CN110470514B - Method for inducing cracks in low-permeability reservoir core through circulating water injection freeze thawing - Google Patents
Method for inducing cracks in low-permeability reservoir core through circulating water injection freeze thawing Download PDFInfo
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Abstract
The invention relates to a method for inducing cracks in a low-permeability reservoir core by circulating water injection, freeze thawing and the like. The invention belongs to the technical field of core analysis in the oil and gas field exploration and development technology, and provides a method for inducing cracks in a low-permeability reservoir core by circulating water injection, freeze thawing, which comprises the following steps: step 1: fracturing the rock core into a crack by adopting fracturing fluid; step 2: continuously introducing water with certain pressure into two ends of the crack until the water pressure of the water outlet end of the crack is stable; and step 3: freezing the rock core by using a refrigerant when water is continuously introduced into two ends of the crack, and freezing the water in the crack of the rock core; and 4, step 4: and unfreezing the rock core to an ice point state, and introducing water with certain pressure into the two ends of the crack again until the water pressure of the water outlet end of the crack is stable. The principle and means of the crack making method provided by the invention are similar to the principle and means of the actual exploitation of the hydraulic fracturing method of the crude oil and gas field, the core cracks are pressed and opened through high-pressure water injection, the generation mechanism of the natural cracks has reliable data reference value, and a credible reference basis is provided for the next research by adopting the crack cores in the subsequent actual production.
Description
Technical Field
The invention belongs to the technical field of core analysis in an oil and gas field exploration and development technology, and particularly relates to a method for inducing cracks in a core of a low-permeability reservoir by circulating water injection, freeze thawing and freezing.
Background
As the development of hypotonic and ultra-hypotonic oil and gas fields gradually becomes an important component of energy supply, the research on the exploration and development technology of the hypotonic and ultra-hypotonic oil and gas fields becomes more important; at present, the development of low-permeability ultra-low-permeability reservoirs in various oil and gas fields is mainly developed by a fracturing modification reservoir technology, so that the indoor research needs to perform crack formation on cores after the cores are obtained so as to simulate the fracturing modification measures of underground reservoirs, the research result is more consistent with the actual underground situation, the best reference data is provided for the actual exploitation, and the existing crack formation method of the cores of the indoor low-permeability reservoirs still has more problems;
for example, the current methods for indoor seam formation of natural cores are mainly divided into two types:
one is that the Brazilian split principle or rock compression principle is utilized to crush the rock through a Brazilian split instrument or a rock mechanics triaxial instrument;
the prior art includes: a manual seam making device for a low-permeability core is disclosed, and the patent application number is as follows: 201520646417.5, respectively; a manual core fracture-making device, application number 201720502213.3; core fracture preparation apparatus and method, application No. 201310231851;
according to the requirement on fracture characteristics, the rock core is cut in advance, a substance similar to a proppant is filled in a cut surface, and then the cut rock cores are combined into a complete fracture rock core;
the prior art includes: a natural core fracture-making method for simulating a tight oil reservoir artificial fracturing fracture, which is applied under the application number of 201710038179.3; a method for manufacturing an artificial fracture core, application number 201610566936. X;
in the process of implementing the embodiment of the invention, the inventor finds that at least the following defects exist in the background art:
both of these methods have significant disadvantages: the rock is broken to a large extent, and the fracture surface is mainly related to the stress property of the rock, so that the fracture surface can have a large difference with the fracture surface of the rock which is subjected to hydraulic fracture in the well; the formed cracks are determined according to manual wishes, irregularity and randomness of a hydraulic fracturing fracture surface cannot be reflected, the distribution and generation rules of the cracks are not consistent with the actual fracturing condition, and the filled proppant has larger instability and gives the closest reference data which are not consistent with the actual exploitation.
Disclosure of Invention
The invention provides a method for inducing cracks in a low-permeability reservoir core by circulating water injection, freezing and thawing, and aims to solve the problems that the degree of rock breakage is large when the rock is broken by a Brazilian split instrument or a rock mechanical triaxial instrument by using a Brazilian split principle or a rock compression resistance principle, and the fracture surface is mainly related to the stress property of the rock, so that the fracture surface possibly has a large difference with the fracture surface of the rock subjected to hydraulic fracture in a well; according to the requirements for fracture characteristics, the rock core is cut in advance, substances similar to proppant are filled in the cut surfaces, then the rock core is directly cut to form the fracture when the cut rock cores are combined into the complete fracture rock core, the formed fractures are determined according to manual wishes, irregularity and randomness of the hydraulic fracturing fracture surface cannot be reflected, the distribution and generation rules of the fractures are not in accordance with the actual fracturing condition, the filled proppant also has greater instability, and the method solves the problem that the most proximate reference data of actual mining are not met.
