Method for judging complex characteristics of crack by using tracer concentration curve

Abstract

The invention discloses a method for judging complex characteristics of a crack by using a tracer concentration curve, which specifically comprises the following steps: step 1, in the multi-section fracturing process of a horizontal well, injecting different oil-water two-phase tracers into a reservoir along with fracturing fluid in the multi-section fracturing process, and then sampling to obtain the output concentration of the tracers; step 2, respectively drawing a dimensionless concentration output curve of the water phase tracer agent of each section; 3, setting a judgment standard according to the curve graph drawn in the step 2; 4, analyzing the characteristics of the fracturing section according to the standard specified in the step 3; according to the method, the application function of tracer testing is further expanded, the hydraulic fracturing crack complex feature identification method which is low in cost, easy to operate and high in reliability is provided, and a new technical means is added for fracturing effect evaluation.

Description

Technical Field

[0001] The invention belongs to the technical field of oil and gas field exploration and development, and relates to a method for judging complex characteristics of fractures by using a tracer concentration curve. Background Art

[0002] Currently, the main method for evaluating the morphology of hydraulic fracturing cracks is downhole microseismic monitoring technology. This technology uses a detector lowered into the well to receive the microseismic event signals generated by the reservoir rock during the hydraulic fracturing process for positioning. By observing the distribution characteristics of all microseismic event signals throughout the entire fracturing process, the morphology of the artificial fractures can be determined. The advantage of this technology is that it can roughly reflect the length, bandwidth, height and complexity of artificial fractures in a relatively intuitive manner; there are three main limitations: First, the microseismic event point may be the vibration sound wave signal generated by the deformation of the rock after the stress is transmitted to a certain position during the fracturing process, and it is not necessarily the formation of artificial fractures. Therefore, the coverage of microseismic event points cannot fully represent the extension range of hydraulic fractures; second, downhole microseismicity cannot characterize the development degree, configuration relationship, fracture aperture, and conductivity differences of primary and secondary fractures; third, downhole microseismic monitoring requires a large number of supporting personnel and equipment, high data interpretation requirements, and high investment costs; although conventional tracer monitoring technology has been widely used in directional wells and horizontal well tests in oil and gas fields, it is mainly used for inter-well connectivity testing, water plugging, and evaluation of the output contribution rate of each section of horizontal wells. It has not yet been used to judge the complex characteristics of artificial fractures after fracturing. Summary of the invention

[0003] The purpose of the present invention is to provide a method for judging complex characteristics of fractures by using tracer concentration curves, which has the characteristics of low cost, easy operation and high reliability in judging complex characteristics of hydraulic fracturing fractures.

[0004] The technical solution adopted by the present invention is a method for judging the complex characteristics of cracks by using a tracer concentration curve, which is specifically implemented in the following steps:

[0005] Step 1, during the multi-stage fracturing of a horizontal well, different oil-water two-phase tracers are injected into the reservoir along with the fracturing fluid, and then samples are taken to obtain the output concentration of the tracer;

[0006] Step 2, drawing the dimensionless concentration output curve of the water phase tracer in each section;

[0007] Step 3, according to the curve graph drawn in step 2, set the judgment standard;

[0008] In step 4, the characteristics of the fracturing stage are analyzed according to the criteria specified in step 3.

[0009] The present invention is also characterized in that:

[0010] Step 1 specifically comprises: during the multi-stage fracturing of the horizontal well, different types of oil-water two-phase tracers are respectively selected for each stage of the multi-stage fracturing process and injected into the reservoir along with the fracturing fluid; after the whole well is fractured, the oil and water are sampled every day during the drainage and production process, and the output concentration of each tracer in the oil and water samples taken every day is tested;

[0011] The output curve in step 2 is specifically as follows: the ordinate of the curve is the dimensionless tracer concentration factor, the value represented by the ordinate is equal to the ratio of the absolute concentration value of the tracer output to the peak concentration of the tracer in the entire test period, and has no unit; the abscissa is the tracer output time, and the unit is day;

[0012] The standard of step 3 is: the point with a scale value of 0.5 on the ordinate axis is called the half-width point, and a straight line parallel to the horizontal axis is drawn from the half-width point to the right. The straight line intersects with the dimensionless concentration output curve, and the width of the intersection is called the half-width; the width of the intersection of the dimensionless concentration output curve and the horizontal axis is called the peak base width;

