CN115288649B - Tracer system for coalbed methane reservoir and coalbed methane horizontal well fracturing monitoring method - Google Patents

Abstract

The invention relates to the technical field of coalbed methane exploitation, in particular to a tracer system for a coalbed methane reservoir and a coalbed methane horizontal well fracturing monitoring method. The tracer system comprises: chlorides of rare earth elements, complexing agents, extractants and saponifying agents. The fracturing monitoring method for the coal bed methane horizontal well comprises the step of monitoring fracturing flowback fluid of the coal bed methane horizontal well by adopting the tracer system. The adsorption rate of the tracer system in the coal seam is only 10-20%, so that the requirements of layered test and multistage fracturing test of the coal seam gas horizontal well can be well met; good compatibility with fracturing fluid, good stability, no interference of trace elements; the use safety is good, the operation is convenient, no obvious foam is generated in the use process, and the problem of sleeve pressure rising caused in the process of pumping the tracer into the stratum can be solved.

Description

Tracer system for coalbed methane reservoir and coalbed methane horizontal well fracturing monitoring method

Technical Field

The invention relates to the technical field of coalbed methane exploitation, and in particular to a tracer system for coalbed methane reservoirs and a coalbed methane horizontal well fracturing monitoring method.

Background technique

After horizontal drilling, segmented multi-cluster perforation operations and large-scale volume fracturing transformation, the conductivity or dominant channel segments of unconventional oil and gas reservoirs (coalbed methane, tight gas, shale gas, etc.) are unclear. Tracer segmented monitoring technology has become an important means to evaluate the effect of horizontal well volume fracturing and the state of fractures. At present, the research on this technology is mainly aimed at oil reservoirs. The tracers used have also evolved from the initial low-precision, large-dosage chemical tracers to radioactive isotope tracers and non-radioactive isotope tracers to high-precision, low-cost, safe and stable trace material tracers, which have been well applied and developed in oil field water injection development.

For the segmented monitoring of coalbed methane reservoirs, existing tracers have the following problems: (1) Traditional chemical tracers have the defects of high construction risk and poor qualitative measurement; (2) As the number of layers for staged fracturing of horizontal wells increases, the number of stable isotope tracers is small, the cost is high, and the detection method is complex, making them not well suited for coalbed methane reservoirs; (3) After the completion of multi-stage fracturing of coalbed methane wells, there is little or even no fracturing fluid return, and the conductivity of each fracturing stage can only be evaluated during the production stage of drainage and gas production. Therefore, the tracer stays in the reservoir for a long time, and the coal seam has a strong adsorption capacity, which causes a sudden increase in the adsorption of trace substance tracers in the coal seam, making it difficult to meet the use requirements.

Currently, there is little research on trace substance tracers for coalbed methane reservoirs. Providing a new trace substance tracer system suitable for highly adsorbed coalbed methane reservoirs has become a key issue that needs to be solved urgently.

Summary of the invention

The purpose of the present invention is to overcome the problem that existing tracers are difficult to meet the requirements of segmented monitoring of coalbed methane reservoirs, and to provide a tracer system for coalbed methane reservoirs and a coalbed methane horizontal well fracturing monitoring method.

In order to achieve the above-mentioned object, the first aspect of the present invention provides a tracer system for coalbed methane reservoirs, comprising: chlorides of rare earth elements, a complexing agent, an extractant and a saponifying agent; wherein:

The rare earth element chloride is at least one selected from the group consisting of yttrium chloride, lanthanum chloride, praseodymium chloride, neodymium chloride, samarium chloride, holmium chloride, erbium chloride and ytterbium chloride;

The complexing agent is selected from at least one of ethylenediaminetetraacetic acid, disodium ethylenediaminetetraacetate and tetrasodium ethylenediaminetetraacetate;

The extractant is an acidic phosphate extractant; the saponifier is selected from at least one of sodium hydroxide, sodium carbonate and sodium bicarbonate.

