Method for determining saturation degree and nuclear magnetic T2 value of irreducible water in production of ultra-low permeability reservoir

Abstract

The invention discloses a method for determining saturation and a nuclear magnetic T2 value of irreducible water produced by an ultra-low permeability reservoir. After a plunger sample is pretreated, basic parameters of the plunger sample are measured, the plunger sample is vacuumized, pressurized and saturated to simulate formation water, a nuclear magnetic resonance signal of the plunger sample is collected by using a low-field nuclear magnetic resonance relaxation analysis instrument and is inverted into a nuclear magnetic resonance T2 spectrum, the nuclear magnetic resonance porosity is obtained, and then according to the centrifugal pressure gradient, the magnetic resonance porosity is calculated. The method comprises the following steps: carrying out centrifugal experiment on a plunger sample under different centrifugal pressures by using a high-speed refrigerated centrifuge, obtaining the water saturation and nuclear magnetic resonance T2 spectrum of the plunger sample after each centrifugation, determining a relation curve between the centrifugal pressure and the water saturation, and obtaining the production irreducible water saturation and the limit irreducible water saturation after treatment by using a second-order derivation method, so as to obtain the production irreducible water saturation and the limit irreducible water saturation. And the accumulated T2 spectrum of the plunger sample when the water is completely contained is calibrated, so that the value range of the nuclear magnetic resonance T2 value of the real production irreducible water saturation is accurately obtained, and reservoir evaluation of an ultra-low permeability reservoir and oil-gas field development are facilitated.

Description

Technical Field

[0001] The present invention relates to the technical field of geophysical well logging, and in particular to a method for determining production irreducible water saturation and nuclear magnetic resonance T2 value of an overpressure low-permeability reservoir. Background Art

[0002] The saturation and mobility of fluids are important parameters for reservoir evaluation and oil and gas field development. By accurately measuring the irreducible water saturation, we can have a more comprehensive understanding of the physical properties of the reservoir, such as the pore structure, which is conducive to accurately evaluating the reserves and production capacity of the reservoir. Overpressured low-permeability reservoirs have the characteristics of low physical properties, poor connectivity, and complex pore structure. The mobility of fluids during development is affected by factors such as reservoir characteristics and production systems, and it is difficult to reach the limit irreducible water state. Therefore, accurately determining the production irreducible water saturation of overpressured low-permeability reservoirs and clarifying the distribution range of their nuclear magnetic resonance T2 values ​​are of great significance for detailed reservoir evaluation, optimization of development plans, and analysis of oil and gas field production fluid properties.

[0003] Experimental analysis is the main means of determining the production bound water. The main methods currently used include centrifugation, mercury injection, displacement, etc. By comparing the mass of the core before and after the experiment or the nuclear magnetic resonance response, the production bound water saturation under experimental conditions can be calculated. However, the calculation results of these methods are limited by the experience of the experimenters, and can only determine the approximate bound water saturation by setting certain parameters through experience. The agreement with the bound water saturation in the actual production process of oil and gas fields is poor, and it is difficult to promote and apply them on a large scale. Summary of the invention

[0004] The present invention aims to solve the problem that it is difficult to accurately determine the production irreducible water saturation and the corresponding nuclear magnetic resonance T2 value range of an overpressured low-permeability reservoir in the prior art, and proposes a method for determining the production irreducible water saturation and the nuclear magnetic resonance T2 value of an overpressured low-permeability reservoir. The method does not need to be limited by the empirical parameter settings of the experimenter, and the distribution range of the real production irreducible water saturation nuclear magnetic resonance T2 value of the overpressured low-permeability reservoir is accurately obtained, which provides a basis for reservoir evaluation and oil and gas field development of overpressured low-permeability reservoirs.

