CN115494102A

CN115494102A - Online nuclear magnetic resonance imaging system and method

Info

Publication number: CN115494102A
Application number: CN202211289272.9A
Authority: CN
Inventors: 姜平; 雷霄; 张恒荣; 骆玉虎; 张群
Original assignee: China National Offshore Oil Corp CNOOC; CNOOC China Ltd Zhanjiang Branch
Current assignee: China National Offshore Oil Corp CNOOC; CNOOC China Ltd Zhanjiang Branch
Priority date: 2022-10-20
Filing date: 2022-10-20
Publication date: 2022-12-20

Abstract

The invention relates to the technical field of oil and gas development experiment measurement, in particular to an online nuclear magnetic resonance imaging system and method, wherein the online nuclear magnetic resonance imaging system comprises a clamping unit, a high-temperature and high-pressure displacement unit, a magnet unit, an oil-gas-water three-phase metering unit, an acquisition unit, a control unit and an imaging unit, the clamping unit is respectively communicated with the high-temperature and high-pressure displacement unit and the oil-gas-water three-phase metering unit, the acquisition unit is respectively communicated with the clamping unit, the high-temperature and high-pressure displacement unit, the magnet unit, the control unit and the imaging unit, and the control unit is also respectively communicated with the clamping unit, the high-temperature and high-pressure displacement unit, the magnet unit and the imaging unit. The invention can realize the high-temperature and high-pressure rock core displacement nuclear magnetic resonance online measurement mode under the stratum condition, obtain the visual spatial distribution characteristics of the fluid in the rock core of the high-temperature and high-pressure reservoir in the displacement direction, and further simulate the dynamic seepage characteristics and the saturation distribution characteristics of crude oil in the actual rock core pores.

Description

Online nuclear magnetic resonance imaging system and method

Technical Field

The invention relates to the technical field of oil and gas development experiment measurement, in particular to an online nuclear magnetic resonance imaging system and method.

Background

In recent years, with the development of most of domestic oil and gas fields entering the middle and later stages, the improvement of the recovery ratio (EOR) becomes an important work for oil and gas development. The measurement of the spatial distribution of the residual oil in the oil and gas development experiment is always the key and difficult point of the EOR experiment, and the traditional experiment technologies such as sand filling pipes, glass etching and the like are difficult to truly simulate the seepage state of a reservoir stratum, thereby causing difficulty in the refined research of the recovery ratio increasing scheme. Since twenty-first century, with the application and popularization of imaging technologies such as CT electronic computed tomography, NMR nuclear magnetic resonance and the like in the field of oil and gas exploration and development, favorable conditions are created for measuring the spatial distribution of the residual oil. The NMR technology has been well-developed in the field of oil exploration and development due to its characteristics of rapidness, no damage, no toxicity, sensitivity only to hydrogen-containing pore fluids, and the like.

Different from the overall NMR signal of a measurement sample in the exploration field, the EOR experiment needs the NMR technology to provide the fluid spatial distribution of the rock core, and the nuclear magnetic resonance imaging technology can meet the requirement in principle. However, for magnetic resonance imaging of an object, a three-dimensional gradient positioning system, most commonly a three-dimensional orthogonal gradient coil, is required to achieve three-dimensional spatial gradient encoding in the sample region. After the gradient field is applied, performing inverse Fourier transform on the nuclear magnetic imaging signal to obtain the three-dimensional space distribution of the core pore fluid.

Common magnetic resonance imaging sequences include MSE, FSE, GRE sequences. The radio frequency excitation pulses of the three imaging sequences are soft pulses which are characterized by small frequency domain bandwidth and good layer selection characteristic, and have the defect of large pulse width in the magnitude of ms. The soft pulse imaging sequence, the radio frequency pulse width and the gradient coding time (also in the order of ms) result in that the TE when the echo signal is generated is at least 5-10 ms, and the small and medium pore signals are seriously lost for compact rock cores with short relaxation time and low signal quantity. To reduce the short relaxation signal loss, the echo interval TE of the imaging sequence has to be shortened, so a hard pulsed three-dimensional imaging sequence HMSE appears, as shown in fig. 2. The HMSE sequence utilizes three-dimensional gradient space coding to realize imaging, simultaneously exerts the great advantage of short duration (us magnitude) of hard pulse, shortens the TE of imaging to 2-3 ms, and ensures the acquisition of short relaxation signals. But core imaging has a greater impact on the limited signal-to-noise ratio (SNR) than does TE. Assuming that 10 layers are selected during rock core imaging, the pixel point of each layer is 100 × 100, and pore fluid is uniformly distributed in the rock core, so that the signal of each pixel point on the image is one hundred thousand of the whole signal. The nuclear magnetic resonance signal of the rock core is a weak signal, and three-dimensional imaging segmentation is carried out, so that the SNR of an imaging pixel point is sharply reduced.