A second aspect of the present invention provides a method for monitoring fracturing of a coalbed methane horizontal well, wherein the method comprises using the tracer system described in the first aspect to monitor fracturing flowback fluid of a coalbed methane horizontal well.

Through the above technical solution, the present invention has the following beneficial effects:

(1) Providing a trace substance tracer system composed of chlorides of rare earth elements, complexing agents, extractants and saponifiers. The tracer system has a small adsorption amount in coal seams, with an adsorption rate of only 10-20%, and can meet the industry standard for tracer screening under the condition of a small tracer dosage;

(2) The tracer system includes a variety of tracers, which can meet the requirements of coalbed methane horizontal well layer testing and multi-stage fracturing testing; it has good compatibility with the fracturing fluid, good stability, and the tracer elements do not interfere with each other;

(3) The tracer system will not generate obvious foam during the process of dissolving into the fracturing fluid, and can overcome the problem of increased casing pressure caused by the tracer being pumped into the formation, and there is less foam in the return fluid;

(4) It is safe to use, easy to operate, and suitable for fracturing construction environment.

Detailed ways

The endpoints and any values of the ranges disclosed in this article are not limited to the precise ranges or values, and these ranges or values should be understood to include values close to these ranges or values. For numerical ranges, the endpoint values of each range, the endpoint values of each range and the individual point values, and the individual point values can be combined with each other to obtain one or more new numerical ranges, which should be regarded as specifically disclosed in this article.

The specific embodiments of the present invention are described in detail below. It should be understood that the specific embodiments described herein are only used to illustrate and explain the present invention, and are not used to limit the present invention.

The first aspect of the present invention provides a tracer system for coalbed methane reservoirs, comprising: chlorides of rare earth elements, complexing agents, extractants and saponifiers; wherein:

The rare earth element chloride is at least one selected from the group consisting of yttrium chloride, lanthanum chloride, praseodymium chloride, neodymium chloride, samarium chloride, holmium chloride, erbium chloride and ytterbium chloride;

The complexing agent is selected from at least one of ethylenediaminetetraacetic acid, disodium ethylenediaminetetraacetate and tetrasodium ethylenediaminetetraacetate;

The extractant is an acidic phosphate extractant; the saponifier is selected from at least one of sodium hydroxide, sodium carbonate and sodium bicarbonate.

According to the present invention, in the tracer system for coalbed methane reservoirs, the selected trace substance tracer is chloride of rare earth elements, wherein the rare earth elements serve as tracer elements.

According to the present invention, specifically, the yttrium chloride, lanthanum chloride, praseodymium chloride, neodymium chloride, samarium chloride, holmium chloride, erbium chloride and ytterbium chloride can be further preferably yttrium chloride hexahydrate, lanthanum chloride hexahydrate, praseodymium chloride hexahydrate, neodymium chloride hexahydrate, samarium chloride hexahydrate, holmium chloride hexahydrate, erbium chloride hexahydrate and ytterbium chloride hexahydrate.

According to the present invention, the yttrium chloride, lanthanum chloride, praseodymium chloride, neodymium chloride, samarium chloride, holmium chloride, erbium chloride and ytterbium chloride can be prepared by conventional methods or obtained through commercial channels, and the purity of the above chlorides is required to be greater than 99%, preferably greater than 99.9%, and further preferably greater than 99.99%.

In the present invention, the tracer system has a variety of trace material tracers that can be selected, has good stability, is not easy to react with underground fluids and minerals in coal seams, and does not interfere with each other, and can well meet the requirements of coalbed methane horizontal well stratification testing and multi-stage fracturing testing.

According to the present invention, the complexing agent is selected from at least one of ethylenediaminetetraacetic acid (EDTA), disodium ethylenediaminetetraacetate (EDTA-2Na) and tetrasodium ethylenediaminetetraacetate (EDTA-4Na). In the process of dissolving the tracer system into the fracturing fluid and performing the coalbed methane well fracturing and gas production operation, the complexing agent can complex the rare earth elements (i.e., tracer elements) in the solution, reduce the activity of the free tracer element ions, make it difficult for the tracer elements to enter the pores of the coal seam, and thus reduce the adsorption rate of the tracer elements in the coal seam.

