Method for evaluating porosity and permeability of tight sandstone reservoir with cracks with different mechanical properties

Abstract

The invention relates to a pore permeability evaluation method for tight sandstone reservoirs containing cracks with different mechanical properties. The method mainly solves the problem that mechanical properties and shape parameters of different types of cracks are not considered in the existing tight sandstone permeability research. The method is characterized by comprising the following steps: S1, dividing mechanical cause types of deep compact sandstone reservoir fractures; s2, respectively preparing tight sandstone samples containing cracks with different mechanical properties; s3, performing overburden pressure pore permeability test on each sandstone sample to obtain the relationship between the confining pressure and the fracture permeability; s4, overburden gravity and pore pressure are calculated through logging, and effective confining pressure is calculated; and S5, evaluating the pore permeability of the cracks by utilizing a fitting formula of confining pressure and permeability of the cracks with different mechanical properties. The evaluation method solves the problem that the pore permeability parameters of multi-scale and multi-type fractures are difficult to quantitatively predict, so that the influence of the fractures with different scales and geometric properties on the permeability of the sandstone reservoir is determined.

Description

Technical Field

[0001] The invention relates to the field of petroleum geological exploration and development, in particular to a porosity and permeability evaluation method for a dense sandstone reservoir containing cracks with different mechanical properties. Background Art

[0002] The main storage and permeability space of tight sandstone reservoirs is fractures, followed by pores. There are usually multiple fractures of different sizes and scales in tight sandstone. Among them, large fractures are very important to the storage and permeability of the reservoir, but pores occupy more storage space and are also indispensable for oil and gas storage. Fractures play a dual role in oil and gas development. Fractures not only provide storage space for oil and gas, but also increase the permeability of the reservoir; however, the current research on the permeability of tight sandstone is mainly aimed at all types of fractures, without considering the mechanical properties and shape parameters of different types of fractures. In the exploration and development of fractured tight sandstone reservoirs, the prediction of reservoir permeability has always been a difficult problem. The porosity and permeability characteristics of tight sandstone reservoirs are closely related to the type and scale of fracture development. When multi-group and multi-scale fractures develop in tight sandstone, the calculation of fracture seepage capacity is a difficult point in the quantitative description of fractures. However, in the evaluation of the permeability of fractured tight sandstone, the evaluation of the conductivity of tight sandstone with fractures of different mechanical properties has rarely been reported. Summary of the invention

[0003] In order to overcome the problem that the existing research on the permeability of tight sandstone does not consider the mechanical properties and shape parameters of different types of cracks, the present invention provides a porosity and permeability evaluation method for tight sandstone reservoirs containing cracks with different mechanical properties. The evaluation method solves the problem that the porosity and permeability parameters of multi-scale and multi-type cracks are difficult to quantitatively predict, thereby clarifying the influence of cracks of different scales and geometric properties on the permeability of sandstone reservoirs.

[0004] The technical solution of the present invention is: a porosity and permeability evaluation method for a dense sandstone reservoir containing cracks with different mechanical properties, characterized by comprising the following steps:

[0005] S1. Classification of mechanical genesis types of fractures in deep tight sandstone reservoirs;

[0006] S2, preparing dense sandstone samples containing cracks with different mechanical properties;

[0007] S3. Conduct overburden pressure porosity test on each sandstone sample to obtain the relationship between confining pressure and fracture permeability;

[0008] S4. Calculate the overburden gravity and pore pressure using well logging and calculate the effective confining pressure;

[0009] S5. Use the fitting formula of fracture confining pressure and permeability with different mechanical properties to evaluate the fracture porosity and permeability.

[0010] Furthermore, in step S1, the sandstone fractures with different mechanical properties include shear fractures, tension fractures and bedding fractures.

[0011] Furthermore, in step S3, firstly, the sandstone sample is saturated with kerosene and then subjected to a saturation test of pore pressure; then, lateral isobaric and triaxial tests with different pore pressures are performed to obtain triaxial porosity and permeability under different pressures, thereby obtaining relationship curves between confining pressure and fracture permeability of different types of rock samples.

[0012] Furthermore, in step S4, the upper shale normally compacted layer section is selected according to the logging curve, and the density curve of the upper section without density logging data is established. By setting the depth range, the overburden pressure gradient is predicted and the curve is drawn. After calculating and quadratically fitting the curve, a smooth curve of the relationship between confining pressure and permeability is formed.

