CN112665987B - Device and method for testing tensile strength of rock core under confining pressure condition based on Brazilian splitting - Google Patents
Device and method for testing tensile strength of rock core under confining pressure condition based on Brazilian splitting Download PDFInfo
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Abstract
The invention discloses a device and a method for testing the tensile strength of a rock core under a confining pressure condition based on Brazilian splitting, wherein the device comprises a cylinder body, wherein the upper end and the lower end of the cylinder body are respectively connected with an upper pressure head and a lower pressure head; the cylinder body is internally provided with a guide cylinder, a rubber sleeve, a pressure rod penetrating through the upper pressure head and the guide cylinder, and a support rod penetrating through the lower pressure head, wherein the bottom end of the pressure rod and the top end of the support rod are inserted into the rubber sleeve; the bottom surface of the guide cylinder and the top surface of the support rod are cambered surfaces matched with the outer diameter of the core to be tested, and when the pressure rod descends to the bottom end of the stroke, an expanding part on the pressure rod is in contact with the top end of the guide cylinder; the bottom of the pressure lever is provided with a splitting cutter head; a closed annular space is formed between the rubber sleeve and the cylinder body, and a confining pressure inlet is formed in the cylinder body. The invention is used for solving the problems that the prior art can not apply confining pressure to the rock core to simulate the geological environment in the development of petroleum and natural gas, and the accurate positioning of the small-volume rock core is difficult to meet, so that the accuracy of the experimental result is insufficient, and the aims of accurately positioning the small-volume rock core and conveniently applying the confining pressure are realized.
Description
Technical Field
The invention relates to the field of petroleum and natural gas, in particular to a device and a method for testing tensile strength of a rock core under a confining pressure condition based on Brazilian splitting.
Background
In the field of petroleum and natural gas, the tensile strength of stratum rock is an important geological parameter, and is closely related to the stability of a well wall in a drilling process and the construction of hydraulic fracturing and the like in a yield increasing and transformation process. Tensile strength is a basic concept in rock mechanics courses, is an important mechanical property of rock, and is an important parameter for rock structure stability analysis. Rock is taken as a typical brittle material, the tensile strength is far less than the compressive strength, when the tensile stress on the rock is higher than the tensile strength of the rock, the rock is subjected to tensile failure, the failure is usually controlled by the tensile strength, and therefore, accurate measurement of the tensile strength of the rock has very important engineering significance for determining reasonable fracture pressure in a drilling process so as to ensure well wall stability and fracturing optimization design in an oil and gas exploitation process. The direct tensile test in the prior art is very difficult to test the tensile strength of the rock in operation, and the Brazilian splitting test is generally adopted internationally to test the tensile strength of the rock.
The existing Brazilian splitting test is realized by adopting a loading plate direct contact method, a pressing die or a filler strip, and the core theory of testing the tensile strength by utilizing the Brazilian splitting method is that a linear load which passes through the direction of a circle center is applied to a round sample, and horizontal tensile stress is generated at the circle center to cause the sample to be damaged, so theoretically, two requirements need to be met in the test: 1. the load applied to the sample by the pressurizing device is a linear load; 2. the center of the test sample and the direction of the vertical load applied by the pressurizing device are collinear, namely, the center is centered, the center crack of the test piece can be ensured, and the accuracy of the measuring result is ensured. However, the existing core brazilian splitting test in the field of oil and gas development has the following defects: (1) the existing Brazilian splitting experiment device cannot simulate the confining pressure environment under the stratum condition, cannot simulate the state that rock is in each side of pressure in the stratum depth, and cannot obtain the strength characteristic of the rock under the real pressure condition. The method is relatively effective for measuring the tensile strength of the rock mass with low or even no confining pressure on the ground or shallow engineering, such as civil engineering, tunnel mines and the like, but cannot meet the requirement of geological operation environment of thousands of meters at all in the field of petroleum and gas, so that the error of the obtained tensile strength is large, and an accurate basis is difficult to provide for engineering operation; (2) the Brazilian splitting requires splitting from the center of a rock sample, the existing Brazilian splitting experimental device clamps the rock sample through a pressing plate or a pressing die and splits through a cushion strip on the pressing plate or the pressing head (as shown in the attached figure 1), the rock sample is difficult to position, and the cushion strip is difficult to ensure to be exactly positioned at the center of the rock sample and meet the stress of line load; (3) different from the acquisition ways of samples of water conservancy, mountain mines, civil engineering and the like, the petroleum engineering samples come from the deep stratum, the stratum depth of oil and gas exploration is continuously increased along with the development of the international petroleum industry and the continuous increase of the demand on petroleum, the drilling scale of deep wells and ultra-deep wells is gradually enlarged, the coring of the stratum is extremely difficult, and the rock core is particularly in short supply and high in price. The disc size required by the conventional Brazilian splitting test tensile strength test sample cannot meet the requirement of increasingly developing petroleum engineering technology under the limitation of core conditions. In conclusion, the Brazilian splitting experiment device in the prior art is difficult to meet the requirement of the tensile strength test of the core in the field of petroleum and natural gas.
Disclosure of Invention
The invention provides a device and a method for testing tensile strength of a rock core under a confining pressure condition based on Brazilian splitting, aiming at solving the technical problems that the geological environment in petroleum and natural gas development cannot be simulated by applying confining pressure on the rock core in the prior art, and the accurate positioning of a small-volume rock core is difficult to meet, so that the accuracy of an experimental result is insufficient, the purposes of accurately positioning the small-volume rock core and conveniently applying the confining pressure are realized.