According to a preferred embodiment of the present invention, the complexing agent is tetrasodium ethylenediaminetetraacetate.

In the present invention, the fracturing fluid is a water-based fracturing fluid, that is, a fracturing fluid prepared with water as a solvent or a dispersion medium.

According to the present invention, the extractant has strong solubility of rare earth elements, and can return most of the rare earth elements adsorbed by the coal seam to the fracturing fluid through extraction, thereby reducing the loss of tracer elements.

According to the present invention, preferably, the extractant is selected from di(2-ethylhexyl) phosphate (HDEHP) and/or monoethylhexyl 2-ethylhexyl phosphate (HEH/EHP).

According to a preferred embodiment of the present invention, the extractant is di(2-ethylhexyl) phosphate.

According to the present invention, the saponifier can greatly improve the solubility of the extractant in the fracturing fluid to better play a role, and enable the tracer system as a whole to be well dissolved in the fracturing fluid. In addition, since the saponifier is alkaline, it can also stabilize the pH value of the fracturing fluid, so that the use of the acidic extractant will not cause acid corrosion to the formation and the construction pipe string.

According to a preferred embodiment of the present invention, the saponifying agent is sodium hydroxide.

According to the present invention, in the tracer system, the quantitative relationship of each component satisfies: the weight ratio of the chloride of the rare earth element: the complexing agent is 1: (0.8-1.2), the weight ratio of the complexing agent: the extracting agent is (8-12): 1, and the weight ratio of the saponifying agent: the extracting agent is 1: (40-70).

Preferably, on the basis of satisfying the above-mentioned quantitative relationship, the components in the tracer system further have a weight ratio of the chloride of the rare earth element: the complexing agent: the extracting agent of (9.5-10.5): 1, and a weight ratio of the saponifying agent: the extracting agent of 1: (63-68), thereby enabling the tracer system to have a further reduced tracer adsorption rate in the coalbed methane reservoir, as well as better stability and compatibility.

According to the present invention, in the tracer system, although the possible effects of the addition of the above-mentioned components can be considered, for example, the complexing agent can reduce the activity of the free tracer element ions in the fracturing fluid, the extractant can return the tracer elements adsorbed by the coal seam to the fracturing fluid, and the saponifier can promote the tracer system to be better dissolved in the fracturing fluid, but the tracer system provided by the present invention can produce a synergistic effect when it contains the above-mentioned specific rare earth element chloride, complexing agent, extractant and saponifier components and satisfies the quantitative relationship of each component at the same time, so that the tracer system has the characteristics of low tracer adsorption rate, good compatibility with fracturing fluid, good stability, and no interference between tracer elements in coalbed methane well fracturing and gas production construction. Outside the above-mentioned limited range, the above-mentioned comprehensive performance of the tracer system provided by the present invention cannot be obtained.

According to a preferred embodiment of the present invention, the tracer system comprises chlorides of rare earth elements, tetrasodium ethylenediaminetetraacetate, di(2-ethylhexyl) phosphate and sodium hydroxide, wherein the weight ratio of chlorides of rare earth elements: tetrasodium ethylenediaminetetraacetate is 1:(1-1.1); the weight ratio of tetrasodium ethylenediaminetetraacetate: di(2-ethylhexyl) phosphate is (9.5-10.5):1; the weight ratio of sodium hydroxide: di(2-ethylhexyl) phosphate is 1:(63-68).

According to a further preferred embodiment of the present invention, the tracer system includes chlorides of rare earth elements, tetrasodium ethylenediaminetetraacetate, di(2-ethylhexyl) phosphate and sodium hydroxide, wherein the weight ratio of chlorides of rare earth elements: tetrasodium ethylenediaminetetraacetate is 1:1.1; the weight ratio of tetrasodium ethylenediaminetetraacetate: di(2-ethylhexyl) phosphate is 10:1; and the weight ratio of sodium hydroxide: di(2-ethylhexyl) phosphate is 1:65.

