A method for determining T2 cutoff value in tight sandstone reservoirs

Abstract

The present invention provides a method for determining the T2 cutoff value of a tight sandstone reservoir; the method comprises: pre-treating a rock sample; obtaining the porosity and permeability of a core sample; classifying the reservoir type according to establishing a porosity-permeability intersection chart; obtaining the distribution data of the nuclear magnetic resonance T2 spectrum in a saturated state; obtaining the distribution data of the nuclear magnetic resonance T2 spectrum in a centrifugal state to determine the T2 cutoff value of the rock sample; determining the main peak on the nuclear magnetic resonance T2 spectrum; and establishing a calculation formula for the T2 cutoff value of the target layer. The present invention realizes a feasible method for predicting the nuclear magnetic resonance T2 cutoff value using multiple parameters such as porosity, permeability, comprehensive physical property index, and T2 spectrum peak position. Compared with the existing conventional methods, the method is not only simple in calculation and can improve the accuracy of calculating the T2 cutoff values ​​of different reservoirs, but also can greatly reduce the various costs consumed in the centrifugal experiment, and is directly applied to nuclear magnetic resonance logging to provide technical support for the exploration and development of oil and gas fields.

Description

Technical Field

[0001] The present invention relates to the field of geophysical well logging; in particular, to a tight sandstone reservoir T 2 Method for determining cut-off value. Background Art

[0002] T 2 Spectroscopy is the basis for the application of nuclear magnetic resonance logging technology. All nuclear magnetic resonance rock properties and fluid analysis parameters are directly or indirectly based on T 2 Spectrum processing and calculation. Where T 2 The cutoff value is T 2 A very critical parameter in the spectrum is the boundary between the movable fluid and the bound fluid. The saturated sample T 2 Spectrum greater than T 2 The part above the cutoff value is the movable fluid, and the part below the cutoff value is the bound fluid. 2 The cutoff value has no effect on the porosity and oil saturation of the sample, but the permeability, movable fluid and bound fluid saturation are all affected by T 2 The cutoff value is directly related, which means that T 2 The determination of the cutoff value directly affects the accuracy of NMR measurement parameters, and further affects the evaluation results of reservoir movable fluid and economic value.

[0003] Currently determined T 2 The cut-off value methods mainly include empirical value method, morphological analysis method, mercury intrusion capillary curve method and centrifugation method.

[0004] The empirical value method is used to convert the clastic rock sample T 2 The cut-off value is defined as 33 ms, and the carbonate sample T 2 The cutoff value is defined as 100ms. This method is simple and single. The actual application of NMR logging data shows that 33ms is taken as the unified T of clastic sandstone. 2 The cutoff value is obviously too high, and the NMR logging results will be affected by factors such as the measurement environment, lithology, and shale content. 2 The cut-off value will also be affected. Therefore, this method is used to determine T for reservoirs in different blocks and layers. 2 Cutoff values ​​can lead to larger errors.

[0005] The morphological analysis method is based on the saturated T 2 Spectral shape and its T 2 The cut-off values ​​were compared and analyzed and it was found that T 2 Cut-off value at T 2 The position on the spectrum and T 2 Spectral shape, T 2 The peak position of the spectrum is closely related. From this, an empirical formula is obtained to calculate T 2This method does not require core experiments and can be directly applied to nuclear magnetic resonance logging. However, this method establishes a set of prediction parameters for different types of reservoirs, and the calculation accuracy is low when the pore structure is complex.

[0006] The mercury injection capillary curve method uses the minimum non-wetting phase saturation in the capillary pressure data as the irreducible water saturation, takes the average value according to the layer segment as the irreducible water saturation of each layer segment, and then multiplies the irreducible water saturation by the porosity to obtain the irreducible fluid content. Finally, the irreducible fluid content is calculated on the saturated sample T. 2 The T corresponding to the bound fluid content is found in the spectrum. 2 value, which is T 2 The capillary pressure method is more accurate, but there is a problem in determining the pressure point. If the pressure point is too large, part of the bound fluid will be calculated as movable fluid, resulting in a lower determined bound water saturation; on the contrary, if the pressure point is too small, the determined bound water saturation will be higher. In addition, this method will damage the core sample, affecting its subsequent experimental use.

