CN115247554A - Multi-step fracturing method for reducing temperature and increasing brittleness - Google Patents
Multi-step fracturing method for reducing temperature and increasing brittleness Download PDFInfo
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- CN115247554A CN115247554A CN202110458086.2A CN202110458086A CN115247554A CN 115247554 A CN115247554 A CN 115247554A CN 202110458086 A CN202110458086 A CN 202110458086A CN 115247554 A CN115247554 A CN 115247554A
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- E—FIXED CONSTRUCTIONS
- E21—EARTH DRILLING; MINING
- E21B—EARTH DRILLING, e.g. DEEP DRILLING; OBTAINING OIL, GAS, WATER, SOLUBLE OR MELTABLE MATERIALS OR A SLURRY OF MINERALS FROM WELLS
- E21B43/00—Methods or apparatus for obtaining oil, gas, water, soluble or meltable materials or a slurry of minerals from wells
- E21B43/25—Methods for stimulating production
- E21B43/26—Methods for stimulating production by forming crevices or fractures
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- E—FIXED CONSTRUCTIONS
- E21—EARTH DRILLING; MINING
- E21B—EARTH DRILLING, e.g. DEEP DRILLING; OBTAINING OIL, GAS, WATER, SOLUBLE OR MELTABLE MATERIALS OR A SLURRY OF MINERALS FROM WELLS
- E21B43/00—Methods or apparatus for obtaining oil, gas, water, soluble or meltable materials or a slurry of minerals from wells
- E21B43/25—Methods for stimulating production
- E21B43/26—Methods for stimulating production by forming crevices or fractures
- E21B43/267—Methods for stimulating production by forming crevices or fractures reinforcing fractures by propping
Abstract
The invention provides a temperature-reducing and brittleness-increasing multi-step fracturing method, wherein the method comprises the steps of repeatedly fracturing a stratum by using low-temperature fluid for n steps to form a fracturing network in a reservoir, supporting the fracturing network by using a fracturing propping agent after fracturing is finished, and n is the frequency of the implementation step of each fracturing. According to the invention, the full volume reformation of reservoirs such as shale and the like can be realized through multi-step reservoir cooling and embrittlement and reciprocating multi-step fracturing. According to the condition of the shale reservoir stratum of Shanxi group of east edge of Ordos basin, the preliminary estimation of the daily output of a single well at the initial stage can be improved by 3-5 times on the basis of the prior art, and the gas well degradation rate is effectively reduced due to the fact that the reservoir stratum is fully reformed in multiple steps, and the effective development of continental facies shale oil and gas resources is expected to be realized.
Description
Technical Field
The invention relates to the field of oil and gas exploitation, in particular to a temperature-reducing and brittleness-increasing multi-step fracturing method.
Background
China continental facies shale oil and gas resources are rich, but continental facies shale reservoirs have low brittle mineral content (20-50%), no bedding development and low gas content (0.5-2.5 square/ton), volume fracturing effect is poor due to low brittleness, single well yield is low (0.1-0.5 ten thousand square/day of vertical well and 0.5-1.0 ten thousand square/day of horizontal well), and economic and effective development cannot be realized at present. Successful experience in effective development of shale oil and gas resources in the United states tells that how to most effectively increase the modification volume of a compact reservoir and establishing a larger-scale artificial oil and gas reservoir is the key to realizing economic and effective development of ultra-compact reservoirs such as shale.
Through years of exploration and development practices at home and abroad, the horizontal well multistage fracturing technology can realize volume modification of a shale reservoir, and is proved to be an effective technology for shale oil and gas resource development. The North America shale reservoirs are all marine phase sedimentary, the transverse distribution is extremely stable, and the later-stage structure is stable, so that the breakthrough of the multi-stage fracturing technology of the horizontal well is the rapid increase of the shale oil and gas yield. The technology mainly improves the modification volume by increasing the length of the horizontal segment, the number of the encrypted fracturing segments and the number of clusters of the horizontal well, and is specifically represented as follows: (1) The horizontal section length of the horizontal well is increased from 1000-1500m to 2500-3000m, and the maximum length is 5652m (PurpleHayes 1H); (2) The interval between fracturing clusters of the horizontal section is encrypted from 20-30m to about 5 m. The development of shale and tight sandstone oil and gas resources in regions such as North America and the like is mainly based on horizontal wells, and the volume improvement of vertical wells is less.