In order to achieve the purpose, the technical scheme adopted by the invention is as follows:
a method for inducing fractures in a low permeability reservoir core by circulating water injection and freeze thawing comprises the following steps:
step 1: fracturing the rock core into a crack by adopting fracturing fluid;
step 2: continuously introducing water with certain pressure into two ends of the crack until the water pressure of the water outlet end of the crack is stable;
and step 3: freezing the rock core by using a refrigerant when water is continuously introduced into two ends of the crack, and freezing the water in the crack of the rock core;
and 4, step 4: and unfreezing the rock core to an ice point state, and introducing water with certain pressure into the two ends of the crack again until the water pressure of the water outlet end of the crack is stable.
The step 4 is followed by:
and 5: continuously repeating the step 3 and the step 4 until the water pressure at the water outlet end of the crack is zero when water is supplied to the two ends of the crack;
the method also comprises the following steps between the step 3 and the step 4:
step 3.1: and injecting gas into two ends of the core crack, and stopping injecting the gas after the gas pressure at the two ends of the crack is stable.
Step 1 is preceded by:
step a: the method comprises the steps of pretreating a rock core, installing the pretreated rock core into a rock core holder, specifically, drilling holes at two ends of the rock core, installing the rock core into the rock core holder, embedding high-pressure pipelines in the drilled holes at the two ends, and fixing ports of the high-pressure pipelines in the drilled holes of the rock core by using cured resin.
In the step 3, the core is frozen by using a refrigerant, specifically, the core is frozen by using liquid nitrogen.
In the step 2, water with certain pressure is continuously introduced into the two ends of the crack until the water pressure at the water outlet end of the crack is stable, specifically:
and continuously introducing water with certain pressure containing the proppant into two ends of the crack, and continuously increasing the pressure of the water with certain pressure until the water pressure at the water outlet end of the crack is stable, wherein the temperature of the water is close to the freezing point.
A core holder, comprising:
the core-removing device comprises a shell, a core-removing device and a core-removing device, wherein the shell is provided with an inner cavity for installing a core, the side surface of the shell is provided with an inner cavity outlet which communicates the inner cavity with the outside, the shell is of a hollow structure, a hollow cavity is arranged in the shell, and the hollow cavity is provided with at least one inlet;
the plugs are arranged at two ends of the shell and used for sealing the inner cavity;
at least two high-pressure water injection pipes extend into the inner cavity from the plugs at the two ends of the shell;
a pump;
the outlet of the pump is connected with the inner cavity through a high-pressure water injection pipe.
The hollow cavity is provided with a hollow cavity inlet and a hollow cavity outlet, the hollow cavity outlet is positioned on one side of the inner cavity outlet, and the hollow cavity inlet is positioned on the other side of the inner cavity outlet.
And the inner cavity is also internally provided with a curing resin for sealing the rock core and the high-pressure water injection pipe, and one section of the curing resin is attached to the plug.
The method comprises the following steps:
a method for inducing cracks in a low-permeability reservoir core by circulating water injection, freeze thawing and adopting a core holder.
The method has the advantages that the method for inducing the cracks in the inner chamber by adopting the circulating water injection freezing and thawing overcomes the problems in the prior method, namely the mechanical integrity of the rock core is influenced due to overlarge rock core crushing degree, the artificial intention of the fractured surface is dominant or is influenced by the rock structure mechanics, and the fracture rule is different from the actual hydraulic fracturing fracture rule; the principle and means of the crack making method provided by the invention are similar to the principle and means of the actual exploitation of the hydraulic fracturing method of the crude oil and gas field, the core cracks are pressed and opened through high-pressure water injection, the generation mechanism of the natural cracks has reliable data reference value, and a credible reference basis is provided for the next research by adopting the crack cores in the subsequent actual production.
Drawings
FIG. 1 is a schematic diagram of the overall construction of a core holder according to the present disclosure;
FIG. 2 is a schematic diagram of core encapsulation in a low permeability reservoir core cyclic water injection freeze-thaw induced fracture method.
FIG. 3 is a schematic diagram of fractures fractured by the method for freeze-thaw induced fracture of low permeability reservoir core by circulating water injection;
FIG. 4 is a schematic diagram of a finished core product in a low permeability reservoir core cyclic water injection freeze-thaw induced fracture method.
In the figure, 1, a housing; 2. a plug; 3. a pseudo core; 4. curing the resin; 5. a high-pressure water injection pipe; 6. a core; 7. a hollow cavity; 8. an outlet of the hollow cavity; 9. an inner cavity outlet; 10. a hollow cavity inlet; 11. a pump; 12. a first control valve; 13. a second control valve; 14. a computer; 15. a third control valve; 16. a fourth control valve; 17. a gas source; 18. a water source.