[0007] The centrifugal method firstly performs a nuclear magnetic resonance experiment on the sample in a saturated state to obtain a saturated sample T 2 Then, the centrifugal force generated by the fluid at different speeds is used to centrifuge the sample under certain conditions to remove the movable fluid in the saturated sample. The centrifuged sample is then subjected to a nuclear magnetic resonance experiment to obtain the T 2 Finally, by comparing the T 2 Spectrum curve acquisition T 2 Cut-off value. This method has the most accurate results and is currently the most widely used method. However, since the centrifugation method requires a large number of centrifugation experiments and nuclear magnetic resonance experiments, the cost is high and it cannot be directly applied to nuclear magnetic resonance logging.

[0008] In the Chinese patent application with application number: CN201811203489.7, a nuclear magnetic resonance logging method is involved. 2 The point-by-point calculation method of the cut-off value is used to calculate the NMR T of the rock sample experimental data in the target work area. 2 The cut-off time geometric mean Gc and pseudo geometric mean G' of the spectrum are constructed. After the Gc and G' data matrices are constructed, the pseudo T 2 Cut-off value T 2 'c. By T 2 'c Calculation of NMR T 2 The pseudo cutoff time geometric mean G'c of the spectrum determines T 2 Cut-off value T 2cutoffThe fitting relationship between G'c and G'c is calculated. The G'c of the measured NMR logging data in the target area is calculated, and then the NMR logging T is realized by using the fitting relationship determined above. 2 Pointwise calculation method for cutoff value.

[0009] In the Chinese patent application with application number: CN200510072232.9, a nuclear magnetic resonance logging method is involved. 2 Spectrum T 2 The method for determining the cutoff value firstly performs the nuclear magnetic resonance logging T 2 The spectra are classified according to their morphological characteristics and according to different types of logging T 2 The eccentric spectrum is obtained by spectrum fitting, and the area of ​​the eccentric spectrum is obtained by area integration, and then the logging T is calculated. 2 The area of ​​the spectrum will be logged by T 2 The area of ​​the spectrum is accumulated. When the accumulated area is equal to or closest to the centrifugal spectrum area, the corresponding T 2 The value is T 2 Cutoff value.

[0010] The above technology still has the following shortcomings: it cannot solve the problem of nuclear magnetic resonance T 2 The problem of determining the cutoff value and its application in nuclear magnetic resonance logging; Based on this problem, the present invention proposes a method for determining the cutoff value of a tight sandstone reservoir T 2 Method for determining cut-off value. Summary of the invention

[0011] The purpose of the present invention is to provide a tight sandstone reservoir T 2 Method for determining cut-off value.

[0012] The present invention is achieved through the following technical solutions:

[0013] The present invention relates to a dense sandstone reservoir T 2 A method for determining a cutoff value, the method comprising:

[0014] Step 1, pre-treating the rock sample;

[0015] Step 2: Conduct physical property measurement experiments on the rock samples to obtain the porosity and permeability of the core samples;

[0016] Step 3, establishing a porosity-permeability intersection chart based on the core porosity-permeability relationship to classify reservoir types;

[0017] Step 4: Perform a nuclear magnetic resonance experiment on the rock sample to obtain the saturated nuclear magnetic resonance T 2 Spectral distribution data;

[0018] Step 5: Perform a nuclear magnetic resonance experiment on the rock sample to obtain the centrifugal nuclear magnetic resonance T 2Spectral distribution data to determine the T 2 Cutoff value;

[0019] Step 6: Calculate the comprehensive physical property index based on the obtained porosity and permeability data and conduct a nuclear magnetic resonance (NMR) 2 Determine the main peak on the spectrum;

[0020] Step 7: Based on the obtained porosity, permeability, comprehensive physical property index and T 2 Peak value, establish T of target layer by reservoir type 2 Cut-off value calculation formula.

[0021] Preferably, in step 1, the rock sample pretreatment includes drilling from a plug sample, washing oil, washing salt and drying.

[0022] Preferably, the drilling of the plunger sample refers to using a rock sample drill to obtain a cylindrical rock sample with a diameter of 2.54 cm and a length of 3-7 cm, and using a cutting machine to cut and polish the end face of the plunger sample, and then wash the oil, wash the salt and dry it.

