale gas is in a slip flow form;

[0086] When 0.01 is less than or equal to Kn and Kn is less than or equal to 1, the shale gas is in a Fick diffusion form;

[0087] When Kn is greater than 1, the shale gas is in a Knudsen diffusion form.

[0088] The formula a is transformed to obtain the throat radius of the connected pore in the bedding direction of the shale sample and the type of the connected pore corresponding to the throat radius.

[0089] In some embodiments of the present application, lambda is determined by formula 1;

[0090]

[0091] In formula 2, λ is the average molecular free path of shale gas, nm;

[0092] K is the Boltzmann constant, J / K;

[0093] T is the in-situ temperature of shale gas, K;

[0094] d is the diameter of shale gas molecules, m;

[0095] P is the in-situ pressure of shale gas, Pa.

[0096] In formula 1, the in-situ temperature of shale gas is the temperature of shale in the formation;

[0097] The in-situ pressure of shale gas is the pressure of shale in the formation.

[0098] And generally, K = 1.380649 x 10 -23 J / K;

[0099] d = 4.14 x 10 -10 m.

[0100] In some embodiments of the present application, the volume of each type of connected pore is obtained according to the volume distribution of the connected pores.

[0101] In the present application, the volume distribution of the connected pores refers to the curve distribution of the volume of the connected pores corresponding to each throat radius and the throat radius.

[0102] Adding the respective volumes corresponding to each type of connected pore can obtain the volume of the connected pore of this type.

[0103] For example, adding the volumes of the connected pores with a throat radius greater than or equal to 500λ can obtain the volume of the Darcy diffusion type pore;

[0104] Adding the volumes of the connected pores with a throat radius less than 500λ and greater than or equal to 50λ can obtain the volume of the slip flow type connected pore;

[0105] Adding the volumes of the connected pores with a throat radius less than 50λ and greater than or equal to 0.5λ can obtain the volume of the Fick diffusion type connected pore;

[0106] Adding the volumes of the connected pores with a throat radius less than 0.5λ can obtain the volume of the Knudsen diffusion type connected pore.

[0107] In some embodiments of the present application, the volume distribution of the connected pores is the product of the total volume of the connected pores and the volume proportion of the connected pores.

[0108] In the present application, the total volume of the connected pores is the sum of the volumes of all the connected pores, i.e., the total volume of the connected pores is the sum of the volume of the Darcy diffusion type connected pores, the volume of the slip flow type connected pores, the volume of the Fick diffusion type connected pores, and the volume of the Knudsen diffusion type connected pores (total volume of connected pores = V1+V2+V3+V4).

[0109] The volume ratio of the connected pores is the ratio of the volume of the connected pores corresponding to each throat radius in the total volume of the connected pores.

[0110] In the present application, the volume distribution of the connected pores can be obtained by multiplying the total volume of the connected pores by the volume ratio of the connected pores.

[0111] The present application does not limit the method for determining the total volume of the connected pores, and a commonly used method in the art can be used for determination.

[0112] In some embodiments of the present application, the total volume of the connected pores can be determined by a helium porosimeter.

[0113] In the present application, the total volume of the connected pores can be determined by a helium porosimeter according to the method in GB / T 34533-2017.

[0114] The present application does not particularly limit the helium porosimeter, and a commonly used helium porosimeter in the art can be selected.

[0115] The present application also does not limit the method for determining the volume ratio of the connected pores, and a commonly used method in the art can be selected for determination.

[0116] In some embodiments of the present application, the volume ratio of the connected pores can be determined by a high-pressure mercury porosimeter.

[0117] In the present application, the volume ratio of the connected pores can be determined by a high-pressure mercury porosimeter according to the method in GB / T 21650.1-2008. When the high-pressure mercury porosimeter is used to test the volume ratio of the connected pores, the mercury will gradually enter the connected pores through the throat as the pressure increases. The mercury will first enter the largest throat, and at this time, the pores connected to the largest throat will be filled with mercury. The volume of this mercury is the volume of the connected pores corresponding to the largest throat. When the pressure increases, the mercury will enter smaller throats, and at this time, the pores connected to the smaller throats will be filled with mercury, and the volume of this mercury is the volume of the connected pores corresponding to the smaller throats. In this way, the volume ratios of the connected pores corresponding to different throats can be obtained.

