CN113984531A - Experimental method and experimental device for researching influence of hydration on fracturing energy storage efficiency - Google Patents
Experimental method and experimental device for researching influence of hydration on fracturing energy storage efficiency Download PDFInfo
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- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N3/00—Investigating strength properties of solid materials by application of mechanical stress
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Abstract
The invention provides an experimental method and an experimental device for researching influence of hydration on fracturing energy storage efficiency, which comprises the following steps: injecting fracturing fluid into the core sample in a point injection mode at a variable injection rate so as to perform variable load fracturing on the core sample; storing energy for the core sample, so that the fracture part before hydration expands to the matrix part before hydration to store energy by self pressure; hydrating the core sample to enable hydration between the fracturing fluid and the core sample; injecting fracturing fluid into the core sample; and (3) storing energy for the rock core sample, so that the hydrated fracture part expands and stores energy to the hydrated matrix part by means of self pressure until the pressure of the hydrated fracture part and the pressure of the hydrated matrix part reach balance, and comparing the hydrated fracturing energy storage efficiency with the pre-hydrated fracturing energy storage efficiency to analyze the influence of the hydration on fracture expansion. The invention can continuously carry out fracturing, energy storage and hydration experiments and can be used for analyzing the influence of hydration on fracture expansion.
Description
Technical Field
The invention relates to the technical field of oil extraction, in particular to an experimental method and an experimental device for researching influence of hydration on fracturing energy storage efficiency.
Background
The permeability of an ultra-low permeability and compact reservoir is poor, an effective displacement gradient is difficult to establish by water injection, the stratum energy supplement efficiency is low, in the process of fracturing a blind well, on one hand, the stratum energy is supplemented by seepage and absorption of fracturing fluid, on the other hand, the fracturing fluid is in long-time contact with reservoir rock, the microstructure and the physical and mechanical properties of the rock are easy to change after interaction, the rock is softened and even damaged, and the water absorption rate dynamically changes along with time due to internal crack expansion, so that the stratum energy supplement efficiency is influenced. In order to research whether new cracks can be generated due to long-time interaction of the fracturing fluid and the rock, so that the imbibition is enhanced, the stratum energy supplement efficiency is improved, and a hydration experiment needs to be developed.
Hydration damage has a time effect, and experiments have a precondition that the contact time of the rock and the fluid is long enough to generate interaction so as to change the microscopic properties of the rock. Most of the existing hydration experiment methods directly soak a rock core sample by using a hydration liquid to cause the rock core sample to spontaneously seep and absorb, the action of a crack after fracturing is not considered, and fracturing, energy storage and hydration experiments cannot be continuously carried out; in addition, most of the existing hydration experiments are directed at shale, the hydration mechanism, namely the change of physical, chemical and mechanical actions of hydration on the properties of the shale is concerned, the research on whether microcracks can be induced by post-compression energy storage of ultra-low permeability and compact sandstone is insufficient, and the influence of the post-compression energy storage on the fracturing energy storage efficiency is not considered. Whether the hydration can promote the expansion of the compact sandstone fracture and further strengthen the energizing mechanism is not clear, and further research is still needed.
Disclosure of Invention
The invention aims to provide an experimental method and an experimental device for researching the influence of hydration on fracturing energy storage efficiency, which aim to solve the problems that the existing hydration experiment does not consider the action of fracturing cracks and can not continuously carry out fracturing, energy storage and hydration experiments, and simultaneously compare the energy storage efficiency before and after fracturing hydration and determine the influence of hydration on the fracturing energy storage efficiency.
In order to achieve the purpose, the invention provides an experimental method for researching the influence of hydration on the fracturing energy storage efficiency, which comprises the following steps: step S110: injecting fracturing fluid into a core sample in a point injection mode at a variable injection rate to perform variable load fracturing on the core sample until a preset fracturing length is reached, and stopping injecting the fracturing fluid, wherein the preset fracturing length is smaller than the whole length of the core sample, and the fractured core sample comprises a fracture part before hydration and a matrix part before hydration; step S120: storing energy for the core sample to enable the pre-hydration fracture part to expand and store energy to the pre-hydration matrix part by means of self pressure until the pressure of the pre-hydration fracture part and the pressure of the pre-hydration matrix part reach balance, recording the time required by stopping injecting the fracturing fluid until the pressure reaches balance in the step S110, the increment of the pressure and the pressure transmission range, and calculating the pre-hydration fracturing energy storage efficiency; step S130: hydrating the core sample to enable hydration between the fracturing fluid and the core sample until a preset hydration time is reached, wherein the hydrated core sample comprises a hydrated fracture part and a hydrated matrix part; step S140: carrying out oil washing and drying treatment on the hydrated rock core sample; step S150: injecting fracturing fluid into the hydrated rock core sample until a preset injection amount is reached, and stopping injecting the fracturing fluid; step S160: storing energy for the hydrated rock core sample to ensure that the hydrated fracture part expands and stores energy to the hydrated matrix part by means of self pressure until the pressure of the hydrated fracture part and the pressure of the hydrated matrix part reach balance, recording the time required by stopping injecting the fracturing fluid until the pressure reaches balance in the step S150, the increment of the pressure and the pressure transmission range, and calculating the fracturing energy storage efficiency after hydration; step S170: and comparing the post-hydration fracturing energy storage efficiency with the pre-hydration fracturing energy storage efficiency to analyze the influence of hydration on fracture propagation.
The experimental method for studying the influence of hydration on the fracturing energy storage efficiency is described above, wherein the preset injection amount in the step S150 is equal to the total amount of the fracturing fluid injected in the step S110.
The experimental method for studying the influence of hydration on the fracturing energy storage efficiency is described above, wherein the fracturing energy storage efficiency before hydration includes an increase of the pressure in the core sample in the step S120 in the unit time and a pressure transmission range in the unit time, and the fracturing energy storage efficiency after hydration includes an increase of the pressure in the core sample in the unit time and a pressure transmission range in the unit time in the step S160.
