A method for activating oil displacement based on double-carbon-source microorganisms

Abstract

The application discloses a double-carbon-source microorganism activation oil displacement method, and relates to the technical field of oil and gas field development and enhanced oil recovery. The method comprises the following steps: injecting an activation liquid slug into an oil reservoir in a first period; the activation liquid comprises a first carbon source, a nitrogen source, a phosphorus source, a target trace element solution and water, the first carbon source comprises water-soluble organic matter, and the water-soluble organic matter is used for providing nutrition for microorganisms in the near-wellbore zone of the oil reservoir; injecting a nutrition maintenance liquid slug into the oil reservoir in a second period; the nutrition maintenance liquid comprises a second carbon source, the second carbon source comprises a nanoscale solid carbon source with a slow-release characteristic; the nutrition maintenance liquid slug can enter different permeable layers based on the flow rate difference of the different permeable layers of the oil reservoir, so as to provide nutrition for the microorganisms in the vertical and horizontal directions of the oil reservoir, promote the microorganisms to produce oil displacement metabolites, and increase the crude oil displaced from the oil reservoir. In this way, the method has the advantages of expanding the action range of the microorganisms, maintaining the continuous production of oil displacement metabolites, prolonging the action period, and improving the oil recovery.

Description

Technical Field

[0001] This application relates to the field of oil and gas field development and enhanced oil recovery technology, and in particular to an oil displacement method based on dual carbon source microbial activation. Background Technology

[0002] Microbial enhanced oil recovery (EOR) technology, as an environmentally friendly and relatively low-cost enhanced oil recovery technique, has received widespread attention in recent years. Its core mechanism involves activating or injecting functional microorganisms, utilizing their metabolic products (such as biosurfactants, organic acids, and gases) to reduce oil-water interfacial tension, improve rock wettability, and increase driving energy, thereby displacing more crude oil. Among these methods, activating the widely existing native microbial communities within the reservoir is considered one of the most promising technological directions due to its strong adaptability and lack of injection blockage risk.

[0003] Currently, mainstream in-situ microbial activation technologies typically employ the injection of single or mixed inexpensive water-soluble nutrients (such as molasses, nitrates, and phosphates) into the reservoir as carbon and nitrogen / phosphorus sources to stimulate microbial growth. However, these technologies suffer from several drawbacks. The injected, easily degradable nutrients are rapidly consumed by microorganisms in the near-wellbore zone, making it difficult for them to migrate to deeper reservoirs, resulting in a limited activation range and short effective period. Furthermore, a single carbon source may cause the microbial community to evolve towards a non-surfactant-producing model, failing to continuously produce effective oil displacement metabolites and leading to low oil displacement efficiency. Summary of the Invention

[0004] In view of this, embodiments of this application provide a method for oil displacement based on dual-carbon-source microbial activation.

[0005] This application provides a method for oil displacement based on dual-carbon-source microbial activation, including: In the first phase, an activation fluid slug is injected into the reservoir. The activation fluid includes a primary carbon source, a nitrogen source, a phosphorus source, a target trace element solution, and water. The primary carbon source includes water-soluble organic matter to provide nutrients to microorganisms in the near-wellbore zone of the reservoir. The target trace element solution contains Mg²⁺. + Ca² + Fe² + ; In the second phase, a nutrient maintenance fluid slug is injected into the reservoir. The nutrient maintenance fluid includes a second carbon source, which includes a nanoscale solid carbon source with slow-release properties. The nutrient maintenance fluid slug can enter different permeable layers based on the flow rate difference between different permeable layers in the reservoir, so as to provide nutrients to the microorganisms in the depth and lateral directions of the reservoir, promote the production of oil displacement metabolites by the microorganisms, and increase the crude oil displaced from the reservoir.

[0006] Furthermore, this application also proposes that the volume ratio of the activating solution to the nutrient maintenance solution is 1:2.

[0007] Furthermore, this application also proposes that the water-soluble organic compound is ethanol.

[0008] Furthermore, this application also proposes that the weight ratio of the first carbon source, nitrogen source, phosphorus source and target trace element solution in the activation solution is 2-4:0.2-0.4:0.04-0.06:0.008-0.012.

[0009] Furthermore, this application also proposes that the nitrogen source is NH4NO3 and the phosphorus source is K2HPO4.

[0010] Furthermore, this application also proposes that the nanoscale solid carbon source is carboxylated nanocellulose crystals.

