CN115124990B

CN115124990B - A clean nutrient-based working fluid and its method for exploiting coalbed methane

Info

Publication number: CN115124990B
Application number: CN202210713417.7A
Authority: CN
Inventors: 周哲; 葛兆龙; 卢义玉; 龚时辉; 盛美玉; 管娅蕊; 张宏伟
Original assignee: Chongqing University
Current assignee: Chongqing University
Priority date: 2022-06-22
Filing date: 2022-06-22
Publication date: 2023-05-23
Anticipated expiration: 2042-06-22
Also published as: CN115124990A

Abstract

The invention relates to a clean nutrient-based working fluid and a method for exploiting coal bed gas. Clean nutrient-based working fluid includes clean fracturing fluid, nutrient solution and water; clean fracturing fluid includes viscoelastic surfactants, viscoelastic activators and counter ion additives; nutrient solution includes sodium bicarbonate, sodium dihydrogen phosphate, chlorine Ammonium Chloride, Calcium Chloride, Magnesium Sulfate, Alcohol, Yeast Extract, and Tryptone. The present invention also provides a method for mining coalbed methane with a clean nutrient-based working fluid, as follows: inject the clean nutrient-based working fluid into the coal seam so that the clean fracturing fluid can fully increase the permeability of the coal seam; Contact the in-situ microorganisms in the coal seam, so that the in-situ microorganisms can transform the coal seam; when the methane gas content exceeds 30%, the coalbed methane is extracted. The invention solves the problems of environmental pollution caused by the introduction of exogenous microorganisms, small nutrient solution injection range, fast closing of cracks in coal seam pores transformed by fracturing fluid, poor flow conductivity, and low coal bed gas output.

Description

Nutrient medium cleaning working solution and method for mining coalbed methane by using same

Technical Field

The invention relates to the technical field of coalbed methane exploitation, in particular to a clean nutrient medium working solution and a method for exploiting coalbed methane.

Background

Coalbed methane (the main component is methane) is an unconventional natural gas stored in a coal bed, and is a low-carbon and clean energy source. CH under the condition of generating the same heat value compared with other fossil energy sources ₄ Greenhouse gas CO generated by combustion ₂ Is minimal, CH ₄ High combustion efficiency and almost no pollutant generation. Therefore, the development and the utilization of the coalbed methane have great significance for improving the greenhouse effect and the environmental pollution.

Coalbed methane is mainly produced by degrading and converting coal bodies by microorganisms present in the coal bed. At present, the number of coal mines is increased year by permanently closing coal mine resources integration and safe exploitation in China. These closed mines store large amounts of coal and coalbed methane resources.

In recent years, after a method for increasing yield of coalbed methane by microorganisms is proposed in coalbed methane exploitation, many scholars at home and abroad aim to improve the current yield increasing technology, for example, CN 101922287 discloses a method for cultivating and activating coalbed methane flora by using a culture solution to convert underground methane flora into methane; CN 102559772 discloses a method for fermenting and culturing coalbed methane-increasing microorganisms by using a fermenter, and injecting fermentation liquor (exogenous microorganisms and nutrient solution) into the coalbed to increase coalbed methane yield; CN 102559772 discloses a method of using in situ electrostimulation of microorganisms in a sedimentary deposit, followed by extraction of coalbed methane from the formation; CN 105063093 discloses a method for producing coalbed methane and the like by injecting mixed microbial inoculum into a coalbed through screening, domestication and enrichment.

The application of culturing and activating the coalbed bacterial colony by using the culture solution and increasing the yield of coalbed methane by injecting exogenous microorganisms into the coalbed is relatively wide. But still have shortcomings, including in particular: 1) The survival rate of the exogenous bacteria in the actual coal seam environment cannot be controlled; 2) The introduction of exogenous bacteria may pollute the local environment and destroy ecology; 3) The methane bacteria have low gas production efficiency due to the difficulty in balancing the quantity of each functional flora, and can not continuously produce gas; 4) The process of degrading and converting coal into coalbed methane by microorganisms is slow, so that the development speed is limited.

