Method for determining complex characteristics of cracks using tracer concentration curves

Abstract

The application discloses a method for judging fracture complex characteristics by using tracer concentration curves, and specifically comprises the following steps: step 1, in the process of multi-stage fracturing of a horizontal well, different oil-water two-phase tracers are injected into a reservoir by a fracturing fluid in the process of multi-stage fracturing, and then the output concentration of the tracers is obtained by sampling; step 2, the dimensionless concentration output curve of the water-phase tracer of each stage is drawn respectively; step 3, a judgment standard is set according to the curve drawn in step 2; and step 4, the characteristics of the fracturing stage are analyzed according to the standard specified in step 3. The method further expands the application function of tracer testing, and proposes a low-cost, easy-to-operate and highly reliable hydraulic fracturing fracture complex characteristic identification method, thereby adding a new technical means for fracturing effect evaluation.

Description

Technical Field

[0001] This invention belongs to the field of oil and gas field exploration and development technology, and relates to a method for judging the complex characteristics of fractures using tracer concentration curves. Background Technology

[0002] Currently, the main method for assessing the morphology of hydraulic fracturing fractures is downhole microseismic monitoring technology. This technology uses geophones installed in the well to receive microseismic event signals generated by the reservoir rock during hydraulic fracturing for localization. By observing the distribution characteristics of all microseismic event signals throughout the fracturing process, the morphology of artificial fractures can be determined. The advantage of this technology lies in its ability to intuitively and roughly reflect the length, bandwidth, height, and complexity of artificial fractures. Its limitations are mainly threefold: First, microseismic event points may be vibrational acoustic signals generated by rock deformation after stress is transmitted to a certain location during fracturing, rather than necessarily the formation of artificial fractures. Therefore, the coverage area of ​​microseismic event points cannot fully represent the extension range of hydraulic fractures. Second, downhole microseismic monitoring cannot characterize the development degree, configuration relationship, fracture aperture, and differences in conductivity between primary and secondary fractures. Third, downhole microseismic monitoring requires a large number of personnel and equipment, has high data interpretation requirements, and incurs high costs. While conventional tracer monitoring technology is widely used in directional and horizontal well testing in oil and gas fields, it is mainly used for inter-well connectivity testing, water shut-off detection, and evaluation of production contribution rates in different sections of horizontal wells, and has not yet been used to determine the complex characteristics of artificial fractures after fracturing. Summary of the Invention

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

[0004] The technical solution adopted in this invention is a method for judging the complex characteristics of cracks using tracer concentration curves, specifically implemented according to the following steps:

[0005] Step 1: During the multi-stage fracturing process of a horizontal well, different oil-water two-phase tracers are injected into the reservoir along with the fracturing fluid, and then the production concentration of the tracers is obtained by sampling.

[0006] Step 2: Plot the dimensionless concentration production curve of the aqueous tracer for each segment;

[0007] Step 3: Based on the curve drawn in Step 2, set the judgment criteria;

[0008] Step 4: Analyze the characteristics of the fracturing section according to the standards specified in Step 3.

[0009] The invention is further characterized by:

[0010] Step 1 specifically involves: during the multi-stage fracturing process of a horizontal well, different types of oil-water two-phase tracers are selected for each stage of the fracturing process and injected into the reservoir along with the fracturing fluid; after the entire well is fracturing is completed, oil and water are sampled daily during the drainage and production process, and the production concentration of each tracer in the oil and water samples taken daily is tested.

[0011] The output curve mentioned in step 2 is specifically as follows: the vertical axis of the curve is the dimensionless tracer concentration factor, and the value represented by the vertical axis is equal to the ratio of the absolute concentration of the tracer output to the peak concentration of the tracer during the entire test period, without units; the horizontal axis is the tracer output time, in days.