[0011] Furthermore, this application also proposes that the diameter of the carboxylated cellulose nanocrystals is 50 nm - 150 nm.

[0012] Furthermore, this application also proposes that the length of the carboxylated cellulose nanocrystals is 200 nm - 500 nm.

[0013] Furthermore, this application also proposes that the nutrient maintenance solution further includes a dispersing stabilizer and water; the weight percentage of the second carbon source is 0.6%-1.0%, the weight percentage of the dispersing stabilizer is 0.08%-0.12%, and the balance is water.

[0014] Furthermore, this application also proposes that the dispersant stabilizer is polyoxyethylene (20) sorbitan monooleate.

[0015] This application provides a dual-carbon-source microbial activation oil recovery method. It involves injecting two plugs containing different types of carbon sources in stages. First, water-soluble carbon sources activate near-wellbore microorganisms, providing a rapid start-up effect. Then, a nutrient maintenance fluid composed of nanoscale solid carbon sources with slow-release properties utilizes the velocity differences between different permeable layers in the reservoir to achieve continuous nutrient supply to microorganisms at different locations both vertically and horizontally within the reservoir. The nanoscale solid carbon sources can penetrate and remain in deeper reservoirs, enabling in-situ fermentation in the deep reservoir and activating areas inaccessible to traditional liquid nutrients. Their slow-release properties ensure a long-term nutrient supply to deep reservoir microorganisms, extending the microbial action cycle from weeks to months, effectively overcoming the limitations of limited activation range and short effective period in existing technologies. This achieves both temporal and long-term nutrient supply, solving the problems of premature nutrient consumption, difficulty in transporting to deep reservoirs, and inability to adapt to reservoir heterogeneity in existing single-carbon-source systems. It has the advantages of expanding the microbial action range, maintaining continuous production of oil displacement metabolites, extending the action cycle, and improving oil recovery. Furthermore, the "start-up-maintenance" dual-carbon-source sequential injection strategy better aligns with the growth and metabolism patterns of microorganisms, more stably guiding them to produce highly efficient oil-displacing metabolites, thereby increasing crude oil utilization and avoiding metabolic pathway deviations that may occur with a single carbon source, thus improving the stability of oil displacement efficiency. Moreover, this oil displacement method exhibits good adaptability to different reservoir environments and native microbial communities, and the solid carbon source poses a low risk of formation damage.

[0016] The above description is only an overview of the technical solution of this application. In order to better understand the technical means of this application and to implement it in accordance with the contents of the specification, and to make the above and other objects, features and advantages of this application more obvious and understandable, the following are specific embodiments of this application. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of a process for a dual-carbon-source microbial activation oil displacement method in an embodiment of this application. Detailed Implementation

[0018] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0019] In microbial enhanced oil recovery (DEER) technology, the injected biodegradable nutrients are assimilated and consumed by reservoir-native microorganisms in the near-wellbore zone, preventing their effective migration to deeper reservoirs and limiting their action cycle. The single carbon source supply method makes the metabolic pathways of the microbial community uncontrollable, resulting in unstable production of oil displacement metabolites. Reservoir heterogeneity causes uneven distribution of nutrients in different permeable layers, leading to insufficient nutrient activation in deeper regions. The rapid depletion of nutrients in the near-wellbore zone and the lack of nutrient supply in deeper regions create a significant contradiction. Furthermore, deviations in metabolic pathways prevent the continuous production of oil displacement metabolites, thereby affecting the waterflood sweep volume and the degree of crude oil mobilization.

[0020] For example, during the development of a sandstone reservoir, which exhibits layered heterogeneity with significant differences in permeability distribution, conventional water-soluble nutrients were rapidly consumed by microorganisms in the high-permeability layers, resulting in a lack of nutrient supply in the low-permeability layers. Monitoring showed that microbial activity in the near-wellbore zone increased initially but then declined, leading to fluctuations in the concentration of oil displacement metabolites, reduced sweep efficiency during waterflooding, and a large amount of residual oil remaining in deep pores. Specifically, the concentration gradient of nutrients changed during migration due to microbial metabolic activity, making it impossible to effectively cover the low-permeability areas.

[0021] If the above problems are not solved, the microbial enhanced oil recovery process will not be able to maintain a long-term active state, and the oil recovery effect will be short-lived and unreliable. The remaining oil in the deep part will be difficult to be effectively utilized due to the lack of continuous microbial metabolic activity, and the potential for improving the oil recovery rate will be limited. In addition, frequent nutrient injection operations may increase the risk of formation blockage and affect the overall development efficiency. Among them, the deviation of metabolic pathways caused by the mismatch of nutrient supply will further aggravate the instability of the oil recovery effect, thereby adversely affecting the economics and sustainability of reservoir development.

