CN106153662A - The measuring method of rock core stress sensitivity - Google Patents

Abstract

The invention discloses a method for measuring rock core stress sensitivity based on nuclear magnetic resonance. Based on conventional displacement experiments and nuclear magnetic resonance test methods, the stress sensitivity of rock core is measured; including: rock core pretreatment; rock core pressurized and saturated to simulate formation water; The non-magnetic core holder is fixed in the NMR coil; set the confining pressure to test the nuclear magnetic resonance T ₂ relaxation spectrum of the core; set the stress value to repeat the test at intervals; reduce the highest static stress value from 20MPa to 2.5MPa in turn, each stress value Keep for a period of time to test the T ₂ spectrum, calculate the current conventional permeability and NMR permeability; draw the conventional stress sensitivity curve of the core. The present invention can understand the change law of rock core stress sensitivity from a microscopic point of view, provide characteristics such as pore size distribution, porosity change, and NMR permeability during the experiment, and can effectively detect the pore size distribution, porosity and permeability of micro-cracked dense matrix limestone rate changes in stress-sensitive tests.

Description

Measurement method of core stress sensitivity

technical field

The invention relates to a rock core stress measurement technology, in particular to a method for measuring rock core stress sensitivity based on nuclear magnetic resonance (NMR), which can be applied in the petroleum industry.

Background technique

The NMR signal mainly comes from the interaction between the hydrogen nuclei and the magnetic field. Limestone cores in a mineral water-saturated state contain a large number of H-1 nuclei. At this time, when the Lamor frequency is used to excite the cores in a magnetic field, a strong NMR signal can be observed. Relaxation is an extremely important physical quantity in hydrogen nuclear magnetic resonance. It characterizes the process of the magnetization vector from deviating from the equilibrium state to returning to the equilibrium state when it is excited by a radio frequency field to generate nuclear magnetic resonance.

The H- ₁ nuclei in pores of different sizes have different contact probabilities with the pore surface, resulting in different T2 relaxation times. Therefore, the structural characteristics of pores in rocks can be reflected by measuring the NMR signal of water that is sensitive to the size of pores. . Because there are pores and fractures of different sizes in the rock core, and the relaxation time of hydrogen nuclei in different pores and fractures is different, so the NMR measurement is not a single value, but a distribution within a certain range, that is, the T ₂ spectrum.

During oil and gas extraction, with the generation of fluid inside the reservoir, the pore pressure of the reservoir decreases, and the force balance state of the rock in the reservoir changes. According to the theory of rock mechanics, the rock undergoes elastic or plastic deformation at this time. Based on the physical principles of reservoirs, rock permeability is controlled by the radius of rock pore throats. The deformation of rock will inevitably cause the change of rock pore structure and pore volume; this change greatly affects the seepage of fluid in it. The purpose of the stress sensitivity evaluation experiment is to understand the extent to which the pore throat deformation and fracture closure lead to changes in rock seepage capacity when the overlying pressure (confining pressure) on the core changes.

Compared with conventional reservoirs, fractured-vuggy limestone reservoirs have complex structures, large differences in fracture-vug scales, and fractures play a major role in communication. The structure of fractured-cavity carbonate reservoirs in my country is complex, and the flow characteristic scales of pores, caves, and fractures in the effective storage space vary greatly, and fractures play a major role in communication. Tight matrix limestone in some areas develops natural micro-fractures. During the mining process, as the formation pressure drops, its fluid distribution and pressure sensitivity are different from ordinary low-permeability sandstone reservoirs.

At present, the prior art mainly conducts laboratory core displacement experiments by simulating formation conditions. During the displacement process, the changes in core stress sensitivity are calculated by measuring the changes in core axial pressure, confining pressure and flow rate. For details, see China Petroleum and Natural Gas Industry Standard SY/T 5358-2010.