[0005] The present invention adopts the following technical solutions:

[0006] A method for determining production irreducible water saturation and nuclear magnetic resonance T2 value of an overpressure low permeability reservoir comprises the following steps:

[0007] Step 1, obtaining a plunger sample of an overpressure low-permeability reservoir, pre-treating the plunger sample and measuring basic parameters of the plunger sample;

[0008] Step 2, saturating the plunger sample with simulated formation water by vacuuming and pressurizing, measuring the liquid-measured porosity, wet weight and floating weight of the plunger sample when it is completely water-containing, setting the operating parameters of the low-field nuclear magnetic resonance relaxation analysis instrument, measuring the plunger sample after it is completely water-containing using the low-field nuclear magnetic resonance relaxation analysis instrument, and inverting the collected nuclear magnetic resonance signal into a nuclear magnetic resonance T2 spectrum to calculate the nuclear magnetic resonance porosity of the plunger sample;

[0009] Step 3, placing the plunger sample saturated with simulated formation water in a high-speed refrigerated centrifuge, setting the centrifugal pressure gradient and the centrifugal time, and performing centrifugal experiments on the plunger sample at different centrifugal pressures according to the centrifugal pressure gradient using the high-speed refrigerated centrifuge. After each centrifugation, the plunger sample is taken out and weighed to calculate the water saturation, and the nuclear magnetic resonance signal of the plunger sample after centrifugation is measured using a low-field nuclear magnetic resonance relaxation analysis instrument and inverted into a nuclear magnetic resonance T2 spectrum to obtain the centrifugal experiment results;

[0010] Step 4, obtaining a relationship curve between centrifugal pressure and water saturation by fitting the centrifugal test results;

[0011] Step 5, processing the relationship curve between centrifugal pressure and water saturation based on the second-order derivative method to obtain the inflection point of the relationship curve between centrifugal pressure and water saturation, obtaining the inflection point value and using the water saturation corresponding to the inflection point as the production irreducible water saturation, and using the water saturation corresponding to the maximum centrifugal pressure in the relationship curve between centrifugal pressure and water saturation as the limit irreducible water saturation;

[0012] Step 6, normalize and accumulate the nuclear magnetic resonance T2 spectrum of the plunger sample when it is completely water-containing to obtain a cumulative T2 spectrum, search the production bound water saturation and the limit bound water saturation determined in step 5 in the cumulative T2 spectrum, and obtain the T2 value corresponding to the production bound water saturation as T 2c1 , the T2 value corresponding to the limit bound water saturation is T 2c2 The range of the NMR T2 value of the real production irreducible water saturation of the overpressure low permeability reservoir is determined to be T 2c2 ~T 2c1 .

[0013] Preferably, in step 1, overpressure low permeability reservoir rock samples are drilled, the overpressure low permeability reservoir rock samples are processed to obtain cylindrical plug samples, the plug samples are washed with oil, salt, gas and dried, and then the basic parameters of the plug samples are measured, the basic parameters including the mass, diameter, length, density, porosity and permeability of the plug samples.

[0014] Preferably, the porosity and permeability of the plug sample are measured using a gas method.

[0015] Preferably, in step 2, the vacuuming time of the plunger sample is set to 24 hours, the saturation pressure of the plunger sample in the process of saturating the simulated formation water is set to 20 MPa, and the saturation time is set to 24 hours.

[0016] Preferably, the operating parameters of the low-field nuclear magnetic resonance relaxation analysis instrument include main frequency, echo interval, waiting time and sampling number, wherein the main frequency is set to 2MHz, the echo interval is set to 0.2ms, the waiting time is set to 6000ms, and the sampling number is set to 64 times.

[0017] Preferably, the collected NMR signal is inverted into a NMR T2 spectrum based on the SIRT inversion method.

[0018] Preferably, in step 3, a plurality of centrifugal pressures are provided in the centrifugal pressure gradient, and the centrifugal pressure is used to simulate the displacement pressure exerted on the rocks in the overpressure low permeability reservoir.

[0019] Preferably, the centrifugal pressure in the centrifugal pressure gradient is set to 0.01MPa, 0.05MPa, 0.19MPa, 0.75MPa, 1.69MPa, and 3MPa in descending order, and the centrifugation time of the plunger sample under each centrifugal pressure condition is set to 40min.

[0020] Preferably, the relationship curve between the centrifugal pressure and the water saturation is:

[0021] S W =a×ln(P C )+b

[0022] In the formula, S W is water saturation, in %; P C is the centrifugal pressure, in MPa; a and b are constants.

[0023] The present invention has the following beneficial effects:

[0024] The present invention proposes a method for determining the production irreducible water saturation and nuclear magnetic resonance T2 value of an overpressure low permeability reservoir. The method performs water saturation and nuclear magnetic resonance T2 spectrum experiments on plunger samples under different centrifugal pressures in a laboratory to obtain the relationship between the centrifugal pressure and the water saturation, processes the relationship curve between the centrifugal pressure and the water saturation based on the second-order derivative method, determines the production irreducible water saturation and the limit irreducible water saturation, and scales the production irreducible water saturation and the limit irreducible water saturation into a cumulative T2 spectrum, thereby achieving a value range of the real production irreducible water saturation nuclear magnetic resonance T2 value of the overpressure low permeability reservoir.