The experimental method for studying the influence of hydration on the fracturing energy storage efficiency is described as follows, wherein in the step S110, the fracturing fluid is injected into the core sample in a punctiform injection mode at a variable injection rate: injecting a fracturing fluid into the core sample in a point injection manner by alternately adopting a higher injection rate and a lower injection rate, wherein the higher injection rate is greater than the lower injection rate.
The experimental method for studying the influence of hydration on the fracturing energy storage efficiency is described above, wherein the step S110 includes: and in the process of injecting the fracturing fluid into the core sample in a point injection mode at a variable injection rate, acquiring a fracture expansion image in the core sample in real time by using an imaging system, and judging whether the fracturing length reaches the preset fracturing length or not according to the fracture expansion image.
The experimental method for studying the influence of hydration on the fracturing energy storage efficiency is described above, wherein the step S120 includes: in the process of storing energy of the core sample, acquiring a diffusion energy storage image in the core sample in real time by using an imaging system; the step S130 includes: in the process of hydrating the core sample, acquiring a hydration process image in the core sample in real time by using an imaging system; the step S160 includes: and in the process of storing the hydrated core sample, acquiring a diffusion energy storage image in the core sample in real time by using an imaging system.
The experimental method for studying the influence of hydration on the fracturing energy storage efficiency is described above, wherein the step S110 includes: under the condition of applying preset triaxial stress and preset formation temperature to the core sample, injecting fracturing fluid into the core sample after the crude oil is saturated in a punctiform injection mode at a variable injection rate; in the step S120, the step S130, and the step S160, the energy storage and hydration are performed on the core sample under the condition that the preset triaxial stress and the preset formation temperature are maintained.
The invention also provides an experimental device for researching the influence of hydration on the fracturing energy storage efficiency, which is used for the experimental method for researching the influence of hydration on the fracturing energy storage efficiency, and the experimental device comprises: the core holder is provided with a core cavity for accommodating a core sample, an inlet end and an outlet end which are respectively communicated with two opposite ends of the core cavity, and a sealing sheet arranged between the inlet end and the core sample, wherein the sealing sheet is provided with an injection hole for communicating the inlet end and the core cavity; the fracturing fluid injection system is connected with the inlet end and used for injecting fracturing fluid into the core sample in the core cavity through the injection hole; and the pressure monitoring system is connected with the inlet end and the outlet end and is used for monitoring the pressure difference between the fracture part and the matrix part of the rock core sample.
The experimental device for researching the influence of hydration on the fracturing energy storage efficiency further comprises an imaging system for acquiring the internal image of the rock core sample, wherein the imaging system is arranged outside the rock core holder.
The experimental device for researching the influence of hydration on the fracturing energy storage efficiency is characterized in that a confining pressure cavity and an axial pressure cavity are arranged in the rock core holder, the confining pressure cavity is arranged around the rock core cavity, and the axial pressure cavity is arranged corresponding to the end part of the rock core cavity; the experimental device further comprises a heating device for heating the core sample, and a confining pressure pump and an axial pressure pump which are arranged outside the core holder, wherein the confining pressure pump is communicated with the confining pressure cavity, and the axial pressure pump is communicated with the axial pressure cavity.
The experimental method and the experimental device for researching the influence of hydration on the fracturing energy storage efficiency have the characteristics and advantages that:
1. the invention adopts the variable injection rate to inject the fracturing fluid into the core sample in a punctiform injection mode, realizes the variable load non-through fracturing of the core sample, ensures that the fracturing length is controllable, realizes the fracturing of the local area of the core sample but not the whole core sample, does not form a through crack, avoids the instantaneous balance of the pressure at the outlet end and the inlet end, ensures that the fractured core sample comprises a crack part and a matrix part, and then continuously carries out energy storage and hydration experiments on the same core sample after the fracturing experiments, forms the effect of the mass transfer and pressure transfer of the crack and the matrix in the energy storage process, ensures that the fracturing fluid directly acts on the matrix through the crack in the hydration process, changes the rock property, compared with the prior art, the experimental method can more truly simulate the hydration working condition in the actual fracturing energy storage, and has more accurate experimental results;
2. according to the invention, on the basis of the fracturing energy storage experiment, the hydration experiment is continuously carried out, and the energy storage experiment is carried out again after the hydration experiment, so that the fracturing energy storage efficiencies required by the energy storage process before hydration and the energy storage process after hydration are respectively obtained, and the change of the energy storage efficiency caused by the change of rock properties due to hydration in the fracturing energy storage process can be fully simulated by comparing the two fracturing energy storage efficiencies, so that the influence of the hydration on the fracturing energy storage efficiency is analyzed;
3. by continuously developing fracturing, energy storage and hydration experiments, the invention ensures that the hydration experiment comprises the fracturing fracture as an influencing factor, ensures that the research on whether the micro fracture can be hydrated and induced in the post-pressure energy storage process of the ultra-low permeability and compact sandstone is more complete, is beneficial to explaining the interaction mechanism of fracturing fluid and a reservoir stratum and the mechanical mechanism influencing the ground stress in the hydration process, and provides an effective research method for judging whether the hydration can promote the energy storage efficiency.
Drawings
The drawings are only for purposes of illustrating and explaining the present invention and are not to be construed as limiting the scope of the present invention. Wherein:
FIG. 1 is a schematic flow chart of an experimental method for studying the influence of hydration on fracturing energy storage efficiency according to an embodiment of the invention;
FIG. 2 is a schematic flow diagram of an experimental process of one embodiment of the present invention prior to fracturing;
FIG. 3 is a schematic flow chart of an experimental method of one embodiment of the present invention after hydration and before injection of a fracturing fluid;
fig. 4 is a schematic structural diagram of an experimental apparatus for studying the influence of hydration on the fracturing energy storage efficiency according to an embodiment of the present invention.