In addition, the bituminous coal accounts for more than 50% of the total coal resources in China, the nutrients available to microorganisms are less, and the increase of the yield of the coalbed methane by the microorganisms is difficult to realize in the prior art. Moreover, most coal beds in China are low-permeability coal beds, and simply injecting the culture solution into the coal beds can cause the culture solution to gather and not to be effectively diffused to the periphery, so that the application range is small, and the gas yield and the gas production rate are very slow. And, the coal seam is because the gas permeability is poor and soft characteristics, even use fracturing fluid fracturing back crack water conservancy diversion ability still relatively poor to can make the extraction rate drop fast, lead to coal seam gas extraction inefficiency. Therefore, whether the problems can be effectively solved is a key point of efficiently increasing the yield of the coalbed methane by utilizing microorganisms.

Disclosure of Invention

The invention aims to provide a clean nutrient medium working solution and a method for mining coal bed gas, which are used for solving the problems that in the prior art, exogenous microorganisms are injected into a coal bed to increase the yield of the coal bed gas, the survival rate of the exogenous microorganisms cannot be controlled and the ecological environment is influenced, and the problems that the nutrient solution of the exogenous microorganisms is easy to gather in a low-permeability coal bed and cannot be effectively diffused to the periphery, so that the action range of the exogenous microorganisms is small, the gas yield and the gas yield rate are slow, and the diversion capability of cracks after fracturing of fracturing fluid is still poor.

In order to achieve the above purpose, the technical scheme adopted by the invention is as follows:

a cleaning nutrient medium working solution, which comprises a cleaning fracturing fluid, a nutrient solution and water;

the clean fracturing fluid comprises a viscoelastic surfactant with the concentration of 4-25 g/L, a viscoelastic activator with the concentration of 0-10 g/L and a counter ion auxiliary agent with the concentration of 10-35 g/L;

the nutrient solution comprises NaHCO with the concentration of 0.1-2 g/L ₃ NaH of 1-3 g/L ₂ PO ₄ NH 0.5-5 g/L ₄ Cl, caCl 0.1-3 g/L ₂ MgSO with 0.1-2 g/L ₄ ·7H ₂ O, 1-25 g/L of alcohol, 0.5-10 g/L of yeast extract and 1-10 g/L of tryptone.

Preferably, the viscoelastic surfactant comprises one or more of cetyltrimethylammonium chloride (CTAC), octadecyltrimethylammonium chloride (sta c), cocamidopropyl betaine (CAB), and Sodium Dodecyl Sulfonate (SDS);

the viscoelastic activator is sodium salicylate (Nasal);

the counter ion auxiliary agent is potassium chloride;

the alcohol is a monohydric alcohol, which includes one or more of methanol, ethanol, and propanol.

Wherein, the monohydric alcohol can promote the microorganism to degrade the coal, convert the coal into green products, and simultaneously can be used as an intermediate substrate to promote the methane generation, and the price is low.

Preferably, the cleaning nutrient medium working solution further comprises a pH buffer solution, wherein the pH buffer solution is hydrochloric acid or sodium hydroxide solution, and the concentration is 1mol/L.

The pH buffer is used for adjusting the pH value of the clean fracturing fluid, so that the pH value of the clean nutrient medium working fluid mixed with the fluid in the coal seam is about 7 after the clean nutrient medium working fluid is injected into the coal seam, and the growth of in-situ microorganisms in the target coal seam is effectively ensured.

The invention also provides a method for mining the coal bed gas by using the clean nutrient medium working solution, which comprises the following steps:

s1, selecting a target coal seam, and injecting a clean nutrient base working solution into the target coal seam by adopting a hydraulic fracturing technology, so that the clean fracturing solution in the clean nutrient base working solution fully enhances the permeability of the target coal seam to form a coal body fracture network;

s2, performing well-flushing operation, namely enabling in-situ microorganisms to reform the target coal seam by enabling nutrient solution in the nutrient medium working solution to be in contact with in-situ microorganisms in the target coal seam, so as to increase hole cracks of the target coal seam and strengthen the crack morphology and strength;

and S3, pumping the coal bed gas when the methane gas content is monitored to be more than 30%.

