Rapid digestion method for detecting heavy metals in water-based drilling cuttings sample

Abstract

The invention relates to a rapid digestion method for detecting heavy metals in a water-based drilling cuttings sample, and belongs to the technical field of environmental monitoring. The method comprises the following steps: A, freeze-drying a to-be-detected sample, and grinding the freeze-dried to-be-detected sample; b, mixing m grams of the to-be-detected sample ground in the step A with V1 milliliters of hydrofluoric acid, and heating at 120-125 DEG C for 30-90 minutes; c, replenishing V2 milliliters of hydrofluoric acid, V3 milliliters of nitric acid and V4 milliliters of hydrochloric acid, keeping the temperature of an acid system at 120-125 DEG C, and heating for 60-120 minutes; d, cooling the to-be-detected sample heated in the step C, fixing the volume to V5ml, uniformly mixing, centrifuging, and carrying out solid-liquid separation to obtain a digested to-be-detected sample solution. The method is short in digestion time and reagent consumption, good in safety and high in accuracy.

Description

Technical Field

[0001] The present invention relates to a rapid digestion method for detecting heavy metals in water-based drilling cuttings samples, and belongs to the technical field of environmental monitoring. Background Art

[0002] Water-based drill cuttings are a type of solid waste generated during the development of shale gas fields. Among all drilling fluid systems, water-based drilling fluids are extremely widely used in the oil and gas industry due to their low price, easy controllability of performance, simple process, and diverse types. With the continuous expansion of the operating area of shale gas fields and the increasing number of gas wells, the generation amount of water-based drill cuttings cannot be underestimated. As a type of solid waste, water-based drill cuttings themselves are only cuttings, but because their surfaces are wrapped with drilling waste liquid, they contain pollutants such as heavy metals and petroleum hydrocarbons, posing environmental safety risks for heavy metals in water-based drilling cuttings. However, in the treatment of drilling waste, the industry has long had a serious tendency of "emphasizing oil-based and neglecting water-based", and a large amount of water-based drilling waste has not been properly disposed of, not only causing serious pollution to soil and water environments, but also occupying land resources, bringing huge challenges to society and the ecological environment. Nowadays, with the increasing emphasis on the environment, the environmental protection requirements for oil and gas fields have been unprecedentedly improved. How to effectively treat the large amount of water-based drilling solid waste generated has become a difficult problem that the industry urgently needs to overcome.

[0003] Accurate determination of heavy metals in water-based drilling cuttings is required for harmless treatment, solidification and landfill, environmental risk assessment, etc. of water-based drilling cuttings. There is no unified standard for the disposal technology and pollution discharge of water-based drilling solid waste, and it is necessary to comprehensively refer to the standards of sludge, soil and industrial solid waste. There are certain differences in pollutant limits, detection methods and technical specifications among different standards. First of all, pollutant limits are one of the important indicators for formulating environmental protection standards. Different standards may have different limit requirements for different types of waste. For example, the allowable limits for heavy metals in soil standards and sludge standards may be different. Therefore, when determining heavy metals in water-based drilling cuttings, it is necessary to refer to the corresponding standards according to specific circumstances to determine whether they meet the environmental protection requirements. Secondly, different standards may adopt different detection methods. The detection methods for heavy metals in soil and sludge usually include atomic absorption spectrometry, inductively coupled plasma mass spectrometry, X-ray fluorescence spectrometry, etc. When selecting a detection method applicable to water-based drilling cuttings, it is necessary to comprehensively consider its characteristics and, in combination with relevant standards and professional knowledge, select an accurate and reliable analysis method. In addition, different standards also have differences in technical specifications and operation guides. For example, different standards have different requirements in aspects such as sampling, sample treatment, reagent dosage and quality control.

[0004] Water-based drilling cuttings are characterized by high alkalinity, high salt content, high silicate content, and high oil content. When using the detection methods for soil to detect heavy metals in them, there are problems such as incomplete digestion, long time consumption, complex procedures, and large reagent consumption, resulting in a long detection cycle and affecting the timeliness and accuracy of sample detection. To improve the accuracy and efficiency of metal detection and analysis in drilling solid waste, achieve rapid detection of samples, reduce human errors in the pretreatment process, and reduce analysis costs, it is of great significance to propose a simple and rapid digestion method for heavy metal indicators in water-based drilling cuttings for production. Summary of the Invention

[0005] The object of the present invention is to provide a rapid digestion method for heavy metal detection of water-based drilling cuttings samples.