Main element number description:
1. a core holder;
11. a core cavity; 12. an inlet end; 121. a first plug; 122. cushion blocks; 123. a first opening;
13. an outlet end; 131. an end cap; 132. a second plug; 133. a second opening;
14. a confining pressure cavity; 15. a shaft pressure chamber; 16. a housing; 17. sealing the sheet; 18. a rubber sleeve;
2. a pressure monitoring system;
21. a first pressure sensor; 22. a second pressure sensor; 23. a differential pressure sensor;
3. a liquid injection system; 31. a crude oil injection system; 32. a fracturing fluid injection system;
301. an injection pump; 302. an injection line; 303. an oil injection line; 304. injecting a fracturing fluid pipeline;
305. a first booster valve; 306. a first container; 307. a first communication valve; 308. a second pressurization valve;
309. a second container; 310. a second communication valve;
4. a pressure loading system; 41. a confining pressure pump; 42. an axial compression pump; 43. a first valve; 44. a second valve;
5. a back pressure device; 51. a back pressure pump; 52. a back pressure line; 53. a back pressure valve;
6. an imaging system; 7. a heating device.
Detailed Description
In order to more clearly understand the technical features, objects, and effects of the present invention, embodiments of the present invention will now be described with reference to the accompanying drawings. Where the terms "first", "second", etc. are used for descriptive purposes only and not to be construed as indicating or implying a relative importance or implicitly indicating the number of technical features indicated, the features defined as "first", "second", etc. may explicitly or implicitly include one or more of such features. In the description of the present invention, "a plurality" means two or more unless otherwise specified. In the description of the present invention, unless otherwise specified, the term "connected" is to be understood broadly, for example, it may be a fixed connection, a detachable connection, a direct connection, or an indirect connection via an intermediate medium, and it is obvious to those skilled in the art that the above terms are used in the patent in a specific sense.
Implementation mode one
In order to solve the problem that fracturing, energy storage and hydration experiments cannot be continuously carried out in the prior art, the invention provides an experimental method for researching the influence of hydration on fracturing energy storage efficiency, as shown in figure 1, the experimental method comprises the following steps:
step S110: and (3) fracturing process: injecting fracturing fluid into the core sample in a dotted injection mode at a variable injection rate to perform variable-load non-penetrating fracturing on the core sample until a preset fracturing length is reached, and stopping injecting the fracturing fluid, wherein the preset fracturing length is smaller than the whole length of the core sample, for example, the preset fracturing length is 1/3-1/2 of the whole axial length of the core sample, the fractured core sample comprises a pre-hydration fracture part and a pre-hydration matrix part, namely, all fractures in the core sample form a pre-hydration fracture part at the moment, and the core body part except the fractures is the pre-hydration matrix part;
step S120: energy storage process before hydration: storing energy for the core sample to ensure that the fracture part before hydration expands and stores energy to the matrix part before hydration by the pressure of the fracture part before hydration until the pressure of the fracture part before hydration and the pressure of the matrix part before hydration are balanced, recording the time required by automatically stopping injecting the fracturing fluid until the pressure reaches the balance, the increment of the pressure and the pressure transmission range, and calculating the fracturing energy storage efficiency before hydration;
step S130: and (3) hydration: continuing to store energy (equivalent to stewing) for the core sample, so that hydration occurs between the fracturing fluid and the core sample until a preset hydration time is reached, wherein the hydrated core sample comprises a hydrated crack part and a hydrated matrix part;
step S140: performing oil washing and drying treatment on the core sample;
step S150: injecting fracturing fluid into the core sample until a preset injection amount is reached, and stopping injecting the fracturing fluid;
step S160: and (3) energy storage process after hydration: storing energy (equivalent to braising) on the hydrated rock core sample to ensure that the hydrated crack part expands and stores energy to the hydrated matrix part by means of self pressure until the pressure of the hydrated crack part and the pressure of the hydrated matrix part reach balance, recording the time required by stopping injecting the fracturing fluid until the pressure reaches balance in the step S150, the increment of the pressure and the pressure transmission range, and calculating the fracturing energy storage efficiency after hydration;
step S170: and comparing the fracturing energy storage efficiency after hydration with the fracturing energy storage efficiency before hydration to analyze the influence of hydration on crack propagation.
The invention adopts the variable injection rate to inject the fracturing fluid into the core sample in a punctiform injection mode, realizes the variable load non-through fracturing of the core sample, ensures that the fracturing length is controllable, realizes the fracturing of the local area of the core sample but not the whole core sample, does not form a through crack, avoids the instantaneous balance of the pressure at the outlet end and the inlet end, ensures that the fractured core sample comprises a crack part and a matrix part, and then continuously carries out the energy storage experiment on the same core sample after the fracturing experiment to form the effect of mass transfer and pressure transfer of the crack and the matrix, but the existing fracturing method can not control the non-through fracturing of the core sample, and can not continuously carry out the fracturing and energy storage experiments, therefore, compared with the prior art, the experimental method can more truly simulate the actual fracturing energy storage working condition, and has more accurate experimental result.
In addition, the hydration experiment is continuously carried out on the basis of the fracturing energy storage experiment, the energy storage experiment is carried out again after the hydration experiment to obtain the fracturing energy storage efficiency respectively needed by the energy storage process before hydration and the energy storage process after hydration, the influence of the hydration on the crack expansion can be analyzed by comparing the two fracturing energy storage efficiencies, for example, if the fracturing energy storage efficiency after hydration is shorter than the fracturing energy storage efficiency before hydration, the hydration can induce new cracks (microcracks), otherwise, the hydration under the experimental condition can not induce the new cracks, the experimental condition needs to be changed (such as prolonging the hydration time, and the like), and further research is carried out.
The method comprises the following steps that fracturing fluid is injected into a core sample in a point injection mode, so that the core sample is fractured in a core holder; by adopting a variable load fracturing mode, the fatigue of the rock is accelerated, the damage of the rock is accelerated, the generation of cracks is accelerated, and the fracturing efficiency is improved.