Detailed Description
It should be noted that, in the embodiments of the present invention, the terms referred to are:
the refrigerant is used for cooling the core, and any conventional refrigerant such as liquid nitrogen, liquid oxygen and liquid nitrogen can be used.
The curing resin 4 is used for connecting the water injection pipeline to the core, the protection scope of the curing resin is not limited by the name of the curing resin in the invention, and the curing resin can adopt the known technologies of curing agent, connecting glue and the like which are known in the prior art and can fixedly connect the water injection pipeline to the core, and the like, and the curing resin is in the protection scope of the invention.
The pump 11, in the present invention, the pump 11 is used for injecting high-pressure water into the core to fracture the fracture on the core, and for supplying water to both ends of the fracture to maintain the gap between the fractures after the fracture is fractured, the number of the pumps in the present invention may be one or more, if the pump can simultaneously pump gas and pump high-pressure water for fracturing the fracture on the core, the pump may be one, and it may also adopt a plurality of pumps, one for pumping gas and the other for pumping high-pressure liquid, of course, the specific design of the pump is a technical means commonly used by those skilled in the art, and will not be described in detail herein.
The scheme of the circulating water injection freeze-thaw induced fracture and the core holder of the core of the hypotonic reservoir provided by the embodiment of the invention is described in detail by several specific embodiments.
Example 1
The invention relates to a specific embodiment of a circulating water injection freeze-thaw induced fracture method for a core of a low-permeability reservoir, which comprises the following steps:
step 1: fracturing the rock core into a crack by adopting fracturing fluid;
step 2: continuously introducing water with certain pressure into two ends of the crack until the water pressure of the water outlet end of the crack is stable;
and step 3: freezing the rock core by using a refrigerant when water is continuously introduced into two ends of the crack, and freezing the water in the crack of the rock core;
and 4, step 4: and unfreezing the rock core to an ice point state, and introducing water with certain pressure into the two ends of the crack again until the water pressure of the water outlet end of the crack is stable.
In the embodiment, high-pressure water is injected into the rock core to fracture the rock core, liquid nitrogen, liquid oxygen or other refrigerants are adopted to freeze the rock core under the condition that the crack of the rock core is kept open, the water in the rock core is quickly frozen to support the crack, the crack opening can be further expanded through volume expansion, and after the rock core is frozen for a certain time, the refrigerants are removed to enable the rock core to start to melt;
when the core is melted, injecting high-pressure water into the core again, wherein the newly-injected high-pressure water can only flow along the hypotonic channel or the original fracture surface because the previous ice blocks the hypertonic channel, and the newly-injected high-pressure water can further enlarge the scale of the fracture along with the increase of the pressure;
and freezing again under the condition of keeping the new crack open, freezing again to block the high-permeability channel and the crack, thawing again after stabilizing for a period of time, and injecting high-pressure water to finish the experiment.
In the embodiment, the method overcomes the problems existing in the prior method, namely the mechanical integrity of the core is influenced due to overlarge core crushing degree, the artificial intention of the fracture surface is dominant or the fracture surface is influenced by the rock structure mechanics, and the fracture rule is different from that of the actual hydraulic fracturing; the principle and means of the crack making method provided by the invention are similar to the principle and means of the actual exploitation of the hydraulic fracturing method of the crude oil and gas field, the core cracks are pressed and opened through high-pressure water injection, the generation mechanism of the natural cracks has reliable data reference value, and a credible reference basis is provided for the next research by adopting the crack cores in the subsequent actual production.
The method overcomes the problems of the existing indoor crack making method by adopting the Brazilian splitting method and the rock triaxial mechanical experiment and the artificial crack core method, can form a crack network caused by hydraulic fracturing in the core, does not damage the mechanical integrity of the core, and does not influence the physicochemical property of the core, the crack forming mechanism is the same as the actual underground hydraulic fracturing forming mechanism, and the influence of human factors generated in the crack making process on subsequent related experimental researches of the core is avoided.
Example 2
Further, according to another embodiment of the method for freeze-thaw induced fracture of core of hypotonic reservoir by circulating waterflooding, the step 4 is followed by:
and 5: and (4) continuously repeating the step (3) and the step (4) until the water pressure at the water outlet end of the crack is zero when water is supplied to the two ends of the crack.
In the above embodiment, the water injection and freezing are repeated to both ends of the rock fracture until the size of the fracture reaches the maximum, and then the operation of inducing the fracture can be finished.
After the treatment of the steps, a fracture network caused by hydraulic fracturing is formed in the rock core, the mechanical integrity of the rock core is not damaged, the physical and chemical properties of the rock core are not influenced, and the influence of human factors generated in the fracture process on subsequent related experimental research of the rock core is avoided.