The above methods cannot provide information such as core pore size distribution and porosity changes in stress-sensitive experiments. For tight limestone reservoirs, there are mainly micro-nano pore throats, micro-fractures, and complex mineral composition. During the seepage process, a considerable part of the fluid is bound and cannot flow; after volume fracturing, artificial fractures and natural fractures are interwoven. The fluid distribution and pressure sensitivity of complex reservoir media are different from ordinary low-permeability sandstone reservoirs. Conventional testing methods and methods are difficult to obtain important reservoir rock seepage physical property parameters such as pore size distribution characteristics and movable fluid distribution in tight limestone reservoirs, and there is a lack of effective theoretical methods to deepen the understanding of multi-parameter coupling of pores, throats, and fractures. Reservoir petrophysical property changes and fluid distribution changes caused by the effect of natural gas. Therefore, it is extremely difficult to understand the flow characteristics and fluid distribution rules of tight limestone ultra-low permeability reservoirs through existing technologies.

Contents of the invention

In order to overcome the deficiencies of the above-mentioned prior art, the present invention provides a method for measuring the stress sensitivity of rock core based on high temperature and high pressure nuclear magnetic resonance, which can measure the stress sensitivity of tight limestone, and obtain the microcosm of the pore size distribution in the core displacement experiment. information.

The principle of the present invention is: by setting up a set of nuclear magnetic resonance online displacement system, carry out the displacement experiment of rock core and the measurement of _T2 spectrum, apply different overlying pressures through rock core holder, simulate the actual condition of stratum to carry out the rock core of laboratory Displacement experiment. Put the core into a special non-magnetic core holder, and put the holder into the center of the magnetic field to achieve the function of using nuclear magnetic resonance equipment to measure the pore size distribution of the core and other information. A high-pressure and high-precision plunger pump (TeledyneIsco 260D) is used in the axial direction to provide fluid displacement control; during the displacement process, the T ₂ spectrum (T ₂ spectrum) of the core in different states can be measured by nuclear magnetic resonance equipment; The fluid flow rate, the pressure at _both ends of the core, the overlying pressure and the T2 spectrum of the core are measured by corresponding calculation formulas to obtain the change rule of the core stress sensitivity.

For the measurement of T2 relaxation time spectrum, due to the complex distribution of pores and micro _- cracks formed inside limestone, it is difficult to determine its internal structure and distribution by conventional methods. When the core is saturated with mineral water, these pores are filled by water, and the distribution of pores inside the core can be measured by _T2 spectrum. According to the principle of magnetic resonance, the NMR _T2 relaxation time and the specific surface area of a single pore Satisfy the following relationship:

Among them, _ρ2 is the surface relaxation intensity. It can be seen from the above _formula that the T2 relaxation time in a single pore is inversely proportional to the specific surface area of the pore, and thus proportional to the pore radius. According to the previous discussion, the inside of the core is a network structure composed of pores of different sizes, and the total relaxation is the superposition of the relaxation of individual pores, that is:

Among them, S(t) is the total NMR signal intensity measured by CPMG sequence, and A _i represents the proportion of components whose relaxation time is T _2i , that is, the pore volume of a certain pore size corresponding to T _2i accounts for the total ratio of pore volume. By mathematically inverting the above _formula , the corresponding T2 relaxation time distribution, ie _T2 spectrum, can be obtained.

The present invention adopts following technical scheme:

A method for measuring rock core stress sensitivity based on nuclear magnetic resonance, based on conventional displacement experiments and nuclear magnetic resonance testing methods, measuring rock core stress sensitivity; comprising the following steps:

The first step, core pretreatment: the core is prepared according to the provisions of China's oil and gas industry standard SY/T 5358, cleaned and dried, and the air permeability and saturated pores are measured according to the provisions of China's oil and gas industry standard SY/T 5336 Determination of volume.

The second step is to prepare simulated formation water, put the core into an intermediate container to evacuate, add formation water and pressurize and saturate. Saturation time is not less than 12h.