By continuously developing fracturing, energy storage and hydration experiments, the invention ensures that the hydration experiments comprise the fracturing fracture as an influencing factor, so that the research on whether the energy storage can induce the microcracks after the fracturing of the ultra-low permeability and compact sandstone is more complete, is beneficial to explaining the interaction mechanism of fracturing fluid and a reservoir and the mechanical mechanism influencing the ground stress in the hydration process, and provides an effective research method on whether the energy storage efficiency can be improved by the hydration action.
For example, when the preset fracture length is 1/3 of the overall axial length of the core sample, the core part of 1/3 length is the modified zone, and the core part of the rest 2/3 length is the non-modified zone, wherein the fracture part and the matrix part reach pressure balance, that is, the fractures of the modified zone, the matrix of the modified zone and the matrix of the non-modified zone all reach pressure balance.
In an embodiment, the preset injection amount in step S150 is equal to the total amount of the fracturing fluid injected in step S110, that is, the injection amount of the fracturing fluid before and after hydration is equal.
Further, in order to ensure that the injection amount of the fracturing fluid is equal before and after hydration, in step S150, the fracturing fluid is injected into the hydrated core sample at a constant injection rate.
In one embodiment, the fracture energy storage efficiency before hydration includes an increase amount (increase amplitude) of the pressure in the core sample per unit time in step S120 and a pressure transmission range per unit time, and the fracture energy storage efficiency after hydration is an increase amount (increase amplitude) of the pressure in the core sample per unit time in step S160 and a pressure transmission range per unit time.
Because hydration may erode some minerals, increase pore channels or cause rock damage, resulting in new fractures, the post-hydration fracturing energy storage efficiency is likely to be greater than the pre-hydration fracturing energy storage efficiency.
In one embodiment, in step S110, the fracturing fluid is injected into the core sample in a point injection manner with a varying injection rate, where: and injecting the fracturing fluid into the core sample in a point injection mode by alternately adopting a higher injection rate and a lower injection rate, wherein the higher injection rate is greater than the lower injection rate.
The upper and lower limits of the injection rate are related to the properties of the rock core used in the experiment, variable load fracturing is that cracks are generated by fatigue damage, and the ranges of the higher injection rate and the lower injection rate can be determined according to rock fatigue damage rock mechanics experiments of different rock core samples.
Preferably, the higher injection rate is 5mL/min to 10mL/min and the lower injection rate is 0.5mL/min to 0.8 mL/min. Wherein higher injection rate and lower injection rate all belong to less discharge capacity, realize little discharge capacity, variable load fracturing, more do benefit to control fracturing length, prevent to form and run through the crack.
In one embodiment, step S110 includes: and in the process of injecting the fracturing fluid into the core sample in a point injection mode at a variable injection rate, acquiring a fracture expansion image in the core sample in real time by using an imaging system so as to judge whether the fracturing length reaches the preset fracturing length or not through the fracture expansion image. The imaging system is, for example, a CT scanner.
In one embodiment, step S120 includes: and in the process of storing energy for the core sample before hydration, acquiring a diffusion energy storage image in the core sample in real time by using an imaging system. Step S160 includes: and in the process of storing energy for the core sample after hydration, acquiring a diffusion energy storage image in the core sample in real time by using an imaging system. In this embodiment, for example, the imaging system is a CT scanner or a nuclear magnetic system, and the nuclear magnetic system may display the oil-water distribution in the core sample.
In the embodiment, the diffusion energy storage image in the core sample is obtained in real time in the energy storage process, so that a mechanism for analyzing the diffusion energy storage from the crack part to the matrix part through the diffusion energy storage image is facilitated, and the influence of hydration on the diffusion energy storage can be analyzed and obtained by comparing the diffusion energy storage image in the energy storage process before hydration and after hydration.
In one embodiment, step S130 includes: in the process of hydrating the core sample, an imaging system is adopted to obtain an image of the hydration process in the core sample in real time, so that whether a new crack is generated in the hydration process can be visually observed through the image of the hydration process. In this embodiment, the imaging system is a CT scanner or a nuclear magnetic system, for example.
In one embodiment, step S110 includes: under the condition of applying preset triaxial stress and preset formation temperature to the core sample, injecting fracturing fluid into the core sample after saturated crude oil in a punctiform injection mode at a variable injection rate. In the steps S120, S130, S150 and S160, the energy storage and hydration are performed on the core sample under the condition that the preset triaxial stress and the preset formation temperature are maintained.
In the embodiment, the preset triaxial stress and the preset formation temperature are applied to the rock core sample in the fracturing process, the energy storage process before hydration, the hydration process and the energy storage process after hydration so as to simulate the formation stress condition and the formation temperature condition, and further improve the authenticity and the accuracy of the experiment.
In one embodiment, as shown in fig. 2, the experimental method further includes the following steps before step S110:
step S101: obtaining a core sample, for example, preparing the core sample by using the existing core preparation method, or sampling from the underground, for example, the core sample has a diameter of 5cm and a length of 7.5 cm-10 cm;
step S102: pretreating the core sample, including but not limited to washing oil and drying the core sample, and measuring basic physical parameters of the core, such as the mass, the length, the diameter, the permeability, the porosity and the like;
step S103: saturating formation water for a core sample, for example, saturating formation water after the core is vacuumized;
step S104: a centrifuge method is adopted to enable the core sample to reach the state of bound water, and the saturation of the bound water can be measured;
step S105: injecting the constant-pressure displacement crude oil into the core sample to saturate the core sample with the crude oil, that is, to make the pressure inside the core sample reach a preset initial pressure, where the preset initial pressure is equivalent to the initial formation pressure, and preferably, the step is also performed under the condition of applying a preset triaxial stress and a preset formation temperature to the core sample.