[0006] To achieve the object of the present invention, the rapid digestion method for heavy metal detection of water-based drilling cuttings samples includes:

[0007] A. Freeze-dry the sample to be tested, and grind the freeze-dried sample to be tested.

[0008] B. Mix m grams of the ground sample to be tested in step A with V1 milliliters of hydrofluoric acid, and heat at 120°C to 125°C for 30 min to 90 min, preferably heat for 30 min.

[0009] C. Add V2 milliliters of hydrofluoric acid, V3 milliliters of nitric acid, and V4 milliliters of hydrochloric acid, and keep the temperature of the acid system at 120°C to 125°C and heat for 60 min to 120 min.

[0010] D. Cool the sample to be tested after heating in step C, make the volume constant to V5 milliliters, mix well, centrifuge, and separate the solid and liquid to obtain the digested sample solution to be tested.

[0011] Wherein, m:V1:V2:V3:V4 = 0.1 to 0.2:0.5 to 1:0.5 to 1:1:3; preferably m:V1:V2:V3:V4 = 0.1:0.5 to 1:0.5 to 1:1:3.

[0012] The digestion time refers to the total heating time of steps B and C. After 2 hours for the Pb element, the concentration continuously decreases with the increase of digestion time, and the concentrations of other elements do not change significantly with the increase of time. Therefore, within 2 hours of digestion time.

[0013] The method of the present invention does not perform the operation of driving off acid.

[0014] The sample obtained by the present invention contains trace amounts of hydrofluoric acid. Do not use instruments with glass devices.

[0015] The sample solution to be tested obtained in step D can be diluted 100 to 1000 times in solid-liquid according to the existing analytical instruments and element concentrations and then detected on the machine.

[0016] The reagents of the present invention can be guaranteed reagent grade hydrofluoric acid, guaranteed reagent grade nitric acid, and guaranteed reagent grade hydrochloric acid.

[0017] In a specific embodiment, the total volume sum of V1, V2, V3, and V4 is Vtotal milliliters, and Vtotal:m is 60 or less.

[0018] In a specific embodiment, the sample to be measured in step A is at least one of soil, clear water cuttings, polymer rock, and polysulfonated cuttings, preferably clear water cuttings.

[0019] In a specific embodiment, the temperature of freeze-drying in step A is -60°C to -50°C; the time of freeze-drying in step A is preferably 12 to 24 h.

[0020] In a specific embodiment, the particle size of the ground sample to be measured in step B is 100 mesh or less.

[0021] In a specific embodiment, the ground sample to be measured in step B is first wetted with water and then mixed with hydrofluoric acid. The mass ratio of water to m is preferably 0.1:1 g / mL. The main purpose of adding water is to wet the sample so that the sample will not adsorb on the wall of the centrifuge tube.

[0022] In a specific embodiment, the heating in steps B and C is performed using a graphite digestion instrument; steps B and C are preferably carried out in a container made of polypropylene material.

[0023] In a specific embodiment, the total time of steps B and C is 4 h or less, preferably 2 h or less.

[0024] In a specific embodiment, m:V5 = 0.1:10 to 50.

[0025] In a specific embodiment, the rotation speed of centrifugation is 3000 to 8000 rpm, preferably 3000 to 6000 rpm, and the centrifugation time is preferably 5 min to 10 min. The speed cannot exceed 8000 rpm.

[0026] Advantageous effects:

[0027] 1. The total digestion time of the whole process of the present invention is reduced from 6 h to 10 h of the standard method to within 4 h, and even within 2 h. The reagent consumption is reduced from 30 mL to within 6 mL. After verification by standard samples and spike recovery rates, the test results are satisfactory, and the effects of rapidity, effectiveness, and accuracy can be achieved.

[0028] 2. Water-based drilling cuttings samples usually have the characteristics of high silicate and high oil content. Generally, the classical "four acids" wet digestion method of hydrofluoric acid, nitric acid, sulfuric acid, and perchloric acid is used. However, perchloric acid is prone to explosion when reacting with high-content organic substances, especially oil substances. Moreover, the boiling point of perchloric acid is relatively high, making it difficult to completely remove, resulting in a high matrix effect. In contrast, the present invention only needs to use a specific ratio of three acids, namely hydrofluoric acid, nitric acid, and hydrochloric acid, for digestion, which has higher safety.