The invention is realized by the following technical scheme:
the device for testing the tensile strength of the rock core under the confining pressure condition based on the Brazilian splitting comprises a cylinder body, wherein the upper end and the lower end of the cylinder body are respectively detachably connected with an upper pressure head and a lower pressure head; the device comprises a barrel body, a guide barrel, a rubber sleeve, a pressure rod and a support rod, wherein the barrel body is internally provided with the guide barrel and a rubber sleeve, the pressure rod axially penetrates through an upper pressure head and the guide barrel, the support rod axially penetrates through a lower pressure head, the pressure rod is in sliding fit with the upper pressure head and the guide barrel, the support rod is detachably and fixedly connected with the lower pressure head, and the bottom end of the pressure rod and the top end of the support rod are inserted into the; the bottom surface of the guide cylinder and the top surface of the support rod are both cambered surfaces matched with the outer diameter of the core to be tested, and when the pressure rod descends to the bottom end of the stroke, an expanding part on the pressure rod is in contact with the top end of the guide cylinder; the bottom of the pressure lever is provided with a splitting tool bit; and a closed annular space is formed between the rubber sleeve and the cylinder body, and a confining pressure inlet communicated with the closed annular space is formed in the cylinder body.
Aiming at the problems that the geological environment in petroleum and natural gas development cannot be simulated by applying confining pressure on a rock core and the accuracy of an experimental result is insufficient due to the fact that accurate positioning of a small-volume rock core is difficult to meet in the prior art, the invention firstly provides a rock core tensile strength testing device based on Brazilian splitting under the confining pressure condition. The depression bar passes pressure head and guide cylinder on along the axial, therefore the depression bar top is located the pressure head outside, is convenient for through the inside transmission splitting load of depression bar, and the depression bar slides from top to bottom with last pressure head and the equal sliding fit of guide cylinder, therefore the depression bar can be followed the axial and carried out. The bracing piece passes pressure head down along the axial, and can dismantle fixed connection between the two, and pressure head is fixed back down, can fix the bracing piece. According to the invention, the core to be detected is coated by the bottom of the guide cylinder and the top of the support rod together, and the bottom surface of the guide cylinder and the top surface of the support rod are cambered surfaces matched with the outer diameter of the core to be detected, so that the core to be detected can be coated from the upper side and the lower side, and the problem that the sample is difficult to effectively position by adopting an upper pressing plate and a lower pressing plate in the prior art is solved. It should be noted that the guide cylinder only has a limiting effect on the core, and the support rod only has a supporting and limiting effect on the core, and neither of the guide cylinder and the support rod can be used as a loading component. The loading part is a splitting tool bit at the bottom of the pressure rod, and the splitting tool bit can directly penetrate out of the guide cylinder to be in contact with the rock core to be tested. In addition, the pressure lever is provided with an expanding part which cannot be inserted into the guide cylinder through the top end of the guide cylinder, so that the pressure lever descends to have a stroke bottom end in the loading process, wherein the stroke bottom end is the position where the expanding part on the pressure lever is contacted with the top end of the guide cylinder. Compared with the prior art, the invention has the following beneficial effects: (1) according to the invention, an experiment mode of external cladding limiting and internal center splitting is adopted, the guide cylinder and the support rod limit the rock core in the loading process, the rock core can be prevented from shaking or displacing left and right, and the rock core to be detected can be effectively positioned, so that the problems of difficult rock sample positioning and large error in the prior art are solved, the rock sample position is relatively fixed during each experiment, and the loading position where the splitting cutter head is located is relatively fixed; in addition, the confining pressure simulation device can effectively load the confining pressure simulation stratum environment, can accurately obtain the real tensile strength parameter under the stratum condition, can obviously reduce the experimental deviation rate, improves the accuracy and stability of the experimental result, and further effectively guides the drilling and fracturing operation; (2) the invention abandons the loading mode through the filler strip in the prior art, loads and splits through the splitting cutter head, and the splitting cutter head is provided with a sharp cutting edge, thereby overcoming the problems of poor splitting effect and inaccurate splitting position caused by overlarge width of the filler strip in the prior art, effectively ensuring the line contact state required in the Brazilian splitting experiment, and improving the experiment precision; (3) the core splitting device is compact in structure and is sleeved layer by layer, the core is positioned by the rubber sleeve, the guide cylinder and the support rod, the pressure rod is limited by the guide cylinder, and the splitting tool bit penetrating through the pressure rod is relatively fixed, so that the relative positions of the core and the splitting tool bit are determined during each operation, the core is effectively cracked from the center of the core, the defects that the requirement on splitting at the central position in a Brazilian splitting experiment is difficult to stably meet in the prior art, and each loading is easy to have large errors are overcome, and the splitting from the central position of the core is ensured in each experiment.
In addition, in order to overcome the problem of difficult sample centering in the prior art, a corresponding method in the field of civil engineering uses a rock sample with a larger volume so as to stabilize the rock sample through an upper pressing plate and a lower pressing plate and reduce the left-right deviation rate of the pad strip, but the means for reducing the deviation rate does not substantially solve the positioning error, and only reduces the relative proportion of the position deviation of the pad strip by increasing the length of the rock sample and changing the phase; but this way of increasing the volume of rock samples is too luxurious for geological research in oil and gas development: the well core taking is a very complex technology in the drilling engineering, and has a large engineering risk, so in order to control the cost and risk, the core taking amount for the same oil gas trap is not too much, and a few cores need to be subjected to various experiments to obtain various underground parameters, such as lithology, porosity, permeability, well wall pollution, triaxial stress, geological fluid media and the like, so that the cores which can be used for tensile strength testing in the field of petroleum and natural gas are very limited, large-size cores are not adopted for testing, stable clamping is further difficult, accurate positioning is difficult, even the width of a filler strip occupies 1/3-1/2 of the length of the core, and accurate experimental results cannot be obtained at all. Therefore, in the field of petroleum and natural gas, the accuracy of Brazilian split experiments cannot be improved by increasing the sample volume like ground or shallow geotechnical engineering, and the method cannot be realized due to the limitation of sample conditions. The invention can just overcome the defect, can obtain accurate splitting effect no matter how small the volume of the core is, by carrying out automatic clamping and accurate positioning through a structure completely different from the prior art, is particularly suitable for the tensile strength test of the core with small volume in the field of petroleum and gas compared with the prior art, obviously improves the utilization rate of the underground rock sample which is difficult to come, and solves the problem of core scrapping caused by clamping position deviation in the prior art.