In one embodiment, as shown in fig. 3, the experimental method further includes the following steps between step S140 and step S150:
step S141: saturating formation water for a core sample, for example, saturating formation water after the core is vacuumized;
step S142: a centrifuge method is adopted to enable the core sample to reach the state of bound water, and the saturation of the bound water can be measured;
step S143: injecting the constant-pressure displacement crude oil into the core sample to saturate the core sample with the crude oil, that is, to make the pressure inside the core sample reach a preset initial pressure, where the preset initial pressure is equivalent to the initial formation pressure, and preferably, the step is also performed under the condition of applying a preset triaxial stress and a preset formation temperature to the core sample.
In the present invention, steps S140 to S143 belong to the treatment of the core sample after hydration, and are basically the same as the steps (steps S102 to S105) of the treatment of the core sample before hydration.
Second embodiment
As shown in fig. 4, the present invention further provides an experimental apparatus for studying an influence of hydration on fracturing energy storage efficiency, which is an experimental apparatus used in the experimental method for studying an influence of hydration on fracturing energy storage efficiency according to the first embodiment, the experimental apparatus includes a core holder 1, a fracturing fluid injection system 32, and a pressure monitoring system 2, the core holder 1 has a core cavity 11 for accommodating a core sample, an inlet end 12 and an outlet end 13 respectively communicated with two opposite ends of the core cavity 11, and a sealing sheet 17 disposed between the inlet end 12 and the core sample, the sealing sheet 17 is provided with an injection hole communicating the inlet end 12 and the core cavity 11, the fracturing fluid injection system 32 is connected with the inlet end 12 and is configured to inject fracturing fluid into the core sample in the core cavity 11 through the injection hole at a variable injection rate, that is, the fracturing fluid injection system 32 adopts a variable load fracturing manner, injecting fracturing fluid into the core sample, wherein a crack is initiated from the end part of the core sample close to the inlet end 12, and when the preset fracturing length is reached, stopping injecting the fracturing fluid to obtain a modified core part close to the inlet end 12 and provided with the crack and an unmodified core part in a closed state (not penetrated by the crack) at the outlet end 13, so as to execute the step S110 and the step S150; the pressure monitoring system 2 is connected to the inlet end 12 and the outlet end 13, and is configured to monitor a pressure difference between the fracture part and the matrix part of the core sample, so as to monitor whether the fracture part and the matrix part reach a pressure balance in real time during the energy storage before hydration in step S120 and the energy storage after hydration in step S160.
In one embodiment, as shown in fig. 4, the pressure monitoring system 2 comprises a first pressure sensor 21 connected to the inlet end 12 and a second pressure sensor 22 connected to the outlet end 13, the two pressure sensors respectively indicating the pressure at the inlet end 12 and the pressure at the outlet end 13; and/or, pressure monitoring system 2 includes differential pressure sensor 23 of connecting between entry end 12 and exit end 13, and differential pressure sensor 23 can show the pressure differential between entry end 12 and exit end 13 in real time, and differential pressure sensor 23's precision is higher, can improve the experiment accuracy. During the experiment, can judge whether crack part and matrix part reach pressure balance through the difference of the readings of two pressure sensor, or directly through the reading of differential pressure sensor 23, can also monitor whether the pressure of entry end drops in the fracturing process in addition for judge whether to extrude the crack.
In one embodiment, as shown in fig. 4, the experimental apparatus includes a liquid injection system 3, the liquid injection system 3 includes an injection pump 301, an injection line 302 connected to the inlet end 12, and an injection line 303 and a fracturing fluid injection line 304 arranged in parallel between the injection pump 301 and the injection line 302; a first pressurizing valve 305, a first container 306 and a first communicating valve 307 are sequentially arranged on the oil injection pipeline 303 along the liquid flow direction, the first container 306 is used for containing crude oil, a second pressurizing valve 308, a second container 309 and a second communicating valve 310 are sequentially arranged on the fracturing fluid injection pipeline 304 along the liquid flow direction, the second container 309 is used for containing fracturing fluid, the injection pump 301, the oil injection pipeline 303, the first pressurizing valve 305, the first container 306 and the first communicating valve 307 form a crude oil injection system 31 for injecting crude oil into the core sample, the injection pump 301, the fracturing fluid injection pipeline 304, the second pressurizing valve 308, the second container 309 and the second communicating valve 310 form a fracturing fluid injection system 32 for injecting fracturing fluid into the core sample in the fracturing step of fracturing before fracturing (step S110) and the step of after fracturing (step S150), wherein the injection pump 301 can control the injection pressure and the injection rate, for example, the injection pump 301 is a plunger pump, preferably an ISCO plunger pump, typically with a maximum injection pressure not exceeding 10 MPa.
When the core sample needs to be saturated with crude oil, the first communication valve 307, the second communication valve 310 and the second pressurization valve 308 are closed, the first pressurization valve 305 is opened, the injection pump 301 is used for enabling the pressure in the first container 306 to reach a preset initial pressure which is equivalent to the initial formation pressure, then the first communication valve 307 is opened, and the crude oil is driven by the injection pump 301 to be driven at a constant pressure to be driven to enter the core sample until the pressure in the core sample reaches the preset initial pressure.
When the fracturing fluid needs to be injected into the core sample, the first communication valve 307, the second communication valve 310 and the first pressurization valve 305 are closed, the second pressurization valve 308 is opened, the pressure in the second container 309 reaches the preset fracturing injection pressure by using the injection pump 301, then the second communication valve 310 is opened, and the fracturing fluid is injected into the core sample until the preset fracturing length is reached.
The liquid injection system 3 in the embodiment can realize the switching between the crude oil injection and the fracturing liquid injection, and has the advantages of simple structure and convenient operation.