[0013] In step 4, the standard curve set in step 3 is divided into single peak type, sharp peak type, wide multi-peak type and sharp multi-peak type, and the characteristics of each fracturing stage are analyzed according to each peak type;

[0014] The single-peak curve is specifically: there is only one peak on the curve, and it is symmetrical about the peak axis, and the half-width is greater than 30% of the peak base width; the single-peak morphological characteristics are: the corresponding fracturing section has a single main fracture, and no tracer enters the branch fracture; the half-width reflects the fracture opening, the larger the fracture opening, the narrower the half-width;

[0015] Specifically, the peak curve is: there is only one peak value greater than the half-width point on the curve, and the half-width is less than 30% of the peak base width, and there are two or more peaks below the half-width point; the morphological characteristics of the peak curve are: there is a short period of concentrated discharge during the fracturing fluid return process, and there are primary and secondary fractures in the corresponding fracturing section, and the conductivity of the primary and secondary fractures is significantly different;

[0016] The wide multi-peak curve is specifically characterized by: the upper half of the curve is wide, the curve on the right side of the peak decreases gently, and there are two or more peaks exceeding the half-width point; the morphological characteristics of the wide multi-peak curve are: the fracture network is developed, and the conductivity of fractures of different levels is relatively small; the wider the right half width, the slower the drainage of the corresponding fracture section, and the longer the tracer discharge time, indicating that the fracture is narrowing and the complexity is increasing;

[0017] The sharp multi-peak curve is specifically as follows: there are two or more peaks exceeding the half-width point on the curve, but the half-width of each peak is small, less than 30% of the peak base width; the morphological characteristics of the sharp multi-peak curve are as follows: the tracer concentration in the corresponding fracturing section is produced in multiple pulses, and each production duration is short; multiple cracks are formed in the fracturing section, and the cracks are relatively simple, and the conductivity of each crack is graded.

[0018] The beneficial effects of the present invention are:

[0019] The method of using tracer concentration curve to judge complex characteristics of fractures of the present invention further expands the application function of tracer testing, and proposes a low-cost, easy-to-operate, and highly reliable method for judging complex characteristics of hydraulic fracturing fractures, adding new technical means to the evaluation of fracturing effects. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 A single-peak dimensionless tracer concentration output curve in a curve diagram of the present invention is drawn;

[0021] Figure 2 A peak-shaped dimensionless tracer concentration output curve in a curve diagram of the present invention is drawn;

[0022] Figure 3 A wide multi-peak dimensionless tracer concentration output curve in the curve diagram of the present invention is drawn;

[0023] Figure 4 The sharp multi-peak dimensionless tracer concentration output curve in the curve diagram of the present invention is drawn;

[0024] Figure 5 This is the dimensionless concentration output curve of the water phase tracer in the second section of a shale oil horizontal well in Example 3 of the present invention. DETAILED DESCRIPTION

[0025] The present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0026] The present invention provides a method for determining complex characteristics of a crack using a tracer concentration curve, as shown in the following examples:

[0027] Example 1, the main idea of ​​the method of using tracer concentration curve to judge the complex characteristics of cracks in the present invention:

[0028] Please follow the steps below to implement:

[0029] Step 1, during the multi-stage fracturing of a horizontal well, different oil-water two-phase tracers are injected into the reservoir along with the fracturing fluid, and then samples are taken to obtain the output concentration of the tracer;

[0030] Step 2, drawing the dimensionless concentration output curve of the water phase tracer in each section;

[0031] Step 3, according to the curve graph drawn in step 2, set the judgment standard;

[0032] Step 4, analyzing the characteristics of the fracturing stage according to the criteria specified in step 3.

[0033] Example 2, specific implementation steps of the method of judging complex characteristics of cracks using tracer concentration curves of the present invention:

[0034] Step 1: During the multi-stage fracturing of a horizontal well, different types of oil-water two-phase tracers are selected for each stage and injected into the reservoir along with the fracturing fluid. After the whole well is fractured, the oil and water are sampled every day during the drainage and production process, and the output concentration of each tracer in the oil and water samples taken every day is tested;

[0035] Step 2, drawing curves: drawing dimensionless concentration output curves of the water phase tracer in each section respectively; the ordinate of the curve is the dimensionless tracer concentration factor, whose value is equal to the ratio of the absolute concentration value of the tracer output to the peak concentration of the tracer in the entire test period, without units; the abscissa is the tracer output time, in days;

[0036] Step 3, curve standard: The point with a scale value of 0.5 on the ordinate axis is called the half-width point. A straight line parallel to the horizontal axis is drawn from this point to the right. The straight line intersects with the dimensionless concentration output curve. The width of the intersection is called the half-width. The width of the intersection of the dimensionless concentration output curve and the horizontal axis is called the peak base width.