Preferably, the method comprises the following steps:

s1, performing geological detection on a coal seam, and taking the coal seam which is not a structural area and has a firmness coefficient f of more than 0.3 as a target coal seam;

selecting a corresponding clean nutrient base working solution according to the deterioration degree of a coal body of a target coal bed, the temperature and the pH value of the coal bed, developing a drilling process according to the requirements of a hydraulic fracturing technology, and then injecting the clean nutrient base working solution into the target coal bed by adopting the hydraulic fracturing technology;

s2, performing well-stewed work, enabling a flow channel to be in an open state and enabling in-situ microorganisms to ferment and reproduce rapidly, then injecting carbon dioxide into the well to provide nutrients for the in-situ microorganisms, and simultaneously displacing methane;

and when the methane gas content is monitored to be more than 30%, installing a coal layer gas extraction system, and extracting coal layer gas.

Wherein the non-structural region comprises a non-corrugated, non-fault region.

Preferably, in the step S1, the hydraulic fracturing technology is a hydraulic volumetric fracturing technology;

the target coal seam includes a low-rank coal seam including lignite and sub-coals and a high-rank coal seam including bituminous coal.

The hydraulic volume fracturing technology is characterized in that the thickness of a coal bed and the occurrence parameters of the coal bed are required to be obtained to select specific parameters of the hydraulic volume fracturing technology during geological exploration.

The construction process of the hydraulic volume fracturing technology mainly comprises the following steps: 1. firstly, drilling a vertical main well from the ground to a fracturing target coal bed; 2. arranging radial fracturing branch wells with the length of 100-150 m from a target coal bed at a vertical main well to form fracturing surfaces; 3. for ultra-thick coal beds with the coal bed thickness d of more than 8m, multi-layer fracturing surfaces are required to be arranged longitudinally, so that volumetric fracturing is formed. Wherein the number of layers k of the fracturing surface is approximately equal to (d-2)/1.5+1, d is the thickness of the coal bed, and the distance between the fracturing surface and the upper and lower top and bottom plates is more than 0.5-1 m.

Preferably, when the target coal seam is a high-order coal seam, cetyltrimethylammonium chloride (CTAC), sodium salicylate and potassium chloride are used as clean fracturing fluid when the temperature of the coal seam is less than or equal to 50 ℃, and octadecyltrimethylammonium chloride (STAC), cocamidopropyl betaine (CAB), sodium salicylate and potassium chloride are used as clean fracturing fluid when the temperature of the coal seam is more than 50 ℃;

when the target coal seam is a low-rank coal seam, cocamidopropyl betaine (CAB), sodium Dodecyl Sulfate (SDS) and potassium chloride are used as clean fracturing fluids.

Preferably, in the step S1, in the drilling process, a high-grade steel casing is required to be adopted, and cement is used to seal the casing and the well wall so as to strengthen the well cementation process, so that clean fracturing fluid circulates in the casing. Ensuring the success of the well-logging operation.

Preferably, in the step S2, the well-soaking time of the low-order coal seam is 25-60 days, and the well-soaking time of the high-order coal seam is 40-90 days;

and (3) filling the clean nutrient medium working solution into the target coal seam every 5-8 days in the well stewing process so as to maintain the pressure of the clean nutrient medium working solution in the cracks, enable the cracks to be in an open state all the time and enable in-situ microorganisms and the nutrient solution to be fully contacted and propagated.

In the process of soaking, the clean nutrient medium working solution is required to be filled into the target coal seam every 5-8 days (the actual days are determined according to the observed underground pressure value) to maintain the pressure of the clean nutrient medium working solution in the cracks, so that the cracks are always in an open state, the clean nutrient medium working solution is enabled to diffuse to surrounding coal bodies, the action range is enlarged, meanwhile, the in-situ microorganisms and the nutrient solution are enabled to be fully contacted and propagated, the coal is converted into methane, and mineral crystals are formed in the crack surfaces and the coal bodies through mineralization.

Preferably, in the step S2, the drainage coalbed methane system includes a drainage pipe, a flowmeter and a valve, and the flow rate of the coalbed methane is reflected by observing the flowmeter.