[0025] The method of the present invention does not need to rely on the experience of experimenters to set the production irreducible water saturation calculation parameters during the experiment process, and can accurately obtain the distribution range of the nuclear magnetic resonance T2 value of the real production irreducible water saturation of the overpressure low permeability reservoir. The determined nuclear magnetic resonance T2 value is consistent with the irreducible water saturation in the actual production process of the oil field. The operation process is simple and easy to promote and apply on a large scale, which is beneficial to the fine reservoir evaluation, development plan optimization and oil and gas field production liquid property analysis of the overpressure low permeability reservoir. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In order to facilitate relevant technical personnel to better understand the patent of the present invention, the following figures are used to better explain the implementation process of the present invention and the example display of the effect.

[0027] Figure 1 The present invention is a flow chart of a method for determining irreducible water saturation and nuclear magnetic resonance T2 value in overpressure and low permeability reservoirs.

[0028] Figure 2 This is the relationship curve between centrifugal pressure and water saturation.

[0029] Figure 3 This is the relationship curve between centrifugal pressure and water saturation after the second-order derivative method is used.

[0030] Figure 4 is the cumulative T2 spectrum of the plunger sample. DETAILED DESCRIPTION

[0031] The specific implementation manner of the present invention is further described below with reference to the accompanying drawings and a certain overpressure low permeability reservoir as an example.

[0032] The present invention proposes a method for determining the production irreducible water saturation and nuclear magnetic resonance T2 value of an overpressure low permeability reservoir. Figure 1 As shown, the specific steps include:

[0033] Step 1, drill overpressure low permeability reservoir rock samples, cut and grind the overpressure low permeability reservoir rock samples to obtain cylindrical plug samples with a length of 3 to 4 cm and a diameter of 2.54 cm, wash the plug samples with oil, salt, gas and dry them, and then measure the mass, diameter, length, density, porosity and permeability of the plug samples, wherein the porosity and permeability of the plug samples are measured by a gas method.

[0034] Step 2, saturate the plunger sample with simulated formation water by vacuuming and pressurizing, measure the liquid-measured porosity, wet weight and floating weight of the plunger sample when it is fully water-containing, set the operating parameters of the low-field nuclear magnetic resonance relaxation analyzer, use the low-field nuclear magnetic resonance relaxation analyzer to measure the plunger sample after it is fully water-containing, collect the low-field nuclear magnetic resonance signal of the plunger sample after it is fully water-containing, and invert the collected low-field nuclear magnetic resonance signal into a nuclear magnetic resonance T2 spectrum based on the SIRT inversion method (Simultaneous Iterative Reconstruction Technique) in the prior art, and calculate the nuclear magnetic resonance porosity of the plunger sample.

[0035] In this embodiment, the plunger sample is evacuated for 24 hours, and the pre-prepared simulated formation water is saturated into the internal vacuum plunger sample by pressurizing. The saturation pressure of the plunger sample in the process of saturating the simulated formation water is set to 20 MPa and the saturation time is set to 24 hours.

[0036] The operating parameters of the low-field nuclear magnetic resonance relaxation analysis instrument include main frequency, echo interval, waiting time and sampling number. In this embodiment, the main frequency of the low-field nuclear magnetic resonance relaxation analysis instrument is set to 2 MHz, the echo interval is set to 0.2 ms, the waiting time is set to 6000 ms, and the sampling number is set to 64 times.