[0012] The standard for step 3 is as follows: the point with a scale value of 0.5 on the vertical axis is called the half-amplitude point. A straight line parallel to the horizontal axis is drawn to the right from the half-amplitude point. The intersection of the straight line and the dimensionless concentration production curve is called the half-amplitude width. The width of the intersection of the dimensionless concentration production curve and the horizontal axis is called the peak base width.

[0013] In step 4, the standard curves set in step 3 are divided into single-peak type, sharp-peak type, broad multi-peak type, and sharp multi-peak type. The characteristics of each fracturing section are analyzed based on each peak type.

[0014] The single-peak curve is characterized by having only one peak, being symmetrical about the peak axis, and having a half-width greater than 30% of the peak base width. The single-peak morphological characteristics are: the corresponding fracturing section has a single main fracture development, with no tracer entering the branch fractures; the half-width reflects the fracture aperture, and the larger the fracture aperture, the narrower the half-width.

[0015] The peak-shaped curve is characterized by: a single peak at the point greater than half the amplitude, with the half-amplitude width being less than 30% of the peak base width, and two or more peaks at the point less than half the amplitude; the morphological characteristics of the peak-shaped curve are: a short period of concentrated fluid discharge during the fracturing fluid flowback process, corresponding to primary and secondary fractures in the fracturing section, and a significant difference in the conductivity of the primary and secondary fractures.

[0016] The wide multi-peak curve is characterized by: a wide upper half of the curve, a gradual decrease in the curve to the right of the peak, and 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 at different levels is relatively small; the wider the right half of the curve, the slower the fluid drainage in the corresponding fracturing section, the longer the tracer drainage time, indicating that the fracture is narrower and more complex.

[0017] The specific characteristics of the sharp multi-peak curve are: there are two or more peaks on the curve that exceed half the amplitude, 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: the tracer concentration in the corresponding fracturing section is produced in multiple pulses, and each production is short in duration; multiple fractures are formed in the fracturing section, and the fractures are relatively simple, with different levels of conductivity in each fracture.

[0018] The beneficial effects of this invention are:

[0019] The present invention expands the application function of tracer testing by using tracer concentration curves to determine the complex characteristics of fractures. It proposes a low-cost, easy-to-operate, and highly reliable method for identifying the complex characteristics of hydraulic fracturing fractures, adding a new technical means to the evaluation of fracturing effects. Attached Figure Description

[0020] Figure 1 To illustrate this invention, a single-peaked dimensionless tracer concentration yield curve was plotted in the graph.

[0021] Figure 2 To illustrate this invention, a peak-shaped dimensionless tracer concentration production curve was plotted in the graph.

[0022] Figure 3 A broad, multi-peaked dimensionless tracer concentration yield curve was plotted for this invention.

[0023] Figure 4 To illustrate this invention, a multi-peaked, dimensionless tracer concentration production curve was plotted in the graph.

[0024] Figure 5 This is the dimensionless concentration production curve of the aqueous tracer in the second stage of a shale oil horizontal well in Example 3 of the present invention. Detailed Implementation

[0025] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments.

[0026] This invention provides a method for determining the complex characteristics of cracks using tracer concentration curves, as shown in the following embodiments:

[0027] Example 1: The main idea of ​​the method for determining the complex characteristics of cracks using tracer concentration curves in this invention:

[0028] The specific steps are as follows:

[0029] Step 1: During the multi-stage fracturing process of a horizontal well, different oil-water two-phase tracers are injected into the reservoir along with the fracturing fluid, and then the production concentration of the tracers is obtained by sampling.

[0030] Step 2: Plot the dimensionless concentration production curve of the aqueous tracer for each segment;

[0031] Step 3: Based on the curve drawn in Step 2, set the judgment criteria;

[0032] Step 4: Analyze the characteristics of the fracturing section according to the standards specified in Step 3.

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

[0034] Step 1: During the multi-stage fracturing of the 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 entire well is fracturing is completed, oil and water samples are taken daily during the drainage and production process, and the production concentration of each tracer in the oil and water samples taken daily is tested.