In summary, the gradient coding time of the existing nuclear magnetic resonance imaging sequence occupies the echo time, and the typical characteristics of short relaxation time and weak signal in the core sample cause that the common three-dimensional nuclear magnetic resonance imaging method is difficult to achieve the ideal effect in the high-temperature and high-pressure reservoir tight core test.

Disclosure of Invention

Aiming at the defects of the prior art, particularly the technical current situation that the nuclear magnetic resonance experiment in the conventional rock core displacement process cannot quickly obtain the internal fluid space imaging characteristics of the rock core, the invention provides an online nuclear magnetic resonance imaging test system for a high-temperature and high-pressure rock core, which can realize the online measurement mode of the nuclear magnetic resonance in the high-temperature and high-pressure rock core displacement under the stratum condition, obtain the visual space distribution characteristics of the fluid in the rock core of a high-temperature and high-pressure reservoir in the displacement direction and further simulate the dynamic seepage characteristics and the saturation distribution characteristics of crude oil in the actual rock core pores.

In order to solve the technical problems, the invention adopts the technical scheme that:

the utility model provides an online nuclear magnetic resonance imaging system, including the centre gripping unit, the high temperature high pressure displacement unit that are used for fixed rock core, be used for producing magnetic field and can make magnetic field act on the magnet unit of centre gripping unit, oil gas water three-phase measurement unit, acquisition unit, the control unit, the imaging unit, the centre gripping unit respectively with the high temperature high pressure displacement unit oil gas water three-phase measurement unit intercommunication, the acquisition unit respectively with centre gripping unit, high temperature high pressure displacement unit, magnet unit, the control unit, imaging unit communication connection, the control unit still respectively with centre gripping unit, high temperature high pressure displacement unit, magnet unit, imaging unit communication connection.

The high-temperature high-pressure displacement unit is used for providing the temperature, overburden pressure, fluid pressure and fluid type environment required by the simulated formation environment when a sample is tested; the magnet unit has the function of providing a magnet environment and a gradient imaging environment for nuclear magnetic resonance imaging; the oil-gas-water three-phase metering unit at least comprises 4 flowmeters with different measuring ranges. After the high-temperature high-pressure displacement unit applies target formation pressure and temperature to the rock core, a magnetic field is formed around the rock core through the magnet unit, only radial layer selection is selected, a two-dimensional scanning space layer selection test is not performed in the layer, and a radial space distribution characteristic quantitative visualization result of the fluid in the rock core is obtained through a new radial space layer selection T2 spectrum technology.

Preferably, the high-temperature high-pressure displacement unit comprises a confining pressure pump, a confining pressure liquid storage device, a displacement pump, a displacement medium storage device, a heating device capable of acting on the confining pressure liquid storage device and heating, a confining pressure liquid circulation pipeline, a displacement pipeline and a back pressure pump, wherein the displacement medium storage device, the displacement pump and the clamping unit, the back pressure pump and the clamping unit, and the oil-gas-water three-phase metering unit and the clamping unit are communicated through the displacement pipeline; and the confining pressure liquid circulating device is respectively communicated with the confining pressure pump and the clamping unit through a confining pressure liquid circulating pipeline.

Preferably, a rock core installation cavity and a confining pressure cavity are arranged in the clamping unit, the rock core installation cavity is communicated with the displacement pipeline, the displacement medium storage device and the oil-gas-water three-phase metering unit, and the confining pressure cavity is communicated with the confining pressure liquid circulation pipeline and the confining pressure liquid storage device.

Preferably, the clamping unit is made of high-density high-strength zirconia, the maximum working temperature is 150 ℃ at the upper limit, the maximum working pressure is 70MPa at the upper limit, and nuclear magnetic resonance measurement under the high-temperature and high-pressure stratum condition can be met. The core is heated by confining pressure liquid and then circulated into the confining pressure cavity of the clamping unit through a confining pressure liquid circulating pipeline. The control unit internally comprises a temperature control system which is connected with the confining pressure cavity, and the core is heated and maintained at the temperature required by the experiment in a circulating confining pressure oil mode by temperature rise heating. The clamping unit is used for clamping the rock core, the displacement medium storage device can adopt an intermediate container or an air storage tank, and the clamping unit is connected with the displacement medium storage device to obtain different displacement fluid sources, so that the recovery ratio simulation experiment of three-phase fluids of oil, gas and water at different stages is realized.

Preferably, the magnet unit includes permanent magnet and magnet support, magnet constant temperature equipment, gradient coil, the magnet support is installed permanent magnet bottom or around to and locate in the magnet constant temperature equipment, the gradient coil encircles the permanent magnet setting, the centre gripping unit sets up at the permanent magnet side.

