An online magnetic resonance imaging system and method

Abstract

The present invention relates to the technical field of experimental measurement of oil and gas development, and more specifically, to an online nuclear magnetic resonance imaging system and method, wherein the online nuclear magnetic resonance imaging system includes a clamping unit, a high temperature and high pressure displacement unit, a magnet unit, an oil, gas and water three-phase metering unit, a collection unit, a control unit, and an imaging unit, wherein the clamping unit is respectively connected to the high temperature and high pressure displacement unit and the oil, gas and water three-phase metering unit, the collection unit is respectively connected to 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 connected to the clamping unit, the high temperature and high pressure displacement unit, the magnet unit, and the imaging unit. The present invention can realize the high temperature and high pressure core displacement nuclear magnetic resonance online measurement mode under formation conditions, obtain the visible spatial distribution characteristics of the fluid in the high temperature and high pressure reservoir core in the displacement direction, and then simulate the dynamic seepage characteristics and saturation distribution characteristics of crude oil in the actual core pores.

Description

Technical Field

[0001] The present invention relates to the technical field of oil and gas development experimental measurement, and more specifically, to an online nuclear magnetic resonance imaging system and method. Background Art

[0002] In recent years, as most domestic oil and gas fields have entered the middle and late stages of development, enhanced oil recovery (EOR) has become an important task in oil and gas development. The measurement of the spatial distribution of remaining oil in oil and gas development experiments has always been the focus and difficulty of EOR experiments. Traditional experimental techniques such as sand filling tubes and glass etching are difficult to truly simulate the seepage state of the reservoir, which makes it difficult to conduct detailed research on enhanced oil recovery plans. Since the beginning of the 21st century, with the application and popularization of imaging technologies such as CT electronic computer tomography and NMR nuclear magnetic resonance in the field of oil and gas exploration and development, favorable conditions have been created for the measurement of the spatial distribution of remaining oil. NMR technology has been maturely applied in the field of petroleum exploration and development due to its characteristics of rapidity, non-destructiveness, non-toxicity, and sensitivity only to hydrogen-containing pore fluids.

[0003] Unlike the measurement of the overall NMR signal of the sample in the exploration field, EOR experiments require NMR technology to provide the spatial distribution of fluid in the core, and nuclear magnetic resonance imaging technology can meet this demand in principle. However, nuclear magnetic resonance imaging of objects requires a three-dimensional gradient positioning system. The most commonly used is a three-dimensional orthogonal gradient coil to achieve three-dimensional spatial gradient encoding in the sample area. After the gradient field is applied, the three-dimensional spatial distribution of the core pore fluid can be obtained by performing an inverse Fourier transform on the nuclear magnetic imaging signal.

[0004] Commonly used nuclear magnetic resonance imaging sequences include MSE sequence, FSE, and GRE sequence. The radio frequency excitation pulses of these three imaging sequences are all soft pulses. The characteristics of soft pulses are small frequency domain bandwidth and good layer selection characteristics. The disadvantage is that the pulse width is large, which is in the ms order. In the soft pulse imaging sequence, the radio frequency pulse width, plus the gradient encoding time (also in the ms order), results in a TE of at least 5 to 10 ms when the echo signal is generated. For dense cores with short relaxation time and low signal amount, there will be serious loss of small and medium pore signals. In order to reduce the loss of short relaxation signals, the echo interval TE of the imaging sequence must be shortened. Therefore, the hard pulse three-dimensional imaging sequence HMSE has emerged, such as Figure 2As shown. The HMSE sequence uses three-dimensional gradient spatial encoding to achieve imaging, while taking advantage of the short duration of hard pulses (in the order of us), shortening the TE of imaging to 2-3ms, ensuring the acquisition of short relaxation signals. However, in addition to higher requirements for TE, core imaging is also more affected by limited signal-to-noise ratio (SNR). Assuming that 10 layers are selected for core imaging, the pixel points of each layer are 100*100, and the pore fluid is evenly distributed inside the core, then the signal of each pixel point on the image is one hundred thousandth of the overall signal. The core nuclear magnetic resonance signal itself is a weak signal, and three-dimensional imaging segmentation will cause the SNR of the imaging pixels to drop sharply.

[0005] In summary, the gradient encoding time of the existing NMR imaging sequence occupies the echo time, and the typical characteristics of short relaxation time and weak signal in core samples make it difficult for the commonly used three-dimensional NMR imaging method to achieve ideal results in high-temperature and high-pressure reservoir tight core testing. Summary of the invention

[0006] In view of the shortcomings of the existing technology, especially the technical status quo that the nuclear magnetic resonance experiment of the existing core displacement process cannot quickly obtain the spatial imaging characteristics of the fluid inside the core, the present invention proposes an online nuclear magnetic resonance imaging test system for high-temperature and high-pressure cores, which can realize the high-temperature and high-pressure core displacement nuclear magnetic resonance online measurement mode under formation conditions, obtain the visible spatial distribution characteristics of the fluid in the high-temperature and high-pressure reservoir core in the displacement direction, and then simulate the dynamic seepage characteristics and saturation distribution characteristics of crude oil in the actual core pores.