[0035] Step 2, plot the curves: plot the dimensionless concentration production curve of the aqueous tracer for each segment; the vertical axis of the curve is the dimensionless tracer concentration factor, which is equal to the ratio of the absolute concentration of the tracer produced to the peak concentration of the tracer throughout the entire test period, without units; the horizontal axis is the tracer production time, in days.

[0036] Step 3, Curve Standard: The point with a scale value of 0.5 on the vertical axis is called the half-amplitude point. Draw a straight line parallel to the horizontal axis to the right from this point. The intersection of this line and the dimensionless concentration production curve is called the half-amplitude width. The width of the intersection of the dimensionless concentration production curve and the horizontal axis is called the peak base width.

[0037] Step 4: Analyze the characteristics of the corresponding fracturing section based on the curves from Step 3, specifically including single-peak type, sharp-peak type, broad multi-peak type, and sharp multi-peak type. Analyze the characteristics of each fracturing section based on each peak type.

[0038] Example 3, characteristic representation of each peak type:

[0039] like Figure 1 As shown in the figure, the double arrows represent the half-width. When the curve is unimodal, with only one peak and basically symmetrical about the peak axis, the half-width is generally greater than 30% of the peak base width. This curve shape characteristic indicates that the corresponding fracturing section has a single main fracture development, with basically no tracer entering the branch fractures; the half-width reflects the fracture aperture, and the larger the fracture aperture, the narrower the half-width.

[0040] like Figure 2 As shown in the figure, the double arrows represent half width. When the curve shape is peak-shaped, there is only one peak above half width, and the half width is less than 30% of the base width. There may be two or more peaks below half width. This curve shape characteristic indicates that there is a short period of concentrated drainage during the fracturing fluid flowback process, the fracturing section has primary and secondary fractures, and there is a significant difference in the conductivity of the primary and secondary fractures.

[0041] like Figure 3As shown in the figure, the double arrows represent half-width. When the curve is a wide multi-peak type, the upper half of the curve is wider, the curve decreases more gently to the right of the peak, and there are two or more peaks exceeding the half-width point. This curve shape characteristic indicates that 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 fluid drainage in the fracturing section, the longer the tracer drainage time, indicating that the fracture is narrower and more complex.

[0042] like Figure 4 As shown in the figure, the double arrows represent the half-width. When the curve is a multi-peaked type, there are two or more peaks exceeding the half-width point, but the half-width of each peak is small, less than 30% of the peak base width. This curve shape indicates that the tracer concentration in the fracturing section is produced in multiple pulses, and each production is short in duration; multiple fractures are formed in the fracturing section, and the fractures are relatively simple, with a certain degree of difference in conductivity among the fractures.

[0043] Example 4, Specific Application:

[0044] The method for determining the complex characteristics of cracks using tracer concentration curves is as follows:

[0045] A shale oil horizontal well has a horizontal section of 1000 meters and an oil layer encounter rate of 88.3%. The design adopts a segmented multi-cluster volumetric fracturing mode to stimulate 12 sections, with 3 to 5 clusters in each section, a fracture density of 9.4 fractures / 100 meters, a sand addition intensity of 3.5 tons / meter, a fluid injection intensity of 20 cubic meters / meter, and an injection displacement of 12 cubic meters / minute.

[0046] Step 1: During the well fracturing process, oil-phase and water-phase liquid tracers are injected into each section along with the fracturing fluid. The types of tracers injected in each section are different, totaling 12 types of water-phase tracers and 12 types of oil-phase tracers. After the well fracturing operation is completed, the well is shut in for 20 days, after which it is reopened for the flowback and production phase. Samples are taken daily during the flowback and production process to analyze the production concentration of various tracers in the samples.

[0047] Step 2: Plot the dimensionless concentration production curve of the aqueous tracer for each segment; the vertical axis of the curve is the dimensionless tracer concentration factor, which is equal to the ratio of the absolute concentration of the tracer produced to the peak concentration of the tracer throughout the entire test period, and has no unit; the horizontal axis is the tracer production time, in days.