The invention has the beneficial effects that:

1) According to the clean nutrient base working solution, the viscoelastic surfactant, the viscoelastic activator and the counter ion auxiliary agent are selected as clean fracturing fluid, meanwhile, the nutrient fluid is added to prepare the clean nutrient base working solution, then the clean nutrient base working solution is injected into a coal bed by utilizing the hydraulic fracturing technology, and the characteristics of high viscosity, low friction resistance, good wettability, clean and pollution-free performance and good joint making effect of the viscoelastic surfactant are utilized to comprehensively enhance the permeability of a target coal bed, so that a coal mass large-range fracture network is formed, and smooth extraction of coal bed gas is ensured; meanwhile, the nutrient solution without exogenous microorganisms is fully contacted with in-situ microorganisms in the coal bed, so that nutrients and places for rapid propagation of the in-situ microorganisms are provided, the efficiency of coal to convert coal bed gas is improved, the in-situ microorganisms rapidly propagate and reform the coal bed, the fracture of the coal bed hole is increased, the fracture morphology, strength and seepage capacity are enhanced, the diversion capacity is improved, and the yield of the coal bed gas is greatly improved through bidirectional coupling of clean fracturing fluid and nutrient solution to the coal bed reformation. The problems that exogenous microorganisms are injected into a coal bed to increase the yield of coal bed gas, the survival rate of the exogenous microorganisms cannot be controlled and the ecological environment is influenced are solved, the problems that the nutrient solution of the exogenous microorganisms is easy to gather in a low-permeability coal bed and cannot be effectively diffused to the periphery, the action range of the exogenous microorganisms is small, the gas yield is low and the gas yield rate is slow are solved, the diversion capacity of cracks is still poor after the existing fracturing fluid is fractured, and the fracture closure of the coal bed hole is fast when the existing fracturing fluid is transformed are solved;

2) Experiments show that after the clean nutrient base working solution is used for mining coal bed gas, the average pore area of the coal bed is greatly increased and the permeability of the coal bed is obviously improved, so that the clean nutrient base working solution provided by the invention can be used for well reforming the coal bed, increasing the coal bed gas extraction effect, and simultaneously has obvious effect on in-situ microbial degradation conversion in the coal bed, and has popularization and application values in the technical field of coal bed mining.

Drawings

FIG. 1 is a schematic diagram of a clean nutrient base working fluid of the invention for mining coal bed gas;

FIG. 2 is a CT scan of a coal seam after fracturing the coal seam with different fracturing fluids;

FIG. 3 is a graph comparing the permeability of different fracturing fluids;

wherein, in FIG. 1, a 1-clean nutrient-based fracturing fluid; 2-a fracturing truck; 3-a gas-liquid separation device; 4-an air storage tank; 5-horizontal fracturing a well; 6-microcrack.

Detailed Description

Further advantages and effects of the present invention will become readily apparent to those skilled in the art from the disclosure herein, by referring to the accompanying drawings and the preferred embodiments. The invention may be practiced or carried out in other embodiments that depart from the specific details, and the details of the present description may be modified or varied from the spirit and scope of the present invention. It should be understood that the preferred embodiments are presented by way of illustration only and not by way of limitation.

Example 1

A cleaning nutrient medium working solution, which comprises a cleaning fracturing fluid, a nutrient solution, water and a pH buffer solution;

the clean fracturing fluid comprises a viscoelastic surfactant with the concentration of 4-25 g/L, a viscoelastic activator with the concentration of 1-10 g/L and a counter ion auxiliary agent with the concentration of 10-35 g/L;

the nutrient solution comprises NaHCO with the concentration of 0.1-2 g/L ₃ NaH of 1-3 g/L ₂ PO ₄ NH 0.5-5 g/L ₄ Cl, caCl 0.1-3 g/L ₂ MgSO with 0.1-2 g/L ₄ ·7H ₂ O, 1-25 g/L of alcohol, 0.5-10 g/L of yeast extract and 1-10 g/L of tryptone;

the pH buffer solution is hydrochloric acid or sodium hydroxide solution, and the concentration is 1mol/L;

viscoelastic surfactants include one or more of cetyltrimethylammonium chloride (CTAC), octadecyltrimethylammonium chloride (sta c), cocamidopropyl betaine (CAB), and Sodium Dodecyl Sulfonate (SDS);

the viscoelastic activator is sodium salicylate (Nasal);

the counter ion auxiliary agent is potassium chloride;

the alcohols are monohydric alcohols, including one or more of methanol, ethanol, and propanol.