According to the present invention, the tracer system may further include a defoamer, a drag reducer and an ethoxylated alcohol on the basis of containing the above components, which is beneficial to the operability and safety of the construction.

According to the present invention, in the process of dissolving the tracer system into the fracturing fluid to form the tracer solution, the complexing agent and the extractant therein will promote the generation of tiny bubbles under the joint action, which brings inconvenience to the on-site construction. For example, it is difficult to determine the amount of tracer added, and it is impossible to prepare a tracer solution that meets the requirements in time. Therefore, the inclusion of a defoamer in the tracer system can prevent obvious foam from being generated during the preparation of the tracer solution, and during the fracturing and gas production construction, the foam in the return fluid is also significantly reduced. Preferably, the defoamer is selected from at least one of a polyether-organosilicon defoamer, a polyether defoamer and an organosilicon defoamer.

In the present invention, there is no particular limitation on the polyether-organosilicon defoamer, polyether defoamer and organosilicon defoamer, and they can be prepared by conventional methods or by using conventional commercially available products.

According to the present invention, considering that the buried depth of the coal seam is relatively shallow (the depth is much smaller than that of the oil layer), the pressure of the formation is small, and the pressure used during fracturing is also relatively small. In this case, the inventor of the present invention believes that the problem of increased casing pressure caused by adding tracers cannot be ignored. The present invention includes a drag reducer in the tracer system provided, which can effectively reduce the turbulence and increased friction caused by the fracturing fluid in the pipe after the chloride (tracer) of the rare earth element, the chelating agent, the extractant and the saponifier are mixed with the proppant of the fracturing fluid through the pipe column and the fracturing fluid is pumped into the formation at high speed, thereby reducing the pressure loss, so that the casing pressure during the fracturing construction is maintained in a relatively stable state, and will not produce large fluctuations with the injection of the tracer. Preferably, the drag reducer is selected from at least one of polyacrylamide, guar gum, xanthan gum and polyethylene oxide.

In the present invention, preferably, the molecular weight of the drag reducer is 200,000-18,000,000 g/mol.

According to the present invention, the ethoxylated alcohol in the tracer system has the effect of reducing the surface tension of the fracturing fluid, improving the flowback rate of the fracturing fluid, and effectively solving the problem of less flowback fluid in the coalbed methane well during the fracturing construction process. Preferably, the ethoxylated alcohol can be ethoxylated-C12-16-alcohol and/or ethoxylated-C12-18-alcohol.

According to the present invention, in the tracer body, preferably, the weight ratio of the defoaming agent to the chloride of the rare earth element is (0.005-0.01):1.

According to the present invention, in the tracer body, preferably, the weight ratio of the drag reducer: chloride of the rare earth element is (1-2):1; preferably, the weight ratio of ethoxylated alcohol: chloride of the rare earth element is (1.5-3.5):1.

According to the present invention, the tracer system is prepared by fully mixing the components in the tracer system according to the above quantitative relationship. In application, multiple tracer systems containing a single type of tracer can be prepared and added to different fracturing stages of a gas well when used. The amount of the tracer system can be determined according to the specific conditions of the fracturing coal seam.

In the present invention, specifically, in each fracturing stage of a coalbed methane well, the amount of each tracer in the tracer system can be calculated according to the following formula (I):

Where M is the amount of tracer required for each fracturing stage, kg;

μ is a correction coefficient for factors such as formation water invasion and coal seam adsorption, with a value range of 800-1200, and is a dimensionless quantity. The specific value of μ can be determined by μ=D _W ×A _t ×N× _Tr ; wherein D _W is a multiple for formation invasion, and is a dimensionless quantity; _At is a multiple for coal seam adsorption, and is a dimensionless quantity; N is the number of stages for staged fracturing; _Tr is a multiple for the residence time of the tracer in the formation, and is a dimensionless quantity.