[0007] In order to solve the above technical problems, the technical solution adopted by the present invention is:

[0008] Provided is an online nuclear magnetic resonance imaging system, comprising a clamping unit for fixing a core, a high-temperature and high-pressure displacement unit, a magnet unit for generating a magnetic field and enabling the magnetic field to act on the clamping unit, an oil-gas-water three-phase metering unit, a collection unit, a control unit, and an imaging unit, wherein the clamping unit is respectively connected to the high-temperature and high-pressure displacement unit and the oil-gas-water three-phase metering unit, the collection unit is respectively connected to 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 connected to the clamping unit, the high-temperature and high-pressure displacement unit, the magnet unit, and the imaging unit.

[0009] The high temperature and high pressure displacement unit of the present invention is used to provide the temperature, overburden pressure, fluid pressure, and fluid type environment required for simulating the formation environment when testing samples; the function of the magnet unit is to provide a magnet environment and a gradient imaging environment for nuclear magnetic resonance imaging; the oil, gas, and water three-phase metering unit includes at least 4 flow meters with different ranges. After the high temperature and high pressure displacement unit applies the target formation pressure and temperature to the core, a magnetic field is formed around the core through the magnet unit, and only radial layer selection is selected, and no two-dimensional scanning spatial layer selection test is performed within the layer. Through the new radial spatial layer selection T2 spectrum technology, the radial spatial distribution characteristics of the fluid in the core are quantitatively visualized.

[0010] Preferably, the high-temperature and high-pressure displacement unit includes a confining pressure pump, a confining pressure liquid storage device, a displacement pump, a displacement medium storage device, a heating device that can act on the confining pressure liquid storage device and heat it, a confining pressure liquid circulation pipeline, a displacement pipeline, and a back-pressure pump. The displacement pipeline connects 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; the confining pressure liquid circulation device is connected to the confining pressure pump and the clamping unit respectively through the confining pressure liquid circulation pipeline.

[0011] Preferably, a core installation cavity and a confining pressure cavity are provided in the clamping unit, the core installation cavity is connected with the displacement pipeline, the displacement medium storage device and the oil, gas and water three-phase metering unit, and the confining pressure cavity is connected with the confining pressure liquid circulation pipeline and the confining pressure liquid storage device.

[0012] Preferably, the clamping unit uses high-density and high-strength zirconia, with a maximum operating temperature limit of 150°C and a maximum operating pressure limit of 70MPa, which can meet the requirements of nuclear magnetic resonance measurements under high-temperature and high-pressure formation conditions. The core heating method is to heat it with confining pressure fluid and then circulate it to the confining pressure chamber of the clamping unit through the confining pressure fluid circulation pipeline. The control unit includes a temperature control system, which is connected to the confining pressure chamber. The heating is to heat the core and maintain it at the temperature required for the experiment by circulating the confining pressure oil. The clamping unit is used to clamp the core, and the displacement medium storage device can use an intermediate container or a gas storage tank. By connecting the clamping unit with the displacement medium storage device, different sources of displacement fluids can be obtained, thereby realizing the recovery rate simulation experiment of oil, gas and water three-phase fluids at different stages.

[0013] Preferably, the magnet unit includes a permanent magnet and a magnet holder, a magnet constant temperature device, and a gradient coil. The magnet holder is installed at the bottom or around the permanent magnet and is installed in the magnet constant temperature device. The gradient coil is arranged around the permanent magnet, and the clamping unit is arranged next to the permanent magnet.

[0014] Preferably, the oil-gas-water three-phase metering unit includes an oil-gas-water separation device, a metering balance, a camera, and multiple flow meters. One end of the oil-gas-water separation device is connected to the clamping unit, and the other end is respectively connected to multiple flow meters and the metering balance. The camera is installed next to the oil-gas-water separation device.