Example 2

As shown in fig. 1, a method for mining coal bed gas by using a clean nutrient-based working fluid comprises the following steps:

s1, selecting a certain domestic non-structural coal seam, wherein the coal seam is bituminous coal, the thickness of the coal seam is 3m, the inclination angle is 10 degrees, the firmness coefficient f is 0.7, the air permeability is poor, and the temperature of the coal seam is 30 ℃;

because the thickness d of the coal bed is less than 4m, the hydraulic fracturing technology has the following requirements: the volume fracturing is arranged on a fracturing surface, the number of fracturing drill holes N is set to be 8, the angles of adjacent fracturing drill holes are θ=360°/8=45°, and the drill hole length is 23m;

the coal bed is the bituminous coal of the high-rank coal, the temperature of the coal bed is less than or equal to 50 ℃, and the selected clean nutrient medium working solution comprises the following components in percentage by weight: 8g/L CTAC, 4g/L Nasal, 12g/L KCl, 0.2g/L NaHCO ₃ 、1.3g/L NaH ₂ PO4、1g/L NH ₄ Cl、0.1g/L CaCl ₂ 、0.2g/L MgSO ₄ ·7H ₂ O, 18g/L ethanol, 5g/L yeast extract, 8g/L tryptone and the balance of pure water;

according to the requirements of the hydraulic volume fracturing technology, a drilling process is developed, and then the hydraulic volume fracturing technology is adopted to inject the clean nutrient base working solution 1 into a target coal seam through the fracturing truck 2;

in the development of the well drilling process, a high-steel-grade casing is required, and the casing and a well wall are sealed by multi-additive cement to strengthen the well cementation process, so that clean fracturing fluid flows in the casing. Ensuring the success of the well-braising work;

s2, after injection of the clean nutrient medium working solution is completed, carrying out well-flushing operation, enabling a flow channel to be in an open state and enabling in-situ microorganisms to ferment and reproduce rapidly, injecting carbon dioxide into a well when the well-flushing time reaches about three fourths of the required well-flushing time, providing nutrients for in-situ microorganisms in a gas production stage, and simultaneously displacing methane to improve gas production efficiency;

wherein, the time of well stewing is 40-90 days; in the process of well stewing, filling clean nutrient medium working solution into a target coal seam every 5-8 days to maintain the pressure of the clean nutrient medium working solution in the cracks, so that the cracks are always in an open state, and in-situ microorganisms and the nutrient solution are fully contacted and propagated;

through the gas monitoring system, when the methane gas content is monitored to be more than 30%, a coal seam gas pumping system is installed, coal seam gas is pumped in a liquid discharging and gas collecting mode, returned gas liquid is separated in the gas-liquid separation device 3, and the coal seam gas is stored in the gas storage tank 4 and is used for flowing to the market.

Example 3

s1, selecting a certain domestic non-structural coal seam, wherein the coal seam is bituminous coal, the thickness of the coal seam is 5m, the inclination angle is 10 degrees, the firmness coefficient f is 0.6, the air permeability is poor, and the temperature of the coal seam is 55 ℃;

because the thickness d of the coal layer is more than 4m, the hydraulic fracturing technical requirements are as follows: the fracturing arrangement multi-layer fracturing surface k= (5-2)/1.5+1=3, the number of fracturing drilling holes N of each layer is set to 10, the angles of adjacent fracturing drilling holes are θ=360°/10=36°, and the drilling length is 20m;

bituminous coal with high rank coal as coal bed and coal bed temperature>The components and the concentrations of the selected cleaning nutrient medium working solution are as follows: 8g/L STAC, 1g/L CAB, 3g/L Nasal, 12g/L KCl, 0.2g/L NaHCO ₃ 、1.3g/L NaH ₂ PO ₄ 、1g/L NH ₄ Cl、 0.1g/L CaCl ₂ 、0.2g/L MgSO ₄ ·7H ₂ O, 20g/L ethanol, 6g/L yeast extract, 9g/L tryptone and the balance of pure water;

according to the requirements of the hydraulic fracturing technology, a drilling process is developed, and then the hydraulic fracturing technology is adopted to inject the clean nutrient base working solution 1 into a target coal seam through the fracturing truck 2;