Preferably, oil gas water three-phase metering unit includes oil gas water separator, measurement balance, camera, a plurality of flowmeter, oil gas water separator one end and centre gripping unit intercommunication, the other end respectively with a plurality of flowmeters and measurement balance intercommunication, the camera is installed in oil gas water separator side.

The invention also provides an online nuclear magnetic resonance imaging method, which is operated by applying the online nuclear magnetic resonance imaging system and specifically comprises the following steps:

s1: installing the vacuumized saturated formation water core into a clamping unit, and collecting a T2 spectrum in a saturated state;

s2: applying confining pressure to the target formation pressure to the clamping unit through the high-temperature high-pressure displacement unit, and keeping the confining pressure greater than the displacement pressure; heating the clamping unit through the high-temperature high-pressure displacement unit to raise the temperature to the target formation temperature; meanwhile, a magnetic field is established by using a magnet unit, a radio frequency pulse sequence is transmitted by using a control unit and an acquisition unit, and radial space phase coding is applied;

s3: driving a displacement medium to the clamping unit through the high-temperature high-pressure displacement unit, driving formation water with saturated rock cores, and enabling the displacement medium and the formation water to enter the oil-gas-water three-phase metering unit;

s4: reading the water yield through an oil-gas-water three-phase metering unit, and simultaneously acquiring a T2 spectral line in a real-time state;

s5, detecting whether the water yield is increased or not, if so, returning to the step S4, and if not, entering the step S6;

s6: calculating the water saturation and the stratum water saturation by combining the saturated water amount, and determining a T2 spectral line under the irreducible water state;

s7: and analyzing and comparing the nuclear magnetic T2 spectral lines in different states, and performing imaging record by using an imaging unit.

Further, the response signal expression of the phase-coded radial spatial layer selection T2 test in step S2 is as follows:

wherein S (N TE, m) is a sampling signal; TE is echo interval, TE1 is first echo interval, TE2 is second echo interval, N is echo serial number, z is layer selection direction, i is echo number, gamma is gyromagnetic ratio, T2 is transverse relaxation time, gmax is maximum gradient value in the layer selection direction, tp is phase encoding time, m is an arithmetic progression corresponding to phase encoding step number, the constant gradient encoding mode is selective excitation sampling, and the radial space layer selection T2 spectrum f (T2, z) can be obtained by inverting the echo signal S (N.TE, z) at different positions.

Further, the method for acquiring the T2 spectral line in step S4 and step S6 is as follows: firstly, performing inverse Fourier transform on a sampling signal S (N.TE, M) to obtain echo attenuation signals M (N.TE, z) at different positions, and performing spectrum decomposition by using a BRD mode smoothing method and a T2 test inversion algorithm to obtain a radial space layer selection T2 spectrum f (T2, z), wherein the formula is as follows:

where M (N · TE, z) is an echo attenuation signal, S (N · TE, M) is a sampling signal, gmax is a maximum gradient value in the slice selection direction, z is the slice selection direction, tp is a phase encoding time, M is an arithmetic progression corresponding to the number of phase encoding steps, N is an echo number, TE1 is a first echo interval, TE2 is a second echo interval, i is the number of echoes, γ is a gyromagnetic ratio, and T2 is a transverse relaxation time.

Further, in the displacement process of the step S3, a constant speed or constant pressure mode is selected for displacement by adjusting the displacement pump.

Further, after the step S6, the method further comprises the step of: combining the T2 spectrum in the saturated state with the T2 spectrum in the bound state to calculate the saturation of the bound water, comparing the saturation with the saturation of the bound water calculated by the water yield meter, and if the difference between the saturation and the saturation is within +/-10%, indicating that the inorganic device of the online nuclear magnetic resonance imaging system fails; if the difference between the two is within +/-10%, the online nuclear magnetic resonance imaging system is firstly subjected to machine detection and then returns to the step S1 to start operation.

The radial space gradient coding technology considers the defects of the three-dimensional nuclear magnetic resonance imaging method in the aspect of SNR, focuses more on the change of residual oil in the displacement direction (radial direction) by combining with an EOR (extreme operating range) experiment, and finally selects a space layer selection testing method which only performs radial layer selection and does not perform two-dimensional scanning in the layer. Compared with the signal dispersion of one ten-thousandth of a three-dimensional nuclear magnetic resonance imaging sequence, the signal dispersion of radial layer selection of several layers to tens of layers can still ensure the SNR of sampled data. Meanwhile, in order to improve the measurement capability of the radial spatial slice selection T2 test sequence on short relaxation signals, a new radial spatial slice selection T2 test technology of a constant gradient mode is used, and the technology is the biggest difference from the traditional method: the echo sampling interval is not subjected to layer selection in a pulse gradient mode, so that the problem of small pore signal loss caused by gradient encoding time is solved, and effective measurement of pore fluid signals with different sizes is fully ensured.