_Ce is the concentration of each tracer (chloride of rare earth element), mg/m ³ ; _Ce can be calculated by the formula _Ce = ( _Cn × _me )/m _a , where _Cn is the minimum detection limit concentration of each tracer element of the instrument, mg/m ³ ; _me is the relative molecular mass of each tracer (chloride of rare earth element); _ma is the relative atomic mass of each tracer element (rare earth element);

V is the volume of fracturing fluid injected into each fracturing stage, m ³ .

On this basis, the dosage of other components can be obtained according to the quantitative relationship of each component in the tracer system.

Aiming at the unique characteristics of coalbed methane reservoirs, namely "intrinsic adsorption and less external flowback", the present invention designs a formula based on a large amount of research to obtain a tracer system for coalbed methane reservoirs containing specific components and component proportions. The tracer system includes many types of tracers, which can exist stably in the fracturing fluid for a long period of time and have good compatibility with the fracturing fluid. The tracers do not interfere with each other, and the adsorption amount in the coal seam is particularly small, which can greatly improve the monitoring and analysis accuracy. In addition, no obvious foam is generated during preparation and use, and the casing pressure increase problem will not be caused.

A second aspect of the present invention provides a method for monitoring fracturing of a coalbed methane horizontal well, wherein the method comprises using the tracer system described in the first aspect to monitor fracturing flowback fluid of a coalbed methane horizontal well.

According to the present invention, preferably, the coalbed methane horizontal well fracturing monitoring method further comprises: adding the tracer system to each fracturing section of the coalbed methane well under layered fracturing construction, sampling the fracturing return fluid during the fracturing construction and gas production process, and detecting the rare earth element content in the sampled samples, evaluating the volume fracturing effect and crack status of each fracturing section of the coalbed methane horizontal well according to the detected rare earth element content, thereby realizing coalbed methane horizontal well fracturing monitoring.

According to the present invention, when the tracer system is added to each fracturing stage of a coalbed methane well in a layered fracturing construction, a tracer system containing one tracer is added to each fracturing stage, and the types of tracers added to each fracturing stage are different.

According to the present invention, preferably, the rare earth element content in the fracturing flowback fluid may be detected by mass spectrometry.

According to the present invention, based on the rare earth element content in the fracturing flowback fluid, a fracturing tracer injection-flowback interpretation model is established to obtain a fracturing tracer curve, which can be used to evaluate the volume fracturing effect and fracture state of each fracturing stage.

The present invention will be described in detail below by way of examples. In the following examples and comparative examples,

Yttrium chloride hexahydrate (purity 99.99 wt %), lanthanum chloride hexahydrate (purity 99.9 wt %), praseodymium chloride hexahydrate (purity 99.9 wt %), neodymium chloride hexahydrate (purity 99.9 wt %), holmium chloride hexahydrate (purity 99.9 wt %), ytterbium chloride hexahydrate (purity 99.9 wt %), samarium chloride hexahydrate (purity 99.9 wt %), and erbium chloride hexahydrate (purity 99.9 wt %) were purchased from Shandong Haoshun Chemical Co., Ltd.;

Polyacrylamide: weight average molecular weight is 3 million to 18 million g/mol, purchased from Gongyi Bibo Water Supply Material Co., Ltd.

Xanthan gum: weight average molecular weight of 2 million to 10 million g/mol, purchased from Sichuan Huatang Jurui Biotechnology Co., Ltd.

Polyether-organosilicon defoamer: purchased from Hefei Xinwancheng Environmental Protection Technology Co., Ltd.;

Silicone defoamer: purchased from Hefei Xinwancheng Environmental Protection Technology Co., Ltd.;

Ethoxylated-C12-16-alcohol, ethoxylated-C12-18-alcohol: purchased from Zhengzhou Aikemu Chemical Co., Ltd.;

Unless otherwise specified, other materials used are common commercially available products.