[0015] The present invention also provides an online nuclear magnetic resonance imaging method, which is operated by using the online nuclear magnetic resonance imaging system as described above, and specifically comprises the following steps:

[0016] S1: Install the vacuumed saturated formation water core into the clamping unit and collect the T2 spectrum under saturation;

[0017] S2: applying confining pressure to the clamping unit to the target formation pressure through the high temperature and high pressure displacement unit, and maintaining the confining pressure greater than the displacement pressure; heating the clamping unit through the high temperature and high pressure displacement unit to raise the temperature to the target formation temperature; at the same time, using the magnet unit to establish a magnetic field, using the control unit and the acquisition unit to transmit a radio frequency pulse sequence and apply radial spatial phase encoding;

[0018] S3: The displacement medium is driven to the clamping unit through the high temperature and high pressure displacement unit to drive the core saturated formation water, and the displacement medium and formation water both enter the oil, gas and water three-phase metering unit;

[0019] S4: Read the water output through the oil, gas and water three-phase metering unit, and collect the T2 spectrum in real time;

[0020] S5 detects whether the water output increases. If so, the process returns to step S4. If not, the process proceeds to step S6.

[0021] S6: Calculate the water saturation and formation water saturation by combining the saturated water volume, and determine the T2 spectrum under the bound water state;

[0022] S7: Analyze and compare the nuclear magnetic T2 spectra under different states, and use the imaging unit to record the images.

[0023] Furthermore, the response signal expression of the phase-encoded radial spatial layer selection T2 test in step S2 is as follows:

[0024]

[0025] Where S(N·TE,m) is the sampling signal; TE is the echo interval, TE1 is the first echo interval, TE2 is the second echo interval, N is the echo number, z is the layer selection direction, i is the number of echoes, γ is the gyromagnetic ratio, T2 is the transverse relaxation time, Gmax is the maximum gradient in the layer selection direction, tp is the phase encoding time, m is the arithmetic progression corresponding to the number of phase encoding steps, and the constant gradient encoding method is selective excitation sampling. The radial space selected T2 spectrum f(T2,z) can be obtained by inverting the echo signal S(N·TE,z) at different positions.

[0026] Furthermore, the method for acquiring the T2 spectrum line in step S4 and step S6 is as follows: first, perform inverse Fourier transform on the sampling signal S(N·TE,m) to obtain the echo attenuation signal M(N·TE,z) at different positions, and use the BRD mode smoothing method and T2 test inversion algorithm to decompose the spectrum to obtain the radial space selected layer T2 spectrum f(T2,z), the formula is as follows:

[0027]

[0028] Where M(N·TE,z) is the echo attenuation signal, S(N·TE,m) is the sampling signal, Gmax is the maximum gradient in the slice selection direction, z is the slice selection direction, tp is the phase encoding time, m is the arithmetic progression corresponding to the phase encoding step number, N is the echo number, TE1 is the first echo interval, TE2 is the second echo interval, i is the number of echoes, γ is the gyromagnetic ratio, and T2 is the transverse relaxation time.

[0029] Furthermore, during the displacement process of step S3, the displacement pump is adjusted to select a constant speed or constant pressure mode for displacement.

[0030] Furthermore, after step S6, a detection step is also included: the bound water saturation is calculated by combining the T2 spectrum in the saturated state with the T2 spectrum in the bound state, and the bound water saturation is compared with the bound water saturation calculated by the water output measurement. If the difference between the two is within ±10%, it means that there is no machine failure in the online nuclear magnetic resonance imaging system; if the difference between the two is outside ±10%, the machine detection of the online nuclear magnetic resonance imaging system is first performed and then the operation is returned to step S1 to start again.

[0031] The radial spatial gradient encoding technology of the present invention takes into account the shortcomings of the three-dimensional nuclear magnetic resonance imaging method in terms of SNR, and combines the EOR experiment to pay more attention to the changes in the remaining oil in the displacement direction (radial). Finally, only radial layer selection is selected, and the spatial layer selection test method of no longer performing two-dimensional scanning within the layer is selected. Compared with the signal discreteness of one hundred thousandth of the three-dimensional nuclear magnetic resonance imaging sequence, the radial layer selection of several to dozens of layers can still guarantee the SNR of the sampled data. At the same time, in order to improve the measurement capability of the radial spatial layer selection T2 test sequence for short relaxation signals, a new radial spatial layer selection T2 test technology with a constant gradient mode is used. The biggest difference between this technology and the traditional method is that the echo sampling interval is not selected by a pulse gradient method, which avoids the problem of small pore signal loss caused by the gradient encoding time, and fully guarantees the effective measurement of pore fluid signals of different sizes.