Further, still including the last sealed head that is located the pressure head below, be located the lower sealed head of pressure head top down, go up sealed head, lower sealed head and be used for the upper and lower both ends of sealed gum cover respectively, the depression bar passes sealed head, the bracing piece passes sealed head down. Go up sealed head, lower sealed head and be used for sealing up both sides about the gum cover respectively for form the sealed interval in the gum cover, avoid all the other external forces to disturb the splitting to the core, improve experimentation stability and security. Certainly, the sealing of the upper sealing head cannot interfere with the pressure rod inserted into the rubber sleeve, and the normal downward loading of the pressure rod penetrating through the upper sealing head cannot be influenced; in the same way, the lower sealing head can not cause interference to the supporting rod inserted into the lower sealing head.
Further, the rubber sleeve comprises an upper sealing part matched with the upper sealing head, a lower sealing part matched with the lower sealing head and a pressurizing part for accommodating the core to be tested; the drift diameter of the pressurizing part is smaller than that of the upper sealing part and that of the lower sealing part;
the bottom of the upper sealing head is matched with the upper sealing part in shape, and the bottom of the upper sealing head is inserted into the upper sealing part and is in contact with a step surface between the upper sealing part and the pressurizing part;
the shape of the top of the lower sealing head is matched with that of the lower sealing part, and the top of the lower sealing head is inserted into the lower sealing part and is in contact with a step surface between the lower sealing part and the pressurizing part.
The rock core in the field of petroleum and natural gas is different from the rock of ground civil engineering, and the fluid medium is underground fluid, and may be crude oil, stratum water or even contain various gas phase media, so that the rock core is coated by using the rubber sleeve in order to prevent the surrounding pressure medium from interfering the rock core, and the rock core and the surrounding pressure medium are isolated by the rubber sleeve in the experimental process so as to ensure the accuracy of the experiment; the scheme is simultaneously suitable for Brazilian splitting experiment requirements of dry samples and saturated samples in the field of petroleum and natural gas. When the pressure-bearing rock core is used, the pressure in the sealed ring space between the rubber sleeve and the barrel is increased to the formation pressure through the confining pressure inlet, and the confining pressure is transmitted to the rock core through the rubber sleeve.
Carry out further optimization to the gum cover structure in this scheme, the gum cover divide into seal portion, pressurization portion, lower seal portion three in proper order, and wherein the latus rectum of pressurization portion is less than the latus rectum of seal portion and lower seal portion, is convenient for hold the core that awaits measuring. In the scheme, the upper sealing head and the upper sealing part of the rubber sleeve can be mutually positioned, and after confining pressure is added into the sealed ring, confining pressure can also act on the upper sealing part, so that the upper sealing part of the rubber sleeve is pressed and held in the upper sealing head inwards and the upper sealing head is pressed and held on the pressure rod penetrating through the upper sealing head, the sealing and positioning effects between the pressure rod and the upper sealing head and between the upper sealing head and the rubber sleeve in the experimental process are obviously improved, and the parts above the rock core are structurally fastened and cannot be loosened or shaken due to splitting interference; in the same way, can fix a position each other between the lower sealing of lower sealing head and gum cover, and after adding the confined pressure to airtight ring aloft, confined pressure still can be used in sealing down, inwards firmly press the lower sealing of gum cover and hold in sealing down the head, simultaneously will seal down the head firmly press hold on the bracing piece that passes from here, thereby showing and improving in the experimentation between bracing piece and the lower sealing, seal and location effect between lower sealing and the gum cover, make the structure of core below part fasten, can not disturbed by the splitting and become flexible or rock. Therefore, the confining pressure adding mode provided by the invention not only plays a role in simulating a stratum pressure environment, but also can improve the overall stability and the sealing property of the testing device in an experimental process, so that the core splitting process is prevented from being interfered by external factors, the accuracy of the splitting result is improved, and the final calculation result of the tensile strength of the core is further improved.
Furthermore, the inner sections of the upper sealing part and the lower sealing part are circular, and the inner section of the pressurizing part is square;
the outer diameter of the bottom of the upper sealing head is equal to the inner diameter of the upper sealing part, the inner diameter of the upper sealing head is equal to the outer diameter of the guide cylinder, and the inner diameter of the guide cylinder is equal to the outer diameter of the bottom of the compression bar;
the outer diameter of the top of the lower sealing head is equal to the inner diameter of the lower sealing part, and the outer diameter of the top of the supporting rod is equal to the inner diameter of the lower sealing head;
the side length of the pressurizing part is equal to the diameter of the rock core to be measured.
The outer diameter of the bottom of the upper sealing head is equal to the inner diameter of the upper sealing part, so that the sufficient matching between the upper sealing head and the upper sealing part is ensured, the mutual positioning capacity is improved, and the lower sealing head and the lower sealing part are the same. The internal diameter of going up the sealed head equals the external diameter of guide cylinder, and the internal diameter of guide cylinder equals the external diameter of depression bar bottom, can ensure that the guide cylinder can just in time insert to last sealed head in, and the depression bar also can just in time pass from the guide cylinder. The top external diameter of bracing piece equals the internal diameter of lower sealing head, ensures that the bracing piece can just in time insert lower sealing head in and receive lower sealing head's spacing fixed. The size of parts through cup jointing each other in this scheme is injectd, ensures that this device installation is accomplished the back wholeness extremely strong, except the depression bar, between each component of inside equal interact, fix a position and spacing mutually, can not take place not hard up in the experimentation, can guarantee the stability of whole experimentation in the core. In addition, the inner section of the pressurizing part is square, wherein the inner section is perpendicular to the axis direction of the rubber sleeve, the structure is suitable for the Brazilian splitting experiment of the rock core with the length-diameter ratio of 1:1, no requirement is imposed on the placing direction of the rock core, the experiment steps are simplified, and the experiment efficiency is improved.