Example 4

s1, selecting a certain domestic non-structural coal seam, wherein the coal seam is low-rank coal, the thickness of the coal seam is 5m, the inclination angle is 10 degrees, the firmness coefficient f is 0.6, the air permeability is poor, and the temperature of the coal seam is 55 ℃;

because the thickness d of the coal layer is more than 4m, the hydraulic fracturing technical requirements are as follows: the fracturing arrangement multi-layer fracturing surface k= (5-2)/1.5+1=3, the number of fracturing holes per layer is set to 9, the angle of adjacent fracturing holes is θ=360°/9=40°, and the drilling length is 20m;

the coal bed is low-rank coal, and the temperature of the coal bed>The components and the concentrations of the selected cleaning nutrient medium working solution are as follows: 17.5g/L CAB, 6g/L SDS, 32g/L KCl, 0.2g/L NaHCO ₃ 、1.3g/L NaH ₂ PO ₄ 、1g/L NH ₄ Cl、0.1g/L CaCl ₂ 、 0.2g/L MgSO ₄ ·7H ₂ O, 10g/L ethanol, 3g/L yeast extract, 6g/L tryptone and the balance of pure water;

wherein the time of well stewing is 40-90 days; in the process of well stewing, filling clean nutrient medium working solution into a target coal seam every 5-8 days to maintain the pressure of the clean nutrient medium working solution in the cracks, so that the cracks are always in an open state, and in-situ microorganisms and the nutrient solution are fully contacted and propagated;

Comparative example 1

In this comparative example, the procedure was as in example 2, except that the cleaning nutrient base working fluid was replaced with deionized water.

Comparative example 2

In this comparative example, the procedure was as in example 2, except that the components and proportions of the cleaning nutrient base working fluid were replaced with 0.8g/L CTAC, 0.2g/L Nasal and 1.0g/L KCl.

Comparative example 3

In this comparative example, except that the composition and ratio of the cleaning nutrient base working fluid was replaced with 0.2g/LNaHCO ₃ 、1g/L NH ₄ Cl、1.3g/L NaH ₂ PO ₄ 、0.5g/L KCl、0.2g/L MgSO ₄ ·7H ₂ O、0.1g/L CaCl ₂ ·2H ₂ O, 0.5g/L yeast extract, 1mol/L HCl andthe remainder was the same as in example 2, except for NaOH.

CT scans were performed on the coal seam fractured by the clean nutrient base working fluid in example 2 and the coal seam fractured by deionized water in comparative example 1, and the results are shown in fig. 2.

The graph (1) in fig. 2 shows a CT scan of a coal seam after fracturing with deionized water, and the graph (2) shows a CT scan of a coal seam after fracturing with a clean nutrient medium working fluid, and it is known from comparative analysis that the coal seam after fracturing with deionized water has no obvious cracks, while the coal seam after fracturing with a clean nutrient medium working fluid has obvious cracks and the cracks are larger.

Meanwhile, as shown in the combination analysis of the graph (2) in the graph (2) and the graph (1), after the clean nutrient medium working solution is used for fracturing a coal seam, a plurality of micro-cracks 6 are formed around the horizontal fracturing well 5, the connectivity of the cracks is good, an effective flow channel is provided for coal bed gas extraction, nutrient solution components in the clean nutrient medium working solution are contacted with the coal seam and in-situ microorganisms in a large range, and the good wettability can enable the nutrient solution to be contacted with the coal body more fully, so that sufficient substrates are provided for in-situ microorganism fermentation propagation, and the efficiency of coal to convert the coal bed gas is greatly improved.