Chinese patents mainly comprise 'an ultra-low permeability reservoir vertical shaft omnibearing three-dimensional fracturing method' (patent number 201609009009054.9), a low permeability reservoir vertical shaft staged fracturing fracture parameter optimization method (patent number 201710946913.6), a vertical shaft fixed point multi-stage fracturing method and application (201611189233.6), a non-fractured compact sandstone reservoir vertical shaft fracture network fracturing process (201410835735.6), a coal bed methane vertical shaft extra-thick continuous oil pipe staged fracturing production-increasing method (201610154844.0), a vertical shaft staged fracturing interval optimization and construction parameter optimization design method (201510716641.1) in the aspect of fracturing technology. The characteristic of low brittleness of the continental facies shale reservoir stratum causes that the fracturing process can not meet the volume transformation requirement, and the characteristic of low brittleness of the continental facies shale reservoir stratum determines that a main crack is usually formed in the fracturing process, so that the volume transformation is difficult to realize. Therefore, by improving the brittleness of the continental facies shale reservoir, the volume modification of the reservoir can be realized, and the scale of the volume modification is optimized and improved through the modification process.
Disclosure of Invention
The invention aims to provide a multi-step fracturing method for reducing temperature and increasing brittleness.
The method aims at the problems that the continental facies shale reservoir and the coal reservoir have relatively high plasticity and low brittleness, and complex volume blocking net is difficult to form in the hydraulic fracturing process, so that the single-well oil and gas yield is relatively low, and the decline is faster. Aiming at the problem of low reservoir brittleness, the low-temperature fracturing fluid is in repeated reciprocating contact with the reservoir to reduce the temperature of the reservoir, so that the reservoir brittleness is improved, and the purpose of volume transformation is achieved.
In order to achieve the above objects, in one aspect, the present invention provides a temperature-reducing and brittleness-increasing multi-step fracturing method, wherein the method comprises repeatedly fracturing a stratum with a low-temperature fluid for n steps in each fracturing to form a fracture network in a reservoir, and propping the fracture network with a fracturing propping agent after fracturing.
Wherein, it is understood that each fracture refers to one fracture in the vertical well fracture; or one 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 fluids) as fracturing fluid, reduces the temperature of a reservoir stratum through the low-temperature fracturing fluid to improve the brittleness, and fully contacts the low-temperature fluid with the reservoir stratum in each step of fracturing process through injecting the fracturing fluid in a step-by-step pulse mode 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 of the cryogenic fluid, J/g; v f Volume of fracturing fluid injected into the formation, m 3 ；ρ f Density of fracturing fluid to be injected into the formation, t/m 3 ；C r Is the heat capacity of the reservoir, J/g; v r Volume of reservoir drawdown, m 3 ；ρ r Is the reservoir density, t/m 3 ；T r0 Is the reservoir initial absolute temperature, K; t is r1 Absolute temperature of the reservoir after cooling, K; t is a unit of f0 Is the initial absolute temperature of the fracturing fluid, K; t is f1 Absolute temperature of fracturing fluid after heat absorption, K.
According to some specific embodiments of the invention, wherein the cryogenic fluid is selected from liquid nitrogen or liquid carbon dioxide.