[0029] 3. The rapid digestion method of the present invention does not require acid expulsion. Therefore, the sample contains a large amount of silicate ions. However, through verification, the heavy metal elements in it can achieve complete digestion effect. At the same time, it saves a large amount of digestion time, manual operation, and acid reagents, and correspondingly reduces the hazardous waste generated by the experiment, avoiding the problems of resource waste and secondary pollution.

[0030] 4. The digestion container of the present invention selects a low-cost disposable polypropylene volumetric pipette, which solves the problems of easy contamination and difficult cleaning of heavy metal sample containers. At the same time, it has a relatively high heat conduction efficiency and good digestion effect. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 It is a graph showing the change of the concentration of each element in the test sample solution obtained by digestion in Examples 4-5 and Comparative Examples 1-2. DETAILED DESCRIPTION OF THE INVENTION

[0032] To achieve the purpose of the present invention, the rapid digestion method for detecting heavy metals in water-based drilling cuttings samples includes:

[0033] A. Freeze-dry the test sample, and grind the freeze-dried test sample.

[0034] B. Mix m grams of the ground test sample in step A with V1 milliliters of hydrofluoric acid, and heat at 120 °C to 125 °C for 30 min to 90 min, preferably heat for 30 min.

[0035] C. Add V2 milliliters of hydrofluoric acid, V3 milliliters of nitric acid, and V4 milliliters of hydrochloric acid, and maintain the temperature of the acid system at 120 °C to 125 °C and heat for 60 min to 120 min.

[0036] D. Cool the test sample after heating in step C, make up the volume to V5 milliliters, mix well, centrifuge, and separate the solid and liquid to obtain the digested test sample solution.

[0037] Wherein, m:V1:V2:V3:V4 = 0.1 to 0.2:0.5 to 1:0.5 to 1:1:3; preferably m:V1:V2:V3:V4 = 0.1:0.5 to 1:0.5 to 1:1:3.

[0038] The digestion time refers to the total heating time of steps B and C. After 2 hours, the concentration of Pb element continuously decreases with the increase of digestion time, while the concentrations of other elements do not change significantly with the increase of time. Therefore, within 2 hours of digestion time.

[0039] The method of the present invention does not perform the operation of removing acid.

[0040] The sample obtained by the present invention contains trace amounts of hydrofluoric acid. Do not use instruments with glass devices.

[0041] The test sample solution obtained in step D can be diluted 100 - 1000 times by solid-liquid according to the existing analytical instruments and element concentrations and then detected on the machine.

[0042] The reagents of the present invention can be hydrofluoric acid of guaranteed reagent grade, nitric acid of guaranteed reagent grade, and hydrochloric acid of guaranteed reagent grade.

[0043] In a specific embodiment, the total volume sum of V1, V2, V3, and V4 is V total milliliters, and V total:m is below 60.

[0044] In a specific embodiment, the test sample in step A is at least one of soil, clear water cuttings, polymer rock, and polysulfonate cuttings, preferably clear water cuttings.

[0045] In a specific embodiment, the temperature of freeze-drying in step A is -60°C to -50°C; the time of freeze-drying in step A is preferably 12 - 24 hours.

[0046] In a specific embodiment, the particle size of the ground test sample in step B is below 100 mesh.

[0047] In a specific embodiment, the ground test sample in step B is first wetted with water and then mixed with hydrofluoric acid. The mass ratio of water to m is preferably 0.1:1 g / mL. The main purpose of adding water is to wet the sample so that the sample will not adsorb on the wall of the centrifuge tube.

[0048] In a specific embodiment, the heating in steps B and C uses a graphite digestion instrument; steps B and C are preferably carried out in a container made of polypropylene material.

[0049] In a specific embodiment, the total time of steps B and C is below 4 hours, preferably below 2 hours.

[0050] The total time of steps B and C being below 4 hours refers to the digestion time plus other times, such as the wetting time and the time for adding reagents, etc., for the total time.

[0051] In a specific embodiment, m:V5 = 0.1:10 - 50.

[0052] In a specific embodiment, the rotational speed of centrifugation is 3000 - 8000 revolutions per minute, preferably 3000 - 6000 revolutions per minute, and the centrifugation time is preferably 5 - 10 minutes. The speed cannot exceed 8000 revolutions per minute.

[0053] The following further describes the specific embodiments of the present invention in conjunction with the examples, and the present invention is not limited to the scope of the described examples.

