CN112883574B

CN112883574B - Method for optimizing shale gas well repeated fracturing by utilizing hydraulic fracturing microseism b value

Info

Publication number: CN112883574B
Application number: CN202110188357.7A
Authority: CN
Inventors: 张丰收; 冯睿; 尹子睿; 王小华; 黄刘科; 赵峦啸
Original assignee: Tongji University
Current assignee: Tongji University
Priority date: 2021-02-18
Filing date: 2021-02-18
Publication date: 2022-08-09
Anticipated expiration: 2041-02-18
Also published as: CN112883574A

Abstract

The invention relates to a method for optimizing shale gas well repeated fracturing by utilizing a hydraulic fracturing micro-earthquake b value, which is characterized in that based on data such as quantity distribution, magnitude of earthquake magnitude and the like of hydraulic fracturing micro-earthquake monitoring events, the magnitude of the micro-earthquake b value in a fracturing stage is calculated by adopting a Gordon-Rickett relation describing frequency-magnitude distribution in seismology, and the relative magnitude of stress difference of a region is identified by utilizing a b-value distribution cloud picture, so that reference is provided for repeated fracturing to find a modified dessert region. The method can promote repeated fracturing to form more complex fracture network to a certain extent, thereby effectively improving the shale gas productivity.

Description

Method for optimizing shale gas well repeated fracturing by utilizing hydraulic fracturing microseism b value

Technical Field

The invention relates to the technical field of shale gas exploitation.

Background

The Changning-Weiyuan shale gas block located in the Sichuan basin is a national grade shale gas demonstration development area in China, and the shale gas yield is gradually obvious under the yield increasing effect of the hydraulic fracturing technology. However, due to the complex geological structure of the Sichuan basin, the initial fracturing often cannot form an ideal complex fracture network due to fracture development and high stress difference, the fracturing yield-increasing effect cannot be expected, and meanwhile, the yield of the shale gas well is too fast to decay after the shale gas well is mined for a period of time, which becomes a bottleneck problem restricting the effective development of the shale gas in China. More potential shale reservoirs can be crushed by developing the repeated fracturing, more complex fracture networks are formed in the repeated fracturing process, and the problems that the primary fracturing effect is not ideal, the productivity is reduced too fast and the like can be solved.

The preferred dessert interval before fracturing repeatedly ("dessert" means a region with high production of oil and gas) is a factor which is important in the horizontal stress difference of the dessert interval. The stress difference is a key factor which is not negligible in the process of forming the complex seam net, and the complex seam net is formed more easily by fracturing under the condition of smaller stress difference, so that the fracturing yield is improved.

The prior art is lack of effective identification of well circumferential stress difference distribution after primary fracturing, so that the optimal sweet spot area of a repeated fracturing design is difficult to define.

Disclosure of Invention

The invention provides a method for optimizing shale gas well repeated fracturing by utilizing a hydraulic fracturing microseism b value, which overcomes the defect that the prior art can not accurately identify the distribution condition of well circumferential stress difference after primary fracturing.

The purpose of the invention is realized by the following technical scheme: a method for optimizing shale gas well repeated fracturing by utilizing a hydraulic fracturing microseism b value includes the steps of respectively calculating the b value by taking a plurality of fracturing sections as different areas, extrapolating the b value to a well periphery area by adopting an interpolation means, obtaining a well periphery b value distribution cloud chart of a horizontal well section, and optimizing a modified sweet spot area of the repeated fracturing by utilizing a region distribution characteristic of the b value.

Specifically, the method comprises the following steps:

s1, determining the fracturing and segmenting condition of the target well;

s2, determining parameters required for calculating the b value of the micro earthquake at the fracturing stage, wherein the parameters comprise the number of the micro earthquakes, the magnitude of the earthquake magnitude of the micro earthquake and the like;

s3, calculating the b value of the micro earthquake at the fracturing stage by utilizing the Goudenberg-Rickett relation in seismology;

s4, drawing a b-value distribution cloud picture based on the calculated b-value of the micro earthquake at each stage;

s5, identifying a transformation area with small stress difference around the well by using the b-value cloud picture;

and S6, optimizing the repeated fracturing design and improving the shale gas productivity.

The invention has the following advantages:

(1) the b value is a fractal dimension which is obtained by inversion based on actually measured micro-seismic data and describes the relationship between seismic frequency and seismic magnitude, can represent a main stress mechanism inducing a micro-seismic event area and a failure mode of the seismic event, and can reflect the distribution condition of stress difference more truly by describing the b value.

(2) A smaller b value is considered to be associated with a retrograde fault that tends to experience higher stress levels with relatively greater stress differences, and thus b can be a measure of the level of regional stress difference. Compare and utilize numerical means to discern regional stress difference size, adopt b value greatly reduced the computational complexity undoubtedly to be applicable to the scene more.