[0032] Compared with the prior art, the present invention has the following significant advantages:

[0033] 1. The present invention has effectively and reasonably broken the upper limits of temperature and pressure conditions in the existing nuclear magnetic resonance measurements of oil and gas development experiments at home and abroad by improving experimental equipment materials and targeted control of conditions, and has realized nuclear magnetic resonance imaging measurements of reservoir cores at high temperature and high pressure (maximum operating temperature upper limit is 150°C, maximum operating pressure upper limit is 70MPa);

[0034] 2. The present invention overcomes the shortcomings of the existing core three-dimensional nuclear magnetic resonance imaging, which has low pixel signal-to-noise ratio and cannot quantitatively characterize the fluid migration state. Through the new technology of radial space layer selection T2 spectrum, the radial space distribution characteristics of the fluid in the core are quantitatively visualized. The present invention ultimately creates the conditions for studying the microscopic mechanism of the remaining oil distribution in oil and gas development experiments, and provides an experimental basis for promoting the refined research of domestic oil and gas development experiments and improving the research level of domestic oil and gas recovery measures. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 This is a structural diagram of a high-temperature and high-pressure displacement online nuclear magnetic resonance measurement system;

[0036] Figure 2 It is a hard pulse multislice spin echo (HMSE) imaging sequence;

[0037] Figure 3 It is a phase-encoded radial space-selected T2 test sequence;

[0038] Figure 4 Comparison results of linear water film and two radial space selected layer T2 spectrum tests;

[0039] Figure 5 The three-dimensional result diagram of T2 spectrum under saturated oil and various displacement conditions;

[0040] Figure 6 The T2 spectrum imaging results under saturated oil and various displacement conditions;

[0041] Figure 7 The overall and layered T2 spectrum signals and saturation relative error results under saturated oil and various displacement conditions.

[0042] The illustrations are as follows:

[0043] 1. Clamping unit; 21. Confining pressure pump; 22. Confining pressure liquid storage device; 23. Displacement pump; 24. Displacement medium storage device; 25. Confining pressure liquid circulation pipeline; 26. Displacement pipeline; 27. Back pressure pump; 3. Magnet unit; 41. Oil-gas-water separation device; 42. Measuring balance; 43. Camera; 44. Flow meter. DETAILED DESCRIPTION

[0044] The present invention is further described below in conjunction with specific implementation methods. The accompanying drawings are only used for exemplary descriptions and are only schematic diagrams, not actual drawings, and cannot be understood as limiting this patent; in order to better illustrate the embodiments of the present invention, some parts of the accompanying drawings may be omitted, enlarged or reduced, and do not represent the size of the actual product; for those skilled in the art, it is understandable that some well-known structures and their descriptions in the accompanying drawings may be omitted.

[0045] In the present invention, unless otherwise clearly specified and limited, the terms "installed", "connected", "connected", "fixed" and the like should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium, it can be the internal connection of two elements or the interaction relationship between two elements, unless otherwise clearly defined. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0046] In the present invention, unless otherwise clearly specified and limited, a first feature being "above" or "below" a second feature may mean that the first and second features are in direct contact, or the first and second features are in indirect contact through an intermediate medium. Moreover, a first feature being "above", "above" or "above" a second feature may mean that the first feature is directly above or obliquely above the second feature, or simply means that the first feature is higher in level than the second feature. A first feature being "below", "below" or "below" a second feature may mean that the first feature is directly below or obliquely below the second feature, or simply means that the first feature is lower in level than the second feature.

[0047] Example 1

[0048] like Figure 1The figure shows an embodiment of an online nuclear magnetic resonance imaging system of the present invention, comprising a clamping unit 1 for fixing a core, a high-temperature and high-pressure displacement unit, a magnet unit 3 for generating a magnetic field and enabling the magnetic field to act on the clamping unit 1, an oil-gas-water three-phase metering unit, a collection unit, a control unit, and an imaging unit. The clamping unit 1 is respectively connected to the high-temperature and high-pressure displacement unit and the oil-gas-water three-phase metering unit, the collection unit is respectively connected to the clamping unit 1, the high-temperature and high-pressure displacement unit, the magnet unit 3, the control unit, and the imaging unit, and the control unit is also respectively connected to the clamping unit 1, the high-temperature and high-pressure displacement unit, the magnet unit 3, and the imaging unit.

[0049] As an embodiment of the present invention, the high-temperature and high-pressure displacement unit includes 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 that can act on the confining pressure liquid storage device 22 and heat it, a confining pressure liquid circulation pipeline 25, a displacement pipeline 26, and a back-pressure pump 27. 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; the confining pressure liquid circulation device is connected to the confining pressure pump 21 and the clamping unit 1 respectively through the confining pressure liquid circulation pipeline 25.