[0037] Step 4, analyzing the characteristics of the corresponding fracturing section according to the curve of step 3, specifically including single peak type, sharp peak type, wide multi-peak type and sharp multi-peak type, and analyzing the characteristics of each fracturing section according to each peak type.

[0038] Example 3, the characteristics of each peak type are shown as follows:

[0039] like Figure 1 As shown in the figure, the double arrows represent the half width. When the curve is unimodal, there is only one peak on the curve and it is basically symmetrical about the peak axis. The half width is generally greater than 30% of the peak base width. The morphological characteristics of this curve indicate that: the corresponding fracturing section has a single main fracture, and basically no tracer enters the branch fracture; the half width reflects the fracture opening. The larger the fracture opening, the narrower the half width;

[0040] like Figure 2 As shown in the figure, the double arrows represent the half width. When the curve shape is a spike type, there is only one peak greater than the half width point on the curve, and the half width is less than 30% of the peak base width. There may be two or more peaks below the half width point. The curve shape characteristics indicate that: during the fracturing fluid backflow process, there is a short period of concentrated discharge, there are primary and secondary fractures in the fracturing section, and the conductivity of the primary and secondary fractures is significantly different;

[0041] like Figure 3As shown in the figure, the double arrows represent the half width. When the curve is wide and multi-peaked, the upper half width of the curve is large, the curve on the right side of the peak decreases slowly, and there are two or more peaks exceeding the half width point. The morphological characteristics of the curve indicate that: the fracture network is developed, and the conductivity of fractures at different levels is relatively small; the wider the half width on the right side, the slower the drainage of the fracture in the fracturing section, and the longer the tracer discharge time, indicating that the fracture is narrowing and the complexity is increasing;

[0042] like Figure 4 As shown in the figure, the double arrows represent the half width. When the curve is sharp and multi-peaked, there are two or more peaks on the curve that exceed the half width point, but the half width of each peak is small, less than 30% of the peak base width. The curve morphology characteristics indicate that: the tracer concentration in the fracturing section is produced in multiple pulses, and each production duration is short; multiple fractures are formed in the fracturing section, and the fractures are relatively simple, and the conductivity of each fracture has a certain difference.

[0043] Embodiment 4, specific application:

[0044] The method of using tracer concentration curve to judge the complex characteristics of fractures is as follows:

[0045] The horizontal section of a shale oil horizontal well is 1,000 meters long, with an oil layer drilling rate of 88.3%. The design adopts a segmented multi-cluster volume fracturing mode to transform 12 sections, with 3 to 5 clusters in a single section, a fracture density of 9.4 per 100 meters, a sand addition intensity of 3.5 tons per meter, a liquid injection intensity of 20 cubic meters per meter, and an injection displacement of 12 cubic meters per minute;

[0046] Step 1: During the fracturing process of the well, oil phase and water phase liquid tracers are injected into each section along with the fracturing fluid. The types of tracers injected into each section are different, with a total of 12 water phase tracers and 12 oil phase tracers. After the fracturing construction of the whole well is completed, the well is shut down for 20 days, and then the well is opened to enter the flowback production stage. Samples are taken every day during the flowback and production process, and the output concentration of various tracers in the test samples is analyzed.

[0047] Step 2, respectively draw the dimensionless concentration output curve of the water phase tracer of each section; the ordinate of the curve is the dimensionless tracer concentration factor, whose value is equal to the ratio of the absolute concentration value of the tracer output to the peak concentration of the tracer in the entire test period, without unit; the abscissa is the tracer output time, in days;

[0048] Step 3, applying the curve judgment criteria;

[0049] Step 4, by analyzing the shape of the dimensionless concentration output curve of the water phase tracer in each section of the well, all of them are wide multi-peaked, indicating that a complex fracture network system has been formed in the fracturing section, and the conductivity of each level of fracture is small, the half width is wide, and the fracture opening is relatively small. The horizontal well has achieved the purpose of volume fracturing as a whole. For example, the dimensionless concentration output curve of the water phase tracer in the second section of the well is as follows Figure 5 shown.