Compared with the prior art, the invention has the following remarkable advantages:

1. according to the invention, through the improvement of experimental equipment materials and the targeted control of conditions, the upper limits of temperature and pressure conditions in the nuclear magnetic resonance measurement of the existing oil and gas development experiments at home and abroad are efficiently and reasonably broken, and the nuclear magnetic resonance imaging measurement of the high-temperature and high-pressure (the upper limit of the maximum working temperature is 150 ℃ and the upper limit of the maximum working pressure is 70 MPa) reservoir core is realized;

2. the invention overcomes the defects that the existing rock core three-dimensional nuclear magnetic resonance imaging pixel signal-to-noise ratio is low and the fluid migration state cannot be quantitatively represented, and obtains the radial spatial distribution characteristic quantitative visualization result of the fluid in the rock core through a new radial spatial slice selection T2 spectrum technology. The invention finally creates the research conditions of the residual oil distribution micro-mechanism in the oil gas development experiment, and provides an experimental basis for promoting the refined research of the domestic oil gas development experiment, improving the research level of domestic oil gas production increasing measures and the like.

Drawings

FIG. 1 is a diagram of a high temperature high pressure displacement on-line NMR measurement system;

FIG. 2 is a hard pulse multi-slice spin echo (HMSE) imaging sequence;

FIG. 3 is a phase encoded radial spatial slice selection T2 test sequence;

FIG. 4 is a comparison result of T2 spectrum test of linear water film and two radial space layer selection;

FIG. 5 is a T2 spectrum three-dimensional result chart under saturated oil and each displacement condition;

FIG. 6 is the T2 spectral imaging results for saturated oil and various displacement cases;

FIG. 7 shows the overall and layered T2 spectrum signals, relative saturation error results for saturated oil and various displacement situations.

The graphic symbols are illustrated as follows:

1. a clamping unit; 21. a confining pressure pump; 22. a confining pressure liquid storage device; 23. a displacement pump; 24. a displacement medium storage device; 25. a confining pressure liquid circulating pipeline; 26. a displacement pipeline; 27. a back pressure pump; 3. a magnet unit; 41. an oil-gas-water separation device; 42. a metering balance; 43. a camera; 44. a flow meter.

Detailed Description

The present invention will be further described with reference to the following embodiments. Wherein the showings are for the purpose of illustration only and not for the purpose of limiting the same, the same is shown by way of illustration only and not in the form of limitation; to better illustrate the embodiments of the present invention, some parts of the drawings may be omitted, enlarged or reduced, and do not represent the size of an actual product; it will be understood by those skilled in the art that certain well-known structures in the drawings and descriptions thereof may be omitted.

In the present invention, unless otherwise explicitly stated or limited, the terms "mounted," "connected," "fixed," and the like are to be construed broadly, e.g., as being permanently connected, detachably connected, or integral; can be mechanically or electrically connected; they may be directly connected or indirectly connected through intervening media, or they may be connected internally or in any other suitable relationship, unless expressly stated otherwise. The specific meanings of the above terms in the present invention can be understood by those skilled in the art according to specific situations.

In the present invention, unless otherwise expressly stated or limited, the first feature "on" or "under" the second feature may be directly contacting the first and second features or indirectly contacting the first and second features through an intermediate. Also, a first feature "on," "over," and "above" a second feature may be directly or diagonally above the second feature, or may simply indicate that the first feature is at a higher level than the second feature. A first feature being "under," "below," and "beneath" a second feature may be directly under or obliquely under the first feature, or may simply mean that the first feature is at a lesser elevation than the second feature.

Example 1

As shown in fig. 1, an embodiment of an online nuclear magnetic resonance imaging system according to the present invention includes a holding unit 1 for fixing a core, a high-temperature high-pressure displacement unit, a magnet unit 3 for generating a magnetic field and applying the magnetic field to the holding unit 1, an oil-gas-water three-phase measurement unit, an acquisition unit, a control unit, and an imaging unit, where the holding unit 1 is respectively communicated with the high-temperature high-pressure displacement unit and the oil-gas-water three-phase measurement unit, the acquisition unit is respectively communicated with the holding unit 1, the high-temperature high-pressure displacement unit, the magnet unit 3, the control unit, and the imaging unit, and the control unit is further respectively communicated with the holding unit 1, the high-temperature high-pressure displacement unit, the magnet unit 3, and the imaging unit.

As an embodiment of the invention, the high-temperature high-pressure displacement unit comprises a confining pressure pump 21, a confining pressure liquid storage device 22, a displacement pump 23, a displacement medium storage device 24, a heating device capable of acting on and heating the confining pressure liquid storage device 22, a confining pressure liquid circulation pipeline 25, a displacement pipeline 26 and a return pressure pump 27, wherein the displacement pipeline 26 communicates the displacement medium storage device 24, the displacement pump 23 and the clamping unit 1, the return pressure pump 27 and the clamping unit 1, and an oil-gas-water three-phase metering unit and the clamping unit 1; the confining pressure liquid circulating device is respectively communicated with the confining pressure pump 21 and the clamping unit 1 through a confining pressure liquid circulating pipeline 25.