According to some specific embodiments of the invention, wherein the method comprises the steps of:
(1) Fracturing the reservoir with cryogenic fluid, closing the well and cooling to make the cryogenic fluid fully contact with the reservoir and realize the cooling effect of the low-temperature flow on the reservoir;
(2) After the well closing and temperature reduction in the step (1) are finished, continuing fracturing the reservoir with low-temperature fluid, and then closing the well and reducing the temperature;
(3) Repeating the step (2) until the fracturing is completed;
(4) And adding a fracturing propping agent to support a fracturing 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 fracturing pressure (reservoir fracturing pressure).
According to some embodiments of the invention, wherein the fracture pressure of step (2) is at least 10% greater than the fracture pressure of step (1).
According to some embodiments of the invention, the fracture pressure of the nth step of step (3) is at least 10% greater than the fracture pressure of the n-1 st step.
Where it is understood that something as described above is at least 10% greater than something, it is meant that something is at least equal to something x (1 + 10%) or greater than this value.
According to some embodiments of the present invention, the time for each (repeated) shut-in and cooling in step (3) is calculated according to the following formulas (2) to (5):
establishing shale reservoir heat conduction equation (2) near the shaft according to reservoir characteristics
Wherein T is reservoir absolute temperature, 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 storage layer, m 2 D; t is the time of closing the well and reducing the temperature, day;
initial conditions: t- ti＝0 ＝T di (3)
(the initial condition of the cooling in the ith step is the reservoir temperature field after the cooling in the ith-1 step)
Wherein ti is the initial time of cooling in the step i, and d; t is di Absolute temperature K of the low-temperature fluid at the initial moment of the cooling in the step i;
inner boundary conditions:
(the radius rdfi of the equivalent radial interface between the reservoir and the cryogenic fluid to be cooled in the ith step is equal to the heat input and output of the reservoir, namely the heat output by the reservoir is the heat absorbed by the cryogenic fluid)
Wherein, lambda is the heat conductivity coefficient of the reservoir, W/(m ℃); rdfi is the radius of the equivalent radial interface where the reservoir is in contact with the cryogenic fluid, m; q. q.s dfi Heat absorbed after the cryogenic fluid is cooled, J;
outer boundary conditions: t- r＝∞ ＝T 0 (5)
(the temperature of the reservoir at infinity remains constant at any cooling time).
Considering the heat diffusion efficiency condition and the time effect of fracturing injection of the low-temperature fluid in each step (i step), if the effective temperature reduction range of the reservoir is basically the same as the temperature of the low-temperature fluid, the required time is too long. Therefore, in order to ensure the heat transfer efficiency, the overlong temperature T of the medium and low temperature fluid after heat absorption and temperature rise is designed fit The temperature is higher than the average temperature T after the reservoir is cooled rit 。
According to some embodiments of the invention, wherein n is 3 to 5.
According to some specific embodiments of the invention, the reservoir is a shale reservoir.
In conclusion, the invention provides a temperature-reducing and brittleness-increasing multi-step fracturing method. The method of the invention has the following advantages:
according to the current production condition of the continental facies shale gas well, the reservoir brittleness is low due to the fact that the content of reservoir clay is 30% -60% and the content of quartz is 20% -50%, and the reservoir is insufficiently reformed due to the fact that a complex gap net system is difficult to achieve in the conventional hydraulic fracturing process. Taking the mud shale reservoir in east and west groups of Oriental mountains and west of an Ordos basin as an example, the daily yield of a single well at the initial stage is 0.1-0.5 ten thousand square/day for a vertical well, and the yield of a horizontal well is 0.5-1.0 ten thousand square/day for a horizontal well, and the yield is decreased rapidly due to insufficient reservoir transformation, so that effective development cannot be realized at present.