[0023] Preferably, the oil washing is achieved by a pressurized oil washing method with a pressure between 68.95 kPa-6894.76 kPa (10-1000 psi); the salt washing is achieved by an organic solvent extraction method, the salt washing solvent is methanol, and the salt washing solution is distilled water; after completing the oil washing and salt washing, the rock sample is placed in a constant temperature and humidity chamber and dried to a constant dry weight.

[0024] Preferably, in step 3, the porosity-permeability intersection diagram is used to divide the reservoir into two categories: low-permeability reservoir and high-permeability reservoir.

[0025] Preferably, in step 4, the saturated fluid in the saturated state is prepared according to the salinity and type of the formation water.

[0026] Preferably, in step 5, the centrifugal force in the centrifugal state is determined according to the reservoir type, the centrifugal force for conventional reservoirs is 100 psi; the centrifugal force for low-porosity and low-permeability reservoirs is 200 psi; the centrifugal force for ultra-low-permeability reservoirs is 300 psi.

[0027] Preferably, in step 6, the calculation formula of the comprehensive physical property index (RQI) is as follows:

[0028]

[0029] Among them, K is the gas permeability, mD; φ is the gas porosity, a decimal.

[0030] Preferably, in step 6, the calculation of the main peak first determines T 2 The index value corresponding to the maximum spectrum amplitude is then calculated by the T 2 The value is determined.

[0031] Preferably, in step 7, the T 2 The cut-off value is calculated as:

[0032] T 2cutoff =a+bφ+cK+dRQI+eT 2f

[0033] Among them, T 2cutoff T 2 Cut-off value, ms; φ is gas-tested porosity, %; K is gas-tested permeability, mD; RQI is comprehensive physical property parameter; T 2f is the peak position; a, b, c, d and e are all constants obtained by regression calculation.

[0034] The present invention has the following advantages:

[0035] The present invention realizes a method of utilizing porosity, permeability, comprehensive physical property index and T 2 Multi-parameter prediction of spectral peak position in nuclear magnetic resonance T 2 A feasible method for calculating the cutoff value is proposed. Compared with the existing conventional method, this method is not only simpler in calculation, but also can improve the calculation of different reservoir T 2 The accuracy of the cut-off value can be greatly reduced, and the various costs consumed in the centrifuge experiment can be directly applied to nuclear magnetic resonance logging, providing technical support for the exploration and development of oil and gas fields. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 A dense sandstone reservoir T in the embodiment of the present invention 2 Flowchart of the cutoff value determination method;

[0037] Figure 2 Schematic diagram of the sample porosity and permeability intersection diagram in the embodiment of the present invention;

[0038] Figure 3 Saturated and centrifugal NMR T of typical samples in the embodiments of the present invention 2 Schematic diagram of the spectrum;

[0039] Figure 4 Typical samples measured T in the embodiment of the present invention 2 Schematic diagram of the cutoff value determination method (centrifugation method);

[0040] Figure 5 In the embodiment of the present invention, T is calculated 2 Cut-off value and measured T 2 Comparison chart of cutoff values. DETAILED DESCRIPTION

[0041] The present invention is described in detail below in conjunction with specific embodiments. It should be noted that the following embodiments are only further descriptions of the present invention, but the protection scope of the present invention is not limited to the following embodiments.

[0042] Example

[0043] This embodiment relates to a tight sandstone reservoir T 2 Methods for determining cut-off values ​​include:

[0044] Step 1, pre-treating the rock sample;

[0045] Step 2: Conduct physical property measurement experiments on the rock samples to obtain the porosity and permeability of the core samples;

[0046] Step 3, establishing a porosity-permeability intersection chart based on the core porosity-permeability relationship to classify reservoir types;

[0047] Step 4: Perform a nuclear magnetic resonance experiment on the rock sample to obtain the saturated nuclear magnetic resonance T 2 Spectral distribution data;

[0048] Step 5: Perform a nuclear magnetic resonance experiment on the rock sample to obtain the centrifugal nuclear magnetic resonance T 2 Spectral distribution data to determine the T 2 Cutoff value;

[0049] Step 6: Calculate the comprehensive physical property index based on the obtained porosity and permeability data and conduct a nuclear magnetic resonance (NMR) 2 Determine the main peak on the spectrum;

[0050] Step 7: Based on the obtained porosity, permeability, comprehensive physical property index and T 2 Peak value, establish T of target layer by reservoir type 2 Cut-off value calculation formula.