In the third step, the core is saturated with simulated formation water and placed in the non-magnetic core holder, and the non-magnetic core holder is fixed to the nuclear magnetic resonance coil to carry out the core displacement experiment.

The core displacement experiment includes the third step to the fifth step.

The fourth step is to set the confining pressure of the holder as the initial static stress, water flood until the flow rate is stable, and keep it for more than 30 minutes, test the nuclear magnetic resonance T2 spectrum of the core, and calculate the current conventional permeability according to formula ₃ .

The fifth step, according to the Chinese petroleum and natural gas industry standard SY/T 5358, the stress value (net confining pressure) interval is set to 2.5MPa, 3.5MPa, 5MPa, 7MPa, 9MPa, 11MPa, 15MPa, 20MPa, repeat the fourth step.

Step 6. According to the Chinese petroleum and natural gas industry standard SY/T 5358, the maximum static stress value is reduced from 20MPa to 2.5MPa in turn, and each stress value is kept for more than 1h, and the T ₂ spectrum is tested, and the current conventional permeability and NMR permeability are calculated. Rate.

Specifically, the current conventional permeability is calculated according to formula 3, and the NMR permeability is calculated according to formula 4.

The seventh step is to draw the conventional stress sensitivity curve of the rock core, and classify the rock core by using the change of T ₂ spectrum under different stress conditions of the rock core.

The fourth and sixth steps involve the measurement methods of conventional permeability as follows: when the core is placed in the holder for pressure displacement, the formula for calculating the permeability of conventional rock samples according to Darcy's law is as follows:

Among them, K ₁ is the rock liquid permeability (10 ^-3 um ² ), μ is the fluid viscosity under test conditions (mPa·s); L is the length of the rock sample (cm); A is the cross-sectional area of the rock sample (cm ² ); Δp is the pressure difference between the two ends of the rock sample (MPa); Q is the volume of the fluid passing through the rock sample per unit time (cm ³ /s). Select the appropriate axial pressure displacement until the axial flow velocity of the core is stable, record the pressure difference Δp and flow velocity Q at both ends, and apply formula (3) to calculate the permeability under different confining pressure conditions.

The sixth step is to calculate the core NMR permeability by formula 4:

In Equation 4, K is the core gas permeability (10- ³ _um2 ); C _SDRE is the SDR expansion model constant; φ _NMR is the NMR porosity (%) of the saturated rock sample; T _2g is the geometric mean of T (ms).

The drawing of the stress sensitivity curve involved in the seventh step is specifically: taking the relative change K _i /K of the conventional permeability (rock sample permeability and initial permeability under different net stresses) as the ordinate, and taking the stress value in the fifth step (Net confining pressure, MPa) is used as the abscissa to draw the stress sensitivity curve.

The seventh step involves classification of rock core evaluation methods based _on T2 spectral changes in detail as follows:

According to the previous discussion, the T ₂ spectrum shows the distribution of pores of different sizes. The T ₂ relaxation time is proportional to the pore radius, and the longer the relaxation time, the larger the pores. Different cores have different T ₂ spectra. According to the shape of T2 spectrum (single peak, _double peak, etc.), the pore composition of different cores can be judged. Combining with the law that the T2 relaxation time is proportional to the pore size, it can be concluded that the distribution of different pore sizes in the _core under the experimental conditions changes in the experiment. At the same time, combined with the stress sensitivity curve, the relationship between permeability change and pore size distribution can be understood macroscopically.

The measurement method of the present invention combines the conventional displacement experiment with the nuclear magnetic resonance test to build a nuclear magnetic resonance online displacement equipment (high temperature and high pressure nuclear magnetic resonance test platform). In the implementation experiment of the present invention, a 0.23T permanent magnetic nuclear magnetic resonance analysis system (product SPEC-PMR-023T provided by Beijing Spike Technology Development Co., Ltd. ₎ is used to measure the T2 spectrum of tight limestone. The system has a horizontally placed magnet (magnetic field strength 0.23T), the magnet gap is 150mm, and the detection coil adopts a common transmitting and receiving coil with an inner diameter of 125mm. The system can be used to measure 1-inch small cores and 4-inch full-diameter cores, which is very suitable for oil flow experiments; the measurement sequence used in the test is CPMG sequence.

