CN112343589B - Test method for researching interaction mechanism of hydraulic fracturing complex fracture network microcracks - Google Patents
Test method for researching interaction mechanism of hydraulic fracturing complex fracture network microcracks Download PDFInfo
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Abstract
The invention discloses a test method for researching interaction mechanism of hydraulic fracturing complex fracture network microcracks. According to the research method, shale is used as a test research object, small-size perforation is installed in a shale sample to construct the micro-cracks at the tips of the hydraulic fracturing, the growth mode of the micro-crack network of the hydraulic fracturing along with time change under different confining pressure conditions is obtained by means of a CT real-time scanning technology, and the interaction mechanism among the micro-cracks and the influence of the gap distance, construction displacement, fracturing fluid viscosity and the like on the effect among the micro-cracks of the crack network are researched. The research method has important theoretical value and economic value for analyzing key factors influencing extension and penetration of the hydraulic fracturing microcracks, disclosing interaction mechanism of the complex fracture network microcracks and designing actual production fracturing parameters.
Description
Technical Field
The invention relates to the technical field of engineering construction, in particular to a test method for researching interaction mechanism of hydraulic fracturing complex fracture network microcracks.
Background
Shale gas reservoirs have low porosity and low permeability characteristics, generally have no natural energy production, and can be efficiently utilized and developed only by carrying out large-scale block integral fracturing. The development degree of microcracks of different shale gas reservoirs and the contents of brittle minerals and clay are different, so that the compressibility of the shale gas reservoirs is greatly different. The shale gas fracturing adopts low-viscosity water-reducing water, so that the filtration loss is increased, natural fractures are communicated as many as possible, and a complex fracture network structure is formed in a reservoir. The more complex the fracture, the larger the reservoir modification volume (SRV), and the higher the well productivity after fracturing. Therefore, the optimization of a proper process and the increase of the complexity of the cracks have important significance for the efficient exploitation and development of the shale gas.
Under certain circumstances, complex fracture networks may be formed. Complex fracture networks may include induced hydraulic fractures and natural fractures that may or may not intersect in multiple planes and directions along multiple azimuth angles, and in multiple zones. In the fracturing process, the original stress field is disturbed through technical measures such as staged fracturing, repeated fracturing and the like, so that the fracture initiation and expansion are not simple and only have tensile damage, and complex mechanical behaviors such as shearing, sliding, dislocation and the like also exist, and further, the multi-class and multi-stage fracture embodiment such as radial expansion, peripheral tension, shearing fracture and the like is formed. As shown in fig. 2.
The hydraulic fracturing technology is to inject fracturing fluid into underground reservoirs at a certain speed and pressure to generate cracks or fractures in target layers to form complex fracture networks. In unconventional oil and gas reservoirs, hydraulic fracturing usually needs to form complex network structure cracks with a certain range to obtain ideal capacity, and the final shape of the complex network depends on the interaction of micro cracks between the network. The mechanism of this interaction is currently relatively poorly studied.
Disclosure of Invention
Aiming at the defects of the prior art, the invention aims to provide a test method for researching the interaction mechanism of the complex fracture network microcracks in hydraulic fracturing. Current hydraulic fracture monitoring methods and systems can map the location where the fracture occurs and the extent of the fracture. The invention starts from the initiation and expansion essence of hydraulic fracturing cracks, and researches the interaction mechanism of complex crack network micro-cracks near the perforation and the influence of the fracture pressure and the crack expansion rule by adopting the CT real-time scanning technology.
In order to achieve the purpose, the test method for researching the interaction mechanism of the micro-cracks of the hydraulic fracturing complex fracture network provided by the invention comprises the following steps:
1) Selecting a shale rock sample;
2) Drilling holes, and setting different perforation intervals;
3) Injecting water and fracturing under the conditions of different perforation intervals and different fracturing sequence combinations; high water pressure and low flow are required to be injected into the hole; the complex network generated varies over time the growth pattern of the hydraulic fracture, the generating comprising: extending hydraulic fracturing fractures from a wellbore and into a fracture network of a subterranean formation to form a hydraulic fracture network comprising natural fractures and hydraulic fracturing fractures; determining hydraulic fracture parameters of the hydraulic fracture after the extension;
4) When water is injected, the change of the internal cracks of the rock sample is recorded by CT real-time scanning:
CT real-time scanning, wherein in the water injection fracturing process, the crack propagation trace is comprehensively tracked, the change of the microcracks in the rock sample is recorded, and the process under the combined condition of different fracturing sequences and different perforation interval arrangements is researched; the process comprises merging, accumulating, communicating, collecting and cracking;
5) Analyzing the influences of the gap between the cracks, the construction discharge capacity and the viscosity of the fracturing fluid on the expansion behavior of the cracks by combining the shapes of the cracks after the fracturing;
6) And describing the interaction mechanism of the complex seam network microcracks according to the CT real-time image.
Preferably, the method also comprises 7) if the hydraulic fracturing fracture meets other fractures, determining the cross characteristics at the other encountered fractures and setting forth a fracture network microcrack interaction mechanism.