Aiming at the characteristic of low brittleness of the continental facies shale reservoir, the invention adopts the low-temperature fluid as the fracturing fluid to reduce the temperature of the shale reservoir to improve the brittleness of the reservoir. The invention adopts multiple pulse fracturing to carry out volume transformation on the reservoir layer step by step, wherein the well is closed after each step of fracturing so as to ensure that the low-temperature fluid is fully contacted with the shale reservoir layer, and the heat in the shale reservoir layer is absorbed to reduce the temperature, thereby improving the brittleness. In the invention, the fracturing is carried out step by step, and the reservoirs subjected to temperature reduction and embrittlement are transformed in the 2 nd to n th fracturing, so that a complex fracture network is more easily formed, and the purpose of volume transformation is further realized. In the invention, the proppant with small particle size (such as 80-100 meshes) is added in the last stage of fracturing (the (n + 1) th time), and then the proppant with large particle size (such as 40-80 meshes) is added in the tail so as to realize the effective support of the fracture network.
According to the invention, the full volume reformation of reservoirs such as shale and the like can be realized through multiple times of reservoir cooling and embrittlement, and reciprocating multi-step fracturing. According to the condition of the shale reservoir stratum of Shanyuan Shanxi group of Ore and Doss basin, the initial daily output of a single well is preliminarily estimated to be improved by 3-5 times on the current basis, and the gas well decrement rate is effectively reduced due to the fact that the reservoir stratum is fully transformed in multiple steps, and the effective development of continental facies shale oil and gas resources is expected to be achieved.
Drawings
FIG. 1 is a schematic representation of a wellbore formation of example 1;
FIG. 2 is a schematic representation of a fractured fracture of example 1;
FIG. 3 is a schematic view of a fracture with proppant added.
Detailed Description
The following detailed description is provided for the purpose of illustrating the embodiments and the advantageous effects thereof, and is not intended to limit the scope of the present disclosure.
Example 1
Taking a sea-land transition facies shale reservoir as an example (such as Shanxi group of Shandong basin), the buried depth of the reservoir is 2500m, the temperature is 76 ℃, the quartz mineral content is 25-30%, the clay mineral content is 40-45%, the fracture pressure of the reservoir is 62-65Mpa, the brittleness of the reservoir is poor, and the difficulty of forming a volume seal net after fracturing is high. According to the invention, low-temperature liquid nitrogen is adopted as the fracturing fluid according to the property that the brittleness of the solid substance can be improved after the temperature is reduced, the fracturing fluid is fully contacted with shale in the reservoir, and the heat of the reservoir is absorbed to reduce the temperature, so that the brittleness of the reservoir is enhanced.
And (3) carrying out volume transformation on the reservoir area after temperature reduction and embrittlement increase 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 specifically comprises the following steps:
(1) As shown in figure 1, the reservoir is fractured by liquid nitrogen, the fracturing pressure is 72Mpa, then the well is shut in for 1.0-1.5 days according to the calculation method, and the average temperature in the range of 2m of the reservoir is reduced to 35-40 ℃.
(2) And (2) after the well closing and temperature reduction in the step (1) are finished, continuing fracturing the reservoir by using liquid nitrogen, closing the well and reducing the temperature, wherein the fracturing rate of each construction is improved by about 10% compared with that of the previous construction, and the well closing time is 1.0-1.5 days.
(3) And (3) repeating the process of the step (2) 4 times (totally 5 times), wherein the fracturing pressure is respectively 79, 87, 96 and 103Mpa, the well closing time is 1.0-1.5 days, and finally the vertical seam height of the fracture to be fractured is about 10m (see fig. 2).
(4) And adding a fracturing propping agent in the last fracturing process to support the fracture network formed by fracturing (see figure 3). Selecting low-density ceramsite as a proppant, and adding the proppant according to the order of 70-100 meshes, 40-70 meshes and 20-40 meshes.
After the treatment, the shale gas horizontal well is fractured at the length of 1000m horizontal section and 10 sections, so that the single well test yield is 2 multiplied by 10 4 -5×10 4 m3/d is increased to 10 x 10 4 m 3 D, the final recoverable reserve of a single well is 0.2 multiplied by 10 8 -0.4×10 8 m 3 Increased to 0.8 × 10 8 m 3 The above.