The NMR online test platform combines conventional displacement experiments with nuclear magnetic resonance tests. It can not only measure rock sensitivity curves, but also make full use of the ability of nuclear magnetic resonance non-destructive testing to obtain changes in the size distribution of pores and fractures in reservoir cores, and to obtain tight ash The change mechanism of rock core sensitivity can be realized, and online measurement under high temperature and high pressure can be realized in the process of oil production flow experiment, and various seepage physical parameters such as porosity, pore size distribution, permeability, oil saturation, etc. of formation conditions can be obtained. The experimental study of the dense matrix limestone with fracture development has opened up a new way. The NMR on-line test can overcome the technical problems in the prior art that only the pressure-sensitive curve of the rock core can be obtained but the pore size distribution during the experiment cannot be obtained.

Compared with prior art, the beneficial effect of the present invention is:

The invention provides a method for measuring the stress sensitivity of rock cores based on high temperature and high pressure nuclear magnetic resonance, which can measure the stress sensitivity of tight limestone and obtain microscopic information such as pore size distribution in core displacement experiments.

_The method of the invention can obtain the stress sensitivity curve of the rock core and the T2 spectrum under different stress conditions of the rock core, and by comparing the conventional pressure sensitive curve with the _T2 spectrum, the change rule of the stress sensitivity of the rock core can be understood from a microcosmic perspective. At the same time, the present invention can provide characteristics such as pore size distribution, porosity change, NMR permeability and the like during the experiment through the _T2 spectrum. In addition, on-line NMR measurement can effectively detect the pore size distribution of micro-fractures in tight matrix limestone, and the change law of porosity and permeability in the stress-sensitive test; through the online measurement of the T2 spectrum of the core in the pressure _- sensitive test, the pores can be observed in real time and the distribution and change of microcracks.

Description of drawings

Fig. 1 is a flow chart of a method for measuring the stress sensitivity of tight limestone based on a high temperature and high pressure NMR test platform provided by an embodiment of the present invention.

Fig. 2 is the structural block diagram of the limestone core nuclear magnetic resonance online displacement system provided by the embodiment of the present invention;

In the figure, the ring pressure pump and displacement pump provide displacement pressure, displacement liquid flow control and overlying pressure setting; nuclear magnetic resonance equipment includes probes, magnets and nuclear magnetic resonance spectrometers, which are used to transmit and collect nuclear magnetic resonance signals to measure T ₂ Spectrum; computer control system: mainly used to control and collect data, including processing of magnetic resonance signals and automatic collection of displacement variables.

Fig. 3 is a comparison chart of the fitting results of gas measured permeability and nuclear magnetic resonance calculated permeability provided by the embodiment of the present invention;

Among them, the ordinate is the permeability calculated by nuclear magnetic resonance; the abscissa is the gas permeability.

Fig. 4 is provided according to the embodiment of the present invention According to the T Spectrum online measurement obtains _three types of typical rock core stress sensitivity T Spectrum change curves _;

Among them, (a) is the T ₂ spectrum during the pressure rise of the 3# core; (b) is the T ₂ spectrum during the pressure drop of the 3# core; (c) is the T ₂ spectrum during the pressure rise of the 5# core; (d) It is the T ₂ spectrum of the 5# core during the pressure drop; (e) is the T ₂ spectrum of the 6# core during the pressure rise; (f) is the T ₂ spectrum of the 6# core during the pressure drop.

detailed description

Below in conjunction with accompanying drawing, further describe the present invention through embodiment, but do not limit the scope of the present invention in any way.

The accompanying drawings in the description are all simplified schematic diagrams, which only illustrate the basic structure of the present invention in a schematic manner, so they only show the configurations related to the present invention.