To better explain the design principle of the present invention, the following design ideas are briefly introduced:
firstly, the method aims at the low-permeability core to make a crack, and the two ends of the core are drilled and embedded into water injection pipelines and sealed by adopting the sealant, so that the channeling is not broken through prematurely when high-pressure water is injected, and the crack can be pressed by the high-pressure water. In addition, a matched experimental device is specially designed for carrying out cyclic water injection freeze thawing to induce cracks;
secondly, the mechanism of inducing the fracture by adopting the cyclic water injection freezing and thawing of the invention is that the core is fractured by injecting high-pressure water into the core and the core is frozen by liquid nitrogen under the state that the fracture of the core is kept open, the water in the core is rapidly frozen to support the fracture, the fracture opening can be further expanded by volume expansion, the liquid nitrogen is removed, injecting high-pressure water into the core again at the stage of the core melting, wherein the newly injected high-pressure water can only flow along the hypotonic channel or the original fracture surface because the previous ice blocks the hypertonic channel, and can further enlarge the scale of the fracture along with the pressure increase again, freezing again under the condition of keeping a new crack open, freezing again to block a high-permeability channel and the crack, thawing again after stabilizing for a period of time, and injecting high-pressure water until the scale of the crack reaches the maximum, thereby forming a fracture network caused by hydraulic fracturing in the rock core;
finally, the injection fluid adopted by the invention is distilled water, and can also be other functional fluids, such as acid, fracturing fluid carrying proppant or several fluids capable of reacting with each other, and the purpose of the functional fluids is finally to improve the seam making effect.
Example 3
Further, according to another embodiment of the method for freeze-thaw induced fracture of core of hypotonic reservoir by circulating waterflooding, the method further comprises the following steps between step 3 and step 4:
step 3.1: and injecting gas into two ends of the core crack, and stopping injecting the gas after the gas pressure at the two ends of the crack is stable.
In the embodiment, after water in a crack of the rock core is frozen, high-pressure gas is injected into the rock core 6, because a high-permeability channel in the rock core is blocked by ice, the gas is forced to flow along a low-permeability channel or the crack surface of the ice and the rock, after the gas is completely mixed, high-pressure water is injected into the rock core 6, and the water temperature at the moment is not too high and should be as close to the freezing point temperature as possible to avoid the injected water from accelerating the melting of original ice; the newly injected water is blocked by the ice in the high-permeability channel and can only flow along the channel communicated with the gas, the pressure of the newly injected water is continuously increased, and the original crack can be further opened.
Example 4
Further, in another embodiment of the method for freeze-thaw induced fracture of core of hypotonic reservoir by circulating waterflooding according to the present invention, before step 1, the method further includes:
step 0: the method comprises the steps of pretreating a rock core, installing the pretreated rock core into a rock core holder, specifically, drilling holes at two ends of the rock core, installing the rock core into the rock core holder, embedding high-pressure pipelines in the drilled holes at the two ends, and fixing ports of the high-pressure pipelines in the drilled holes of the rock core by using cured resin.
In the above-described embodiment of the present invention,
the core in the first step needs to be pretreated, and the method comprises the following steps:
core packaging: drilling holes at the left end and the right end of the rock core, embedding a high-pressure pipeline, filling by adopting compounded curing resin, and applying certain pressure to cure the resin;
as a preferred embodiment, the compounded curing resin adopts: the weight proportion of the epoxy resin/curing agent is 4/1;
the specification of the core is as follows: the core adopted by the invention is a low permeability reservoir core, and can be an underground coring, outcrop core and artificial core; the core size can be selected from a variety of experimental devices, including: 1 inch, 1.5 inches, 4 inches;
as an example, the number of the drilled holes is determined according to the size of the core or the requirement of the experiment on the number of cracks, and may be one or more, when the size of the core is 1 inch and 1.5 inches, the number of the drilled holes is one, when the size is 4 inches, the number of the drilled holes is more than one, the positions of the holes should be uniformly distributed on the end face of the core, the depth of the drilled holes is specifically determined according to the length of the core, and the deeper the drilled holes are, the better the sealing effect is, the higher the pressure of the injected water is, in the experimental device, the preferred drilling depth is: 3-10 cm; (ii) a
Installing a core model and connecting pipelines: placing a rock core in a rock core holder, ensuring that the rock core is placed in the center of the holder as far as possible, supplementing a false rock core when the rock core is short, screwing a plug, and fixing a rock core model; and connecting the high-pressure pipeline with a pump, preferably, the pump is a Quizix pump, the name of the instrument is: a high pressure precision displacement pump; instrument brand: quizix; the instrument model is as follows: a Q5000 series; the production area of the instrument: the united states; the manufacturer: chandler.