As shown in fig. 1, the displacement pump 23 is an ISCO double-cylinder displacement pump 23, the displacement medium is fluorine oil, the intermediate container filled with fluorine oil is connected to the clamping unit 1 through the first section, the second section and the third section of the displacement pipeline 26, oil, gas and water reach the oil-gas-water three-phase metering unit through the fourth section and the fifth section of the displacement pipeline 26, the displacement pressure is controlled by combining an injection end pressure gauge and a fluid inlet end pressure under the action of the confining pressure pump 21 and the displacement pump 23 through the displacement fluid, the pressure at a fluid outlet end is controlled by the back pressure pump 27, the displacement pressure difference is generated by combining the two, a back pressure valve is further arranged between the clamping unit 1 and the back pressure pump 27, and the confining pressure pump 21 provides pressure equivalent to the pressure under the formation condition by pressurizing and controlling the back pressure valve at the outlet end. The temperature control system is connected with the confining pressure, the temperature is controlled by the confining pressure oil during heating, and the temperature can be raised to the temperature of the target stratum by the temperature control system. Furthermore, the working process of a software interface can be controlled fully automatically, and the interface is convenient for experimenters to realize the integrated and automatic control of NMR measurement and displacement experiment operation.

As an embodiment of the invention, a core installation cavity and a confining pressure cavity are arranged in the clamping unit 1, the core installation cavity is communicated with the displacement pipeline 26, the displacement medium storage device 24 and the oil-gas-water three-phase metering unit, and the confining pressure cavity is communicated with the confining pressure liquid circulation pipeline 25 and the confining pressure liquid storage device 22.

The rock core is placed in the rock core installation cavity, the confining pressure cavity is arranged at the bottom or around the rock core installation cavity, and heated confining pressure liquid circulates in the confining pressure cavity, so that the temperature of the rock core in the rock core installation cavity can be increased. The oil-gas-water three-phase metering unit can be used for metering saturated formation water in a rock core and also can be used for metering the amount of a displacement medium.

As an embodiment of the present invention, the magnet unit 3 includes a permanent magnet, a magnet holder installed at the bottom of the permanent magnet and installed in the magnet thermostat, a magnet thermostat, and a gradient coil installed around the permanent magnet, and the clamping unit 1 is installed at the side of the permanent magnet.

The magnet support provides support for the permanent magnet, and the permanent magnet and the gradient coil provide a magnetic field for the rock core.

As an embodiment of the invention, the oil-gas-water three-phase metering unit comprises an oil-gas-water separation device 41, a metering balance 42, an image pickup display camera 43 and a plurality of flow meters 44, wherein one end of the oil-gas-water separation device 41 is communicated with the clamping unit 1, the other end of the oil-gas-water separation device is respectively communicated with the plurality of flow meters 44 and the metering balance 42, and the image pickup display camera 43 is arranged beside the oil-gas-water separation device.

The plurality of flow meters 44 respectively comprise a gas flow meter and a liquid flow meter, wherein the liquid flow meter comprises a flow meter which can be used for measuring displacement oil liquid and a flow meter which can be used for measuring water, the flow meters with different measuring ranges can be adopted according to requirements, the oil-gas-water separation device 41 can be combined with the flow meters, and the oil-gas-water separation device 41 is provided with scales for measurement; the metering balance 42 is used for metering the weight (volume) of water and oil produced by core displacement, and then the produced oil volume is removed according to the scale of the oil-gas-water separation device 41, so that the volume of the produced water is obtained.

Example 2

The first embodiment of the online mri method according to the present invention is implemented by the above online mri system, and specifically includes the following steps:

washing the core with oil, drying, and carrying out basic physical property test;

s1: installing the vacuumized saturated formation water core into the clamping unit 1, and collecting a T2 spectrum in a saturated state;

s2: applying confining pressure to the target formation pressure to the clamping unit 1 through the high-temperature high-pressure displacement unit, and keeping the confining pressure greater than the displacement pressure; heating the clamping unit 1 through a high-temperature high-pressure displacement unit to raise the temperature to the target formation temperature; meanwhile, a magnetic field is established by using the magnet unit 3, a radio frequency pulse sequence is transmitted by using the control unit and the acquisition unit, and radial space phase gradient encoding is applied;

conventional mri technology usually adopts a hard pulse sequence (HMSE sequence), as shown in fig. 2, the sequence exerts the great advantage of short duration (us magnitude) of a hard pulse while realizing imaging by using three-dimensional gradient spatial coding, shortens TE of imaging to 2-3 ms, and ensures acquisition of short relaxation signals. But core imaging has a greater impact on the limited signal-to-noise ratio (SNR) than does TE. Assuming that 10 layers are selected during rock core imaging, the pixel point of each layer is 100 × 100, and pore fluid is uniformly distributed in the rock core, so that the signal of each pixel point on the image is one hundred thousand of the whole signal. The nuclear magnetic resonance signal of the rock core is a weak signal, and three-dimensional imaging segmentation is carried out, so that the SNR of the imaging pixel point is reduced sharply.