The invention provides a method for measuring the stress sensitivity of a rock core. Taking the measurement of tight limestone as an example, the stress sensitivity curve measurement of the tight limestone is realized by using a high-temperature and high-pressure nuclear magnetic resonance online test platform. In the embodiment, the NMR permeability is also calculated according to the T ₂ spectrum obtained by the nuclear magnetic resonance test adopted in the present invention, and compared with the conventional test results.

The method for measuring the stress sensitivity of rock core provided by the invention comprises the steps:

The first step, core pretreatment: the core is prepared, cleaned and dried according to the provisions of China's oil and gas industry standard SY/T 5358;

According to the Chinese petroleum and natural gas industry standard SY/T 5336, the determination of air permeability and saturated pore volume is measured. When the core is saturated with mineral water, these pores are filled by water, and the distribution of pores inside the core can be measured by _T2 spectrum. NMR _T2 relaxation time and individual pore specific surface area Satisfy the above formula 1.

The second step is to prepare experimental brine (simulated formation water) similar to that of the formation water, and put the rock core into an intermediate container to evacuate and pressurize to saturate;

Saturation time is not less than 12h.

In the third step, the core is saturated with simulated formation water and placed in the non-magnetic core holder, and the non-magnetic core holder is fixed on the nuclear magnetic resonance coil.

The invention specifically adopts the non-magnetic rock core holder to carry out the rock core displacement experiment. This experiment adopts the core holder recorded in CN201110304182.8, including left adjustment core plug, right adjustment core plug, depletion mining inlet, depletion mining outlet, left pressure cap, right pressure cap, left plug, right plug, metal Gasket, PTFE O-ring, PTFE rubber sleeve, ring pressure chamber, coil support, coil, ring pressure chamber outlet, ring pressure chamber inlet, core chamber, cylinder, back pressure valve, bypass valve . Considering the environment of the strong magnetic field of NMR, the pressure resistance value of the non-magnetic core holder used in this experiment is 30Mpa. In order to prevent the core holder from destroying the magnetic field distribution and reducing the quality of the NMR measurement signal, the cylinder of the holder is made of polyether ether (PEEK) resin, and this material also ensures the holder's portability and pressure. Hydrogen-free oil is selected as the confining pressure fluid. If the confining pressure fluid contains hydrogen, it will affect the accuracy of NMR measurement.

In the embodiment, the parameters of the tight limestone core nuclear magnetic resonance measurement involved in the third step are set as follows: the experimental measurement sequence is CPMG sequence, the parameters are resonance frequency 10.11MHz, echo time 300us, scanning accumulation times 16 times, sampling interval 1us, The number of sampling points is 2048.

Step 4: Set the confining pressure of the holder as the initial static stress, water flood until the flow rate is stable, keep it for more than 30 minutes, and test the nuclear magnetic resonance T ₂ spectrum;

The T ₂ spectrum of the core is measured by a high temperature and high pressure nuclear magnetic resonance test platform. The measurement of the T ₂ spectrum satisfies the above formula 2, and the corresponding T ₂ spectrum can be obtained by mathematically inverting the formula 2.

The current permeability can be further calculated. Select the appropriate axial pressure displacement until the axial flow velocity of the core is stable, record the pressure difference Δp and flow velocity Q at both ends, and apply Equation 3 to calculate the conventional permeability under different confining pressure conditions.

The NMR permeability of tight limestone cores can be calculated by Equation 4:

In Equation 4, K is the core gas permeability (10 ^-3 um2); C _SDRE is the SDR expansion model constant; φ _NMR is the NMR porosity of the saturated rock sample (%); T _2g is the geometric mean value of the T ₂ spectrum (ms).