Furthermore, the upper pressure head and the lower pressure head are both screwing pressure heads in threaded connection with the cylinder body, the bottom surface of the upper pressure head is matched with the top surface of the upper sealing head, and the top surface of the lower pressure head is matched with the bottom surface of the lower sealing head; the upper sealing head, the lower sealing head and the cylinder body are sealed through a plurality of sealing rings. The upper pressure head and the lower pressure head are screwing pressure heads in threaded connection with the cylinder body, after the upper sealing head is installed, the upper pressure head is installed and screwed downwards, the bottom surface of the upper pressure head is matched with the top surface of the upper sealing head, and the upper sealing head is locked in the device through the upper pressure head. Similarly, after the lower sealing head is installed, the lower pressing head is installed and screwed upwards, the top surface of the lower pressing head is matched with the bottom surface of the lower sealing head, and the lower sealing head is locked in the device through the lower pressing head.
The outer wall of the annular piston is in contact with the inner wall of the upper pressure head, and the annular piston and the pressure rod are relatively fixed along the axial direction; the annular piston is driven to move downwards by fluid applied to the piston fluid inlet. The compression bar is loaded by external equipment in a laboratory, and the loading by equipment such as a universal testing machine belongs to the prior art for technicians in the field, however, the loading equipment cannot be used or does not have the traditional loading environment in some special environments, if the traditional loading experiment conditions cannot be met in an engineering operation field, the experiment cannot be carried out at the moment, and the rock core needs to be transferred to a special laboratory for operation, so that the efficiency is extremely low. Therefore, the scheme is further optimized to reduce the condition limitation of the Brazilian splitting experiment in the oil field. Specifically, still set up annular piston in the device is inside, and annular piston establishes along depression bar outer wall movable sleeve, and annular piston outer wall and last pressure head inner wall contact, the piston fluid entry on barrel and/or last pressure head, when squeeze into high-pressure fluid to piston fluid entry, the fluid can promote annular piston and descend along the axial, because annular piston and depression bar are relatively fixed along the axial, consequently can drive the depression bar in step and descend, the realization is to the loading operation of splitting tool bit. When the scheme is used, the test can be realized only by utilizing the pressure pump and the pressurized fluid, the test is not limited by laboratory equipment, the Brazilian splitting experiment on the core can be completed on the site, the requirement on the Brazilian splitting condition of the core in the field of petroleum and natural gas is obviously reduced, and the test efficiency is favorably improved.
Furthermore, the top of the pressure lever is sleeved with a hollow screwing bolt, the pressure lever comprises an expanding part, and the screwing bolt is positioned above the expanding part and cannot pass through the expanding part; the outer wall of the screwing bolt is connected with the inner wall of the annular piston through threads. The tightening bolt is in threaded connection with the annular piston, so that the tightening bolt and the annular piston are axially fixed, when the annular piston moves downwards, the tightening bolt is driven to synchronously move downwards, and the tightening bolt is located above the diameter expanding part and cannot pass through the diameter expanding part, so that the downward movement of the tightening bolt can be abutted against the diameter expanding part, the pressing rod is driven to synchronously move downwards, and the effect of driving the pressing rod to load downwards through the annular piston is achieved.
Further, the bottom of the splitting tool bit is a prism with one side facing downwards. The filler strip or other splitting parts used for Brazilian splitting in the prior art have the mode of adopting the blunt surface with radian, and the original intention of this mode is in order to satisfy the line contact relation between loading part and the measured core in the brazilian splitting experiment. However, in the course of extensive research, the inventor found that the cambered blunt surface pad strip forms two contact lines when the core splits (as shown in fig. 1), so that the subsequent loading not only does not meet the experimental requirements of central splitting, but also is contrary to the original purpose of line contact. Therefore, the specific structure of the loading part is improved in the scheme, the bottom of the splitting tool bit is a prism with one side facing downwards, the sharp side of the prism is used as the splitting loading part, and the line contact relation between the loading part and the rock core in the experiment process can be guaranteed all the time, so that the experiment requirement is met, the theoretical calculation formula is met, and the accurate tensile strength parameter of the rock core is obtained.
The method for testing the tensile strength of the rock core under the condition of confining pressure based on Brazilian splitting comprises the following steps:
s1, sequentially loading a lower sealing head and a lower pressure head from the bottom of the cylinder, fixing the lower pressure head, loading a support rod from the top of the cylinder and contacting with the lower pressure head, and enabling the lower sealing head and the support rod to be located on the lower pressure head;
s2, loading the cylindrical rock core with the length-diameter ratio of 1: 1-1: 0.5 into the rubber sleeve, so that the rock core is positioned in the pressurizing part, and the axis of the rock core is perpendicular to the axis of the rubber sleeve; the rubber sleeve with the core is integrally arranged from the top of the cylinder body, the direction is adjusted, the core is located on the cambered surface of the top of the supporting rod, the lower sealing part is sleeved on the outer side of the top of the lower sealing head, and the lower sealing head is in surface contact with the step between the lower sealing part and the pressurizing part;
s3, sequentially loading a guide cylinder, an upper sealing head and an upper pressure head from the top of the cylinder body to enable the bottom cambered surface of the guide cylinder to be attached to the core, inserting the bottom end of the upper sealing head into a gap between the guide cylinder and the upper sealing part and enabling the bottom end of the upper sealing head to be in contact with a step surface between the upper sealing part and the pressurizing part;
s4, inserting a pressure lever from the top of the cylinder body, enabling the pressure lever to sequentially penetrate through the upper pressure head, the upper sealing head and the guide cylinder from top to bottom, and enabling the splitting cutter head at the bottom of the pressure lever to be in contact with the core;
s5, applying confining pressure to the closed annulus through the confining pressure inlet;
s6, the pressing rod is loaded downwards, and the splitting cutter head is made to move downwards to split the core.
The method is suitable for the cylindrical rock core with the length-diameter ratio of 1: 1-1: 0.5, can be seen to fully meet the requirement of Brazilian splitting experiments on the small-volume rock core in the field of petroleum and natural gas, and achieves the purposes of accurately positioning the small-volume rock core, effectively applying confining pressure and improving the accuracy of the experiments.