Permeability test

To analyze the effect of cleaning a nutrient-based working fluid to reform a coal seam, a permeability test was performed with the clean fracturing fluid, nutrient fluid, and clean nutrient-based working fluid of example 1 as fracturing fluids. The method comprises the following steps: cylindrical coal samples with the diameter of 25mm and the height of 50mm are selected, the coal samples are respectively placed in three fracturing fluids and soaked for 40 days, and the permeability of the coal samples is tested by using a permeability measuring instrument. The test results were all referenced to deionized water. The results are shown in FIG. 3.

The permeability change of the coal sample of fig. 3 is the permeability of the coal sample in the fracturing fluid compared to the permeability of the coal sample in deionized water. As can be seen from the analysis in FIG. 3, when the clean fracturing fluid is adopted alone as the fracturing fluid, the permeability change rate of the coal sample is about 50%, when the nutrient fluid is adopted alone as the fracturing fluid, the permeability change rate of the coal sample is 3-4 times, and when the clean nutrient-based working fluid is adopted as the fracturing fluid, the permeability is 5-6 times, so that the effects of synergistic effect on the permeability of the coal bed are proved when the clean fracturing fluid and the nutrient fluid are mixed and treated.

The clean fracturing fluid in the clean nutrient medium working fluid has the characteristics of high viscosity, small fluid loss, good wettability, good joint making effect and the like. Experiments show that the clean fracturing fluid in the clean nutrient medium working fluid is only used for fracturing the bituminous coal, compared with deionized water used for fracturing the bituminous coal, the average pore area is increased by 3-4 times, the average pore diameter is increased by 2-3 times, the nutrient fluid is only used for the bituminous coal, nutrition is provided for in-situ microorganisms, compared with deionized water used for fracturing the bituminous coal, the total pore volume in a coal bed is increased by about 26%, the porosity is increased by about 42%, the permeability is increased by 3-4 times, the stress sensitivity coefficient is reduced by about 31%, and the clean nutrient medium working fluid is used for fracturing the bituminous coal, the average pore area is increased by more than 10 times, and the permeability is increased by about 5-6 times, so that the clean fracturing fluid and the nutrient fluid in the clean nutrient medium working fluid have a synergistic effect, the coal bed can be well transformed, and the effect of the nutrient fluid on biodegradation conversion is very remarkable. Thereby providing reliable reference for effectively solving the problem of high-efficiency increase of the production of the coalbed methane by microorganisms and effectively promoting the development and utilization of clean energy. Meanwhile, experiments show that when the clean nutrient medium working solution is applied to a soft coal seam, a complex fracture network can be formed, meanwhile, a large number of micro-fractures are accompanied, the gas extraction pure quantity is improved by 26.1%, the extraction concentration is kept above 70%, and the effective extraction time is increased, so that the clean nutrient medium working solution can well reform the coal seam, and the coal seam gas extraction effect is improved.

The above embodiments are merely preferred embodiments for fully explaining the present invention, and the scope of the present invention is not limited thereto. Equivalent substitutions and modifications will occur to those skilled in the art based on the present invention, and are intended to be within the scope of the present invention.

Claims

1. A clean nutrient-based working fluid, characterized in that it comprises clean fracturing fluid, nutrient solution and water;

The clean nutrient-based working solution includes a concentration of 4-25g/L viscoelastic surfactant, 0-10g/L viscoelastic activator and 10-35g/L counter ion additive;

The nutrient solution includes NaHCO ₃ with a concentration of 0.1-2g/L, NaH ₂ PO ₄ with a concentration of 1-3g/L, NH ₄ Cl with a concentration of 0.5-5g/L, CaCl ₂ with a concentration of 0.1-3g/L, 0.1-2g/L L MgSO ₄ ·7H ₂ O , 1~25g/L alcohols, 0.5~10g/L yeast extract and 1~10g/L tryptone.