In the embodiment, after the core is accurately pretreated, the accuracy of the experiment is enhanced during the experiment.
Example 5
Further, in another embodiment of the method for freeze-thaw inducing fracture of core of hypotonic reservoir by circulating water injection, in the step 3, the core is frozen by using a refrigerant, specifically, by using liquid nitrogen.
In the above embodiment, liquid nitrogen is adopted for refrigeration, so that the refrigeration effect is good, the price is substantial, and the liquid nitrogen is stable, so that accidents can not be caused.
Example 6
Further, in step 2, water with a certain pressure is continuously introduced into two ends of the fracture until the water pressure at the water outlet end of the fracture is stable, specifically:
and continuously introducing water containing a propping agent and having certain pressure into two ends of the crack, and continuously increasing the water pressure at the two ends of the crack until the water pressure at the water outlet end of the crack is stable, wherein the temperature of the water is close to the freezing point.
In the above embodiment, when high-pressure water is injected into the core 6, the temperature of the water should not be too high, and should be as close to the freezing point temperature as possible, so as to prevent the injected water from accelerating the melting of the original ice, and the injected water can be used for re-fracturing the relatively low-permeability position in the core, and a new crack is generated at the relatively low-permeability position of the core.
Example 7
Referring to fig. 1 to 4, which are schematic diagrams illustrating an overall structure, a core encapsulation, a fracture and a fractured finished core product in an embodiment of a core holder according to the present invention, the core holder includes:
the core installing device comprises a shell 1, wherein the shell 1 is provided with an inner cavity for installing a core, the side surface of the shell 1 is provided with an inner cavity outlet 9, the inner cavity is communicated with the outside through the inner cavity outlet 9, the shell 1 is of a hollow structure, a hollow cavity is arranged in the shell 1, and the hollow cavity is provided with at least one inlet;
the plugs 2 are arranged at two ends of the shell 1, and the plugs 2 are used for sealing the inner cavity;
the high-pressure water injection pipes 5 are arranged, and at least two high-pressure water injection pipes 5 penetrate into the inner cavity from the plugs 2 at two ends of the shell 1;
a pump 11;
the outlet of the pump 11 is connected with the inner cavity through a high-pressure water injection pipe 5.
In the above embodiment, the core 6 is placed in the inner cavity of the core holder, the plug 2 is fixedly connected with the shells 1 at the two ends of the core holder, and optionally, the shells 1 and the plug 2 can be in threaded connection, bolted connection or inter-clamping connection of internal clamping blocks;
a common rock core 6 is fixed at the central position of the rock core holder so as to stabilize the whole device, and optionally, the plug 2 is an annular cylinder so as to facilitate the introduction of a high-pressure water injection pipe 5 on the rock core 6;
an inner cavity in the shell 1 and a hollow cavity in the middle of the shell 1 are completely separated, the hollow cavity is used for containing a refrigerant and used for freezing a rock core 6 in the inner cavity of the shell 1, and the hollow cavity is provided with at least one inlet and used for filling the refrigerant; the rock core 6 in the shell 1 is fractured by injecting water from two ends of the inner cavity of the shell 1, so that the inner cavity outlets 9 of the inner cavity in the shell 1 are arranged below the rock core 6, the number of the inner cavity outlets 9 is designed and arranged according to the length of the rock core, and one outlet is arranged at intervals of 10-20 cm, so that the rock core can be rapidly discharged after the fracture by injecting water is broken through; the high-pressure water injection pipes 5 at two ends of the rock core 6 in the rock core holder are respectively connected with the output end of the pump 11 after being connected with the valve.
When the core holder is used, holes are punched at two ends of the core 6 in advance, then the core 6 is installed in an inner cavity of the holder, the high-pressure water injection pipes 5 are inserted into the holes punched at two ends of the core 6, the plugs 2 are used for fixing the high-pressure water injection pipes 5 and the holes at two ends of the core 6 relatively, and assembling and fixing are completed.
Example 8
Further, referring to fig. 1, in another embodiment of a core holder according to the present invention, the hollow chamber has a hollow chamber inlet 10 and a hollow chamber outlet 8, the hollow chamber outlet 8 is located at one side of the chamber outlet 9, and the hollow chamber inlet 10 is located at the other side of the chamber outlet 9.
In the above embodiment, cavity 7 concrete function holds the coolant for the cooling rock core, and consequently cavity 7 has cavity entry 10 and cavity export 8, and when cooling rock core 6, pour into the coolant from cavity entry 10, and when needs unfreeze the rock core, discharge the coolant from cavity export 8 in, unfreeze rock core 6 fast, because cavity export 8 is the coolant of discharging, because gravity reason, cavity export 8 needs establish same one side with cavity export 9.