In one embodiment, as shown in fig. 4, a confining pressure cavity 14 and an axial pressure cavity 15 are arranged in the core holder 1, the confining pressure cavity 14 is arranged around the core cavity 11, the axial pressure cavity 15 is arranged corresponding to the end of the core cavity 11, and the confining pressure cavity 14 and the axial pressure cavity 15 are used for applying the preset triaxial stress to the core sample so as to simulate a formation stress condition, preferably, the axial pressure cavity 15 is located at the outlet end 13 of the core holder 1 instead of the inlet end 12; the experimental device further comprises a pressure loading system 4 arranged outside the core holder 1, the pressure loading system 4 comprises a confining pressure pump 41 and an axial pressure pump 42, the confining pressure pump 41 is communicated with the confining pressure cavity 14 and used for injecting liquid into the confining pressure cavity 14 so as to apply confining pressure to the core sample through the liquid, and the axial pressure pump 42 is communicated with the axial pressure cavity 15 and used for injecting liquid into the axial pressure cavity 15 so as to apply axial pressure to the core sample through the liquid. For example, the confining pressure pump 41 and the axial pressure pump 42 are plunger pumps, preferably ISCO plunger pumps, and a first valve 43 is provided between the confining pressure pump 41 and the confining pressure chamber 14, and a second valve 44 is provided between the axial pressure pump 42 and the axial pressure chamber 15 for control.
Further, as shown in fig. 4, the core holder 1 includes a housing 16, a first plug 121 fixed at one end of the housing 16, an end cap 131 fixed at the other end of the housing 16, and a cushion block 122 and a second plug 132 arranged in the housing 16, preferably, the first plug 121 is detachably and fixedly connected with the housing 16, the cushion block 122 abuts against the first plug 121, a core cavity 11 is formed between the cushion block 122 and the second plug 132, the second plug 132 is axially and movably arranged in the housing 16, an axial pressure cavity 15 is formed between the second plug 132 and the end cap 131, a confining pressure cavity 14 is formed between the housing 16 and a rubber sleeve 18 for wrapping a core sample in the core cavity 11, the cushion block 122 is in sealing engagement with the core sample in the core cavity 11 through a sealing sheet 17, the sealing sheet 17 is provided with an injection hole, for example, the diameter of the injection hole is 3mm, the injection hole communicates the inlet end 12 with the core cavity 11 to realize the punctiform injection of fracturing fluid through the injection hole, instead of face injection, the seal plate 17 seals the end of the pad 122 and the core sample, preventing the fracturing fluid from flowing between the pad 122 and the core sample.
In this embodiment, specifically, as shown in fig. 4, a first opening 123 sequentially passes through the first plug 121 and the spacer 122, the first opening 123 is connected and communicated with an injection hole on the sealing sheet 17, the first opening 123, the first plug 121 and the spacer 122 together form the inlet end 12 of the core holder 1, a second opening 133 sequentially passes through the end cover 131 and the second plug 132, and the second opening 133, the end cover 131 and the second plug 132 together form the outlet end 13 of the core holder 1.
In an embodiment, as shown in fig. 4, the experimental apparatus further includes a back pressure device 5, the back pressure device 5 includes a back pressure pump 51, a back pressure pipeline 52 communicating the back pressure pump 51 and the outlet end 13 of the core holder 1, and a back pressure valve 53 disposed on the back pressure pipeline 52, during an experiment, during crude oil saturation of a core sample, the back pressure valve 53 is kept open, and a back pressure is set by the back pressure pump 51, and the back pressure is equal to the preset initial pressure so as to simulate an initial formation pressure; during the fracturing and energy storage processes, the back-pressure valve 53 is kept closed to suppress the pressure of the core sample.
In an embodiment, as shown in fig. 4, the experimental apparatus further includes an imaging system 6 for obtaining an image of an inside of the core sample, the imaging system 6 obtains a fracture propagation image and a diffusion energy storage and hydration image in the core sample by scanning the core holder 1, the imaging system 6 is disposed outside the core holder 1, for example, the imaging system 6 is a CT scanner and/or a nuclear magnetic system, the CT scanner may be used during fracturing and hydration, and the nuclear magnetic system may be used during energy storage, so as to display oil-water distribution in the core sample through the nuclear magnetic system.
When a CT scanner is used as the imaging system 6, the core holder 1 is made of aluminum alloy or carbon fiber, so that X-rays are allowed to pass through the core holder 1, the CT scanner can scan and obtain structural images of different sections of a core sample in real time, the internal fracture form and the fracture extension condition of the core are monitored in fracturing and hydration experiments, and the processes of fluid loss from the fracture to a matrix part and energy storage through diffusion are monitored in a diffusion energy storage experiment.
In an embodiment, as shown in fig. 4, the experimental apparatus further includes a heating device 7, the heating device 7 is configured to apply the preset formation temperature to the core sample to simulate a formation temperature condition, the heating device 7 includes a heating pipe wound around the periphery of the core holder 1, a temperature sensor configured to measure a temperature of the core sample in the core cavity 11, and a temperature control module electrically connected to the heating pipe and the temperature sensor, the heating temperature is set by the temperature control module, the actual temperature is monitored by the temperature sensor in real time, the temperature control module receives a temperature signal sent by the temperature sensor, and controls a heating temperature of the heating pipe according to the temperature signal, so that the actual temperature of the core sample reaches the set temperature, and therefore the heating device 7 of this embodiment not only has a heating function, but also has a temperature control function. For example, the temperature sensor is arranged between the heating pipe and the outer wall of the core holder, and when the actual temperature of the core holder 1 reaches the set temperature and is stabilized for 3-5 hours, the temperature of the core sample is considered to reach the set temperature. Wherein the heating pipe can be a water bath heating pipe. However, the present invention is not limited thereto, and in other embodiments, other existing heating devices may be used.