[0050] like Figure 1 As shown, the displacement pump 23 adopts an ISCO double-cylinder displacement pump 23, and the displacement medium adopts fluorine oil. The intermediate container filled with fluorine oil is connected to the clamping unit 1 through the first, second and third sections of the displacement pipeline 26. The oil, gas and water reach the oil, gas and water three-phase metering unit through the fourth and fifth sections of the displacement pipeline 26. The displacement pressure is controlled by the displacement fluid under the action of the confining pressure pump 21 and the displacement pump 23, and the pressure at the fluid inlet end is controlled by the injection end pressure gauge. The back pressure pump 27 controls the pressure at the fluid outlet end. The combination of the two creates a displacement pressure difference. A back pressure valve is also provided between the clamping unit 1 and the back pressure pump 27, and the confining pressure pump 21 controls the back pressure valve by pressurizing the outlet end to provide a pressure equivalent to the formation conditions. The temperature control system is connected with the confining pressure, and the temperature is controlled by the confining pressure oil. The temperature control system can raise the temperature to the target formation temperature. Furthermore, the fully automatic control software interface workflow can also be used, which is convenient for experimenters to realize the integrated and automated control of NMR measurement and displacement experiment operations.

[0051] As an embodiment of the present invention, a core installation chamber and a confining pressure chamber are provided in the clamping unit 1. The core installation chamber is connected to the displacement pipeline 26, the displacement medium storage device 24 and the oil-gas-water three-phase metering unit, and the confining pressure chamber is connected to the confining pressure liquid circulation pipeline 25 and the confining pressure liquid storage device 22.

[0052] The core is placed in the core installation cavity, and the confining pressure cavity is set at the bottom or around the core installation cavity. The temperature of the core in the core installation cavity is increased by circulating the heated confining pressure fluid in the confining pressure cavity. The oil, gas and water three-phase metering unit can measure the saturated formation water in the core and the amount of displacement medium.

[0053] As one embodiment of the present invention, the magnet unit 3 includes a permanent magnet and a magnet holder, a magnet constant temperature device, and a gradient coil. The magnet holder is installed at the bottom of the permanent magnet and installed in the magnet constant temperature device. The gradient coil is arranged around the permanent magnet, and the clamping unit 1 is arranged next to the permanent magnet.

[0054] The magnet bracket provides support for the permanent magnet, and the permanent magnet and the gradient coil provide a magnetic field for the core.

[0055] As an embodiment of the present invention, the oil-gas-water three-phase metering unit includes an oil-gas-water separation device 41, a metering balance 42, a video display camera 43, and multiple flow meters 44. One end of the oil-gas-water separation device 41 is connected to the clamping unit 1, and the other end is respectively connected to the multiple flow meters 44 and the metering balance 42. The video display camera 43 is installed next to the oil-gas-water separation device.

[0056] The multiple flow meters 44 include gas flow meters and liquid flow meters, among which the liquid flow meters include flow meters that can be used to measure the displacement oil and flow meters that can be used to measure water. Flow meters with different ranges can be used as needed. The oil-gas-water separation device 41 can also be combined with the flow meter, and measurement is performed by installing a scale on the oil-gas-water separation device 41; the measuring balance 42 is used to measure the weight (volume) of water and oil produced by core displacement, and then the volume of produced oil is eliminated according to the scale of the oil-gas-water separation device 41, so as to obtain the volume of produced water.

[0057] Example 2

[0058] A first embodiment of an online nuclear magnetic resonance imaging method of the present invention is operated using the online nuclear magnetic resonance imaging system as described above, and specifically comprises the following steps:

[0059] After washing the oil from the core, the core is dried and basic physical property tests are carried out;

[0060] S1: Install the vacuumed saturated formation water core into the clamping unit 1 to collect the T2 spectrum under saturation;

[0061] S2: applying confining pressure to the clamping unit 1 to the target formation pressure through the high temperature and high pressure displacement unit, and maintaining the confining pressure greater than the displacement pressure; heating the clamping unit 1 to raise the temperature to the target formation temperature through the high temperature and high pressure displacement unit; and establishing a magnetic field by using the magnet unit 3 at the same time, and transmitting a radio frequency pulse sequence and applying radial spatial phase gradient encoding by using the control unit and the acquisition unit;

[0062] Conventional MRI techniques usually use hard pulse sequences (HMSE sequences), such as Figure 2 As shown in the figure, while using three-dimensional gradient spatial encoding to achieve imaging, this sequence takes advantage of the short duration of hard pulses (in the order of us), shortens the TE of imaging to 2-3ms, and ensures the acquisition of short relaxation signals. However, in addition to higher requirements for TE, core imaging is also more affected by the limited signal-to-noise ratio (SNR). Assuming that 10 layers are selected for core imaging, the pixel points of each layer are 100*100, and the pore fluid is evenly distributed inside the core, then the signal of each pixel point on the image is one hundred thousandth of the overall signal. The core nuclear magnetic resonance signal itself is a weak signal, and three-dimensional imaging segmentation will cause the SNR of the imaging pixels to drop sharply.