[0013] Furthermore, after drawing the overburden pressure gradient curve, the curve is calculated and processed by formula (3), and the calculation of formula (4) is performed on it to perform quadratic fitting, thereby forming a smooth curve of the relationship between confining pressure and permeability;

[0014] P i =0.11(P i-2 +P i+2 )+0.24(P i-1 +P i+1 )+0.3P i· (3)

[0015] Where P i is the pressure at a certain depth in the formation;

[0016] ρ=ρ0+AH B (4)

[0017] Where A and B are fitting parameters (obtained by least squares regression); ρ is the formation density, g / cm3; ρ0 is the density of the upper mud surface formation, g / cm3, which is 2.0 on land and 1.03 in deep water; H is the well depth below the upper mud surface formation, m.

[0018] Furthermore, the fitting function formula of the overburden pressure gradient is:

[0019] G o =A+BH-Ce -DH (5)

[0020] Where G0 is the pressure gradient of the overburden at a certain depth (confining pressure), g / cm; H is the depth, km; A, B, C, and D are model coefficients.

[0021] Furthermore, the confining pressure of different types of rock samples is determined by formula (1) and formula (2), and the compressive strength of the rock sample is determined by the differential stress Sd To express:

[0022] S d =σ1-σ2 (1)

[0023] Where: σ1 is the axial pressure (MPa);

[0024] P c =σ2=σ3 (2)

[0025] Where: P c is the confining pressure, MPa.

[0026] The present invention has the following beneficial effects: due to the adoption of the above scheme, the present invention uses physical simulation means to determine the evaluation method of the porosity and permeability of sandstones of different mechanical types, clarifies the influence of cracks of different scales and geometric properties on the permeability of sandstone reservoirs, and provides guidance for the comprehensive evaluation of fractured tight sandstone reservoirs and the study of seepage laws. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 is a flow chart of the present invention;

[0028] Figure 2 It is a flow chart for the classification of mechanical properties of tight sandstone reservoir fractures;

[0029] Figure 3 is a fitting relationship diagram between confining pressure and porosity and permeability of the rock sample containing bedding fractures in Example 1;

[0030] Figure 4 is a fitting relationship diagram between confining pressure and porosity and permeability of the rock sample containing shear fractures in Example 1;

[0031] Figure 5 is a fitting relationship diagram between confining pressure and porosity and permeability of the rock sample containing tensile fractures in Example 1;

[0032] Figure 6 It is a fitting relationship diagram between confining pressure and porosity and permeability of the rock sample without fractures in Example 1. DETAILED DESCRIPTION

[0033] The present invention will be further described below in conjunction with the accompanying drawings:

[0034] Depend on Figures 1 to 6 As shown, a porosity and permeability evaluation method for a tight sandstone reservoir containing cracks with different mechanical properties comprises the following steps:

[0035] S1. Classify the mechanical genesis types of cracks in deep tight sandstone reservoirs. When classifying cracks in sandstone reservoirs, first take photos of the cores, perform imaging logging, prepare rock thin sections, and classify the mechanical genesis types according to the characteristics of the rock thin sections. Sandstone cracks with different mechanical properties include shear fractures, tension fractures, and bedding fractures. The classification criteria are as follows: for shear fracture cores, thin sections show conjugate groups, straight fracture surfaces, fracture surface scratches, cutting particles, longitudinal interlayers, and smooth friction mirrors or scratches on the fracture surface. For tension fracture cores, thin sections show parallel groups, rough fracture surfaces, and grain-wrapped features, and imaging logging shows complete bright or dark sinusoidal images. Bedding fractures are a type of horizontal fractures related to weak surfaces such as bedding, foliation, and lamination. They are caused by overlying stress unloading, diagenetic shrinkage, or horizontal strong stress sliding. Cores and thin sections show development along structural planes, calcite or organic matter filling, closed and unfilled, bedding scratches or mirrors, and parallel to lamination features.

[0036] S2. Dense sandstone samples containing cracks of different mechanical properties are prepared respectively. The sandstone column sample is cylindrical in shape with a diameter of 25.32 mm and a height of 53.70 mm. Each sandstone sample contains cracks of one mechanical property.