The experimental device for researching the influence of hydration on the fracturing energy storage efficiency is adopted to carry out the fracturing energy storage hydration integrated experiment, for example, the experimental device comprises the following specific operation steps which are carried out in sequence:
a. preparing a full-diameter underground core;
b. washing oil and drying the core sample;
c. saturating formation water for a core sample;
d. enabling the core sample to reach a bound water state under the condition of formation temperature by adopting a centrifuge method, and simultaneously measuring the saturation of the bound water;
e. putting a core sample into a core cavity 11 of the core holder 1, and starting a heating device 7 to apply a preset formation temperature to the core sample so as to simulate the formation temperature; then, opening a first valve 43 and a second valve 44, and applying preset triaxial stress on the rock core sample through a confining pressure pump 41 and an axial pressure pump 42 so as to simulate three-dimensional stress under a triaxial condition; opening a back pressure valve 53, and applying a preset initial pressure to the rock core sample through a back pressure pump 51 to simulate an initial formation pressure;
f. saturated crude oil: closing the first communication valve 307 and the second communication valve 310, opening the first pressurizing valve 305, enabling the pressure in the first container 306 to reach the preset initial pressure by using the injection pump 301, then opening the first communication valve 307, driving crude oil to enter the core sample at constant pressure under the driving of the injection pump 301 until the pressure in the core sample reaches the preset initial pressure so as to simulate the initial formation pressure and stabilize for more than 24 hours, simultaneously monitoring the pressure by using the first pressure sensor 21 and the second pressure sensor 22, and then closing the first communication valve 307;
g. variable load fracturing: closing the back pressure valve 53, continuing to apply the preset triaxial stress and the preset formation temperature to the core sample, closing the first communication valve 307 and the second communication valve 310, opening the second pressurization valve 308, and enabling the pressure of the fracturing fluid in the second container 309 to reach a preset fracturing injection pressure by using the injection pump 301, wherein the preset fracturing injection pressure is less than the maximum pressure bearing of the core holder; then opening a second communication valve 310, injecting fracturing fluid into the core sample in a punctiform injection mode at a variable injection rate, for example, the initial injection rate is set to be 5 mL/min-8 mL/min, when the pressure reaches the preset fracturing injection pressure, changing the injection rate, gradually reducing the injection rate to 0.5 mL/min-0.8 mL/min, then gradually increasing the injection rate to 5 mL/min-10 mL/min, when the pressure reaches the preset fracturing injection pressure, changing the injection rate again, repeating the injection process, simultaneously monitoring the pressure change at an inlet end 12 and scanning the core sample in real time by using a CT scanner until the preset fracturing length is reached, and stopping injecting the fracturing fluid, wherein the core sample comprises a pre-hydration fracture part containing fractures and a pre-hydration matrix part containing no fractures;
h. diffusion energy storage before hydration: the second communication valve 310 is closed, the whole core system simulates the energy storage state after fracturing, the fracture part before hydration of the core sample transfers mass and pressure to the matrix part before hydration, the pressure of the fracture part before hydration gradually decreases and the pressure of the matrix part before hydration gradually increases along with the flowing of fracturing fluid from the fracture part before hydration to the matrix part before hydration, a CT scanner is adopted to scan the core sample in real time in the process, the pressure of the fracture part before hydration, the pressure of the matrix part before hydration and the pressure difference between the fracture part and the matrix part before hydration are monitored in real time by using the first pressure sensor 21, the second pressure sensor 22 and the pressure difference sensor 23 until the reading of the pressure difference sensor 23 is less than 30psi or the pressure reading of the second pressure sensor 22 does not change any more, which indicates that the fracture part before hydration and the matrix part before hydration reach pressure balance, and the pressure expansion energy storage process before hydration is finished, calculating the fracturing energy storage efficiency before hydration;
i. and (3) hydration: continuing to store energy for the core sample, so that hydration is generated between the fracturing fluid and the core sample until the preset hydration time is reached, wherein the hydrated core sample comprises a hydrated crack part and a hydrated matrix part, and scanning the core sample in real time by adopting a CT (computed tomography) scanner;
j. taking out the core sample, washing oil and drying the hydrated core sample, and treating the hydrated core sample with binding water, and then putting the core sample into the core holder again;
k. saturated crude oil: closing the first communication valve 307 and the second communication valve 310, opening the first pressurizing valve 305, enabling the pressure in the first container 306 to reach the preset initial pressure by using the injection pump 301, then opening the first communication valve 307, driving crude oil to enter the core sample at constant pressure under the driving of the injection pump 301 until the pressure in the core sample reaches the preset initial pressure so as to simulate the initial formation pressure and stabilize for more than 24 hours, simultaneously monitoring the pressure by using the first pressure sensor 21 and the second pressure sensor 22, and then closing the first communication valve 307;
m, injecting fracturing fluid: continuously keeping the preset triaxial stress and the preset formation temperature applied to the core sample, closing the first communication valve 307 and the second communication valve 310, opening the second pressurization valve 308, and enabling the pressure of the fracturing fluid in the second container 309 to reach a preset fracturing injection pressure by using the injection pump 301, wherein the preset fracturing injection pressure is less than the maximum bearing pressure of the core holder; then, opening a second communication valve 310, injecting fracturing fluid into the core sample at a constant injection rate, monitoring pressure change at an inlet end 12, scanning the core sample in real time by adopting a CT (computed tomography) scanner until a preset injection amount is reached, and stopping injecting the fracturing fluid;
n, diffusion energy storage after hydration: and (3) storing energy for the hydrated rock core sample, so that the hydrated fracture part expands and stores energy to the hydrated matrix part by means of self pressure until the pressure of the hydrated fracture part and the pressure of the hydrated matrix part reach balance, and recording the energy storage efficiency of the hydrated fracture.
The above description is only an exemplary embodiment of the present invention, and is not intended to limit the scope of the present invention. Any equivalent changes and modifications that can be made by one skilled in the art without departing from the spirit and principles of the invention should be considered within the scope of the invention. It should be noted that the components of the present invention are not limited to the above-mentioned whole application, and various technical features described in the present specification can be selected to be used alone or in combination according to actual needs, so that the present invention naturally covers other combinations and specific applications related to the invention.