Furthermore, the hydraulic fracturing is performed by adopting high water pressure and low flow, and micro cracks in the rock sample are shot in real time by CT scanning; the hydraulic fracturing modes comprise different fracturing sequences and hole dividing modes.
Still further, the hydraulic fracture network has a fracture pressure greater than the stress acting on the encountered fracture along which the fracture growth mode is propagating.
Still further, the fracture growth mode continues to propagate along the encountered fracture until the end of the natural fracture is reached.
Still further, the fracture growth mode changes direction at the end of the natural fracture where the fracture growth mode extends in a direction perpendicular to the direction of the minimum stress.
Furthermore, CT scanning results of different rock samples under different crack spacing conditions are observed, and the influence of the crack spacing on stress interference between complex crack nets is analyzed; and (5) observing CT scanning results under different construction discharge capacity conditions, and analyzing the influence of the construction discharge capacity on the micro-fracture morphology of the complex fracture network.
The invention has the following advantages and beneficial effects:
at present, most of hydraulic fracturing experiments are focused on researching the initiation and fracturing of main hydraulic fracturing cracks, the initiation and propagation paths and the interaction and communication process and mechanism with natural weak surfaces, and systematic research has not been carried out on the interaction mechanism of micro cracks at the tips of a hydraulic fracturing network. The testing method provided by the invention can accurately obtain the form of the microcracks at the tips of the fractured fractures, research the interaction mechanism of geological parameters and the fracturing parameters on the microcracks, optimize the construction parameters for forming a complex fracture network, increase the complexity of the fractures, improve the yield of the unconventional oil and gas exploitation, and have important significance for promoting the commercial exploitation process of the unconventional oil and gas.
Drawings
FIG. 1 is a schematic illustration of hydraulic fracturing;
FIG. 2 is a hydraulic fracture network reconstruction mechanism;
FIG. 3 is a schematic view of a fracturing test piece;
fig. 4 is a schematic diagram of shale reservoir seam network formation.
Detailed Description
The technical solution of the present invention is further described in detail below with reference to the specific embodiments and the accompanying drawings.
The invention relates to a test method for researching interaction mechanism of hydraulic fracturing complex fracture network microcracks, which comprises the following specific steps;
1) And drilling holes, and setting different perforation intervals. As shown in fig. 3.
2) And water injection and fracturing require high water pressure and low flow (fracture initiation is guaranteed) to be injected into the hole.
Adopting different fracturing modes: and fracturing the perforation simultaneously, and fracturing another perforation after fracturing of one perforation is finished. The test sequence is simple and complex, the influence between the micro-crack interaction after the crack initiation and the expansion process of the perforation distribution fracturing crack is proved, and then the perforation and fracturing test is carried out.
Although rock mass stress near a drilling hole fluctuates during porous fracturing, obvious stress concentration does not exist, so that cracks in each fracturing section uniformly germinate and extend, similar crack nets can be generated in each fracturing section during porous fracturing, and the integral permeability of the rock mass is obviously increased.
Mechanical conditions for forming the seamed net: the mechanical conditions for natural fracture opening during fracturing were analyzed as shown in figure 4.
(1) The fracture net pressure required for the fracture to generate tensile fracture is as follows:
when in use
Sometimes with a maximum value of σ H -σ h 。
(2) The net fracture pressure required to produce shear fracture is:
when in use
Sometimes with a maximum value, maximum value P max Comprises the following steps: />
Natural fracture tau 0 =0, so that the maximum value is P when shear fracture occurs max ＝(σ H -σ h )。
The method can be used for opening the natural fracture to form the complex fracture when the net pressure in the fracture exceeds the maximum and minimum horizontal principal stress difference of the reservoir during construction.
The horizontal principal stress difference can be calculated by:
Δσ＝2σ h -P i -P f +S t (4)
in the formula: delta sigma is the horizontal principal stress difference, MPa; sigma h Is the minimum principal stress of the stratum, MPa; pi is the initial pore pressure of the stratum, MPa; p f Is the formation fracture pressure, MPa; s t Tensile strength of rock, MPa. The net pressure in the seam is mainly controlled by two aspects of reservoir characteristics and human factors, and the reservoir characteristics comprise: young modulus, poisson's ratio, horizontal and vertical formation stress, fracture toughness, etc. The human factors include:discharge capacity, fracturing fluid viscosity, friction resistance, average sand ratio, fracture plugging condition and the like.
3) And (3) CT scanning, wherein the change of the micro-cracks of the internal fracture network of the rock sample is recorded in real time while water injection fracturing is carried out.
4) And analyzing the influences of the seam spacing, the construction discharge capacity and the viscosity of the fracturing fluid on the crack form expansion behavior by combining the pressed crack form.
5) According to the CT real-time image, the interaction mechanism of the complex crack network micro-cracks near the perforation and the influence of the fracture pressure and the crack propagation rule are described.