[0048] Step 3: Apply the curve judgment criteria;

[0049] Step 4: By analyzing the morphology of the dimensionless concentration production curves of the aqueous tracer in each section of the well, all curves were found to be broad and multi-peaked, indicating that a complex fracture network system had been formed in the fracturing section, with relatively small differences in conductivity between fracture levels, a wide half-width, and a relatively small fracture aperture. The horizontal well as a whole achieved the purpose of volumetric fracturing. For example, the dimensionless concentration production curve of the aqueous tracer in the second section of the well is shown below. Figure 5 As shown.

Claims

1. A method for determining fracture complexity characteristics using tracer concentration profiles, characterized in that, The method is implemented according to the following steps: Step 1, in the process of multi-stage fracturing of horizontal wells, different oil-water two-phase tracers are injected into the reservoir with fracturing fluid in the multi-stage fracturing process, and then the concentration of the tracer is sampled and obtained; Step 2, the dimensionless concentration production curve of the water phase tracer of each stage is drawn respectively; Step 3, the judgment standard is set according to the curve drawn in step 2; Step 4, the characteristics of the fracturing stage are analyzed according to the standard set in step 3; 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; The wide multi-peak type curve is specifically: the half-width on the curve is wide, the right side curve of the peak decreases gently, and there are two or more than two peaks beyond the half-width point; the wide multi-peak type curve shape characteristic is: the fracture network is developed, and the difference of the conductivity of each level of the fracture is small; the wider the right side half-width, the slower the fracture drainage of the corresponding fracturing stage, the longer the tracer drainage time, indicating that the fracture is narrowed and the complexity is increased; The sharp multi-peak type curve is specifically: there are two or more than two peaks beyond 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 sharp multi-peak type curve shape characteristic is: the tracer concentration of the corresponding fracturing stage is output in a multi-pulse mode, and each output lasts for a short time; the fracturing stage forms multiple fractures, and the fractures are relatively simple, and there is a difference in the conductivity of each fracture.

2. The method for determining fracture complexity characteristics using tracer concentration profiles of claim 1, wherein, Step 1 is specifically: in the process of multi-stage fracturing of horizontal wells, different types of oil-water two-phase tracers are selected for each stage in the multi-stage fracturing process and injected into the reservoir with fracturing fluid; after the well is completely 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 respectively.

3. The method for determining fracture complexity characteristics using tracer concentration profiles of claim 1, wherein, The output curve in step 2 is specifically: the vertical coordinate of the curve is the dimensionless tracer concentration factor, and the value represented by the vertical coordinate 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, which has no unit; the horizontal coordinate is the tracer output time, which has a unit of day.

4. The method for determining fracture complexity characteristics using tracer concentration profiles of claim 3, wherein, The standard of step 3 is: the point with a scale value of 0.5 on the vertical coordinate axis is called the half-width point, a straight line parallel to the horizontal axis is drawn to the right of the half-width point, the straight line intersects with the dimensionless concentration production curve, and the width of the intersecting part is called the half-width; the width of the intersection part of the dimensionless concentration production curve and the horizontal axis is called the peak base width.

5. The method for determining fracture complexity characteristics using tracer concentration profiles of claim 1, wherein, The single-peak type curve is specifically: there is only one peak value 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 type shape characteristic is: the corresponding fracturing stage is a single main fracture development, and no tracer enters the branch fracture; the half-width reflects the fracture opening, and the larger the fracture opening, the narrower the half-width.

6. The method for determining fracture complexity characteristics from tracer concentration profiles of claim 1, wherein, The sharp-peak 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 sharp-peak type curve shape characteristic is: there is a short time of concentrated drainage during the fracturing fluid flowback process, the corresponding fracturing stage has primary and secondary fractures, and there is a significant difference in the conductivity of the primary and secondary fractures.

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