CN115247554B

CN115247554B - A multi-step fracturing method for reducing temperature and increasing brittleness

Info

Publication number: CN115247554B
Application number: CN202110458086.2A
Authority: CN
Inventors: 杨慎; 王红岩
Original assignee: Petrochina Co Ltd
Current assignee: Petrochina Co Ltd
Priority date: 2021-04-27
Filing date: 2021-04-27
Publication date: 2024-05-28
Anticipated expiration: 2041-04-27
Also published as: CN115247554A

Abstract

The invention provides a cooling and embrittlement multi-step fracturing method, which comprises the steps of repeatedly fracturing a stratum by using low-temperature fluid to form a fracture network in a reservoir, supporting the fracture network by using a fracturing propping agent after the fracturing is finished, wherein n is the number of implementation steps of each fracturing. According to the invention, through multi-step reservoir cooling and embrittlement and reciprocating multi-step fracturing, the full volume transformation of reservoirs such as shale and the like can be realized. According to the conditions of the mud shale reservoirs of the east-west group of the Erdos basin, the initial daily output of a single well can be estimated to be improved by 3-5 times on the basis of the prior art, and the gas well reducing rate is effectively reduced due to the fact that the reservoirs are fully transformed in multiple steps, so that the effective development of land shale oil and gas resources is expected to be realized.

Description

Multi-step fracturing method for cooling and embrittlement

Technical Field

The invention relates to the field of oil and gas exploitation, in particular to a multi-step fracturing method for cooling and embrittlement.

Background

The land shale oil and gas resources in China are rich, but the land shale reservoir has low brittle mineral content (20% -50%), irrvelation and low gas content (0.5-2.5 square/ton), the volume fracturing effect is poor due to low brittleness, the single well yield is low (0.1-0.5 square/day for a straight well and 0.5-1.0 square/day for a horizontal well), and the economic and effective development cannot be realized at present. Successful experience of effective development of shale oil and gas resources in the United states tells us how to most effectively improve the transformation volume of a tight reservoir, and establishment of a larger-scale artificial oil and gas reservoir is a key for realizing economic and effective development of ultra-tight reservoirs such as shale.

Through the years of exploration and development practice at home and abroad, the horizontal well multistage fracturing technology can realize the volume reconstruction of shale reservoirs, and has proved to be an effective technology for shale oil and gas resource development. The North American shale reservoir is all sea-phase sediments, the transverse distribution is extremely stable, the later-stage structure is stable, and therefore the breakthrough of the horizontal well multistage fracturing technology is the rapid increase of shale oil and gas yield. The technology is mainly characterized in that the improvement volume is improved by increasing the length of a horizontal section of a horizontal well and the number of encrypted fracturing sections and clusters, and the technology is specifically characterized in that: (1) The horizontal section length of the horizontal well has been increased from 1000-1500m to 2500-3000m, up to 5652m (PurpleHayes H); (2) The fracturing cluster spacing of the horizontal section is encrypted from 20-30m to about 5 m. The shale and tight sandstone oil-gas resource development in North America and other areas is mainly performed by the characteristics of the reservoir, and the volume transformation of the vertical well is less.

The Chinese patent mainly comprises an ultra-low permeability reservoir vertical well omnibearing three-dimensional fracturing method (patent number 201610909054.9), a low permeability reservoir vertical well staged fracturing fracture parameter optimization method (patent number 201710946913.6), a vertical well fixed-point multi-stage fracturing method and application (201611189233.6), a non-fractured tight sand shale reservoir vertical well fracture network fracturing process (201410835735.6), a coalbed methane vertical well super-thick coal seam continuous oil pipe staged fracturing yield increasing method (201610154844.0) and a vertical well layered fracturing interval optimization and construction parameter optimization design method (201510716641.1) in the aspect of fracturing technology. The characteristic of low brittleness of the land shale reservoir makes the above fracturing process unable to meet the requirement of volume transformation, and the characteristic of low brittleness of the land shale reservoir determines that main cracks are usually formed in the fracturing process, so that the volume transformation is difficult to realize. Therefore, by improving the brittleness of the land shale reservoir, the volume transformation of the reservoir can be realized, and the scale of the volume transformation can be improved through the transformation process optimization.