Specific steps are as follows:

In order to calculate the NMR permeability, it is necessary to fit m and n. For this purpose, 15 cores were selected. The core information is shown in Table 1. According to the fitting calculation of formula 4, m=2.41, n=0.45, C _SDRE =6862, and the fitting The result is shown in Figure 3. Among them, the R2 value is ^0.8244 , indicating that the permeability measured by NMR is quite accurate, but the result still has a certain deviation from the conventional gas permeability. The main reason is that the limestone pore structure is complex, containing fractures and dead pores , and permeability is an extremely complex quantity, T ₂ spectrum NMR measurement can not reflect the anisotropy of permeability.

The fifth step, according to the Chinese oil and gas industry standard SY/T 5358, the stress value interval is set to 2.5MPa, 3.5MPa, 5MPa, 7MPa, 9MPa, 11MPa, 15MPa, 20MPa, repeat the fourth step.

Step 6: According to the Chinese oil and gas industry standard SY/T 5358, the highest static stress value from 20MPa is reduced to 2.5MPa in turn, and each stress value is kept for more than 1h. Test the _T2 spectrum and calculate the current permeability.

The seventh step is to use the relative change of permeability K _i /K (rock sample permeability and initial permeability under different net stresses) as the ordinate, and the net confining pressure (MPa) as the abscissa to draw the stress sensitivity curve, and draw the core stress Sensitivity Curve.

According to the pressure-sensitive curve changes of tight limestone cores, the cores can be divided into different categories.

The change of T ₂ spectrum during the experiment shows the law of pressure-sensitive curve and permeability change. According to the change of T2 spectrum during the experiment, the cores are divided into _three categories: Type I: higher pressure sensitivity during pressurization and better recovery during depressurization; this type of core has better permeability recovery; Type II: High pressure sensitivity during pressurization, poor recovery during depressurization; this type of core has poor permeability recovery; Type III: low pressure sensitivity during pressurization, poor recovery during depressurization, The permeability recovery of such cores is poor.

The following preferred embodiments take the stress sensitivity experiment of tight limestone cores in Tahe Oilfield as an example to measure the stress sensitivity of tight limestone cores. Table 1 is the core sample information of a tight limestone reservoir in Xinjiang, China used in this example.

Table 1 Information of core samples from a tight limestone reservoir in Xinjiang

Fig. 1 is a flow chart of a method for measuring the stress sensitivity of compact limestone based on a high temperature and high pressure nuclear magnetic resonance test platform provided by an embodiment of the present invention, which mainly includes the following steps:

First, perform rock sample pretreatment according to the SY/T 5358 standard, and perform operations A) to C):

A) rock sample preparation

The drilling direction of the plunger sample should be consistent with the flow direction of the reservoir fluid; the length of the core should not be less than 1.5 times the diameter.

B) Rock sample cleaning

All fluids originally present in the rock sample must be completely cleaned.

C) Rock sample drying

Each rock sample should be dried to constant weight, and the drying time should not be less than 48h.

D) Then use the oil and gas standard SY/T 5336 to measure the air permeability and pore volume of the core, and perform operations D) to E): measure the air permeability of the core according to the provisions of the oil and gas standard SY/T 5336.

E) Core saturation and pore volume measurement

Vacuumize the core to saturate the formation water; soak the rock sample in the saturated liquid for at least 40 hours, and measure the quality of the rock sample after the saturated liquid.

Put the sample into the test platform, search for suitable parameters to measure the T2 spectrum of the core in different states, and perform operations F ₎ ～G):

F) Put the standard sample on the NMR online test platform, turn on the T ₂ spectrum test equipment, and search for suitable test parameters.

G) The specific steps are as follows: the rock core is put into the holder, and the holder is placed in the center of the magnet. Turn on the nuclear magnetic resonance equipment and debug the equipment to obtain the NMR signal of the rock core. Main steps: a. Search for a suitable RF frequency for exciting the core sample in a magnetic field; b. Search for P90 and P180 to obtain the correct pulse width. As shown in Figure 2, the core was fixed in a non-magnetic core holder, and its T2 spectrum was tested according to the steps shown in Figure ₁ .