Claims (10)
1. A temperature-reducing and brittleness-increasing multi-step fracturing method comprises the steps that in each fracturing, a stratum is repeatedly fractured by n steps with low-temperature fluid to form a fracturing fracture network in a reservoir layer, and the fracturing fracture network is propped by fracturing propping agents after fracturing is finished.
2. The method of claim 1, wherein the heat capacity of the cryogenic fluid satisfies the following equation (1):
wherein, C f Is the heat capacity of the cryogenic fluid, J/g; v f Volume of fracturing fluid, m, injected into the formation 3 ；ρ f Density of fracturing fluid to be injected into formation, t/m 3 ；C r Is the heat capacity of the reservoir, J/g; v r For reservoir cooling volume, m 3 ；ρ r Is the reservoir density, t/m 3 ；T r0 Is the reservoir initial absolute temperature, K; t is a unit of r1 The absolute temperature of the reservoir after temperature reduction, K; t is f0 Is the initial absolute temperature of the fracturing fluid, K; t is f1 Absolute temperature of fracturing fluid after heat absorption, K.
3. The method of claim 2, wherein the cryogenic fluid is selected from liquid nitrogen or liquid carbon dioxide.
4. A method according to any one of claims 1 to 3, wherein the method comprises the steps of:
(1) Fracturing the reservoir with the low-temperature fluid, closing the 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 and cooling in the step (1) are finished, continuing fracturing the reservoir with the low-temperature fluid, and then closing the well and cooling;
(3) Repeating the step (2) until fracturing is completed;
(4) And adding a fracturing propping agent to prop a fracturing fracture network formed by fracturing.
5. The method of any one of claims 1-4, wherein the fracturing pressure of step (1) is greater than the maximum horizontal principal stress of the reservoir and at least 10% greater than the fracture pressure.
6. The method of claim 4 or 5, wherein the fracture pressure of step (2) is at least 10% greater than the fracture pressure of step (1).
7. The method according to any one of claims 4 to 6, wherein the fracture pressure of the nth step of step (3) is at least 10% greater than the fracture pressure of the n-1 st step.
8. The method according to any one of claims 4 to 7, wherein the time for each shut-in cooling in step (3) is obtained according to the following equations (2) to (5):
establishing shale reservoir heat conduction equation near pitshaft according to reservoir characteristics (2)
Wherein T is reservoir absolute temperature, K; r is the distance from a certain point in the reservoir to the center of the shaft, m; alpha is reservoir thermal diffusivity, m 2 D; t is the time of closing the well and reducing the temperature, day;
initial conditions: t- ti＝0 ＝T di (3)
Wherein ti is the initial time of cooling in the step i, and d; t is di Absolute temperature of the cryogenic fluid at the initial moment of the cooling in the ith step, K;
inner boundary conditions:
wherein, lambda is the heat conductivity coefficient of the reservoir, W/(m ℃); rdfi is the radius of the equivalent radial interface where the reservoir is in contact with the cryogenic fluid, m; q. q.s dfi Heat absorbed after the cryogenic fluid is cooled, J;
outer boundary conditions: t is critical to r＝∞ ＝T 0 (5)。
9. The method according to any one of claims 2 to 8, wherein n is 3 to 5.
10. The method of any one of claims 1 to 9, wherein the reservoir is a shale reservoir.
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| US20080087426A1 (en) * | 2006-10-13 | 2008-04-17 | Kaminsky Robert D | Method of developing a subsurface freeze zone using formation fractures |
| US20180266227A1 (en) * | 2015-06-03 | 2018-09-20 | Geomec Engineering Ltd. | Thermally Induced Low Flow Rate Fracturing |
| CN105134158A (en) * | 2015-08-26 | 2015-12-09 | 中国石油天然气股份有限公司 | Fracturing method for supplementing stratum energy of dense oil reservoir |
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