Claims (10)
1. An experimental method for researching influence of hydration on fracturing energy storage efficiency is characterized by comprising the following steps:
step S110: injecting fracturing fluid into a core sample in a point injection mode at a variable injection rate to perform variable load fracturing on the core sample until a preset fracturing length is reached, and stopping injecting the fracturing fluid, wherein the preset fracturing length is smaller than the whole length of the core sample, and the fractured core sample comprises a fracture part before hydration and a matrix part before hydration;
step S120: storing energy for the core sample to enable the pre-hydration fracture part to expand and store energy to the pre-hydration matrix part by means of self pressure until the pressure of the pre-hydration fracture part and the pressure of the pre-hydration matrix part reach balance, recording the time required by stopping injecting the fracturing fluid until the pressure reaches balance in the step S110, the increment of the pressure and the pressure transmission range, and calculating the pre-hydration fracturing energy storage efficiency;
step S130: hydrating the core sample to enable hydration between the fracturing fluid and the core sample until a preset hydration time is reached, wherein the hydrated core sample comprises a hydrated fracture part and a hydrated matrix part;
step S140: washing the hydrated rock core sample with oil, drying and treating with saturated crude oil;
step S150: injecting fracturing fluid into the hydrated rock core sample until a preset injection amount is reached, and stopping injecting the fracturing fluid;
step S160: storing energy for the hydrated rock core sample to ensure that the hydrated fracture part expands and stores energy to the hydrated matrix part by means of self pressure until the pressure of the hydrated fracture part and the pressure of the hydrated matrix part reach balance, recording the time required by stopping injecting the fracturing fluid until the pressure reaches balance in the step S150, the increment of the pressure and the pressure transmission range, and calculating the fracturing energy storage efficiency after hydration;
step S170: and comparing the post-hydration fracturing energy storage efficiency with the pre-hydration fracturing energy storage efficiency to analyze the influence of hydration on fracture propagation.
2. The experimental method for studying the effect of hydration on the fracturing energy storage efficiency as claimed in claim 1,
the preset injection amount in the step S150 is equal to the total amount of the fracturing fluid injected in the step S110.
3. The experimental method for studying the effect of hydration on the fracturing energy storage efficiency as claimed in claim 2, wherein the fracturing energy storage efficiency before hydration comprises the increase of the pressure in the core sample in the step S120 in the unit time and the pressure transmission range in the unit time, and the fracturing energy storage efficiency after hydration comprises the increase of the pressure in the core sample in the unit time and the pressure transmission range in the unit time in the step S160.
4. The experimental method for studying the effect of hydration on the fracturing energy storage efficiency as claimed in claim 1, wherein in the step S110, the injecting the fracturing fluid into the core sample by using the varying injection rate in a punctiform injection manner is:
injecting a fracturing fluid into the core sample in a point injection manner by alternately adopting a higher injection rate and a lower injection rate, wherein the higher injection rate is greater than the lower injection rate.
5. The experimental method for studying the effect of hydration on the fracturing energy storage efficiency as claimed in claim 1,
the step S110 includes: and in the process of injecting the fracturing fluid into the core sample in a point injection mode at a variable injection rate, acquiring a fracture expansion image in the core sample in real time by using an imaging system, and judging whether the fracturing length reaches the preset fracturing length or not according to the fracture expansion image.
6. The experimental method for studying the effect of hydration on the fracturing energy storage efficiency as claimed in claim 1,
the step S120 includes: in the process of storing energy of the core sample, an imaging system is adopted to obtain a diffusion energy storage image in the core sample before hydration in real time;
the step S130 includes: in the process of hydrating the core sample, acquiring a hydration process image in the core sample in real time by using an imaging system;
the step S160 includes: and in the process of storing energy of the hydrated core sample, acquiring a diffusion energy storage image in the hydrated core sample in real time by using an imaging system.
7. The experimental method for studying the effect of hydration on the fracturing energy storage efficiency as claimed in claim 1,
the step S110 includes: under the condition of applying preset triaxial stress and preset formation temperature to the core sample, injecting fracturing fluid into the core sample after the crude oil is saturated in a punctiform injection mode at a variable injection rate;
in the step S120, the step S130, and the step S160, the energy storage and hydration are performed on the core sample under the condition that the preset triaxial stress and the preset formation temperature are maintained.
8. An experimental facility for researching the influence of hydration on the fracturing energy storage efficiency, which is used for the experimental method for researching the influence of hydration on the fracturing energy storage efficiency according to any one of claims 1 to 7, and comprises:
the core holder is provided with a core cavity for accommodating a core sample, an inlet end and an outlet end which are respectively communicated with two opposite ends of the core cavity, and a sealing sheet arranged between the inlet end and the core sample, wherein the sealing sheet is provided with an injection hole for communicating the inlet end and the core cavity;
the fracturing fluid injection system is connected with the inlet end and used for injecting fracturing fluid into the core sample in the core cavity through the injection hole;
and the pressure monitoring system is connected with the inlet end and the outlet end and is used for monitoring the pressure difference between the fracture part and the matrix part of the rock core sample.
9. The experimental device for studying the effect of hydration on fracturing energy storage efficiency as recited in claim 8, further comprising an imaging system for obtaining an image of the interior of the core sample, wherein the imaging system is disposed outside the core holder.
10. The experimental device for researching the influence of hydration on the fracturing energy storage efficiency as claimed in claim 8, wherein a confining pressure cavity and an axial pressure cavity are arranged in the core holder, the confining pressure cavity is arranged around the core cavity, and the axial pressure cavity is arranged corresponding to the end part of the core cavity;
the experimental device further comprises a heating device for heating the core sample, and a confining pressure pump and an axial pressure pump which are arranged outside the core holder, wherein the confining pressure pump is communicated with the confining pressure cavity, and the axial pressure pump is communicated with the axial pressure cavity.
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