Drawings

FIG. 1 is a schematic diagram of the number and magnitude distribution of micro-earthquakes at the 1 st fracturing stage of an example well;

FIG. 2 is a schematic illustration of a microseismic b-value calculation for an example well at stage 1 fracturing;

FIG. 3 is a schematic diagram of an exemplary well-perimeter b-value distribution;

FIG. 4 is a schematic diagram of an example well utilizing b-values to identify optimal reconstruction zones;

FIG. 5 is a flow chart of the present invention;

Detailed Description

The invention will be further described with reference to the accompanying drawings, without limiting the scope of the invention to the following:

in the seismic field, the calculation of the b value is usually only performed for a certain specific area, and the fracturing construction of a horizontal well section of a shale gas well in the oil and gas exploitation field is divided into a plurality of sub-well sections (fracturing sections) to be respectively performed.

S1, determining the fracturing and segmenting conditions of the target well, selecting a Changning block well for example analysis, transforming the well in a segmented fracturing mode, and dividing the horizontal well segment into 18 fracturing segments.

And S2, determining basic parameters for calculating the microseismic b value, including the number N of microseismic signals of each fracture section and the magnitude M of the magnitude of the seismic magnitude. The microseismic data of the 1 st fracture section of the well in the embodiment is shown in fig. 1, the rectangular blocks represent microseismic signals generated by the 1 st fracture section of the sample well, and the star symbols represent microseismic signals with larger magnitude.

S3, in the seismic field, the b value can be used as a measure of the stress difference level of the region, and a smaller b value can generally indicate a region with a relatively larger stress difference, whereas a larger b value can indicate a region with a smaller stress difference. Then applying this feature based on the b-value to the engineering field may help identify the reformed sweet spot region of the re-fracture very well. Calculating the b value of the 1 st stage fracturing micro earthquake, wherein the b value calculation formula is a Gordoni-Rickett relation describing frequency-seismic distribution in seismology:

log ₁₀ N(m≥M)＝a-b*M-----------------------------------------(1)

wherein N (M ≧ M) represents the number of earthquakes with magnitude M greater than or equal to M, b is the absolute value of the slope of the linear portion of the frequency-magnitude distribution, a represents the seismic activity of the area, and when M is 0, represents the total number of earthquakes in the area, i.e., 10 ^a And (4) respectively. Referring to the frequency-magnitude graph shown in fig. 2, N corresponds to the number of the 1 st fracture stage microseismic determined in step S2, M corresponds to the 1 st fracture stage microseismic magnitude determined in step S2, and M denotes the x-axis in the frequency-magnitude graph. The b value of the 1 st fracture microseismic is about 0.72 as shown in figure 2, which can be found by using the maximum curvature solution.

S4, based on the fracturing segmentation condition determined in the step S1, repeating the steps S2 and S3 according to the number of segments, sequentially calculating the b value of the subsequent fracturing segment, extrapolating the b value to the peri-well area by adopting an interpolation means, and drawing a peri-well b value distribution cloud chart of all fracturing segments of the sample well, as shown in FIG. 3. It can be seen from fig. 3 that the large-magnitude signals of the microseismic monitoring of the well of this example are all in the region with a low b value, the large-magnitude signals often correspond to the shear fracture zone activated by the fault, and the fault is prone to shear failure under the condition with a large stress difference, which shows that there is a significant correlation between the b value and the stress difference. In addition, the observation of uniformly distributed small seismic level signals can find that most of the signals are located in a region with a higher b value, and the fracturing is easier to form a uniformly distributed complex fracture network under the condition of smaller stress difference, so that the b value well reflects the distribution condition of the stress difference around the well.

S5, effective identification of well periphery stress difference distribution after primary fracturing is lacked in the prior art, and the invention innovatively provides that the b value calculated by microseism data is used for identifying the well periphery stress difference distribution so as to make up the defects of the prior art and further improve the productivity of repeated fracturing. And (4) based on the well periphery b value distribution cloud chart drawn in the step S4, a region with a larger b value in the graph is circled, and the region is the optimal transformation region of the repeated fracturing. Fig. 4 illustrates how a b-value distribution cloud can be used to identify areas of optimal reformation of a re-fracture. From the monitoring condition of the micro earthquake, the initial fracturing effect of the well is not ideal, and particularly, the micro earthquake signals are hardly generated in the middle part, which indicates that the initial fracturing does not realize effective reconstruction of the reservoir in the middle part. From the b-value distribution of this example well, it can be seen that the middle portion near the root has a region with a greater b-value (b >1), indicating that the stress difference in this region is relatively small, and is a region of the sweet spot that is repeatedly fractured and reformed.

And S6, carrying out repeated fracturing design and optimization aiming at the determined repeated fracturing dessert well section, improving the repeated fracturing effect and improving the shale gas productivity.

Claims

1. A method for optimizing shale gas well repeated fracturing by utilizing a hydraulic fracturing microseism b value is characterized in that a plurality of fracturing sections are regarded as different areas to calculate the b value respectively, the fracturing sections are extrapolated to a well periphery area by adopting an interpolation means, so that a well periphery b value distribution cloud chart of a horizontal well section is obtained, and a modified sweet spot area of the repeated fracturing is optimized by utilizing the area distribution characteristic of the b value;

the method comprises the following steps:

s1, determining the fracturing and segmenting condition of the target well;

s2, determining parameters required for calculating the b value of the micro earthquake at the fracturing stage, wherein the parameters comprise the number of the micro earthquakes and the magnitude of the earthquake magnitude of the micro earthquake;