[0118] The present application does not particularly limit the high-pressure mercury porosimeter, and a commonly used high-pressure mercury porosimeter in the art can be selected.

[0119] The shale sample can be further selected, so as to simplify the operation steps and improve the accuracy of the determination method.

[0120] In some embodiments of the present application, the shale sample is cylindrical, and the diameter of the shale sample is 2.5 cm, and the length is 3-7 cm.

[0121] Further, the length of the shale sample is 5 cm.

[0122] In the present application, the length direction of the cylinder is the bedding direction.

[0123] In some embodiments of the present application, the cylindrical shale sample with the above-mentioned size can be drilled along the bedding direction of the shale, the side surface of the cylinder is wrapped with epoxy resin, the top surface and the bottom surface of the cylinder are not wrapped, the total volume of the connected pores in the bedding direction of the shale sample is tested by using a helium porosimeter, and then the volume ratio of the connected pores in the bedding direction of the shale sample is obtained by using a high-pressure mercury porosimeter.

[0124] In the following, the technical solutions of the present application will be further described in combination with specific embodiments.

[0125] Embodiments

[0126] The determination method of shale gas in the present embodiment comprises the following steps:

[0127] The shale in the present embodiment is the Quaternary shale in the eastern Qaidam Basin;

[0128] 1) A cylindrical shale sample with a diameter of 2.5 cm and a length of 5 cm is drilled along the bedding direction of the shale, the side surface of the cylinder is wrapped with epoxy resin, and the top surface and the bottom surface of the cylinder are not wrapped;

[0129] The total volume of the connected pores in the bedding direction of the shale sample is obtained by using a helium porosimeter, which is 0.32 mL / g (referring to GB / T 34533-2017);

[0130] The volume ratio of the connected pores in the bedding direction of the shale sample is obtained by using a high-pressure mercury porosimeter (referring to GB / T 21650.1-2008), as shown in Figure 1 ;

[0131] The volume distribution of the connected pores is obtained by multiplying the volume ratio of the connected pores by the total volume of the connected pores, as shown in Figure 2 ;

[0132] 2) According to the throat radius of the connected pores in the bedding direction of the shale sample, the connected pores are classified, which specifically comprises:

[0133] The average free path of shale gas molecules under in-situ conditions of the shale sample is calculated according to formula 1;

[0134] The in-situ temperature of shale gas is 338.15 K.

[0135] The in-situ pressure of shale gas is 18 MPa;

[0136] The diameter d of shale gas molecules is 0.414 nm;

[0137] The average molecular path of freedom λ of shale gas is 0.341 nm;

[0138] If r ≥ 170 nm, then the interconnected pores are Darcy diffusion type interconnected pores;

[0139] If 170nm>r≥17nm, then the connected pores are slip flow type connected pores;

[0140] If 17nm > r ≥ 0.17nm, then the interconnected pores are Fick diffusion type interconnected pores;

[0141] If r < 0.17n, then the connected pores are Knudsen diffusion type connected pores;

[0142] 3) Based on the volume distribution of the connected pores, obtain the volume of each type of connected pore;

[0143] Will Figure 2 In the figure, the volumes of the connected pores with r≥170nm are added together (the ordinates of each point with r≥170nm are added together), and the volume of the Darcy diffusion type connected pores is 0.029mL / g.

[0144] Will Figure 2 In the above, the volumes of the connected pores with 170nm>r≥17nm are added together (the ordinates of each point with 170nm>r≥17nm are added together), and the volume of the slip flow type connected pores is 0.011mL / g.

[0145] Will Figure 2 In the figure, the volumes of the interconnected pores with 17nm>r≥0.17nm are added together (the ordinates of each point with 17nm>r≥0.17nm are added together), and the volume of the Fick diffusion-type interconnected pores is 0.274mL / g.

[0146] Will Figure 2 In the figure, the volumes of the interconnected pores with r < 0.17 nm are added together (the ordinates of all points with r < 0.17 nm are added together), and the volume of the Knudsen diffusion-type interconnected pores is 0.007 mL / g.