[0063] The high temperature and high pressure displacement online nuclear magnetic resonance imaging technology provided by the present invention abandons the hard pulse sequence commonly used in three-dimensional nuclear magnetic resonance imaging and adopts a radial space selected layer T2 test sequence, as shown in the attached Figure 3 As shown, a phase encoding gradient is applied between the 90° and the first 180° pulses, and subsequent echo acquisition uses the shortest echo interval TE for continuous sampling. The sampling mode is the same as the traditional CPMG sequence. Continuous SE echoes acquire echo attenuation signals. During the sampling period, there is a constant gradient field to realize the gradient encoding layer selection function. The biggest difference from the traditional phase encoding method: gradient encoding is completed during the entire sampling period, so echo acquisition always uses the shortest TE sampling; only the selected frequency position will be excited for sampling, and samples at other positions are in a state to be excited for sampling, so there is no waiting time for changing position sampling. Change the phase encoding gradient to achieve complete signal acquisition in the radial space of the sample. Therefore,

[0064] like Figure 3 , the response signal expression of phase-encoded radial space-selective T2 test is as follows:

[0065]

[0066] Where S(N·TE,m) is the sampling signal; TE is the echo interval, TE1 and TE2 are different echo intervals, TE1 is the first echo interval, TE2 is the second echo interval, and N is the echo number; z is the slice selection direction, i is the number of echoes, γ is the gyromagnetic ratio, T2 is the transverse relaxation time, Gmax is the maximum gradient in the slice selection direction, tp is the phase encoding time, and m is the arithmetic progression corresponding to the number of phase encoding steps. The constant gradient encoding method is selective excitation sampling. The radial space slice selection T2 spectrum f(T2,z) can be obtained by inverting the echo signal S(N·TE,z) at different positions;

[0067] S3: The displacement medium is driven to the clamping unit 1 through the high temperature and high pressure displacement unit to drive the core saturated formation water, and the displacement medium and formation water both enter the oil, gas and water three-phase metering unit;

[0068] By adjusting the displacement pump 23, a constant speed or a constant pressure mode is selected for displacement; preferably, the displacement medium is fluorine oil;

[0069] S4: Read the water output through the oil, gas and water three-phase metering unit, and collect a T2 spectrum line in real time;

[0070] First, perform inverse Fourier transform on the sampling signal S(N·TE,m) to obtain the echo attenuation signal M(N·TE,z) at different positions. Use BRD and T2 test inversion algorithm to decompose the spectrum to obtain the radial space selected layer T2 spectrum f(T2,z). The formula is as follows:

[0071]

[0072] Where M(N·TE,z) is the echo attenuation signal, S(N·TE,m) is the sampling signal, Gmax is the maximum gradient in the slice selection direction, z is the slice selection direction, tp is the phase encoding time, m is the arithmetic progression corresponding to the phase encoding step number, N is the echo number, TE1 is the first echo interval, TE2 is the second echo interval, i is the number of echoes, γ is the gyromagnetic ratio, and T2 is the transverse relaxation time;

[0073] S5 detects whether the water output increases. If so, the process returns to step S4. If not, the process proceeds to step S6.

[0074] S6: Calculate the water saturation and formation water saturation by combining the saturated water volume, and determine the T2 spectrum under the bound water state;

[0075] S7: Analyze and compare the nuclear magnetic T2 spectra under different states, and use the imaging unit to record the images.

[0076] Example 3

[0077] like Figures 4 to 7 As shown, the present invention is a second embodiment of an online nuclear magnetic resonance imaging method. This embodiment is similar to embodiment 2, except that the saturated formation water in the core is replaced by saturated kerosene.

[0078] In order to verify the accuracy of the radial space layer selection T2 test method, the following design is made: Figure 4 a shows a linear water film with a volume decreasing linearly along the length direction, and the water film is filled with a copper sulfate aqueous solution with a relaxation time T2 = 20 ms; Figure 4 b shows the linear radial space selected layer T2 spectrum and layered signal profile tested by the phase encoding method. Figure 4c shows the linear water film radial space selected layer T2 spectrum and layered signal profile tested by the constant gradient layer selection method proposed in the present invention. Figure 4 It is explained that the layer selection method proposed in the present invention has the same spatial positioning function as the traditional nuclear magnetic resonance imaging method (phase encoding method).

[0079] The above steps can realize the distinction and quantitative calculation of water phase and oil and gas phase signals during the oil and gas development displacement experiment, and then obtain the spatial distribution of the overall recovery factor and the remaining oil saturation in the displacement direction.