The measurement parameters of this example are as follows: main frequency (SF): 10.11MHz; 90° pulse width (P90): 20us; 180° pulse width (P180): 40us; number of sampling points (nech): 2048; number of scan accumulations (NS) : 16, sampling interval (DW) 1us; probe dead time (Dead1): 40us, pulse interval: 150us.

H) draw the core stress sensitivity curve according to the evaluation method of stress sensitivity in the oil and gas standard SY/T5336;

Fig. 2 is a structural block diagram of the limestone core nuclear magnetic resonance online displacement system used in this embodiment. In the figure, the ring pressure pump and displacement pump provide displacement pressure, displacement liquid flow control and overlying pressure setting; nuclear magnetic resonance equipment includes probes, magnets and nuclear magnetic resonance spectrometers, which are used to transmit and collect nuclear magnetic resonance signals to measure T ₂ Spectrum; computer control system: mainly used to control and collect data, including processing of magnetic resonance signals and automatic collection of displacement variables. In this experiment, a 0.23T permanent magnet NMR analysis system (SPEC-PMR-023T, product of Beijing Spike Technology Development Co., Ltd. ₎ was used to measure the T2 spectrum of tight limestone. The system has a horizontally placed magnet (magnetic field strength 0.23T), the magnet gap is 150mm, and the detection coil adopts a common transmitting and receiving coil with an inner diameter of 125mm. The system can measure both small 1-inch cores and 4-inch full-diameter cores, which is very suitable for oil flow experiments. The measurement sequence used in the test is the CPMG sequence. In order to prevent the core holder from destroying the magnetic field distribution and reducing the quality of the NMR measurement signal, a non-magnetic core holder was used in the experiment with a withstand voltage of 30Mpa. The displacement pump is a high-pressure and high-precision plunger pump (Teledyne Isco 260D); in order to reduce the influence of confining pressure fluid on the accuracy of NMR signals, dehydrofluorinated oil is selected as the confining pressure fluid. The computer control and data acquisition unit is used to acquire and process nuclear magnetic resonance signals.

Fig. 3 is a comparison chart of the fitting results of gas permeability and nuclear magnetic resonance calculated permeability provided by the embodiment of the present invention; wherein, the vertical axis is the nuclear magnetic resonance calculated permeability; the abscissa is the gas measured permeability.

I) According to the oil and gas standard SY/T ₅₃₅₈ , the stress sensitivity curve of tight limestone is measured, and the T2 spectrum in different states is measured at the same time.

Fig. 4 is _three kinds of typical rock core stress sensitivities T Spectrum change curves that the present embodiment provides according to T Spectrum online measurement _; Wherein (a) is T in the process of ₃ # core pressure rise; (b) is 3 #Core T ₂ spectrum during pressure drop; (c) T ₂ spectrum during pressure rise of 5# core; (d) T _{2 spectrum during pressure drop of 5# core; (e) T 2} spectrum during pressure rise of 6# core T ₂ spectrum; (f) T ₂ spectrum during pressure drop of 6# core.

It should be noted that the purpose of the disclosed embodiments is to help further understand the present invention, but those skilled in the art can understand that various replacements and modifications are possible without departing from the spirit and scope of the present invention and the appended claims of. Therefore, the present invention should not be limited to the content disclosed in the embodiments, and the protection scope of the present invention is subject to the scope defined in the claims.

Claims (10)

1. a measuring method for rock core stress sensitivity, comprises the following steps:

The first step, core pretreatment: prepared by rock sample, clean and dry, and measures rock core air permeability and pore volume；

Second step, preparation simulated formation water, put into rock core evacuation in intermediate receptacle, add described simulated formation water and pressurize Saturated；

3rd step, rock core saturation simulation formation water is placed on without in magnetic core holding unit, and will fix without magnetic core holding unit Rock core displacement test is carried out in malcoils；

4th step, to arrange clamper confined pressure be initial static stress, water drive to flow speed stability, the nuclear magnetic resonance, NMR T of testing rock core₂Spectrum, and It is calculated current conventional permeability；

5th step, by stress value interval be set to 2.5MPa, 3.5MPa, 5MPa, 7MPa, 9MPa, 11MPa, 15MPa, 20MPa, weight Perform the 4th step again；

6th step, the highest static stress value being down to by 20MPa 2.5MPa successively, each stress value keeps a period of time to test T₂Spectrum, And it is calculated current conventional permeability and nuclear magnetic resonance, NMR permeability；

7th step, drafting rock core routine stress sensitive linearity curve.