[0037] Step 3, placing the plunger sample saturated with simulated formation water in a high-speed refrigerated centrifuge, setting the centrifugal pressure gradient and centrifugal time, wherein a plurality of centrifugal pressures are set in the centrifugal pressure gradient, and the centrifugal pressures are set in order from small to large to 0.01MPa, 0.05MPa, 0.19MPa, 0.75MPa, 1.69MPa, and 3MPa, and the centrifugal time for the plunger sample under each centrifugal pressure is 40min, and the centrifugal pressure is used to simulate the displacement pressure of the rock in the overpressure low-permeability reservoir. According to each centrifugal pressure set in the centrifugal pressure gradient, a high-speed refrigerated centrifuge is used to perform centrifugal experiments on the plunger sample at different centrifugal pressures, i.e., to simulate the displacement process of the fluid in the rock under the influence of the displacement pressure in the overpressure low-permeability reservoir. After each centrifugation, the plunger sample is taken out and weighed to calculate the water saturation of the plunger sample, and the nuclear magnetic resonance signal of the plunger sample after centrifugation is measured using a low-field nuclear magnetic resonance relaxation analysis instrument with the same operating parameters as in step 2. The collected nuclear magnetic resonance signal is inverted into a nuclear magnetic resonance T2 spectrum based on the SIRT inversion method to obtain the centrifugation experiment results, which include the water saturation and nuclear magnetic resonance T2 spectrum of the plunger sample under different centrifugal pressures.

[0038] Step 4, by fitting and analyzing the water saturation of the plunger sample under different centrifugal pressures in the centrifugal experiment results, a relationship curve between centrifugal pressure and water saturation is obtained, such as Figure 2As shown, the relationship between the centrifugal pressure and the water saturation in this embodiment is:

[0039] S W =-12.88ln(P C )+43.313

[0040] In the formula, S W is water saturation, in %; P C is the centrifugal pressure in MPa.

[0041] analyze Figure 2 It can be seen that there is a logarithmic function relationship between water saturation and centrifugal pressure (i.e., displacement pressure), and there is a good correlation between the two.

[0042] Step 5: Process the relationship curve between centrifugal pressure and water saturation based on the second-order derivative method. Figure 3 For the relationship curve processed by the second-order derivative method, the inflection point in the relationship curve between centrifugal pressure and water saturation processed by the second-order derivative method is determined, the value of the inflection point and its corresponding water saturation are obtained, the water saturation corresponding to the inflection point is used as the production irreducible water saturation, and the water saturation corresponding to the maximum centrifugal pressure in the relationship curve between centrifugal pressure and water saturation processed by the second-order derivative method is used as the limiting irreducible water saturation.

[0043] Depend on Figure 3 It can be obtained that the production irreducible water saturation is about 42% and the limit irreducible water saturation is about 11%.

[0044] Step 6, normalize and accumulate the nuclear magnetic resonance T2 spectrum of the plunger sample when it is completely hydrated in step 2, draw the cumulative T2 spectrum, and obtain the saturation accumulation corresponding to different durations, such as Figure 4 In the cumulative T2 spectrum, the production bound water saturation and the limit bound water saturation determined in step 5 are searched to obtain the T2 cutoff value T corresponding to the production bound water saturation. 2c1 The T2 cutoff value T corresponding to the limit bound water saturation is 15.19ms. 2c2 It is 1.71ms, so the relaxation time range between the production irreducible water and the ultimate irreducible water saturation is 1.71ms~15.19ms, and the actual production irreducible water saturation NMR T2 value range of the overpressure low permeability reservoir is 1.71ms~15.19ms.

[0045] Of course, the above description is not a limitation of the present invention, and the present invention is not limited to the above examples. Changes, modifications, additions or substitutions made by technicians in this technical field within the essential scope of the present invention should also fall within the protection scope of the present invention.