Example 1

Yttrium chloride hexahydrate (tracer), EDTA-4Na (complexing agent), HDEHP (extractant), NaOH (saponifying agent), polyacrylamide (drag reducer), polyether-organosilicon defoamer and ethoxylated-C12-18-alcohol are fully mixed to obtain a mixture, that is, a tracer system for coalbed methane reservoirs (denoted as S1).

The components and contents in S1 are shown in Table 1.

Example 2-18

According to the method of Example 1, the difference is that different tracers, complexing agents, extractants, saponifiers, drag reducers, defoamers and ethoxylated alcohols are used, as well as the quantitative relationship of each component to prepare a tracer system for coalbed methane reservoirs (denoted as S2-S18).

The components and contents of S2-S18 are shown in Table 1.

Comparative Example 1

According to the method of Example 1, except that EDTA-4Na is not included in the raw materials, other conditions are the same as Example 1, and a tracer system (denoted as D1) is prepared.

The components and contents in D1 are shown in Table 1.

Comparative Example 2

According to the method of Example 1, except that the raw materials do not contain HDEHP and NaOH, other conditions are the same as Example 1, to prepare a tracer system (denoted as D2).

The components and contents of D2 are shown in Table 1.

Comparative Example 3

According to the method of Example 1, except that the raw material does not contain NaOH, other conditions are the same as Example 1, to prepare a tracer system (denoted as D3).

The components and contents of D3 are shown in Table 1.

Table 1

Note: In Table 1, A represents ethoxylated-C12-16-alcohol, and B represents ethoxylated-C12-18-alcohol

Test Case

The performance tests of the tracer systems S1-S18 and D1-D3 prepared in Examples 1-18 and Comparative Examples 1-3 were performed as follows:

1. Stability test

The following steps were used to conduct the experiment:

(1) Formation water from a coalbed methane fracturing reservoir is taken and mixed with tracer systems S1-S18 to prepare tracer solutions with a tracer element (i.e., rare earth element) concentration of 1 mg/L (denoted as C ₀ );

(2) The above tracer solution was sealed and stored in a constant temperature water bath (water bath temperature was 43°C to simulate reservoir temperature) for 60 days, and then the concentration of the tracer element in the tracer solution was tested using an ICP-MS/Agilent 7900 inductively coupled plasma mass spectrometer (denoted as C ₆₀ );

(3) The concentration retention rate η of the tracer solution was calculated according to formula (II) to evaluate the stability of the tracer system. The results are shown in Table 2.

η＝C ₆₀ /C ₀ ×100％ (Ⅱ)

Table 2

As can be seen from Table 2, in the tracer system for coalbed methane reservoirs of the present invention, the concentration retention rate of the tracer solution in the stability experiment is higher than 96%, indicating that the use of each component has little effect on the stability of the tracer element, and the stability of the tracer system meets the requirements for segmented monitoring of coalbed methane reservoirs. In addition, the complexing agent used in S18 is oxalic acid, which affects the stability of the tracer element.

2. Compatibility test

The following steps were used to conduct the experiment:

(1) using the tracer systems S1-S18 and the fracturing fluid sample M without tracer to prepare tracer mixed solutions with a tracer element (i.e., rare earth element) concentration of 15 mg/L;

(2) The tracer mixed solution was placed in a water bath constant temperature oscillator for oscillation (the water bath temperature was 43°C to simulate the reservoir temperature), and sealed and stored for 90 days. Afterwards, the tracer mixed solution was observed to see whether it became turbid or precipitated, and the temperature resistance and shear resistance tests were performed according to the method specified in SY/T 5107-2005;

(3) comparing the temperature resistance and shear resistance of the tracer mixed solution sealed and stored for 90 days with that of a fracturing fluid sample M without a tracer;

Wherein, based on the total weight of the above-mentioned fracturing fluid sample M without tracer, its composition is: 1% potassium chloride+0.025% drag reducer+0.01% ammonium persulfate.