Further, in step S6, the downward loading manner of the pressing rod includes mechanical loading and hydraulic loading;
the method of mechanical loading comprises: the testing device is arranged on the testing machine, and the press rod is loaded downwards through the machine head until the diameter expansion part on the press rod reaches the stroke low position when the diameter expansion part on the press rod is abutted against the top end of the guide cylinder;
the hydraulic loading method comprises the following steps: pumping fluid to a piston fluid inlet through pumping equipment, driving an annular piston to descend by fluid pressure until the bottom of the annular piston reaches a stroke low position when the top surface of an upper sealing head is abutted by the bottom of the annular piston; and in the descending process of the annular piston, the screwing bolt is driven to synchronously descend, and the screwing bolt applies thrust to the diameter expanding part, so that the pressing rod is driven to integrally descend. The pumping device may be any existing fluid pump, such as a common booster pump, and the pumped fluid is preferably hydraulic oil.
Compared with the prior art, the invention has the following advantages and beneficial effects:
1. according to the device and the method for testing the tensile strength of the rock core under the confining pressure condition based on the Brazilian splitting, an experiment mode of external cladding limiting and internal center splitting is adopted, the guide cylinder and the support rod limit the rock core in the loading process, the rock core can be prevented from shaking or displacing left and right, the rock core to be tested can be effectively positioned, and therefore the problems that in the prior art, the rock sample is difficult to position and has a large error are solved, the position of the rock sample is relatively fixed during each experiment, and the loading position of the splitting cutter head is relatively fixed; in addition, the confining pressure simulation formation environment can be effectively loaded, the experiment deviation rate can be obviously reduced, and the accuracy and the stability of the experiment result can be improved.
2. The device and the method for testing the tensile strength of the rock core under the confining pressure condition based on the Brazilian splitting abandon the mode of loading through the backing strip in the prior art, and the splitting tool bit is used for loading and splitting and is provided with the sharp cutting edge, so that the problems of poor splitting effect and inaccurate splitting position caused by too large width of the backing strip in the prior art are solved, the line contact state required in the Brazilian splitting experiment can be effectively ensured, and the experiment precision is improved.
3. The device and the method for testing the tensile strength of the rock core under the confining pressure condition based on the Brazilian splitting have the advantages that the whole device is compact in structure and is sleeved layer by layer, the rock core is positioned by the rubber sleeve, the guide cylinder and the support rod, meanwhile, the pressure rod is limited by the guide cylinder, and the position of the splitting tool bit penetrating through the pressure rod is relatively fixed, so that the relative positions of the rock core and the splitting tool bit with the same size are determined during each operation, the splitting from the center of the rock core can be effectively ensured, the defect that the requirement for the central position splitting in the Brazilian splitting experiment is difficultly and stably met in the prior art, and further, each loading is easy to have a large error is overcome, and the splitting from the central position of the rock core is ensured in each experiment.
4. The device and the method for testing the tensile strength of the rock core under the confining pressure condition based on Brazilian splitting perform automatic clamping and accurate positioning through a structure completely different from the prior art, and can obtain an accurate splitting effect no matter how small the volume of the rock core is, compared with the prior art, the device and the method overcome the problem that the size of a disc required by a conventional Brazilian splitting test tensile strength sample cannot meet the technical requirement of developing petroleum engineering day by day, are particularly suitable for the tensile strength test of the rock core with small volume in the petroleum and gas field, obviously improve the utilization rate of an underground rock sample which is difficult to come, and solve the problem of scrapping of the rock core caused by clamping position deviation in the prior art.
Drawings
The accompanying drawings, which are included to provide a further understanding of the embodiments of the invention and are incorporated in and constitute a part of this application, illustrate embodiment(s) of the invention and together with the description serve to explain the principles of the invention. In the drawings:
FIG. 1 is a schematic diagram of the principle of Brazilian splitting in the prior art;
FIG. 2 is a sectional view of embodiment 1 of the present invention;
FIG. 3 is a sectional view of embodiment 1 of the present invention;
FIG. 4 is a sectional view of embodiment 2 of the present invention;
FIG. 5 is a sectional view of embodiment 2 of the present invention;
FIG. 6 is a schematic partial structure diagram of an embodiment of the present invention;
FIG. 7 is a partial schematic view of an embodiment of the present invention;
fig. 8 is a schematic structural diagram of a rubber sleeve in an embodiment of the invention.
Reference numbers and corresponding part names in the drawings:
1-cylinder body, 2-upper pressure head, 3-lower pressure head, 4-pressure rod, 401-expanding part, 5-support rod, 6-guide cylinder, 7-rubber sleeve, 701-upper sealing part, 702-lower sealing part, 703-pressurizing part, 8-splitting cutter head, 9-closed annular space, 10-confining pressure inlet, 11-upper sealing head, 12-lower sealing head, 13-sealing ring, 14-annular piston, 15-piston fluid inlet, 16-screwing bolt and 17-core.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is further described in detail below with reference to examples and accompanying drawings, and the exemplary embodiments and descriptions thereof are only used for explaining the present invention and are not meant to limit the present invention.
Example 1:
the device for testing the tensile strength of the core under the confining pressure condition based on brazilian splitting is shown in fig. 2 to 3 and fig. 6 to 7, wherein fig. 2 and 3 are schematic diagrams of the same structure in different sections; the device comprises a cylinder body 1, wherein the upper end and the lower end of the cylinder body 1 are respectively detachably connected with an upper pressure head 2 and a lower pressure head 3; the barrel body 1 is internally provided with a guide cylinder 6 and a rubber sleeve 7, and further comprises a pressure lever 4 which axially penetrates through the upper pressure head 2 and the guide cylinder 6, and a support rod 5 which axially penetrates through the lower pressure head 3, wherein the pressure lever 4, the upper pressure head 2 and the guide cylinder 6 are in sliding fit, the support rod 5 and the lower pressure head 3 are detachably and fixedly connected, and the bottom end of the pressure lever 4 and the top end of the support rod 5 are inserted into the rubber sleeve 7; the bottom surface of the guide cylinder 6 and the top surface of the support rod 5 are both cambered surfaces matched with the outer diameter of the core to be tested, and when the pressure rod 4 descends to the bottom end of the stroke, the diameter expanding part 401 on the pressure rod 4 is in contact with the top end of the guide cylinder 6; the bottom of the pressure lever 4 is provided with a splitting cutter head 8; a closed annular space 9 is formed between the rubber sleeve 7 and the cylinder body 1, and an ambient pressure inlet 10 communicated with the closed annular space 9 is formed in the cylinder body 1.