[0037] S3. Perform an overburden porosity test on each sandstone sample to obtain the relationship between confining pressure and fracture permeability. The overburden porosity test simulates the process of pore throat deformation, fracture closure or opening when the net overburden pressure changes. Before stress loading, the overburden porosity test first needs to conduct a pore pressure saturation experiment. The rock sample is saturated with kerosene. The BH-1 core (oil, water) evacuation and pressurization saturation experimental device is used to evacuate the core and inject kerosene at high pressure to overcome the capillary pressure of the dense core and make it fully saturated.

[0038] Then carry out triaxial tests with lateral isobaric pressure and different pore pressures. The specific experimental steps are as follows: 1. Connect the power supply and gas source of the CMS core automatic analyzer, and preheat the whole machine for 30 minutes; 2. Adjust each pressure regulator to the required pressure in the order required by the instrument; 3. Start the test program, run the self-test or leak detection program, the pore volume of the self-test standard block is less than or equal to the standard number 0.02cm3, and the leak detection standard is less than or equal to 2412.9Pa-cm3 / min; 4. Run the sample test program, first input the atmospheric pressure and experimental temperature data under the test environment, and then input the test sample and its basic parameters; 5. The highest experimental confining pressure is selected as half of the overburden pressure, and the following are 4-8 pressure points; 6. Load the samples to be measured in the sample turntable in order for testing; 7. The computer automatically collects and stores data to obtain the triaxial porosity and permeability; 8. After the test is completed, turn off the gas source and instrument power in order.

[0039] The data interpretation method of the experimental results is as follows: The triaxial compressive strength of rock refers to the ultimate strength of rock that reaches failure under triaxial pressure, which is numerically equal to the maximum differential stress when the rock fails. The triaxial porosity and permeability under different pressures are obtained through triaxial tests, thereby obtaining the relationship curve between the confining pressure and fracture permeability of different types of rock samples. The confining pressure of different types of rock samples is determined by formula (1) and formula (2), and the compressive strength of the rock sample is determined by the differential stress S d To express:

[0040] S d =σ1-σ2 (1)

[0041] Where: σ1 is the axial pressure (MPa);

[0042] P c =σ2=σ3 (2)

[0043] Where: P c is the confining pressure, MPa.

[0044] S4. Use logging to calculate the overburden gravity and pore pressure, and calculate the effective confining pressure. Select the upper shale normal compaction layer according to the logging curve, establish the upper density curve of the well section without density logging data, and predict the overburden pressure gradient and draw the curve by setting the depth range. After calculating and quadratically fitting the curve, a smooth curve of the relationship between confining pressure and permeability is formed.

[0045] After drawing the overburden pressure gradient curve, the curve is calculated and processed by formula (3), and then the curve is calculated by formula (4) and a quadratic fit is performed to form a smooth curve of the relationship between confining pressure and permeability;

[0046] P i =0.11(P i-2 +P i+2 )+0.24(P i-1 +P i+1 )+0.3P i· (3)

[0047] Where P i is the pressure at a certain depth in the formation;

[0048] ρ=ρ0+AH B (4)

[0049] Where A and B are fitting parameters (obtained by least squares regression); ρ is the formation density, g / cm3; ρ0 is the density of the upper mud surface formation, g / cm3, which is 2.0 on land and 1.03 in deep water; H is the well depth below the upper mud surface formation, m.

[0050] After the overburden pressure gradient data is obtained from the logging density scatter data, the overburden pressure gradient can be directly interpolated from the depth data when applied. However, sometimes due to the shallow drilling depth or short density logging section, the overburden pressure gradient data of the shallow or deep non-density logging formation section cannot be obtained. At this time, it is necessary to regress the existing data into a function of depth for extrapolation (upward or downward extrapolation). The fitting function formula of the overburden pressure gradient is:

[0051] G o =A+BH-Ge -DH (5)

[0052] Where G0 is the pressure gradient of the overburden at a certain depth (confining pressure), g / cm; H is the depth, km; A, B, C, and D are model coefficients.

[0053] S5. Evaluate the size of the crack porosity and permeability using the fitting formula of the fracture confining pressure and permeability of different mechanical properties in step S4.

[0054] Embodiment 1:

[0055] The cores used in the experiment were taken from tight sandstone reservoirs. The experimental temperature was set at 70°C, and the confining pressure was set at 3.4MPa, 5MPa, 7.5MPa, 10Mpa, 12.5MPa, 15MPa, 20MPa, 25MPa, and 35MPa. In order to take into account the influence of the bedding plane, before the compression and tensile strength tests, the sampling direction was controlled to ensure that the final applied minimum principal stress was perpendicular to the bedding plane direction and follow the mechanical principle of "compression is positive and tension is negative". When preparing samples for uniaxial and triaxial rock mechanics experiments, standard cylindrical specimens were drilled along the bedding plane direction. The sample size was φ25.3mm×53.70mm, the error did not exceed ±0.5mm, and the end face parallelism error was less than ±0.02mm.