Claims

1. A method for determining the production irreducible water saturation and nuclear magnetic resonance T2 value of an overpressured low-permeability reservoir, characterized in that: The following steps are involved: Step 1, obtaining a plunger sample of an overpressure low-permeability reservoir, pre-treating the plunger sample and measuring basic parameters of the plunger sample; Step 2, saturating the plunger sample with simulated formation water by vacuuming and pressurizing, measuring the liquid-measured porosity, wet weight and floating weight of the plunger sample when it is completely water-containing, setting the operating parameters of the low-field nuclear magnetic resonance relaxation analysis instrument, measuring the plunger sample after it is completely water-containing using the low-field nuclear magnetic resonance relaxation analysis instrument, and inverting the collected nuclear magnetic resonance signal into a nuclear magnetic resonance T2 spectrum to calculate the nuclear magnetic resonance porosity of the plunger sample; Step 3, placing the plunger sample saturated with simulated formation water in a high-speed refrigerated centrifuge, setting the centrifugal pressure gradient and the centrifugal time, and performing centrifugal experiments on the plunger sample at different centrifugal pressures according to the centrifugal pressure gradient using the high-speed refrigerated centrifuge. After each centrifugation, the plunger sample is taken out and weighed to calculate the water saturation, and the nuclear magnetic resonance signal of the plunger sample after centrifugation is measured using a low-field nuclear magnetic resonance relaxation analysis instrument and inverted into a nuclear magnetic resonance T2 spectrum to obtain the centrifugal experiment results; Step 4, obtaining a relationship curve between centrifugal pressure and water saturation by fitting the centrifugal test results; Step 5, processing the relationship curve between centrifugal pressure and water saturation based on the second-order derivative method to obtain the inflection point of the relationship curve between centrifugal pressure and water saturation, obtaining the inflection point value and using the water saturation corresponding to the inflection point as the production irreducible water saturation, and using the water saturation corresponding to the maximum centrifugal pressure in the relationship curve between centrifugal pressure and water saturation as the limit irreducible water saturation; Step 6, normalize and accumulate the nuclear magnetic resonance T2 spectrum of the plunger sample when it is completely water-containing to obtain a cumulative T2 spectrum, search the production bound water saturation and the limit bound water saturation determined in step 5 in the cumulative T2 spectrum, and obtain the T2 value corresponding to the production bound water saturation as T 2c1 , the T2 value corresponding to the limit bound water saturation is T 2c2 The range of the NMR T2 value of the real production irreducible water saturation of the overpressure low permeability reservoir is determined to be T 2c2 ~T 2c1 .

2. The method for determining the production irreducible water saturation and nuclear magnetic T2 value of an overpressured low-permeability reservoir according to claim 1, characterized in that: In the step 1, an overpressure low-permeability reservoir rock sample is drilled, and the overpressure low-permeability reservoir rock sample is processed to obtain a cylindrical plunger sample. After the plunger sample is washed with oil, salt, gas, and dried, the basic parameters of the plunger sample are measured, and the basic parameters include the mass, diameter, length, density, porosity, and permeability of the plunger sample.

3. The method for determining the production irreducible water saturation and nuclear magnetic T2 value of an overpressured low-permeability reservoir according to claim 2, characterized in that: The porosity and permeability of the plug samples were measured using a gas method.

4. The method for determining the production irreducible water saturation and nuclear magnetic T2 value of an overpressured low-permeability reservoir according to claim 1, characterized in that: In step 2, the vacuuming time of the plunger sample is set to 24 hours, and the saturation pressure of the plunger sample in the process of saturating the simulated formation water is set to 20 MPa and the saturation time is set to 24 hours.

5. The method for determining the production irreducible water saturation and nuclear magnetic T2 value of an overpressured low-permeability reservoir according to claim 4, characterized in that: The operating parameters of the low-field nuclear magnetic resonance relaxation analysis instrument include main frequency, echo interval, waiting time and sampling number, wherein the main frequency is set to 2 MHz, the echo interval is set to 0.2 ms, the waiting time is set to 6000 ms, and the sampling number is set to 64 times.

6. The method for determining production irreducible water saturation and nuclear magnetic resonance T2 value of an overpressured low-permeability reservoir according to claim 1, characterized in that: The collected NMR signals are inverted into NMR T2 spectra based on the SIRT inversion method.

7. The method for determining production irreducible water saturation and nuclear magnetic resonance T2 value of an overpressured low-permeability reservoir according to claim 1, characterized in that: In step 3, a plurality of centrifugal pressures are provided in the centrifugal pressure gradient, and the centrifugal pressures are used to simulate the displacement pressure exerted on the rocks in the overpressure low-permeability reservoir.

8. The method for determining the production irreducible water saturation and nuclear magnetic T2 value of an overpressured low-permeability reservoir according to claim 7, characterized in that: The centrifugal pressures in the centrifugal pressure gradient are set to 0.01 MPa, 0.05 MPa, 0.19 MPa, 0.75 MPa, 1.69 MPa, and 3 MPa in descending order, and the centrifugal time of the plunger sample under each centrifugal pressure condition is set to 40 min.

9. The method for determining production irreducible water saturation and nuclear magnetic resonance T2 value of an overpressured low-permeability reservoir according to claim 1, characterized in that: The relationship curve between the centrifugal pressure and water saturation is: S W =a×ln(P C )+b In the formula, S W is water saturation, in %; P C is the centrifugal pressure, in MPa; a and b are constants.

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