Disclosure of Invention

The invention aims to provide a multi-step fracturing method for cooling and embrittlement.

Aiming at the problems of relatively high plasticity and low brittleness of a land shale reservoir and a coal reservoir, complex volume sealing net is difficult to form in the hydraulic fracturing process, so that the oil gas yield of a single well is relatively low and the decrease speed is faster. Aiming at the problem of low reservoir brittleness, the low-temperature fracturing fluid is in repeated contact with the reservoir for multiple times, so that the temperature of the reservoir is reduced, the reservoir brittleness is further improved, and the purpose of volume transformation is achieved.

In order to achieve the above purpose, in one aspect, the invention provides a cooling and embrittlement multi-step fracturing method, wherein the method comprises the steps of repeatedly fracturing a stratum with a low-temperature fluid in each fracturing to form a fracture network in a reservoir, and supporting the fracture network with a fracturing propping agent after the fracturing is finished.

Wherein it is understood that each fracturing refers to one of the vertical well fractures; or a single fracture per section of a horizontal well fracture.

The invention mainly adopts low-temperature fluid (such as liquid nitrogen, liquid carbon dioxide or other low-temperature fluid and the like) as fracturing fluid, reduces the temperature of a reservoir through the low-temperature fracturing fluid to improve brittleness, and fully contacts the reservoir in the fracturing process of each step through step-by-step pulse injection of the fracturing fluid in the operation process so as to achieve better volume fracturing effect.

According to some embodiments of the invention, wherein the heat capacity of the cryogenic fluid satisfies the following formula (1):

Wherein C _f is the heat capacity, J/g, of the low-temperature fluid; v _f is the volume of fracturing fluid injected into the formation, m ³;ρ_f is the density of fracturing fluid injected into the formation, t/m ³;C_r is the heat capacity of the reservoir, J/g; v _r is the reservoir cooling volume, m ³;ρ_r is the reservoir density, t/m ³;T_r0 is the reservoir initial absolute temperature, K; t _r1 is the absolute temperature of the reservoir after cooling, K; t _f0 is the initial absolute temperature of the fracturing fluid, K; t _f1 is the absolute temperature of the fracturing fluid after heat absorption, K.

According to some embodiments of the invention, the cryogenic fluid is selected from liquid nitrogen and liquid carbon dioxide.

According to some embodiments of the invention, the method comprises the steps of:

(1) Fracturing a reservoir with low-temperature fluid, closing a well, and cooling to ensure that the low-temperature fluid is fully contacted with the reservoir and realize the cooling effect of low-temperature flow on the reservoir;

(2) After the well closing cooling of the step (1) is finished, continuing to fracture the reservoir stratum by using low-temperature fluid, and then closing the well for cooling;

(3) Repeating the step (2) until the fracturing is completed;

(4) And adding a fracturing propping agent to support a fracture network formed by fracturing.

According to some embodiments of the invention, wherein the fracturing pressure of step (1) is greater than the reservoir maximum horizontal principal stress and at least 10% greater than the fracture pressure (reservoir fracture pressure).

According to some embodiments of the invention, the fracturing pressure of step (2) is at least 10% greater than the fracturing pressure of step (1).

According to some embodiments of the invention, the fracturing pressure of step (3) in step n is at least 10% greater than the fracturing pressure of step n-1.

It is understood that something at least 10% greater than something as described above means something at least equal to something x (1 + 10%), or greater than this value.

According to some embodiments of the invention, the time of each (each repetition) shut-in and cooling in step (3) is calculated according to formulas (2) - (5):

Establishing a shale reservoir heat transfer equation (2) near a shaft according to reservoir characteristics

Wherein T is the absolute temperature of the reservoir, K; r is the distance from a certain point in the reservoir to the center of the shaft, m; alpha is the thermal diffusion coefficient of the reservoir, m ²/d; t is the cooling time of closing the well and is the day;

Initial conditions: t| _ti＝0＝T_di (3)

(The initial condition of the i-th step cooling is the reservoir temperature field after the i-1 th step cooling)

Wherein ti is the initial time of the i-th cooling, d; t _di is the absolute temperature, K, of the low-temperature fluid at the initial time of the i-th cooling;

Inner boundary conditions:

(the heat input and output of the radius rdfi of the equivalent radial interface of the reservoir cooled in the ith step and the contact of the cryogenic fluid are equal, namely the heat output by the reservoir is the heat absorbed by the cryogenic fluid)

Wherein lambda is the thermal conductivity of the reservoir, W/(mdeg.C); rdfi is the radius, m, of the equivalent radial interface of the reservoir in contact with the cryogenic fluid; q _dfi is the heat quantity absorbed by the low-temperature fluid after cooling, J;

outer boundary conditions: t| _r＝∞＝T₀ (5)

(Any one cooling down, the temperature at infinity in its reservoir remains constant).