[0080] In order to verify the practicability and accuracy of the method provided by the present invention, a water-displacement oil experiment of a sandstone was conducted, and the experimental process is as follows:

[0081] (1) Wash the core oil, dry it, and conduct basic physical property tests;

[0082] (2) Vacuum and pressurize the core to soak it in kerosene until it is saturated;

[0083] (3) Raise the temperature of the high-temperature and high-pressure holder to 150°C and place it between the magnets to measure the overall T2 spectrum of the core and the radial space selected layer T2 spectrum as the basement signal;

[0084] (4) The saturated kerosene sample was placed in a high-temperature and high-pressure holder and nitrogen was injected at the inlet and outlet of the holder simultaneously to gradually increase the pore pressure to 62 MPa and the confining pressure to 70 MPa. During the process, heating was turned on to gradually increase the sample temperature to 150°C. Then, the back pressure at the outlet was reduced to 61 MPa, and the overall T2 spectrum and radial space selected layer T2 spectrum of the saturated oil core were measured before displacement began.

[0085] (5) Subtracting the basement signal from the overall T2 spectrum and radial space selected layer T2 spectrum of the oil-saturated core to obtain the initial distribution overall T2 spectrum and radial space selected layer T2 spectrum under the oil-saturated state;

[0086] (6) Using heavy water to prepare a displacement fluid with properties close to those of formation water, start the displacement pump 23 and set it to a constant flow mode with a flow rate of 0.2 ml / min. After displacing 0.3 PV, 0.5 PV, 1 PV, 2 PV, 5 PV, 8 PV, and 10 PV, test the overall T2 spectrum of the core and the T2 spectrum of the radial space selected layer respectively;

[0087] (7) The stratified saturation is calculated using the spatially selected layer T2 spectrum under saturated conditions, and the saturation measured by the overall T2 spectrum is used as a benchmark to calculate the stratified saturation of all layers to obtain the relative error of the overall saturation. The corresponding experimental results under saturated kerosene and various displacement amounts are shown in the attached figure. Figure 5-7 shown.

[0088] By the attached Figure 5-7It can be seen that through heavy water displacement, the water phase signal is shielded and the oil signal is retained. Therefore, the decrease of the overall T2 spectrum measurement signal and the radial space selected layer T2 spectrum signal are changes caused by the decrease of oil saturation during the heavy water displacement process; Figure 5 and Figure 6 As shown, the radial space selected layer T2 spectrum provides the distribution of oil saturation at different positions along the displacement direction. According to the different relaxation times corresponding to pores of different sizes, the information on the utilization rules of oil in pores of different sizes can be obtained. It can be seen from the two figures that before the displacement begins, the core is saturated with oil. Due to the gravity differentiation, the oil is mainly concentrated in the large pores in the lower part of the core. As the displacement proceeds, the oil saturation gradually decreases, and the displacement efficiency reaches a peak after the displacement reaches 2PV. The subsequent displacement no longer has a significant impact on the degree of oil utilization. Thanks to the constant gradient layer selection technology's ability to measure small pore signals, the relative error of the overall saturation calculated by layered measurement and the saturation obtained by the overall T2 measurement is controlled within 5%, as shown in the figure. Figure 7 As shown, the accuracy requirements of application tests are fully met.

[0089] In summary, through theoretical and practical tests, it has been confirmed that the method and technology provided by the present invention can realize the measurement function of recovery rate and spatial distribution of remaining oil in oil and gas development experiments, and has the conditions for industry promotion and application.

[0090] Obviously, the above 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. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the embodiments here. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the claims of the present invention.

Claims

1. An online magnetic resonance imaging method, characterized in that: The invention is applied to an online nuclear magnetic resonance imaging system, the system comprising a clamping unit (1) for fixing a core, a high temperature and high pressure displacement unit, a magnet unit (3) for generating a magnetic field and enabling the magnetic field to act on the clamping unit (1), an oil, gas and water three-phase metering unit, a collection unit, a control unit and an imaging unit, the clamping unit (1) being respectively connected to the high temperature and high pressure displacement unit and the oil, gas and water three-phase metering unit, the collection unit being respectively connected to the clamping unit (1), the high temperature and high pressure displacement unit, the magnet unit (3), the control unit and the imaging unit for communication, and the control unit being respectively connected to the clamping unit (1), the high temperature and high pressure displacement unit, the magnet unit (3) and the imaging unit for communication; the method specifically comprises the following steps: S1: After washing the oil from the core, the core is dried and helium porosity and permeability tests are carried out. The vacuumed saturated formation water core is installed in the clamping unit (1) and the T2 spectrum under saturation is collected; S2: applying confining pressure to the clamping unit (1) to the target formation pressure through the high temperature and high pressure displacement unit, and maintaining the confining pressure greater than the displacement pressure; heating the clamping unit (1) through the high temperature and high pressure displacement unit to raise the temperature to the target formation temperature; and simultaneously establishing a magnetic field using the magnet unit (3), and transmitting a radio frequency pulse sequence and applying radial spatial phase encoding using the control unit and the acquisition unit; S3: The displacement medium is driven to the clamping unit (1) through the high temperature and high pressure displacement unit to drive the core saturated formation water, and the displacement medium and formation water both enter the oil, gas and water three-phase metering unit; S4: Read the water output through the oil, gas and water three-phase metering unit, and collect the T2 spectrum in real time; S5: Detect whether the water output increases. If it increases, return to step S4; if it does not change, go to step S6; S6: Calculate the water saturation and formation water saturation by combining the saturated water volume, and determine the T2 spectrum under the bound water state; S7: Analyze and compare the nuclear magnetic T2 spectra under different states, and record them through an imaging unit.