[0054] Example 1

[0055] The silicate content of the sample used in Example 1 is shown in Table 1 in detail:

[0056] Table 1 Silicate content of the sample

[0057] Sample Fresh water cuttings Polymer cuttings Poly-sulfonate cuttings Silicate content (%) 62 77 82

[0058] (1) Pre-cool the freeze dryer for 30 minutes to -52°C, control the vacuum degree of the freeze dryer to be 20 ± 2 Pa, and freeze-dry the sample to be measured at -50 to -55°C for 12 to 24 hours to obtain the freeze-dried sample. Grind the above sample, then pass it through a 100-mesh sieve, and put it into a brown glass bottle for measurement.

[0059] (2) Accurately weigh 0.1000 ± 0.0002 g of the sample below 100 mesh into a 50 mL polypropylene quantitative tube, and add a little ultrapure water to moisten the sample;

[0060] (3) Set the graphite digestion instrument to 120°C;

[0061] (4) Add 1 mL of high-grade pure hydrofluoric acid to the polypropylene quantitative tube containing the sample, put it into the graphite digestion instrument, use a thermometer to measure the acid system temperature, adjust the temperature setting of the graphite digestion instrument, and keep the acid system temperature at 120°C and heat for 30 minutes;

[0062] (5) Add 1 mL of high-grade pure hydrofluoric acid, 1 mL of high-grade pure nitric acid, and 3 mL of high-grade pure hydrochloric acid to the polypropylene quantitative tube containing the sample, and keep the acid system temperature at 120°C and heat for 60 minutes;

[0063] (6) Remove the polypropylene quantitative tube from the graphite digestion instrument, do not perform the acid expulsion operation, cool it and then make the volume up to 50 mL, shake well and centrifuge at 6000 revolutions for 5 minutes to obtain the digested sample solution to be measured by solid-liquid separation;

[0064] (7) Dilute the solid and liquid by 100 - 1000 times according to the analytical instrument and element concentration and then perform on-machine detection. The sample contains trace hydrofluoric acid, and do not use instruments with glass devices.

[0065] The digestion method of this invention was verified for three indicators: detection limit, precision, and accuracy using ultrapure water, water-based cuttings, water-based polymer cuttings, water-based polysulfonate cuttings, and soil standard samples:

[0066] ① Detection limit: Copper: 2.8 mg / kg, Zinc: 2.7 mg / kg, Lead: 3.0 mg / kg, Cadmium: 0.3 mg / kg, Nickel: 2.6 mg / kg, Chromium: 2.5 mg / kg.

[0067] ② Precision: Six determinations were made on water-based cuttings, water-based polymer cuttings, and water-based polysulfonate cuttings. The relative standard deviation within the laboratory was as follows: for copper: 3.2% - 6.4%, for zinc: 1.3% - 4.5%, for lead: 3.3% - 6.8%, for cadmium: 4.5% - 8.6%, for nickel: 2.2% - 5.6%, for chromium: 3.0 - 4.9%.

[0068] ③ The digestion method of this invention was used to digest the soil standard sample GSS-14 of the Sichuan Basin soil and conduct machine detection. The detection results are shown in Table 2:

[0069] Table 2 Detection Results of Soil Standard Sample GSS-14

[0070]

[0071]

[0072] After verification, the detection limit, precision, and accuracy of this rapid digestion method all meet the requirements of the national standard method.

[0073] Example 2

[0074] Take water-based cuttings, water-based polymer cuttings, and water-based polysulfonate cuttings and digest them according to the method of the example, and conduct a standard addition recovery experiment. The results are shown in Table 3:

[0075] Table 3 Results of Standard Addition Recovery of Water-Based Cuttings

[0076] Unit: % Copper Zinc Lead Cadmium Nickel Chromium Fresh water based drilling cuttings 92.3 104.6 92.4 102.3 94.1 90.3 Water-based polymer drilling cuttings 90.1 92.4 94.4 93.3 92.1 89.6 Water-based poly-sulfonate drilling cuttings 88.3 91.4 88.7 86.3 90.2 88.7

[0077] From the results of the standard addition recovery, it can be concluded that the standard addition recovery rates of this digestion method are all between 80 - 120%. The pollution rate and loss rate of the target substances in the experiment are relatively low, and the feasibility and reliability of the method are relatively high.