The upper sealing head 11 and the lower sealing head 12 are respectively used for sealing the upper end and the lower end of the rubber sleeve 7, the compression bar 4 penetrates through the upper sealing head 11, and the support bar 5 penetrates through the lower sealing head 12.
As shown in fig. 8, the rubber sleeve 7 comprises an upper sealing part 701 for matching with the upper sealing head 11, a lower sealing part 702 for matching with the lower sealing head 12, and a pressurizing part 703 for accommodating the core to be measured; the drift diameter of the pressurizing part 703 is smaller than the drift diameters of the upper sealing part 701 and the lower sealing part 702; the phantom part in fig. 8 shows the state of the core encased in the rubber casing 7.
Wherein, the bottom shape of the upper sealing head 11 is matched with the upper sealing part 701, the bottom of the upper sealing head 11 is inserted into the upper sealing part 701 and is contacted with the step surface between the upper sealing part 701 and the pressurizing part 703; the shape of the top of the lower seal head 12 is matched with that of the lower seal part 702, and the top of the lower seal head 12 is inserted into the lower seal part 702 and contacts with the step surface between the lower seal part 702 and the pressurizing part 703.
Preferably, the bottom of the cleaving bit 8 is a prism with a side facing downward.
In a more preferred embodiment, the inner cross sections of the upper seal part 701 and the lower seal part 702 are circular, and the inner cross section of the pressurizing part 703 is square; the outer diameter of the bottom of the upper sealing head 11 is equal to the inner diameter of the upper sealing part 701, the inner diameter of the upper sealing head 11 is equal to the outer diameter of the guide cylinder 6, and the inner diameter of the guide cylinder 6 is equal to the outer diameter of the bottom of the pressure rod 4; the outer diameter of the top of the lower sealing head 12 is equal to the inner diameter of the lower sealing part 702, and the outer diameter of the top of the support rod 5 is equal to the inner diameter of the lower sealing head 12; the side length of the pressurizing part 703 is equal to the diameter of the core to be measured.
More preferably, the upper pressure head 2 and the lower pressure head 3 are both screwing pressure heads in threaded connection with the cylinder body 1, the bottom surface of the upper pressure head 2 is matched with the top surface of the upper sealing head 11, and the top surface of the lower pressure head 3 is matched with the bottom surface of the lower sealing head 12; the upper sealing head 11, the lower sealing head 12 and the cylinder 1 are sealed by a plurality of sealing rings 13.
The application method of the embodiment is specifically as follows:
s1, sequentially loading a lower sealing head 12 and a lower pressing head 3 from the bottom of a cylinder 1, fixing the lower pressing head 3, loading a support rod 5 from the top of the cylinder 1 and contacting the lower pressing head 3, and enabling the lower sealing head 12 and the support rod 5 to be located on the lower pressing head 3;
s2, loading the cylindrical core 17 with the length-diameter ratio of 1:1 into the rubber sleeve 7, so that the core 17 is positioned in the pressurizing part 703, and the axis of the core 17 is vertical to the axis of the rubber sleeve 7; the rubber sleeve 7 with the core 17 is integrally arranged from the top of the barrel body 1, the direction is adjusted, the core 17 is located on the arc surface of the top of the support rod 5, the lower sealing part 702 is sleeved on the outer side of the top of the lower sealing head 12, and the lower sealing head 12 is in contact with the step surface between the lower sealing part 702 and the pressurizing part 703;
s3, sequentially loading the guide cylinder 6, the upper sealing head 11 and the upper pressure head 2 from the top of the cylinder body 1, attaching the bottom cambered surface of the guide cylinder 6 to the core 17, and inserting the bottom end of the upper sealing head 11 into the gap between the guide cylinder 6 and the upper sealing part 701 and contacting the step surface between the upper sealing part 701 and the pressurizing part 703;
s4, inserting a pressure lever 4 from the top of the cylinder body 1, enabling the pressure lever 4 to sequentially penetrate through the upper pressure head 2, the upper sealing head 11 and the guide cylinder 6 from top to bottom, and enabling the splitting tool bit 8 at the bottom of the pressure lever 4 to be in contact with the rock core 17;
s5, applying confining pressure to the closed annulus 9 through the confining pressure inlet 10;
s6, the compression bar 4 is loaded downwards, so that the splitting tool bit 8 moves downwards to split the rock core 17; the loading method comprises the following steps: the testing device is arranged on a universal testing machine, the pressing rod 4 is loaded downwards through a machine head of the universal testing machine until the stroke is low when the diameter expanding part 401 on the pressing rod 4 is abutted against the top end of the guide cylinder 6.
The diameter of the core 17 used in this embodiment is 25.4mm, the length is 25.4mm, the side length of the pressing portion 703 is 25.4mm, and the overall length of the rubber sleeve 7 is 121.33 mm; the outer diameter of the cylinder 1 is 153.58 mm.
Example 2:
the device for testing the tensile strength of the core under the confining pressure condition based on brazilian splitting is shown in fig. 4 to 7, wherein fig. 4 and 5 are schematic diagrams of the same structure in different sections; this example differs from example 1 in that: the outer wall of the annular piston 14 is in contact with the inner wall of the upper pressure head 2, and the annular piston 14 and the pressure rod 4 are relatively fixed in the axial direction; and a piston fluid inlet 15 arranged on the cylinder body 1 and/or the upper pressure head 2, wherein fluid applied to the piston fluid inlet 15 pushes the annular piston 14 to move downwards. The top of the compression bar 4 is sleeved with a hollow tightening bolt 16, the compression bar 4 comprises an expanding part 401, and the tightening bolt 16 is positioned above the expanding part 401 and cannot pass through the expanding part 401; the outer wall of the tightening bolt 16 is in threaded connection with the inner wall of the annular piston 14.