[0056] The study selected sandstone samples containing bedding fractures, shear fractures and tension fractures, divided them into three categories for triaxial mechanical experiments, and obtained the porosity and permeability experimental parameters of dense sandstone containing fractures with different mechanical properties under different pressures. The experimental results were fitted by type, see Figures 3 to 6 .

[0057] According to the relationship diagram between confining pressure and permeability of three different types of fractures, the overburden gravity and pore pressure are calculated based on well logging to obtain the effective confining pressure parameters, thereby estimating the permeability of the core, as shown in Table 1. The fitting formula of confining pressure and permeability of fractures with different mechanical properties can be used to estimate the size of fracture porosity and permeability in the area.

[0058] Table 1

[0059]

Claims

1. A porosity and permeability evaluation method for tight sandstone reservoirs containing fractures with different mechanical properties, characterized in that The following steps are involved: S1. Classification of mechanical genesis types of fractures in deep tight sandstone reservoirs; S2, preparing dense sandstone samples containing cracks with different mechanical properties; S3. Conduct overburden pressure porosity test on each sandstone sample to obtain the relationship between confining pressure and fracture permeability; S4. Calculate the overburden gravity and pore pressure using well logging and calculate the effective confining pressure; S5. Use the fitting formula of fracture confining pressure and permeability with different mechanical properties to evaluate the fracture porosity and permeability.

2. The porosity and permeability evaluation method for tight sandstone reservoirs containing cracks with different mechanical properties according to claim 1, characterized in that: In step S1, the sandstone fractures with different mechanical properties include shear fractures, tension fractures and bedding fractures.

3. The porosity and permeability evaluation method for tight sandstone reservoirs containing cracks with different mechanical properties according to claim 1, characterized in that: In step S3, firstly, the sandstone sample is saturated with kerosene and then subjected to a saturation test of pore pressure; then, lateral isobaric and triaxial tests with different pore pressures are performed to obtain triaxial porosity and permeability under different pressures, thereby obtaining relationship curves between confining pressure and fracture permeability of different types of rock samples.

4. The porosity and permeability evaluation method for tight sandstone reservoirs containing cracks with different mechanical properties according to claim 1, characterized in that: In step S4, the upper shale normally compacted layer section is selected according to the logging curve, and the density curve of the upper well section without density logging data is established. By setting the depth range, the overburden pressure gradient is predicted and the curve is drawn. After calculating and quadratically fitting the curve, a smooth curve of the relationship between confining pressure and permeability is formed.

5. The porosity and permeability evaluation method for tight sandstone reservoirs containing cracks with different mechanical properties according to claim 4, characterized in that: After drawing the overburden pressure gradient curve, the curve is calculated and processed by formula (3), and then the curve is calculated by formula (4) and a quadratic fit is performed to form a smooth curve of the relationship between confining pressure and permeability; P i =0.11(P i-2 +P i+2 )+0.24(P i-1 +P i+1 )+0.3P i . (3) Where P i is the pressure at a certain depth in the formation; ρ=ρ0+AH B (4) Where A and B are fitting parameters (obtained by least squares regression); ρ is the formation density, g / cm3; ρ0 is the density of the upper mud surface formation, g / cm3, which is 2.0 on land and 1.03 in deep water; H is the well depth below the upper mud surface formation, m.

6. The porosity and permeability evaluation method for tight sandstone reservoirs containing cracks with different mechanical properties according to claim 5, characterized in that: The fitting function formula of the overburden pressure gradient is: G o =A+BH-Ce -DH (5) Where G0 is the pressure gradient of the overburden at a certain depth (confining pressure), g / cm; H is the depth, km; A, B, C, and D are model coefficients.

7. The porosity and permeability evaluation method for tight sandstone reservoirs containing cracks with different mechanical properties according to claim 3, characterized in that: The confining pressure of different types of rock samples is determined by formula (1) and formula (2), and the compressive strength of the rock sample is determined by the differential stress S d To express: S d =σ1-σ2 (1) Where: σ1 is the axial pressure (MPa); P c =σ2=σ3 (2) Where: P c is the confining pressure, MPa.

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