[0078] Example 3

[0079] Take the soils of Jianyang, Jiangyou, and Suining in the Sichuan Basin and digest them according to the digestion method of Example 1, and conduct a standard addition recovery experiment. The results are shown in Table 4:

[0080] Table 4 Results of Standard Addition Recovery

[0081] Unit: % Copper Zinc Lead Cadmium Nickel Chromium Jianyang soil 94.6 102.3 96.4 103.1 92.3 95.5 Jiangyou soil 91.7 98.3 95.8 92.3 93.1 94.2 Suining soil 104.4 101.4 90.2 96.5 96.5 95.6

[0082] From the results of the spike recovery rate, it can be concluded that the spike recovery rates of this digestion method are all between 90% and 110%. The contamination rate and loss rate of the target substances in the experiment are relatively low. The method has strong operability and high reliability, and can also be used as a digestion method for heavy metals in soil.

[0083] Examples 4-5 and Comparative Examples 1-2

[0084] Take a certain water-based polysulfonate cuttings sample as the sample to be tested. Other digestion methods are the same as those in Example 1, except that the heating digestion time is different. The test solution obtained by digestion is detected. The digestion times of Examples 4 and 5 are 1 h and 2 h respectively, and those of Comparative Examples 1 and 2 are 3 h and 4 h respectively. The results are shown in Figure 1 , from Figure 1 It can be seen that except for the Pb element, the concentration continuously decreases with the increase of the digestion time after 2 h, and the concentrations of other elements do not change significantly with the increase of time. Therefore, the digestion time is below 2 h.

Claims

1. A rapid digestion method for detecting heavy metals in water-based drilling cuttings samples, characterized in that, The method includes: A. Freeze-drying the sample to be tested, and grinding the freeze-dried sample to be tested; B. Mixing m grams of the ground sample to be tested in step A with V1 milliliters of hydrofluoric acid, and heating at 120°C to 125°C for 30 min to 90 min, preferably heating for 30 min; C. Adding V2 milliliters of hydrofluoric acid, V3 milliliters of nitric acid, and V4 milliliters of hydrochloric acid, and maintaining the temperature of the acid system at 120°C to 125°C and heating for 60 min to 120 min; D. Cooling the sample to be tested after heating in step C, diluting to a volume of V5 milliliters, mixing evenly, centrifuging, and separating the solid and liquid to obtain the digested sample solution to be tested; Wherein, m:V1:V2:V3:V4 = 0.1 - 0.2:0.5 - 1:0.5 - 1:1:3; preferably m:V1:V2:V3:V4 = 0.1:0.5 - 1:0.5 - 1:1:

3.

2. The rapid digestion method for detecting heavy metals in water-based drilling cuttings samples according to claim 1, characterized in that, The total volume sum of V1, V2, V3, and V4 is V total milliliters, and V total:m is below 60.

3. The rapid digestion method for detecting heavy metals in water-based drilling cuttings samples according to claim 1 or 2, characterized in that, The sample to be tested in step A is at least one of soil, clear water cuttings, polymer rock, and polysulfonate cuttings, preferably clear water cuttings.

4. The rapid digestion method for detecting heavy metals in water-based drilling cuttings samples according to claim 1 or 2, characterized in that, The temperature of freeze-drying in step A is -60°C to -50°C; the time of freeze-drying in step A is preferably 12 to 24 h.

5. The rapid digestion method for detecting heavy metals in water-based drilling cuttings samples according to claim 1 or 2, characterized in that, The particle size of the ground sample to be tested in step B is below 100 mesh.

6. The rapid digestion method for detecting heavy metals in water-based drilling cuttings samples according to claim 1 or 2, characterized in that, The ground sample to be tested in step B is first wetted with water and then mixed with hydrofluoric acid, and the mass ratio of the water to m is preferably 0.1:1 g / mL.

7. The rapid digestion method for detecting heavy metals in water-based drilling cuttings samples according to claim 1 or 2, characterized in that, The heating in steps B and C is carried out using a graphite digestion instrument; steps B and C are preferably carried out in a container made of polypropylene material.

8. The rapid digestion method for detecting heavy metals in water-based drilling cuttings samples according to claim 1 or 2, characterized in that, The total time of steps B and C is below 4 h, preferably below 2 h.

9. The rapid digestion method for detecting heavy metals in water-based drilling cuttings samples according to claim 1 or 2, characterized in that, m:V5 = 0.1:10 - 50.

10. The rapid digestion method for detecting heavy metals in water-based drilling cuttings samples according to claim 1 or 2, characterized in that, The rotation speed of the centrifugation is 3000 to 8000 revolutions per minute, preferably 3000 to 6000 revolutions per minute, and the time of centrifugation is preferably 5 to 10 minutes.

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