Example 9
Further, referring to fig. 1, according to another embodiment of the core holder of the present invention, a curing resin 4 for sealing the core and the high-pressure water injection pipe is further provided in the inner cavity, and a section of the curing resin 4 is attached to the plug 2.
In the above embodiment, as the cured resin 4 for sealing the core and the high-pressure water injection pipe, the function of the cured resin 4 is to fix the high-pressure water injection pipe 5 and the core 6, in order to make the high-pressure water injection pipe 5 and the core 6 more tightly fixed, the thickness of the cured resin 4 needs to be increased, the cured resin 4 is used for filling the gap between the plug 2 and the core 6, and the cured resin 4 with increased thickness makes the high-pressure water injection pipe 5 and the core 6 more tightly fixed.
Example 10
Referring to fig. 1, another embodiment of a method for freeze-thaw induced fracture by circulating waterflooding of a core of a hypotonic reservoir according to the present invention includes:
example 1-example 6 any of the core holders of examples 7-9 were used in a method of freeze-thaw inducing fracture in a core of a hypotonic reservoir using cyclic waterflooding.
In the above embodiment, in order to make the core 6 more breakable during hydraulic fracturing, in this embodiment, the core holder does not need to apply confining pressure to the core, so the rubber sleeve of the conventional core holder is removed, and instead, the core holder is a hollow cavity of the casing 1, and the hollow cavity is mainly used for containing a coolant to freeze the core.
Example 11
Further, as shown in fig. 1, a pseudo core 3 is further arranged between the plug 2 and the core 6, and the pseudo core 3 is used for lengthening the core 6, so that the core 6 is more stable under the fixation of the plug 2; the high-pressure water injection pipes 5 at the two ends of the pump 11 are respectively provided with a first control valve 12 and a second control valve 13 which are used for controlling the high-pressure water injection pipes 5 at the two ends of the pump 11;
the inlet of the pump 11 is connected with a water source 18 and an air source 17 in parallel, a fourth control valve 16 is arranged between the water source 18 and the pump 11, and a third control valve 15 is arranged between the air source 17 and the pump 11;
pump 11 is a Quizix pump that is in electrical signal connection with computer 14.
The displacement pump is a high-precision Quizix pump, but the displacement pump is not limited to the high-precision Quizix pump, the higher the output pressure of the selected pump is, the more beneficial the pump is, and the highest pump pressure of the Quizix pump used by the invention can reach 90 MPa.
The design of the false core 3 facilitates the positioning of the core model in the core chamber or the supplement of the core length.
Example 12
The specific operation of the experiment is as follows:
core packaging: drilling holes at two ends of a rock core 6, embedding a high-pressure water injection pipe 5, filling by adopting compound curing resin 4 (epoxy resin: curing agent is 4:1) and curing under certain pressure;
installing a core model and connecting pipelines: placing the rock core 6 in a rock core holder, ensuring that the rock core 6 is placed in the middle as much as possible, supplementing a false rock core 3 when the length of the rock core 6 is not enough, and finally screwing a plug 2 to fix the rock core 6; connecting the high-pressure water injection pipe 5 with a Quizix pump 11;
thirdly, water injection and fracturing: closing the third control valve 15, opening the fourth control valve 16 and the first control valve 12 and the second control valve 13, pumping high-pressure water into the rock core 6 at a constant flow rate, recording the pumped pressure and flow rate by the computer 14, observing the change of the pressure and flow rate, and determining whether the fracture of the rock core 6 is pressed open; when the flow rate suddenly increases and the pressure suddenly decreases with the gradually increasing injection pressure of the Quizix pump 11, the core 6 fracture is opened, and high-pressure water is continuously injected at a constant pressure for keeping the fracture open. (Note: to ensure that the core 6 can be fractured at both ends, the valves at one end, such as the first control valve 12, are opened, the second control valve 13 at the other end is closed, the pressure P1 is recorded after the core at the end of the first control valve 12 is fractured, the first control valve 12 is closed, the second control valve 13 is opened to measure the pressure P2 after the core at the end of the second control valve 13 is fractured, finally the first control valve 12 and the second control valve 13 are opened simultaneously, high-pressure water is injected at the higher pressure of the pressure P1 and the pressure P2 when the first control valve 12 and the second control valve 13 are opened, the fractures at both ends are ensured to be opened simultaneously, and so on, when a plurality of high-pressure water injection pipes 5 for drilling exist, the high-pressure fluid injected into each drilling hole can be ensured to be opened in the same way.)