2. The online magnetic resonance imaging method according to claim 1, characterized in that: The high-temperature and 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 the confining pressure liquid storage device (22) and heating it, a confining pressure liquid circulation pipeline (25), a displacement pipeline (26), and a back-pressure pump (27); 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); the confining pressure liquid circulation device is respectively connected to the confining pressure pump (21) and the clamping unit (1) through the confining pressure liquid circulation pipeline (25).

3. The online magnetic resonance imaging method according to claim 2, characterized in that: A core installation chamber and a confining pressure chamber are provided in the clamping unit (1); the core installation chamber is in communication with a displacement pipeline (26), a displacement medium storage device (24) and an oil, gas and water three-phase metering unit; and the confining pressure chamber is in communication with a confining pressure liquid circulation pipeline (25) and a confining pressure liquid storage device (22).

4. The online magnetic resonance imaging method according to any one of claims 1 to 3, characterized in that: The magnet unit (3) comprises a permanent magnet and a magnet support, a magnet constant temperature device, and a gradient coil. The magnet support is installed at the bottom or around the permanent magnet and is installed in the magnet constant temperature device. 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 method according to claim 4, characterized in that: The oil-gas-water three-phase metering unit comprises an oil-gas-water separation device (41), a metering balance (42), a video display camera (43), and a plurality of flow meters (44); one end of the oil-gas-water separation device (41) is connected to the clamping unit (1), and the other end is respectively connected to the plurality of flow meters (44) and the metering balance (42); the video display camera (43) is installed beside the oil-gas-water separation device.

6. The online magnetic resonance imaging method according to claim 1, characterized in that: In step S2 The response signal expression of phase-encoded radial space-selective T2 test is as follows: Where S(N·TE,m) is the sampling signal, TE is the echo interval, TE1 is the first echo interval, TE2 is the second echo interval, N is the echo number, z is the slice selection direction, i is the number of echoes, γ is the gyromagnetic ratio, T2 is the transverse relaxation time, Gmax is the maximum gradient in the slice selection direction, tp is the phase encoding time, m is the arithmetic progression corresponding to the phase encoding step number, and the constant gradient encoding method is selective excitation sampling. The radial space selected T2 spectrum f(T2,z) can be obtained by inverting the echo signal S(N·TE,z) at different positions.

7. The online magnetic resonance imaging method according to claim 6, characterized in that: The method for obtaining the T2 spectrum line in step S4 and step S6 is as follows: first perform inverse Fourier transform on the sampling signal S(N·TE,m) to obtain the echo attenuation signal M(N·TE,z) at different positions, and use the BRD mode smoothing method and T2 test inversion algorithm to decompose the spectrum to obtain the radial space selected layer T2 spectrum f(T2,z), the formula is as follows: Where M(N·TE,z) is the echo attenuation signal, S(N·TE,m) is the sampling signal, Gmax is the maximum gradient in the slice selection direction, z is the slice selection direction, tp is the phase encoding time, m is the arithmetic progression corresponding to the phase encoding step number, N is the echo number, TE1 is the first echo interval, TE2 is the second echo interval, i is the number of echoes, γ is the gyromagnetic ratio, and T2 is the transverse relaxation time.

8. The online magnetic resonance imaging method according to claim 6, characterized in that: During the displacement process of step S3, the displacement is performed by adjusting the displacement pump (23) to select a constant speed or constant pressure mode.

9. The online magnetic resonance imaging method according to any one of claims 1 to 8, characterized in that: After step S6, a detection step is also included: the bound water saturation is calculated by combining the T2 spectrum in the saturated state with the T2 spectrum in the bound state, and the bound water saturation is compared with the bound water saturation calculated by the water output measurement. If the difference between the two is within ±10%, it means that there is no machine failure in the online nuclear magnetic resonance imaging system; if the difference between the two is outside ±10%, the machine detection of the online nuclear magnetic resonance imaging system is first performed, and then the process returns to step S1 to start the operation.

Images