The results are shown in Table 3.

table 3

Tracer system Becomes turbid or produces precipitation S1 The solution is clear without precipitation S2 The solution is clear without precipitation S3 The solution is clear without precipitation S4 The solution is clear without precipitation S5 The solution is clear without precipitation S6 The solution is clear without precipitation S7 The solution is clear without precipitation S8 The solution is clear without precipitation S9 The solution is clear without precipitation S10 The solution is clear without precipitation S11 The solution is clear without precipitation S12 The solution is clear without precipitation S13 The solution is clear without precipitation S14 The solution is clear without precipitation S15 The solution is clear without precipitation S16 The solution is clear without precipitation S17 The solution is clear without precipitation S18 A little white precipitate

It can be seen from Table 3 that the tracer system for coalbed methane reservoirs of the present invention can be well compatible with the fracturing fluid system (including chemical agents), and the fracturing fluid does not become turbid or precipitate (the complexing agent used in S18 is oxalic acid, which has a certain impact on the compatibility).

In addition, before and after the tracer system of the present invention is added, the temperature resistance and shear resistance of the fracturing fluid can be effectively maintained and are not affected, which meets the fracturing construction requirements of the coalbed methane reservoir.

3. Interference experiment

The following steps were used to conduct the experiment:

(1) ultrapure water and tracer systems S1, S2, S3, S4, S5, S6, S7 and S8 are used to prepare a mixed solution in which the concentration of each tracer element (i.e., rare earth element) is 200 mg/L (i.e., the prepared concentration);

(2) The content of each tracer element in the mixed solution was tested by liquid chromatography, the prepared concentration of each tracer element in the mixed solution was compared with the measured concentration, and the difference between the prepared concentration and the measured concentration was calculated. The results are shown in Table 4.

Table 4

Tracer elements Difference between the prepared concentration of the tracer element in the mixed solution and the measured concentration/% yttrium 4.35 lanthanum 5.46 praseodymium 5.62 neodymium 5.79 holmium 5.32 ytterbium 4.96 samarium 5.02 erbium 4.58

It can be seen from Table 4 that after the tracer systems S1-S8 for coalbed methane reservoirs of the present invention are mixed, the measured concentrations of the tracer elements contained in the mixed solution are close to the prepared concentrations (the difference is less than 6%), indicating that there is basically no interference between them and the requirements for segmented monitoring of coalbed methane reservoirs can be met.

4. Static adsorption experiment

According to the industry standard SY/T 5925-2012 for tracer screening, the experiment was carried out using the following steps:

(1) using the tracer systems S1-S18, D1-D3 and the aforementioned fracturing fluid sample M without tracer, respectively, a tracer mixed solution with an initial concentration (denoted as _Cinitial ) of 500C _n of the tracer element (i.e., rare earth element) was prepared (where C _n is the minimum detection limit concentration of each tracer by the instrument, mg/m ³ );

(2) The tracer mixed solution was mixed with the screened coal sample (taken from Baode block, Shanxi, with a particle size of 40-60 mesh, and the coal sample was washed and dried) in a weight ratio of 1:1, sealed and placed in a constant temperature water bath (water bath temperature was 43°C to simulate reservoir temperature) for standing;

(3) Sampling each of the above-mentioned static tracer mixed solutions every day, and using CP-MS/Agilent 7900 inductively coupled plasma mass spectrometer to test the concentration of the tracer element in the sample until adsorption equilibrium is reached, that is, the concentration of the tracer element in the sample no longer changes, and recording the concentration of the tracer element in the tracer mixed solution at the adsorption equilibrium (referred to as C _{adsorption equilibrium} );

(4) The adsorption rate of the tracer was calculated according to formula (III). The results are shown in Table 5.

Tracer adsorption rate % = (C _initial - C _{adsorption equilibrium} ) / C _initial × 100% (III)

table 5

It can be seen from Table 5 that the tracer system for coalbed methane reservoirs of the present invention has a small adsorption amount in the coal seam, and the adsorption rate is only 10-20%. It can meet the industry standard for tracer screening under the condition of a smaller tracer dosage, and when other conditions in the system are the same, the tracer system containing rare earth elements yttrium, lanthanum, praseodymium and neodymium has a relatively smaller adsorption rate in the coal seam.