The high-temperature high-pressure displacement-based online nuclear magnetic resonance imaging technology provided by the invention abandons a hard pulse sequence usually adopted by three-dimensional nuclear magnetic resonance imaging, adopts a radial space layer selection T2 test sequence, as shown in figure 3, a phase coding gradient is applied between 90-degree and first 180-degree pulses, the subsequent echo acquisition uses the shortest echo interval TE for continuous sampling, the sampling mode is the same as that of the traditional CPMG sequence, the continuous SE echo acquires echo attenuation signals, and a constant gradient field exists during the sampling period to realize the function of gradient coding layer selection. The most different from the traditional phase encoding method is that: gradient encoding is done during the entire sampling period, so echo acquisition always uses the shortest TE sampling; only the selected frequency position is excited to sample, and the samples at other positions are in a state to be excited to sample, so that waiting time is not needed for sampling at the change position. And changing the phase encoding gradient to realize complete signal acquisition on the radial space of the sample. Therefore, the temperature of the molten metal is controlled,

as shown in fig. 3, the response signal of the phase-coded radial spatial layer selection T2 test is expressed as follows:

wherein S (N TE, m) is a sampling signal; TE is an echo interval, TE1 and TE2 are different echo intervals, TE1 is a first echo interval, TE2 is a second echo interval, and N is an echo serial number; z is the layer selection direction, i is the echo number, gamma is the gyromagnetic ratio, T2 is the transverse relaxation time, gmax is the maximum gradient value in the layer selection direction, tp is the phase encoding time, m is the arithmetic progression corresponding to the phase encoding step number, the constant gradient encoding mode is selective excitation sampling, and the radial space layer selection T2 spectrum f (T2, z) can be obtained by inverting the echo signal S (N.TE, z) at the unused position;

s3: the displacement medium is driven to the clamping unit 1 through the high-temperature high-pressure displacement unit, the formation water with saturated rock cores is driven, and the displacement medium and the formation water both enter the oil-gas-water three-phase metering unit;

the displacement is carried out by adjusting the displacement pump 23 to select a constant speed or constant pressure mode; preferably, the displacement medium is fluorine oil;

s4: reading the water yield through an oil-gas-water three-phase metering unit, and simultaneously collecting a T2 spectral line in a real-time state;

firstly, performing inverse Fourier transform on a sampling signal S (N.TE, M) to obtain echo attenuation signals M (N.TE, z) at different positions, and performing spectrum decomposition by using a BRD and T2 test inversion algorithm to obtain a radial space layer selection T2 spectrum f (T2, z), wherein the formula is as follows:

wherein M (N.TE, z) is an echo attenuation signal, S (N.TE, M) is a sampling signal, gmax is a gradient maximum value in the layer selection direction, z is the layer selection direction, tp is phase encoding time, M is an arithmetic progression corresponding to the phase encoding step number, N is an echo serial number, TE1 is a first echo interval, TE2 is a second echo interval, i is the echo number, gamma is a gyromagnetic ratio, and T2 is a transverse relaxation time;

s5, detecting whether the water yield is increased, if so, returning to the step S4, and if not, entering the step S6;

Example 3

As shown in fig. 4 to 7, the present invention is a second embodiment of an online mri method, and this embodiment is similar to embodiment 2 except that saturated formation water in the core is replaced with saturated kerosene.

In order to verify the accuracy of the radial spatial layer selection T2 testing method, a linear water film with linearly decreasing volume along the length direction as shown in FIG. 4a is designed, and a copper sulfate aqueous solution with relaxation time T2=20ms is filled in the water film; fig. 4b shows a linear radial spatial layer selection T2 spectrum and a layered signal profile tested by the phase encoding method, and fig. 4c shows a linear water film radial spatial layer selection T2 spectrum and a layered signal profile tested by the constant gradient layer selection method of the present invention. Fig. 4 illustrates that the slice selection method proposed by the present invention has the same spatial localization function as the conventional mri method (phase encoding method).

The operation of the steps can realize the distinguishing and quantitative calculation of water phase and oil gas phase signals in the oil and gas development displacement experiment process, and further obtain the space distribution of the integral recovery ratio and the residual oil saturation in the displacement direction.