Fourthly, freezing by liquid nitrogen: the pump 11 continuously injects fluid into the rock core 6 at the pressure for keeping the crack open, liquid nitrogen is poured into the hollow cavity inlet 10 after the stability, water in the rock core 6 is rapidly frozen and frozen due to the cooling effect of the liquid nitrogen until the water in the high-pressure water injection pipe 5 is also frozen, at the moment, the flow rate of the pump 11 is 0, and water injection is stopped; the water in the core 6 is frozen and then plays a role of a propping agent, so that the crack of the core 6 is kept in an open state, and the frozen volume of the water expands to further expand the crack; keeping liquid nitrogen frozen for a certain time (1-2 hours), maintaining the opening state of the crack, and reducing the re-closing degree of the crack after ice melts;
fifthly, injecting water again after freeze thawing: after keeping freezing for a certain time, discharging liquid nitrogen to melt the rock core 6; at this time, the high-pressure water injection pipe 5 is heated to rapidly melt the ice in the high-pressure water injection pipe 5 and the ice at the end of the pipeline embedded in the core 6, then, the fourth control valve 16 is closed, the third control valve 15 is opened, the displacement pump 11 is started, and high-pressure gas is injected into the core 6; because the high-permeability channel in the rock core is blocked by ice, gas is forced to flow along the low-permeability channel or along the crack surface of the ice and the rock, after the gas is completely communicated, the third control valve 15 is closed, the fourth control valve 16 is opened, high-pressure water is returned again to be injected into the rock core 6, the water temperature at the moment is not too high (should be close to the freezing point as much as possible), and the injected water is prevented from accelerating the melting of the original ice. The newly injected water is blocked by the ice of the high-permeability channel and can only flow along the channel communicated with the gas, the pressure of the newly injected water is continuously improved, new cracks can be generated by observing the change of flow and pressure, and meanwhile, the original cracks are further opened so as to keep the pressure for opening the new cracks to continuously inject high-pressure water;
sixthly, freezing again: after the newly injected water presses open a new crack again or the original crack is further expanded, constant-pressure water injection is kept; then, pouring liquid nitrogen into the hollow cavity inlet 10 again, freezing the rock core 6 again, and freezing water in the rock core again to keep a new crack in an open state, wherein the frozen volume of the water expands to further expand the crack; continuously freezing for a certain time (1-2 hours), and repeating the fifth step and the sixth step again until no new crack is generated;
as an example, the failure to generate a new fracture means that the displacement pressure is too high to ensure the sealing integrity of the core package, or the core is completely fractured;
obtaining a fracture core section: after the freeze-thaw water injection process is cycled for multiple times, when the crack cannot be further expanded, stopping injecting water, discharging liquid nitrogen, taking out the core 6, and after ice is completely melted, cutting off hole sections at two ends of the core and the solidified resin 4, wherein the rest core 6 is a core section with the crack generated by high-pressure water injection;
as an illustration, the gas used in the experiment is one or a combination of nitrogen or air; the method has the main effects that after the core is frozen and begins to melt, the gas forms a new dominant channel, so that liquid is easier to inject in subsequent experiments, and liquid injection is avoided or the required hydraulic pressure is prevented from being too high;
as an illustration, the liquid used in the experiment is distilled water, and can also be various functional water;
as an application example, the functional water includes: adding a liquid of an acidic substance to water; the configuration is convenient for acid liquor to corrode the rock core in the water injection process, and cracks are more easily formed or the scale of the cracks is enlarged;
as an application example, the functional water includes: the liquid of the propping agent is added into the water, the water carrying the propping agent cannot flow after entering the rock core and being frozen, and the propping agent can be mostly left in the cracks of the rock core after being melted, so that the phenomenon that part of the propping agent is discharged due to the release of the pressure in the rock core during water injection fracturing is avoided, and the defect that the propping agent is less remained in the rock core is overcome;
as an application example, the functional water includes: injecting several different, but interactable fluids one after the other; for example, fluid a and fluid B may react to form a new substance which may have a beneficial effect on the seam (may act as a proppant), but such a substance is normally difficult to suspend in water and therefore fluid a may be injected first, fluid a frozen, fluid B injected again, and frozen, and the ice melted, and fluid a and fluid B react to form a new substance which may be beneficial to the seam;
the method overcomes the problems of the existing indoor crack making method by adopting the Brazilian splitting method and the rock triaxial mechanical experiment and the artificial crack core method, can form a crack network caused by hydraulic fracturing in the core, does not damage the mechanical integrity of the core, and does not influence the physicochemical property of the core, the crack forming mechanism is the same as the actual underground hydraulic fracturing forming mechanism, and the influence of human factors generated in the crack making process on subsequent related experimental researches of the core is avoided.