Considering the heat diffusion efficiency and time effect of the fracturing injection of the cryogenic fluid at each step (i-th step), if the effective cooling range of the reservoir is substantially the same as the temperature of the cryogenic fluid, the time required is too long. Therefore, in order to ensure the heat transfer efficiency, the temperature T _fit of the overlong medium-low temperature fluid after heat absorption and temperature rise is designed to be larger than the average temperature T _rit of the reservoir after temperature reduction.

According to some embodiments of the invention, n is 3-5.

According to some embodiments of the invention, the reservoir is a shale reservoir.

In summary, the invention provides a multi-step fracturing method for cooling and embrittlement. The method of the invention has the following advantages:

According to the current production condition of the land shale gas well, the reservoir has low brittleness due to the fact that the clay content of the reservoir is 30% -60% and the quartz content is 20% -50%, a complex net-stitching system is difficult to realize in the conventional hydraulic fracturing process, and the reservoir is not fully transformed. Taking the eastern Shanxi group shale reservoirs of the Erdos basin as an example, the daily output of a vertical well at the initial stage of a single well is 0.1-0.5 square/day, the daily output of a horizontal well is 0.5-1.0 square/day, the output of the shale reservoirs is reduced rapidly due to insufficient reservoir transformation, and the effective development cannot be realized at present.

Aiming at the characteristic of low brittleness of the land shale reservoir, the invention adopts low-temperature fluid as fracturing fluid to reduce the temperature of the shale reservoir so as to improve the reservoir brittleness. According to the invention, multiple pulse fracturing is adopted to perform volume transformation on the reservoir step by step, and the well is closed after each step of fracturing, so that low-temperature fluid is fully contacted with the shale reservoir, the heat in the shale reservoir is absorbed to reduce the temperature, and the brittleness is improved. According to the invention, fracturing is carried out step by step, and the reservoir after cooling and embrittlement is reformed in the 2 nd to n th fracturing steps, so that a complex fracture network is easier to form, and the purpose of volume reformation is further realized. In the invention, the propping agent with small particle size (such as 80-100 meshes) is firstly added in the last stage of fracturing (n+1st time), and then the propping agent with large particle size (such as 40-80 meshes) is tracked at the tail, so that the effective support of a fracture network is realized.

According to the invention, through repeated cooling and embrittlement of the reservoir, and repeated multi-step fracturing, the full volume transformation of the reservoir such as shale can be realized. According to the conditions of the mud shale reservoirs of the east-west group of the Erdos basin, the initial daily output of a single well can be estimated to be improved by 3-5 times on the basis of the prior art, and the gas well reducing rate is effectively reduced due to the fact that the reservoirs are fully transformed in multiple steps, so that the effective development of land shale oil and gas resources is expected to be realized.

Drawings

FIG. 1 is a schematic view of a wellbore formation of example 1;

FIG. 2 is a schematic representation of a fracture of example 1;

FIG. 3 is a schematic of a fracture with proppant added.

Detailed Description

The following detailed description of the invention and the advantages achieved by the embodiments are intended to help the reader to better understand the nature and features of the invention, and are not intended to limit the scope of the invention.

Example 1

Taking a sea-land transition phase shale reservoir (for example, a mountain-western group of a Huidos basin land phase) as an example, the reservoir burial depth is 2500m, the temperature is 76 ℃, the quartz mineral content is 25% -30%, the clay mineral content is 40% -45%, the reservoir fracture pressure is 62-65Mpa, the reservoir brittleness is poor, and the difficulty of forming a volume seal after fracturing is high. According to the invention, according to the attribute that brittleness can be improved after solid substances are cooled, low-temperature liquid nitrogen is adopted as fracturing fluid, the fracturing fluid is fully contacted with shale in a reservoir, heat of the reservoir is absorbed, the temperature is reduced, and therefore the brittleness of the reservoir is enhanced.