When loading is carried out in the use process of the embodiment, the loading can be carried out in the following mode except that the machine head driving pressure rod of the universal testing machine is used for descending: pumping fluid to a piston fluid inlet 15 through pumping equipment, driving an annular piston 14 to move downwards by fluid pressure until the bottom of the annular piston 14 reaches a stroke low position when the top surface of an upper sealing head 11 is abutted by the bottom of the annular piston; in the process of descending the annular piston 14, the tightening bolts 16 are driven to synchronously descend, and the tightening bolts 16 apply thrust to the diameter-expanding part 401, so that the whole compression bar 4 is driven to descend. The loading method can be realized only by utilizing the pressure pump and the pressurized fluid when in use, is not limited by laboratory equipment, can complete Brazilian splitting experiments on the cores on site, obviously reduces the Brazilian splitting condition requirements on the cores in the field of petroleum and natural gas, and is beneficial to improving the experiment efficiency.
That is, in the specific use of this embodiment, it is only necessary to manually connect two pumps to communicate with the confining pressure inlet 10 and the piston fluid inlet 15, respectively, one pump is used for applying confining pressure, and the other pump is used for loading.
Preferably, the maximum travel distance between the annular piston and the travel low position from the initial position is 1-2 cm.
Preferably, the embodiment may further include a displacement sensor for monitoring a downward distance of the pressing rod, so that an experimenter can estimate the splitting degree.
The above-mentioned embodiments are intended to illustrate the objects, technical solutions and advantages of the present invention in further detail, and it should be understood that the above-mentioned embodiments are merely exemplary embodiments of the present invention, and are not intended to limit the scope of the present invention, and any modifications, equivalent substitutions, improvements and the like made within the spirit and principle of the present invention should be included in the scope of the present invention.
It should be noted that, in this document, terms such as "comprises," "comprising," or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. Further, the term "connected" used herein may be either directly connected or indirectly connected through other components without being particularly described.
Claims (8)
1. The device for testing the tensile strength of the rock core under the confining pressure condition based on Brazilian splitting is characterized by comprising a cylinder body (1), wherein the upper end and the lower end of the cylinder body (1) are respectively detachably connected with an upper pressure head (2) and a lower pressure head (3); the barrel body (1) is internally provided with a guide cylinder (6) and a rubber sleeve (7), and further comprises a pressure rod (4) which axially penetrates through the upper pressure head (2) and the guide cylinder (6), and a support rod (5) which axially penetrates through the lower pressure head (3), wherein the pressure rod (4), the upper pressure head (2) and the guide cylinder (6) are in sliding fit, the support rod (5) and the lower pressure head (3) are detachably and fixedly connected, and the bottom end of the pressure rod (4) and the top end of the support rod (5) are inserted into the rubber sleeve (7); the bottom surface of the guide cylinder (6) and the top surface of the support rod (5) are cambered surfaces matched with the outer diameter of the core to be tested, and when the pressure rod (4) descends to the bottom end of the stroke, an expanding part (401) on the pressure rod (4) is in contact with the top end of the guide cylinder (6); the bottom of the pressure lever (4) is provided with a splitting tool bit (8); a closed annular space (9) is formed between the rubber sleeve (7) and the cylinder body (1), and a confining pressure inlet (10) communicated with the closed annular space (9) is formed in the cylinder body (1);
the device is characterized by further comprising an upper sealing head (11) positioned below the upper pressure head (2) and a lower sealing head (12) positioned above the lower pressure head (3), wherein the upper sealing head (11) and the lower sealing head (12) are respectively used for sealing the upper end and the lower end of the rubber sleeve (7), the pressure lever (4) penetrates through the upper sealing head (11), and the support rod (5) penetrates through the lower sealing head (12);
the rubber sleeve (7) comprises an upper sealing part (701) matched with the upper sealing head (11), a lower sealing part (702) matched with the lower sealing head (12) and a pressurizing part (703) used for containing a rock core to be tested; the drift diameter of the pressurizing part (703) is smaller than the drift diameters of the upper sealing part (701) and the lower sealing part (702);
the bottom of the upper sealing head (11) is matched with the upper sealing part (701) in shape, and the bottom of the upper sealing head (11) is inserted into the upper sealing part (701) and is in contact with a step surface between the upper sealing part (701) and the pressurizing part (703);
the shape of the top of the lower sealing head (12) is matched with that of the lower sealing part (702), and the top of the lower sealing head (12) is inserted into the lower sealing part (702) and is in contact with the step surface between the lower sealing part (702) and the pressurizing part (703).
2. The device for testing the tensile strength of the core under the confining pressure condition based on Brazilian splitting as claimed in claim 1, wherein the inner sections of the upper sealing part (701) and the lower sealing part (702) are circular, and the inner section of the pressurization part (703) is square;
the outer diameter of the bottom of the upper sealing head (11) is equal to the inner diameter of the upper sealing part (701), the inner diameter of the upper sealing head (11) is equal to the outer diameter of the guide cylinder (6), and the inner diameter of the guide cylinder (6) is equal to the outer diameter of the bottom of the pressure rod (4);
the outer diameter of the top of the lower sealing head (12) is equal to the inner diameter of the lower sealing part (702), and the outer diameter of the top of the supporting rod (5) is equal to the inner diameter of the lower sealing head (12);
the side length of the pressurizing part (703) is equal to the diameter of the rock core to be measured.