The preferred embodiments of the present invention are described in detail above, but the present invention is not limited thereto. Within the technical concept of the present invention, the technical solution of the present invention can be subjected to a variety of simple modifications, including the combination of various technical features in any other suitable manner, and these simple modifications and combinations should also be regarded as the contents disclosed by the present invention and belong to the protection scope of the present invention.

Claims (16)

1. A tracer system for a coal-bed gas reservoir, comprising: chlorides of rare earth elements, complexing agents, extractants and saponifying agents; wherein,

the rare earth element chloride is selected from at least one of yttrium chloride, lanthanum chloride, praseodymium chloride, neodymium chloride, samarium chloride, holmium chloride, erbium chloride and ytterbium chloride;

the complexing agent is at least one of ethylenediamine tetraacetic acid, disodium ethylenediamine tetraacetate and tetrasodium ethylenediamine tetraacetate;

the extractant is an acidic phosphate extractant; the saponifier is at least one of sodium hydroxide, sodium carbonate and sodium bicarbonate;

chlorides of the rare earth elements: the weight ratio of the complexing agent is 1: (0.8-1.2);

the complexing agent: the weight ratio of the extractant is (8-12): 1, a step of;

the saponification agent: the weight ratio of the extractant is 1: (40-70).

2. A tracer system according to claim 1, wherein the complexing agent is tetrasodium ethylenediamine tetraacetate.

3. A tracer system according to claim 1, wherein the extractant is selected from di (2-ethylhexyl) phosphate and/or monoethylhexyl 2-ethylhexyl phosphate.

4. A tracer system according to claim 3, wherein the extractant is di (2-ethylhexyl) phosphate.

5. A tracer system according to any of claims 1-4, wherein the saponifying agent is sodium hydroxide.

6. A tracer system according to any of claims 1-4, wherein the chlorides of rare earth elements: the weight ratio of the complexing agent is 1: (1-1.1).

7. A tracer system according to any of claims 1-4, wherein the complexing agent: the weight ratio of the extractant is (9.5-10.5): 1.

8. a tracer system according to any of claims 1-4, wherein the saponifying agent: the weight ratio of the extractant is 1: (63-68).

9. The tracer system of any of claims 1-4, wherein the tracer system further comprises an antifoaming agent, a drag reducer, and an ethoxylated alcohol.

10. The tracer system of claim 9, wherein the defoamer is selected from at least one of a polyether-silicone defoamer, a polyether defoamer, and a silicone defoamer.

11. The tracer system of claim 9, wherein the drag reducing agent is selected from at least one of polyacrylamide, guar gum, xanthan gum, and polyethylene oxide.

12. The tracer system of claim 9, wherein the defoamer: the weight ratio of the chlorides of rare earth elements is (0.005-0.01): 1.

13. the tracer system of claim 9, wherein the drag reducer: the weight ratio of the chlorides of the rare earth elements is (1-2): 1.

14. a tracer system according to claim 9, wherein the ethoxylated alcohol: the weight ratio of the chlorides of rare earth elements is (1.5-3.5): 1.

15. a method of monitoring the fracturing flow-back of a horizontal well of coalbed methane, wherein the method comprises monitoring the fracturing flow-back of a horizontal well of coalbed methane with the tracer system of any one of claims 1-14.

16. The method of claim 15, wherein the method comprises: and adding the tracer system into each fracturing stage of the coal-bed gas well in the layered stage fracturing construction, sampling fracturing flowback fluid in the fracturing construction and gas production process, detecting the content of rare earth elements in the sampled sample, and evaluating the volume fracturing effect and the fracture state of each fracturing stage of the coal-bed gas horizontal well according to the detected content of the rare earth elements to realize the fracturing monitoring of the coal-bed gas horizontal well.