Claims

1. A method for determining complex crack characteristics using a tracer concentration curve, characterized in that: Please follow the steps below to implement: Step 1, during the multi-stage fracturing of a horizontal well, different oil-water two-phase tracers are injected into the reservoir along with the fracturing fluid, and then samples are taken to obtain the output concentration of the tracer; Step 2, drawing the dimensionless concentration output curve of the water phase tracer in each section; Step 3, according to the curve graph drawn in step 2, set the judgment standard; Step 4, analyzing the characteristics of the fracturing stage according to the criteria specified in step 3.

2. The method for determining complex characteristics of cracks using tracer concentration curves according to claim 1, characterized in that: The step 1 is specifically as follows: during the multi-stage fracturing process of a horizontal well, different types of oil-water two-phase tracers are respectively selected for each stage of the multi-stage fracturing process and injected into the reservoir along with the fracturing fluid; after the whole well is fractured, the oil and water are sampled every day during the drainage and production process, and the output concentration of each tracer in the oil and water samples taken every day is tested separately.

3. The method for determining complex characteristics of cracks using tracer concentration curves according to claim 1, characterized in that: The output curve in step 2 is specifically as follows: the ordinate of the curve is the dimensionless tracer concentration factor, the value represented by the ordinate is equal to the ratio of the absolute tracer output concentration value to the tracer peak concentration during the entire test period, and has no unit; the abscissa is the tracer output time, in days.

4. The method for determining complex characteristics of cracks using tracer concentration curves according to claim 3, characterized in that: The standard of step 3 is: the point with a scale value of 0.5 on the vertical axis is called the half-width point, and a straight line parallel to the horizontal axis is drawn to the right from the half-width point. The straight line intersects with the dimensionless concentration output curve, and the width of the intersection is called the half-width; the width of the intersection of the dimensionless concentration output curve and the horizontal axis is called the peak base width.

5. The method for determining complex characteristics of cracks using tracer concentration curves according to claim 4, characterized in that: In step 4, the standard curve set in step 3 is divided into single peak type, sharp peak type, wide multi-peak type and sharp multi-peak type, and the characteristics of each fracturing stage are analyzed according to each peak type.

6. The method for determining complex characteristics of cracks using tracer concentration curves according to claim 5, characterized in that: The single-peak curve is specifically as follows: there is only one peak on the curve, and it is symmetrical about the peak axis, and the half-width is greater than 30% of the peak base width; the single-peak morphological characteristics are represented by: a single main crack develops in the corresponding fracturing section, and no tracer enters the branch crack; the half-width reflects the crack opening, and the larger the crack opening, the narrower the half-width.

7. The method for determining complex characteristics of cracks using tracer concentration curves according to claim 5, characterized in that: The spike-type curve is specifically: there is only one peak value greater than the half-width point on the curve, and the half-width is less than 30% of the peak base width, and there are two or more peaks below the half-width point; the morphological characteristics of the spike-type curve are expressed as: there is a short period of concentrated discharge during the fracturing fluid return process, there are primary and secondary cracks in the corresponding fracturing section cracks, and the conductivity of the primary and secondary cracks is significantly different.

8. The method for determining complex characteristics of cracks using tracer concentration curves according to claim 5, characterized in that: The wide multi-peak curve is specifically characterized by: a wide upper half of the curve, a gentle decrease in the curve on the right side of the peak, and two or more peaks exceeding the half-width point; the morphological characteristics of the wide multi-peak curve are represented by: a fracture network is developed in the cracks, and the difference in conductivity of cracks at various levels is small; the wider the right half width, the slower the drainage of the corresponding fracturing section cracks, and the longer the tracer discharge time, indicating that the cracks are narrowing and the complexity is increasing.

9. The method for determining complex characteristics of cracks using tracer concentration curves according to claim 5, characterized in that: The sharp multi-peak curve is specifically as follows: there are two or more peaks on the curve that exceed the half-width point, but the half-width of each peak is small, less than 30% of the peak base width; the morphological characteristics of the sharp multi-peak curve are as follows: the tracer concentration of the corresponding fracturing section is produced in multiple pulses, and each production duration is short; multiple cracks are formed in the fracturing section, and the cracks are relatively simple, and the conductivity of each crack has a graded difference.

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