In order to verify the practicability and accuracy of the method provided by the invention, a sandstone water flooding experiment is carried out, and the experimental flow is as follows:

(1) Washing oil from the core, drying and carrying out basic physical property test;

(2) Vacuumizing and pressurizing to soak the rock core into kerosene until the rock core is saturated;

(3) Raising the temperature of the high-temperature and high-pressure holder to 150 ℃, placing the holder between magnets, and measuring the whole T2 spectrum of the rock core and the radial space stratum selection T2 spectrum as base signals;

(4) Loading a saturated kerosene sample into a high-temperature high-pressure clamp holder, injecting nitrogen into an inlet and an outlet of the clamp holder simultaneously to enable the pore pressure to be gradually increased to 62MPa, enabling the confining pressure to be increased to 70MPa, and starting heating to enable the temperature of the sample to be gradually increased to 150 ℃ in the process; then, reducing the back pressure of an outlet to 61MPa, and measuring the integral T2 spectrum and the radial space bed selection T2 spectrum of the saturated oil core before the displacement does not start;

(5) Subtracting the base signal from the integral T2 spectrum and the radial space selective layer T2 spectrum when the oil core is saturated to obtain an initial distribution integral T2 spectrum and a radial space selective layer T2 spectrum in a saturated oil state;

(6) Preparing a displacement fluid with the property close to that of formation water by using heavy water, starting the displacement pump 23, setting the flow rate to be 0.2ml/min in a constant flow mode, and testing the integral T2 spectrum and the radial space stratum selection T2 spectrum of the rock core after displacing 0.3PV, 0.5PV, 1PV, 2PV, 5PV, 8PV and 10PV respectively;

(7) And calculating the layered saturation by using the spatial layer selection T2 spectrum in a saturated state, and calculating the layered saturation of all the layers by taking the saturation measured by the whole T2 spectrum as a reference to obtain the relative error of the whole saturation. The results of the saturated kerosene and the corresponding experiments at each displacement are shown in the attached figures 5-7.

As can be seen from fig. 5-7, the shielding of the water phase signal and the retention of the oil signal are realized by the heavy water displacement, so that the integral T2 spectrum measurement signal and the decrease of the radial spatial selective layer T2 spectrum signal are both changes caused by the decrease of the oil saturation in the heavy water displacement process; as shown in fig. 5 and 6, the distribution of oil saturation at different positions along the displacement direction is provided by radial spatial slice selection T2 spectra. According to the different relaxation time corresponding to the pore with different size, the information of the oil using rule in the pore with different size can be obtained. It can be seen from the two figures that before the displacement is started, the core is saturated with oil, the oil is mainly concentrated in large pores at the lower part of the core due to the gravity differentiation, the oil saturation is gradually reduced along with the displacement, the displacement efficiency reaches the peak value after the displacement reaches 2PV, and the oil consumption degree of the subsequent displacement is no longer obvious. Thanks to the measurement capability of the constant gradient layer selection technology on small pore signals, the relative error between the integral saturation calculated by the layered measurement and the saturation obtained by the integral T2 measurement is controlled within 5 percent, as shown in figure 7, and the precision requirement of the application test is completely met.

In conclusion, theoretical and practical tests prove that the method and the technology provided by the invention can realize the measurement function of the recovery rate and the residual oil spatial distribution in an oil and gas development experiment, and have industrial popularization and application conditions.

It should be understood that the above-described embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the embodiments of the present invention. Other variations and modifications will be apparent to persons skilled in the art in light of the above description. And are neither required nor exhaustive of all embodiments. Any modification, equivalent replacement, and improvement made within the spirit and principle of the present invention should be included in the protection scope of the claims of the present invention.

Claims

1. The utility model provides an online nuclear magnetic resonance imaging system, its characterized in that, including centre gripping unit (1) that is used for fixed rock core, high temperature high pressure displacement unit, be used for producing magnetic field and can make magnetic field act on magnet unit (3), oil gas water three-phase measurement unit, acquisition unit, the control unit, the imaging unit of centre gripping unit (1), centre gripping unit (1) respectively with high temperature high pressure displacement unit oil gas water three-phase measurement unit intercommunication, the acquisition unit respectively with centre gripping unit (1), high temperature high pressure displacement unit, magnet unit (3), the control unit, imaging unit communication connection, the control unit still respectively with centre gripping unit (1), high temperature high pressure displacement unit, magnet unit (3), imaging unit communication connection.

2. The online magnetic resonance imaging system according to claim 1, wherein the high-temperature high-pressure displacement unit comprises a confining pressure pump (21), a confining pressure liquid storage device (22), a displacement pump (23), a displacement medium storage device (24), a heating device capable of acting on and heating the confining pressure liquid storage device (22), a confining pressure liquid circulation pipeline (25), a displacement pipeline (26) and a back pressure pump (27), wherein the displacement pipeline (26) connects the displacement medium storage device (24), the displacement pump (23) and the clamping unit (1), the back pressure pump (27) and the clamping unit (1), and the oil-gas-water three-phase metering unit and the clamping unit (1) are communicated; the confining pressure liquid circulating device is respectively communicated with the confining pressure pump (21) and the clamping unit (1) through a confining pressure liquid circulating pipeline (25).