[0147] 4) Determine the shale gas accumulation type based on the volume of each type of connected pore, specifically including:

[0148] V1 < 1 / 2 (V2 + V3 + V4), therefore the shale gas in the Quaternary shale in the eastern Qaidam Basin is of the non-buoyancy type.

[0149] V1 is the volume of Darcy diffusion type of interconnected pores;

[0150] V2 is the volume of slip flow type of interconnected pores;

[0151] V3 is the volume of Fick diffusion type of interconnected pores;

[0152] V4 is the volume of Knudsen diffusion type of interconnected pores.

[0153] The well T18 in the area has no shale gas productivity in natural state, has productivity after taking stimulation measures (fracturing), and has daily production of 20,000 square meters of shale gas. According to the productivity, it can be proved that the shale gas in the area is a non-buoyancy accumulation type, which shows that the determination method of the shale gas is reliable.

[0154] Finally, it should be noted that: the above examples are only used to illustrate the technical solutions of the present application, but not to limit it; although the present application has been described in detail with reference to the foregoing examples, those skilled in the art should understand that: it can still modify the technical solutions recorded in the foregoing examples, or make equivalent replacement for part or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present application.

Claims

1. A method for determining shale gas accumulation types, characterized in that, include: The interconnected pores are classified and assigned according to the throat radius of the interconnected pores in the bedding direction of the shale sample; The type of shale gas accumulation is determined based on the volume of each type of interconnected pores; The bedding direction refers to the direction perpendicular to the shale deposition direction; the types of interconnected pores include Darcy diffusion interconnected pores, slip flow interconnected pores, Fick diffusion interconnected pores, and Knudsen diffusion interconnected pores. The determination of the shale gas accumulation type based on the volume of each type of interconnected pores includes: If V1≥2(V2+V3+V4), then the shale gas is of the buoyancy-accumulated type; If 2(V2+V3+V4)>V1≥1 / 2(V2+V3+V4), then the shale gas is a composite accumulation type of non-buoyancy accumulation and buoyancy accumulation; If V1 < 1 / 2 (V2 + V3 + V4), then the shale gas is of the non-buoyancy-based accumulation type. Wherein, V1 is the volume of Darcy diffusion-type connected pores, V2 is the volume of slip flow-type connected pores, V3 is the volume of Fick diffusion-type connected pores, and V4 is the volume of Knudsen diffusion-type connected pores.

2. The method for determining shale gas accumulation type according to claim 1, characterized in that, The classification of interconnected pores based on the throat radius of the interconnected pores along the bedding direction in shale samples includes: If r ≥ 500λ, then the connecting pores are Darcy diffusion type connecting pores; If 500 >r≥50 Then the connecting pores are slip-flow type connecting pores; If 50 >r≥0.5 Then the connecting pores are Fick diffusion type connecting pores; If r < 0.5 Then the connecting pores are Knudsen diffusion type connecting pores; Where r is the throat radius of the connecting pore, in nm; denoted as the mean molecular free path of the shale gas, in nm.

3. The method for determining shale gas accumulation type according to claim 2, characterized in that, Determined by Equation 1; Formula 1; In Equation 1, The mean molecular free path of shale gas is given in nm. K is the Boltzmann constant, J / K; T represents the in-situ temperature of the shale gas, in K. d is the diameter of a shale gas molecule, in meters (m). P is the in-situ pressure of shale gas, in Pa.

4. The method for determining the shale gas accumulation type according to any one of claims 1-3, characterized in that, The volume of each type of connected pore is obtained based on the volume distribution of the connected pores.

5. The method for determining shale gas accumulation type according to claim 4, characterized in that, The volume distribution of the connected pores is the product of the total volume of the connected pores and the volume ratio of the connected pores.

6. The method for determining shale gas accumulation type according to claim 5, characterized in that, The total volume of the connected pores was determined using a helium porosimeter.

7. The method for determining shale gas accumulation type according to claim 5 or 6, characterized in that, The volume percentage of the interconnected pores was determined using a high-pressure mercury porosimeter.

8. The method for determining the shale gas accumulation type according to any one of claims 1-7, characterized in that, The shale sample was cylindrical.

9. The method for determining shale gas accumulation type according to claim 8, characterized in that, The shale sample had a diameter of 2.5 cm and a length of 3-7 cm.

Images