It should be noted that all the directional indications (such as up, down, left, right, front, and rear … …) in the present embodiment are only used to explain the relative positional relationship between the components, the movement, and the like in a specific posture (as shown in the drawings), and if the specific posture is changed, the directional indication is changed accordingly.
In addition, descriptions related to "first", "second", and the like are used for descriptive purposes only and are not to be construed as indicating or implying relative importance or implicit to the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one such feature.
Technical solutions between various embodiments may be combined with each other, but must be realized by those skilled in the art, and when the technical solutions are contradictory or cannot be realized, such a combination should not be considered to exist, and is not within the protection scope of the present invention.
Claims (9)
1. A method for inducing cracks in a core of a low-permeability reservoir through circulating water injection and freeze thawing is characterized by comprising the following steps of:
step 1: fracturing the rock core into a crack by adopting fracturing fluid;
step 2: continuously introducing water with certain pressure into two ends of the crack until the water pressure of the water outlet end of the crack is stable;
and step 3: freezing the rock core by using a refrigerant when water is continuously introduced into two ends of the crack, and freezing the water in the crack of the rock core;
and 4, step 4: and unfreezing the rock core to an ice point state, and introducing water with certain pressure into the two ends of the crack again until the water pressure of the water outlet end of the crack is stable.
2. The method for freeze-thaw inducing fracture in low permeability reservoir core by cyclic waterflooding according to claim 1, wherein the step 4 is followed by further comprising:
and 5: and (4) continuously repeating the step (3) and the step (4) until the water pressure at the water outlet end of the crack is zero when water is supplied to the two ends of the crack.
3. The method for freeze-thaw inducing fracture in low permeability reservoir core by cyclic waterflooding according to claim 1, further comprising between step 3 and step 4:
step 3.1: and injecting gas into two ends of the core crack, and stopping injecting the gas after the gas pressure at the two ends of the crack is stable.
4. The method for freeze-thaw induced fracture in low permeability reservoir core by cyclic waterflooding as claimed in claim 1, wherein the step 1 is preceded by:
step a: the method comprises the steps of pretreating a rock core, installing the pretreated rock core into a rock core holder, specifically, drilling holes at two ends of the rock core, installing the rock core into the rock core holder, embedding high-pressure pipelines in the drilled holes at the two ends, and fixing ports of the high-pressure pipelines in the drilled holes of the rock core by using cured resin.
5. The method for freeze-thaw inducing fracture in hypotonic reservoir core by cyclic waterflooding according to claim 1, wherein in the step 3, the core is frozen by using a refrigerant, specifically, liquid nitrogen.
6. The method for freeze-thaw inducing fracture in hypotonic reservoir core by means of circulating water injection according to claim 1, wherein in the step 2, water with a certain pressure is continuously introduced into two ends of the fracture until the water pressure at a water outlet end of the fracture is stable, specifically:
and continuously introducing water with certain pressure containing the proppant into two ends of the crack, and continuously increasing the pressure of the water with certain pressure until the water pressure at the water outlet end of the crack is stable, wherein the temperature of the water is close to the freezing point.
7. A method for inducing cracks in a core of a low-permeability reservoir through circulating water injection and freeze thawing is characterized by comprising the following steps of:
the method for inducing the fracture of the low permeability reservoir core by circulating water injection, freeze thawing and according to any one of claims 1 to 6, wherein a core holder is adopted;
the core holder comprises:
the core installing device comprises a shell (1), wherein the shell (1) is provided with an inner cavity for installing a core, an inner cavity outlet (9) is formed in the side surface of the shell (1), the inner cavity is communicated with the outside through the inner cavity outlet (9), the shell (1) is of a hollow structure, a hollow cavity is arranged in the shell (1), and the hollow cavity is provided with at least one inlet;
the plugs (2) are arranged at two ends of the shell (1), and the plugs (2) are used for sealing the inner cavity;
the high-pressure water injection pipes (5) are arranged, and at least two high-pressure water injection pipes (5) penetrate into the inner cavity from the plugs (2) at the two ends of the shell (1);
a pump (11);
the outlet of the pump (11) is connected with the inner cavity through a high-pressure water injection pipe (5).
8. The method for freeze-thaw inducing fracture in low permeability reservoir core by circulating waterflood as recited in claim 7, wherein the hollow cavity has a hollow cavity inlet (10) and a hollow cavity outlet (8), the hollow cavity outlet (8) is located at one side of the cavity outlet (9), and the hollow cavity inlet (10) is located at the other side of the cavity outlet (9).
9. The method for freeze-thaw inducing fracture by circulating water injection of the core of the hypotonic reservoir according to claim 7, wherein a cured resin (4) for sealing the core and the high-pressure water injection pipe is further arranged in the inner cavity, and one section of the cured resin (4) is attached to the plug (2).
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