3. The on-line nuclear magnetic resonance imaging system according to claim 2, wherein a core installation cavity and a confining pressure cavity are arranged in the clamping unit (1), the core installation cavity is communicated with the displacement pipeline (26), the displacement medium storage device (24) and the oil-gas-water three-phase metering unit, and the confining pressure cavity is communicated with the confining pressure liquid circulation pipeline (25) and the confining pressure liquid storage device (22).

4. On-line MRI system according to any of the claims 1-3, characterized in that the magnet unit (3) comprises a permanent magnet and a magnet holder, a magnet thermostat, a gradient coil, the magnet holder is mounted at the bottom or around the permanent magnet and in the magnet thermostat, the gradient coil is arranged around the permanent magnet, the clamping unit (1) is arranged beside the permanent magnet.

5. The online magnetic resonance imaging system according to claim 4, wherein the oil-gas-water three-phase metering unit comprises an oil-gas-water separating device (41), a metering balance (42), a camera display camera (43) and a plurality of flow meters (44), one end of the oil-gas-water separating device (41) is communicated with the clamping unit (1), the other end of the oil-gas-water separating device is communicated with the plurality of flow meters (44) and the metering balance (42), and the camera display camera (43) is installed beside the oil-gas separating device.

6. An online magnetic resonance imaging method, characterized in that the online magnetic resonance imaging system according to any one of claims 1 to 5 is applied, and the method specifically comprises the following steps:

s1: washing the core with oil, drying, carrying out helium porosity and permeability tests, installing the vacuumized saturated formation water core into a clamping unit (1), and collecting a T2 spectrum in a saturated state;

s2: applying confining pressure to the target formation pressure to the clamping unit (1) through the high-temperature high-pressure displacement unit, and keeping the confining pressure greater than the displacement pressure; heating the clamping unit (1) through a high-temperature high-pressure displacement unit to raise the temperature to the target formation temperature; meanwhile, a magnetic field is established by using the magnet unit (3), a radio frequency pulse sequence is transmitted by using the control unit and the acquisition unit, and radial space phase encoding is applied;

s3: driving a displacement medium to the clamping unit (1) through the high-temperature high-pressure displacement unit, driving formation water with saturated rock cores, and enabling the displacement medium and the formation water to enter an oil-gas-water three-phase metering unit;

s5: detecting whether the water yield is increased or not, if so, returning to the step S4, and if not, entering the step S6;

s7: and analyzing and comparing the nuclear magnetic T2 spectral lines in different states, and recording the nuclear magnetic T2 spectral lines through imaging of the imaging unit.

7. The on-line MRI method according to claim 6, wherein the response signal of the phase-coded radial spatial slice T2 test in step S2 is expressed as follows:

wherein S (N.TE, m) is a sampling signal, TE is an echo interval, TE1 is a first echo interval, TE2 is a second echo interval, N is an echo serial number, z is a layer selection direction, i is the number of echoes, gamma is a gyromagnetic ratio, T2 is transverse relaxation time, gmax is a maximum gradient value in the layer selection direction, tp is phase encoding time, m is an arithmetic progression corresponding to the phase encoding step number, the constant gradient encoding mode is selective excitation sampling, and a radial space layer selection T2 spectrum f (T2, z) can be obtained by inverting the echo signal S (N.TE, z) at different positions.

8. The on-line mri method of claim 7, wherein the T2 spectral line is obtained in steps S4 and S6 as follows: firstly, performing inverse Fourier transform on a sampling signal S (N.TE, M) to obtain echo attenuation signals M (N.TE, z) at different positions, and performing spectrum decomposition by using a BRD mode smoothing method and a T2 test inversion algorithm to obtain a radial space layer selection T2 spectrum f (T2, z), wherein the formula is as follows:

9. An on-line mri method according to claim 7, characterized in that the displacement is performed by adjusting the displacement pump (23) to select a constant speed or a constant pressure during the displacement of step S3.

10. The on-line mri method of any one of claims 6-9, further comprising, after step S6, a detection step of: combining the T2 spectrum in the saturated state with the T2 spectrum in the bound state to calculate the saturation of the bound water, comparing the saturation with the saturation of the bound water calculated by the water yield meter, and if the difference between the saturation and the saturation is within +/-10%, indicating that the inorganic device of the online nuclear magnetic resonance imaging system fails; if the difference between the two is within +/-10%, the online nuclear magnetic resonance imaging system is firstly subjected to machine detection and then returns to the step S1 to start operation.