[0051] like Figure 1 As shown, Figure 1 The nuclear magnetic resonance T 2 Flow chart of the fitting calculation method of the cut-off value, comprising the following steps:

[0052] The samples in this embodiment were taken from a deep tight sandstone reservoir in a well group in the X well area, with a reservoir porosity of 8.9%-11% and an air permeability of 0.077mD-0.818mD.

[0053] In step 1, the samples are pre-processed according to the process specified in the standard "Core Analysis Method" GB / T29172-2012, including drilling columnar rock samples, washing salt, washing oil and drying.

[0054] In step 2, according to the process specified in the GB / T29172-2012 standard "Core Analysis Method", conventional physical property tests are carried out on the samples to determine the porosity and permeability of the samples. The permeability of the tight sandstone reservoir samples is determined by the pulse decay method. The porosity and permeability data of the reservoir samples in this example are shown in Table 1.

[0055] Table 1

[0056] Serial number Porosity(%) Permeability (mD) 1 11.2 0.173 2 9.5 0.077 3 8.9 0.266 4 9.1 0.328 5 12.8 0.392 6 11.8 0.318 7 12.8 0.467 8 13.4 0.62 9 11.5 0.382 10 14.1 0.818 11 11.1 0.287

[0057] In step 3, based on the porosity and permeability data of the samples, a porosity-permeability relationship cross-plot is established to classify the samples into reservoirs, see Figure 2 As shown; the classification standard table of reservoir samples is shown in Table 2, those with good physical properties are classified as Type 1, and those with poor physical properties are classified as Type 2.

[0058] Table 2

[0059]

[0060]

[0061] In step 4, according to the process specified in the standard SY / T 6490-2014, a saturated sample nuclear magnetic resonance experiment is carried out to obtain the sample saturated state T 2 Distribution spectrum, see Figure 3 As shown; wherein the saturated fluid is prepared according to the salinity and type of formation water.

[0062] In step 5, according to the process specified in the standard SY / T 6490-2014, the centrifugal sample nuclear magnetic resonance experiment is carried out to obtain the centrifugal sample T 2 Distribution spectrum, see Figure 3 As shown; the optimal centrifugal force for conventional reservoirs is 100psi; the optimal centrifugal force for low-porosity and low-permeability reservoirs is 200psi; and the optimal centrifugal force for ultra-low-permeability reservoirs is 300psi.

[0063] In step 6, the porosity data and permeability data obtained in step 2 are used to calculate the comprehensive physical property index of each sample. The calculation formula of the comprehensive physical property index is:

[0064]

[0065] Then, according to the process specified in the standard SY / T 6490-2014 "Laboratory Measurement Specifications for Nuclear Magnetic Resonance Parameters of Rock Samples", the saturated state T of each sample was measured. 2 The peak position is obtained in the distribution spectrum. The comprehensive physical property index, T 2 The peak position data are shown in Table 3.

[0066] Table 3

[0067]

[0068]

[0069] In step 7, the saturated sample and the centrifuged sample T obtained in steps 3 and 4 are used to 2 Distribution data calculation measured T 2 Cut-off value, using centrifuged sample T 2 The cumulative porosity of the centrifuged sample was calculated from the distribution data and the porosity of the saturated sample T 2 Find a point on the distribution spectrum where the cumulative porosity on its left side is equal to the total cumulative porosity of the centrifuged sample. This point corresponds to T 2 The value is T 2 Cut-off value, see Figure 4 shown.

[0070] In step 7, according to the reservoir classification in step 3 and the porosity, permeability, comprehensive physical property index and T obtained in steps 2 and 6, 2 Peak position parameters, establish different reservoir porosity, permeability, comprehensive physical property index, peak position and T 2 Using mathematical regression analysis method, a number of conventional physical property parameters and T 2 Cutoff value fitting calculation formula.

[0071] In step 7, in this embodiment, the type 1 reservoir NMR T 2 The cutoff value fitting formula is shown in formula (1), type 2 reservoir NMR T 2 The cutoff value fitting formula is shown in formula (2):

[0072] T 2cutoff =22.7065-0.5912φ+4.70706K-65.522RQI+0.18134T 2f (1)

[0073] T 2cutoff =169.3807-7.07814φ+450.7602K-1273.47RQI+0.156853T 2f (2)

[0074] Where T 2cutoff T 2 Cut-off value, ms; φ is gas-tested porosity, %; K is gas-tested permeability, mD; RQI is comprehensive physical property parameter; T 2f is the peak position.