And (3) carrying out volume transformation on the reservoir region subjected to temperature reduction and embrittlement by adopting a multi-step pulse fracturing mode, improving the complexity of a fracture network, and adding a propping agent in the last step of fracturing to realize the propping of the fracture network. The method comprises the following steps:

(1) As shown in fig. 1, fracturing the reservoir with liquid nitrogen at a fracturing pressure of 72Mpa, and then shutting in the well for 1.0-1.5 days according to the calculation method, wherein the average temperature of the reservoir is reduced to 35-40 ℃ within the range of 2 m.

(2) After the well closing cooling of the step (1) is finished, the reservoir is continuously fractured by liquid nitrogen, then the well closing is carried out, the fracturing is improved by about 10% compared with the last time in each construction, and the well closing time is 1.0-1.5 days.

(3) Repeating the step (2) for 4 times (5 times), wherein each time the fracturing pressure is 79, 87, 96 and 103Mpa respectively, each time the well is closed for 1.0-1.5 days, and finally the vertical joint of the fracture is about 10m (see figure 2).

(4) And adding a fracturing propping agent in the last fracturing process to prop a fracture network formed by fracturing (see figure 3). Selecting low-density ceramsite as a propping agent, and adding the propping agent according to the sequence of 70-100 meshes, 40-70 meshes and 20-40 meshes.

Through the treatment, the shale gas horizontal well has a 1000m horizontal section length of 10 sections of fracturing, so that the single well test yield is improved from 2X 10 ⁴-5×10⁴ m3/d to 10X 10 ⁴m³/d, and the single well final recoverable reserve is improved from 0.2X 10 ⁸-0.4×10⁸m³ to more than 0.8X 10 ⁸m³.

Claims

1. A multi-step fracturing method for reducing temperature and increasing embrittlement, wherein the method comprises, in each fracturing, repeatedly fracturing the formation with a low-temperature fluid for n steps to form a fracture network in the reservoir, and supporting the fracture network with a fracturing proppant after the fracturing is completed; the low-temperature fluid is selected from liquid nitrogen or liquid carbon dioxide; the method comprises the following steps:

(1) Fracturing the reservoir with a low-temperature fluid, then shutting down the well to cool it down, so that the low-temperature fluid is fully in contact with the reservoir and the low-temperature flow has a cooling effect on the reservoir; the fracturing pressure is greater than the maximum horizontal principal stress of the reservoir and is at least 10% greater than the fracture pressure;

(2) After the shut-in and cooling of step (1) is completed, the reservoir is continuously fracturing with a low-temperature fluid, and then the well is shut down and cooled; the fracturing pressure of step (2) is at least 10% greater than the fracturing pressure of step (1);

(3) Repeat step (2) until fracturing is completed; the fracturing pressure of the nth step of step (3) is at least 10% greater than the fracturing pressure of the n-1th step; the time for each shut-in cooling is calculated according to formulas (2)-(5):

Establish the heat conduction equation of shale reservoir near the wellbore based on reservoir characteristics

Where, T is the absolute temperature of the reservoir, K; r is the distance from a point in the reservoir to the center of the wellbore, m; α is the thermal diffusion coefficient of the reservoir, m ² /d; t is the shut-in cooling time, day;

Initial condition: T| _ti＝0 ＝ _Tdi (3)

Wherein, ti is the initial time of the i-th step cooling, d; T _di is the absolute temperature of the cryogenic fluid at the initial moment of the i-th step cooling, K;

Internal boundary conditions:

Where, λ is the thermal conductivity of the reservoir, W/(m℃); rdfi is the radius of the equivalent radial interface between the reservoir and the cryogenic fluid, m; _qdfi is the heat absorbed by the cryogenic fluid after cooling, J;

External boundary condition: T| _r＝∞ ＝T ₀ (5)

(4) Adding fracturing proppant to support the fracture network formed by fracturing.

2. The method according to claim 1, wherein n is 3-5.

The method according to claim 1 , wherein the reservoir is a shale reservoir.