2. the measuring method of rock core stress sensitivity as claimed in claim 1, is characterized in that, core pretreatment tool described in the first step Body carries out pretreatment according to China National Petroleum industry standard SY/T 5358 regulation.

3. the measuring method of rock core stress sensitivity as claimed in claim 1, is characterized in that, described in second step, saturation time is the lowest In 12 hours.

4. the measuring method of rock core stress sensitivity as claimed in claim 1, is characterized in that, clamps without magnetic rock core described in the 3rd step Implement body uses number of patent application to be the core holding unit that CN201110304182.8 records.

5. the measuring method of rock core stress sensitivity as claimed in claim 1, is characterized in that, the nuclear-magnetism of rock core described in the 4th step is altogether Shake T₂Spectrum, records especially by following method:

Nuclear magnetic resonance, NMR T₂Relaxation time and single void ratio surface areaMeet relation described in formula 1:

In formula 1, ρ₂For surface relaxation intensity；

The internal network structure for different size hole composition of rock core, is overlapped single hole relaxation by formula 2, obtains total Relaxation:

In formula 2, S (t) is the total nuclear magnetic resonance signal intensity utilizing CPMG sequence to record；A_iThe expression relaxation time is T_2iComponent Shared ratio, is and T_2iThe pore volume of corresponding certain pore size accounts for the ratio of total pore size volume；

By formula 2 is carried out mathematical inversion, obtain corresponding T₂Distribution of relaxation times, i.e. T₂Spectrum.

6. the measuring method of rock core stress sensitivity as claimed in claim 1, is characterized in that, conventional permeability tool described in the 6th step Body is calculated by formula 3:

In formula 3, K₁For rock Test Liquid Permeability of Core (10^-3um2)；μ is the fluid viscosity (mPa s) under test condition；L is rock sample Length (cm)；A is rock sample cross-sectional area, and unit is cm²；Δ p is rock sample two ends pressure reduction (MPa)；Flow velocity Q is that fluid is when unit Interior by the volume of rock sample, unit is cm³/s。

7. the measuring method of rock core stress sensitivity as claimed in claim 1, is characterized in that, nuclear magnetic resonance, NMR infiltration described in the 6th step Rate is calculated especially by formula 4:

In formula 4, K is rock core perm-plug method (10^-3um2)；C_SDREFor SDR extended model constant；φ_NMRFor saturated rock sample nuclear-magnetism Resonance holes porosity (%)；T_2gFor T₂The geometrical mean of spectrum, unit is ms.

8. the measuring method of rock core stress sensitivity as claimed in claim 1, is characterized in that, draws rock core conventional described in the 7th step Stress sensitive linearity curve, specifically:, will as vertical coordinate using the ratio of rock sample permeability and original permeability under different net impact Stress value in 5th step draws stress sensitive linearity curve as abscissa.

9. the measuring method of rock core stress sensitivity as claimed in claim 1, is characterized in that, the 7th step is also with rock core not With the nuclear magnetic resonance, NMR T under stress condition₂Rock core is classified by spectrum change.

10. the measuring method of rock core stress sensitivity as claimed in claim 1, is characterized in that, the ginseng of described Nuclear Magnetic Resonance Measurement Number is arranged particularly as follows: it is CPMG sequence that sequence is measured in experiment；Parameter is resonant frequency 10.11MHz；Echo time is 300us；Sweep Retouch accumulative frequency 16 times；Sampling interval 1us；Sampling number is 2048.