2. The cleaning nutrient-based working fluid according to claim 1, wherein the viscoelastic surfactant comprises cetyltrimethylammonium chloride (CTAC), octadecyltrimethylammonium chloride One or more of Ammonium (STAC), Cocamidopropyl Betaine (CAB), and Sodium Dodecyl Sulfate (SDS);

The viscoelastic activator is sodium salicylate;

The counter ion aid is potassium chloride;

The alcohols are monohydric alcohols, and the monohydric alcohols include one or more of methanol, ethanol and propanol.

3. The cleaning nutrient-based working solution according to claim 1, characterized in that, the cleaning nutrient-based working solution also includes a pH buffer, and the pH buffer is hydrochloric acid or sodium hydroxide solution, and the concentration is 1mol/L .

4. The method for mining coalbed methane using the clean nutrient-based working fluid as claimed in any one of claims 1 to 3, is characterized in that it comprises the following steps:

S1. Select the target coal seam, and inject the clean nutrient-based working fluid into the target coal seam by using hydraulic fracturing technology, so that the clean fracturing fluid in the clean nutrient-based working fluid can fully increase the permeability of the target coal seam and form a coal fracture network;

S2. Carrying out the braising work, contacting the in-situ microorganisms in the target coal seam with the nutrient solution in the clean nutrient-based working fluid, so that the in-situ microorganisms can transform the target coal seam, so as to increase the pores and fissures of the target coal seam, and strengthen the shape and strength of the fissures;

S3. When it is detected that the methane gas content exceeds 30%, the coalbed methane can be extracted.

5. the method for exploiting coalbed methane according to claim 4, is characterized in that, comprises the following steps:

S1. Carry out geological exploration on the coal seam, and use the coal seam in the non-structural area and the firmness coefficient f>0.3 as the target coal seam;

According to the degree of coal metamorphism, coal seam temperature and pH value of the target coal seam, select the corresponding clean nutrient-based working fluid, carry out the drilling process according to the technical requirements of hydraulic fracturing, and then use the hydraulic fracturing technology to inject the clean nutrient-based working fluid In the target coal seam;

S2. Carry out the work of braising the well, and make the flow channel open and the in-situ microorganisms rapidly ferment and multiply, and then inject carbon dioxide into the well to provide nutrients for the in-situ microorganisms and displace methane at the same time;

When the monitored methane gas content reaches more than 30%, install the coalbed methane extraction system and extract the coalbed methane.

6. The method according to claim 5, characterized in that, in said S1, the hydraulic fracturing technique is a hydraulic volume fracturing technique;

The target coal seam includes a low-rank coal seam and a high-rank coal seam, the low-rank coal seam includes lignite and sub-coal, and the high-rank coal seam includes bituminous coal.

7. The method according to claim 6, characterized in that, when the target coal seam is a high-rank coal seam and the temperature of the coal seam is ≦50°C, cetyltrimethylammonium chloride (CTAC), sodium salicylate and Potassium chloride is used as a clean fracturing fluid. When the coal seam temperature is >50°C, use stearyltrimethylammonium chloride (STAC), cocamidopropyl betaine (CAB), sodium salicylate and potassium chloride as a clean fracturing fluid;

When the target coal seam is a low-rank coal seam, cocamidopropyl betaine (CAB), sodium dodecyl sulfonate (SDS) and potassium chloride are used as clean fracturing fluids.

8. The method according to claim 5, characterized in that in said S1, in the drilling process, it is necessary to use high-grade steel casing, and use cement to seal the casing and the well wall to strengthen the cementing process, so that Clean fracturing fluid circulates through the casing.

9. The method according to claim 5, characterized in that, in the S2, the soaking time of the low-rank coal seam is 25 to 60 days, and the soaking time of the high-rank coal seam is 40 to 90 days;

During the braising process, every 5 to 8 days, it is necessary to refill the clean nutrient-based working fluid into the target coal seam to maintain the pressure of the clean nutrient-based working fluid in the fracture, so that the fracture is always open and the in-situ microorganisms Propagate in full contact with nutrient solution.

10. The method according to claim 5, characterized in that, in said S2, the coalbed methane extraction system includes a drainage pipe, a flow meter and a valve, and the flow rate of the coalbed methane is reflected by observing the flow meter.