Claims (7)
1. A test method for researching interaction mechanism of hydraulic fracturing complex fracture network microcracks is characterized in that: the method comprises the following steps:
1) Selecting a shale rock sample;
2) Drilling holes, and setting different perforation intervals;
3) Injecting water and fracturing under the conditions of different perforation intervals and different fracturing sequence combinations; high water pressure and low flow are required to be injected into the hole; the complex network generated varies over time the growth pattern of the hydraulic fracture, the generating comprising: extending hydraulic fracturing fractures from a wellbore and into a fracture network of a subterranean formation to form a hydraulic fracture network comprising natural fractures and hydraulic fracturing fractures; determining hydraulic fracture parameters for the hydraulic fracture after the extending;
4) When water is injected, the change of the internal cracks of the rock sample is recorded by CT real-time scanning:
CT real-time scanning, in the water injection fracturing process, comprehensively tracking a crack propagation trace, recording the change of the internal micro-cracks of the rock sample, and researching the process under the combined condition of different fracturing sequences and different perforation interval arrangements; the process comprises merging, accumulating, communicating, collecting and cracking;
5) Analyzing the influences of the gap between the cracks, the construction discharge capacity and the viscosity of the fracturing fluid on the expansion behavior of the cracks by combining the shapes of the cracks after the fracturing;
6) Describing the interaction mechanism of the complex seam network microcracks according to the CT real-time image;
also included is 7) if the hydraulic fracture encounters other fractures, determining cross-over characteristics at the other fractures encountered, elucidating the fracture network microfracture interaction mechanism.
2. The test method for researching the interaction mechanism of the hydraulic fracturing complex fracture network microcracks, according to claim 1, is characterized in that: the hydraulic fracture network has a fracture stress greater than the stress acting on the encountered fracture along which the fracture growth pattern propagates.
3. The test method for researching the interaction mechanism of the hydraulic fracturing complex fracture network microcracks according to claim 2, characterized in that: the fracture growth mode continues to propagate along the encountered fracture until the end of the natural fracture is reached.
4. The test method for researching the interaction mechanism of the hydraulic fracturing complex fracture network microcracks according to claim 3, characterized in that: the fracture growth mode changes direction at the ends of the natural fracture where it extends in a direction perpendicular to the direction of minimum stress.
5. The test method for researching the interaction mechanism of the hydraulic fracturing complex fracture network microcracks according to claim 1, 3 or 4, is characterized in that: observing CT scanning results of different rock samples under different crack spacing conditions, and analyzing the influence of the crack spacing on stress interference between complex crack nets; and (5) observing CT scanning results under different construction discharge capacity conditions, and analyzing the influence of the construction discharge capacity on the micro-fracture morphology of the complex fracture network.
6. The test method for researching the interaction mechanism of the hydraulic fracturing complex fracture network microcracks, according to claim 2, is characterized in that: observing CT scanning results of different rock samples under different crack spacing conditions, and analyzing the influence of the crack spacing on stress interference between complex crack nets; and (5) observing CT scanning results under different construction discharge capacity conditions, and analyzing the influence of the construction discharge capacity on the micro-fracture morphology of the complex fracture network.
7. The test method for researching interaction mechanism of hydraulic fracturing complex fracture network microfractures according to claim 1, 3, 4 or 6, characterized in that:
adopting different fracturing modes: simultaneously fracturing two perforations, and fracturing the other perforation after fracturing of one perforation is finished; the test sequence is simple and then complex, the influence between the micro-crack interaction after the crack initiation and the expansion process of the fracture distributed by the two perforations is proved, and then the test of simultaneous fracture of the two perforations is carried out;
during porous fracturing, rock stress near a drilling hole fluctuates but no obvious stress concentration exists, so that cracks in each fracturing section uniformly germinate and extend, during porous fracturing, similar crack nets can be generated in each fracturing section, and the integral permeability of the rock is obviously increased;
mechanical conditions for forming the seamed net: the mechanical conditions for natural fracture opening during fracturing were analyzed as follows:
(1) The fracture net pressure required for the fracture to generate tensile fracture is as follows:
when in use
Sometimes with a maximum value of σ H -σ h ；
(2) The net fracture pressure required to produce shear fracture is:
when in use
Sometimes with a maximum value, maximum value P max Comprises the following steps:
natural fracture τ 0 =0, so that the maximum value is P when shear fracture occurs max ＝(σ H -σ h )；
The method can obtain that when the net pressure in the fracture exceeds the maximum and minimum horizontal principal stress difference of the reservoir during construction, the natural fracture can be opened to form a complex fracture;
the horizontal principal stress difference can be calculated by:
Δσ＝2σ h -P i -P f +S t (4)
in the formula: delta sigma is the horizontal principal stress difference, MPa; sigma h Is the minimum principal stress of the stratum, MPa; pi is the initial pore pressure of the stratum, MPa; p f Is the formation fracture pressure, MPa; s t Tensile strength of rock, MPa; the net pressure in the seam is mainly controlled by two aspects of reservoir characteristics and human factors, and the reservoir characteristics comprise: young modulus, poisson's ratio, horizontal stress and vertical stress of stratum, fracture toughness; the human factors include: discharge capacity, fracturing fluid viscosity, friction resistance, average sand ratio and fracture plugging condition;
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