3. The device for testing the tensile strength of the core under the confining pressure condition based on the Brazilian split is characterized in that the upper pressure head (2) and the lower pressure head (3) are both screwing pressure heads in threaded connection with the barrel body (1), the bottom surface of the upper pressure head (2) is matched with the top surface of the upper sealing head (11), and the top surface of the lower pressure head (3) is matched with the bottom surface of the lower sealing head (12); the upper sealing head (11), the lower sealing head (12) and the cylinder body (1) are sealed through a plurality of sealing rings (13).
4. The device for testing the tensile strength of the core under the confining pressure condition based on Brazilian splitting as claimed in claim 1, further comprising an annular piston (14) positioned outside the pressure rod (4), wherein the outer wall of the annular piston (14) is in contact with the inner wall of the upper pressure head (2), and the annular piston (14) and the pressure rod (4) are relatively fixed in the axial direction; the device also comprises a piston fluid inlet (15) arranged on the cylinder body (1) and/or the upper pressure head (2), and fluid applied to the piston fluid inlet (15) pushes the annular piston (14) to move downwards.
5. The device for testing the tensile strength of the core under the confining pressure condition based on Brazilian splitting as claimed in claim 4, wherein the top of the compression rod (4) is sleeved with a hollow screwing bolt (16), the compression rod (4) comprises an expanded diameter part (401), and the screwing bolt (16) is positioned above the expanded diameter part (401) and cannot pass through the expanded diameter part (401); the outer wall of the screwing bolt (16) is in threaded connection with the inner wall of the annular piston (14).
6. The device for testing the tensile strength of the core under the confining pressure condition based on Brazilian splitting as claimed in claim 1, wherein the bottom of the splitting tool bit (8) is a prism with a downward side.
7. The testing method of the device for testing the tensile strength of the core under the confining pressure condition based on the Brazilian split as claimed in any one of claims 1 to 6, is characterized by comprising the following steps:
s1, sequentially loading a lower sealing head (12) and a lower pressing head (3) from the bottom of a cylinder body (1), fixing the lower pressing head (3), loading a support rod (5) from the top of the cylinder body (1) and contacting the lower pressing head (3), and enabling the lower sealing head (12) and the support rod (5) to be located on the lower pressing head (3);
s2, taking a cylindrical rock core (17) with the length-diameter ratio of 1: 1-1: 0.5, and filling the cylindrical rock core (17) into the rubber sleeve (7), so that the rock core (17) is positioned in the pressurizing part (703), and the axis of the rock core (17) is perpendicular to the axis of the rubber sleeve (7); the rubber sleeve (7) provided with the rock core (17) is integrally arranged from the top of the barrel body (1), the direction is adjusted, the rock core (17) is located on the cambered surface of the top of the support rod (5), the lower sealing part (702) is sleeved on the outer side of the top of the lower sealing head (12), and the lower sealing head (12) is in contact with the step surface between the lower sealing part (702) and the pressurizing part (703);
s3, sequentially loading a guide cylinder (6), an upper sealing head (11) and an upper pressure head (2) from the top of a cylinder body (1) to enable the bottom cambered surface of the guide cylinder (6) to be attached to a rock core (17), and inserting the bottom end of the upper sealing head (11) into a gap between the guide cylinder (6) and an upper sealing part (701) and enabling the bottom end of the upper sealing head to be in contact with a step surface between the upper sealing part (701) and a pressurizing part (703);
s4, inserting a pressure lever (4) from the top of the cylinder body (1), enabling the pressure lever (4) to sequentially penetrate through the upper pressure head (2), the upper sealing head (11) and the guide cylinder (6) from top to bottom, and enabling a splitting tool bit (8) at the bottom of the pressure lever (4) to be in contact with the rock core (17);
s5, applying confining pressure to the closed annulus (9) through the confining pressure inlet (10);
s6, the compression bar (4) is loaded downwards, and the splitting tool bit (8) moves downwards to split the rock core (17).
8. The test method according to claim 7, wherein in step S6, the downward loading of the pressure rod (4) comprises mechanical loading, hydraulic loading;
the method of mechanical loading comprises: the testing device is arranged on the testing machine, the pressure lever (4) is loaded downwards through the machine head until the stroke is low when the diameter expanding part (401) on the pressure lever (4) is abutted against the top end of the guide cylinder (6);
the hydraulic loading method comprises the following steps: pumping fluid to a piston fluid inlet (15) by pumping equipment, driving an annular piston (14) to descend by fluid pressure until the bottom of the annular piston (14) is close to the top surface of an upper sealing head (11) and reaches a stroke low position; in the descending process of the annular piston (14), the tightening bolts (16) are driven to synchronously descend, and the tightening bolts (16) apply thrust to the diameter expanding part (401), so that the pressing rod (4) is driven to integrally descend.
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| CN202011505872.5A CN112665987B (en) | 2020-12-18 | 2020-12-18 | Device and method for testing tensile strength of rock core under confining pressure condition based on Brazilian splitting |
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| CN202011505872.5A CN112665987B (en) | 2020-12-18 | 2020-12-18 | Device and method for testing tensile strength of rock core under confining pressure condition based on Brazilian splitting |
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| CN100588945C (en) * | 2008-01-08 | 2010-02-10 | 中国科学院武汉岩土力学研究所 | Rock mechanics experiment loading unit for bittern flow field corrosion condition |
| CN101936851B (en) * | 2010-09-02 | 2011-07-13 | 中国科学院武汉岩土力学研究所 | Specimen clamp for measuring tensile strength of rock by splitting method |
| CN103604702B (en) * | 2013-11-28 | 2016-06-22 | 中国科学院武汉岩土力学研究所 | A kind of assay device testing rock fracture toughness |
| KR101936877B1 (en) * | 2016-11-03 | 2019-01-16 | 강릉건설(주) | Rock splitting using discontinuity face force relaxation method |
| CN206410979U (en) * | 2017-01-25 | 2017-08-15 | 重庆地质矿产研究院 | Core holder for simulating hydraulic fracturing and permeability test |
| CN206960232U (en) * | 2017-06-23 | 2018-02-02 | 防灾科技学院 | Rock Under Uniaxial Compression tensile strength test system |
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