[0075] The T2 Cut-off value and calculated T 2 The cut-off value data are shown in Table 4.

[0076] Table 4

[0077]

[0078]

[0079] Figure 5 The calculation T of the reservoir for this example 2 Cut-off value and measured T 2 The cut-off value comparison chart shows that the method of the present invention has a better application effect.

[0080] The present invention realizes a method of utilizing porosity, permeability, comprehensive physical property index and T 2 Multi-parameter prediction of spectral peak position in nuclear magnetic resonance T 2 A feasible method for calculating the cutoff value is proposed. Compared with the existing conventional method, this method is not only simpler in calculation, but also can improve the calculation of different reservoir T 2 The accuracy of the cut-off value can be greatly reduced, and the various costs consumed in the centrifuge experiment can be directly applied to nuclear magnetic resonance logging, providing technical support for the exploration and development of oil and gas fields.

[0081] The above describes the specific embodiments of the present invention. It should be understood that the present invention is not limited to the above specific embodiments, and those skilled in the art may make various modifications or variations within the scope of the claims, which will not affect the essence of the present invention.

Claims

1. A method for determining the T2 cutoff value of a tight sandstone reservoir, characterized in that: The method comprises: Step 1, pre-treating the rock sample; Step 2: Conduct physical property measurement experiments on the rock samples to obtain the porosity and permeability of the core samples; Step 3, establishing a porosity-permeability intersection chart based on the core porosity-permeability relationship to classify reservoir types; Step 4, performing a nuclear magnetic resonance experiment on the rock sample to obtain nuclear magnetic resonance T2 spectrum distribution data in a saturated state; Step 5, performing a nuclear magnetic resonance experiment on the rock sample, obtaining nuclear magnetic resonance T2 spectrum distribution data in a centrifugal state, and determining the T2 cutoff value of the rock sample; Step 6, calculating the comprehensive physical property index based on the acquired porosity and permeability data, and determining the main peak on the nuclear magnetic resonance T2 spectrum; Step 7, according to the obtained porosity, permeability, comprehensive physical property index and T2 peak value, establish a T2 cutoff value calculation formula for the target layer according to the reservoir type; In step 1, rock sample pretreatment includes drilling, oil washing, salt washing and drying from a plunger sample. The drilling of the plunger sample refers to using a rock sample drill to obtain a cylindrical rock sample with a diameter of 2.54 cm and a length of 3-7 cm, and using a cutting machine to cut and polish the end face of the plunger sample, and then washing oil, washing salt and drying. The oil washing is achieved by a pressurized oil washing method with a pressure between 68.95 kPa and 6894.76 kPa; the salt washing is achieved by an organic solvent extraction method, the salt washing solvent is methanol, and the salt washing solution is distilled water; after completing the oil washing and salt washing, the rock sample is placed in a constant temperature and humidity chamber and dried to a constant dry weight; In step 3, the porosity-permeability intersection chart is used to divide the reservoir into two categories: low-permeability reservoir and high-permeability reservoir; In step 4, the saturated fluid in the saturated state is prepared according to the salinity and type of the formation water; In step 5, the centrifugal force in the centrifugal state is determined according to the reservoir type. The centrifugal force of a conventional reservoir is 100 psi; the centrifugal force of a low-porosity and low-permeability reservoir is 200 psi; and the centrifugal force of an ultra-low-permeability reservoir is 300 psi. In step 6, the calculation formula of the comprehensive physical property index is as follows: Where K is the gas permeability, mD; φ is the gas porosity, decimal; The calculation of the main peak is to first determine the index value corresponding to the maximum value of the T2 spectrum amplitude, and then determine it from the T2 value at the same index position; In step 7, the calculation formula of the T2 cutoff value is: T 2cutoff =a+bφ+cK+dRQI+eT 2f Among them, T 2cutoff is T2 cutoff value, ms; φ is gas-tested porosity, %; K is gas-tested permeability, mD; RQI is comprehensive physical property parameter; T 2f is the peak position; a, b, c, d and e are all constants obtained by regression calculation.

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