[0022] In response, this application proposes a method for oil displacement based on dual-carbon-source microbial activation, such as... Figure 1 As shown, including; S1, In the first phase, an activation fluid slug is injected into the reservoir. The activation fluid includes a first carbon source, a nitrogen source, a phosphorus source, a target trace element solution, and water. The first carbon source includes water-soluble organic matter to provide nutrients to microorganisms in the near-wellbore zone of the reservoir. The target trace element solution contains Mg²⁺. + Ca² + Fe² + .

[0023] S2, a nutrient maintenance fluid slug is injected into the reservoir during the second time period; the nutrient maintenance fluid includes a second carbon source, which includes a nanoscale solid carbon source with slow-release properties; the nutrient maintenance fluid slug can enter different permeable layers based on the flow rate difference of different permeable layers in the reservoir, so as to provide nutrients to the microorganisms in the depth and lateral of the reservoir, promote the microorganisms to produce oil displacement metabolites, and increase the crude oil displaced from the reservoir.

[0024] For ease of understanding, the following explains some key terms in this embodiment: An oil reservoir is a porous underground formation that stores oil and natural gas. During oil and gas extraction, the characteristics of the reservoir, such as permeability, porosity, formation temperature, and pressure, all affect the effectiveness of oil displacement methods.

[0025] Microorganisms refer to various tiny life forms such as bacteria and archaea that exist in oil reservoir strata. Under suitable nutritional and environmental conditions, these microorganisms can grow and reproduce and produce a variety of metabolic products, which play a role in the oil displacement process.

[0026] Oil displacement metabolites are substances produced by microorganisms during metabolism that have oil displacement functions. These substances may include biosurfactants, organic acids, gases, or biopolymers, which can alter the properties of the oil-water interface, improve fluid mobility, or increase formation pressure, thereby promoting the displacement of crude oil.

[0027] An activating fluid slug is a fluid slug injected into the reservoir during the first stage of an oil displacement method. This slug is primarily used to rapidly activate the activity of native microorganisms in the near-wellbore zone, providing them with the nutrients necessary for initial growth.

[0028] The primary carbon source refers to the main carbon source component in the activation solution. This carbon source is usually a water-soluble organic matter, which is easily and rapidly absorbed and utilized by microorganisms to achieve rapid activation and biomass increase of microorganisms in the near-wellbore zone.

[0029] A nutrient maintenance fluid slug is a fluid slug injected into the reservoir during the second phase of an oil displacement process. This slug is designed to provide a continuous nutrient supply to deep and lateral reservoir microorganisms to maintain their activity and promote the long-term production of displacement metabolites.

[0030] The second carbon source refers to the main carbon source component in the nutrient maintenance fluid. This carbon source is usually a nanoscale solid carbon source, characterized by its ability to slowly release nutrients, thereby achieving long-term, slow-release nutrient supply to microorganisms, and being able to enter the deep reservoir area with the fluid.

[0031] Specifically, in the first phase, an activation fluid slug is injected into the reservoir. This activation fluid slug can be injected via conventional water injection wells or through specialized injection equipment at a constant pressure or flow rate. The purpose of injection is to ensure the activation fluid quickly reaches the near-wellbore zone and comes into contact with the native microorganisms in that area. The activation fluid consists of a primary carbon source, a nitrogen source, a phosphorus source, a target trace element solution, and water. Water, as the primary solvent, can be formation water, treated produced water, or fresh water. The primary carbon source is designed to be a water-soluble organic compound, whose main function is to provide nutrients to the microorganisms in the near-wellbore zone. Water-soluble organic compounds are characterized by their small molecular weight, ease of diffusion, and rapid assimilation by microorganisms. For example, when the injected activation fluid slug reaches the near-wellbore zone, the water-soluble organic compounds within it can be rapidly absorbed and utilized by the microorganisms in that area, thereby increasing the activity and biomass of the microorganisms in a short period, laying the biological foundation for the subsequent oil displacement process. The primary carbon source can be a variety of water-soluble organic compounds readily utilized by microorganisms. For example, simple sugars such as glucose and sucrose, small-molecule alcohols such as methanol, ethanol, and isopropanol, or organic acids such as acetic acid and citric acid can be used. The nitrogen source can be nitrates, ammonium salts, or urea. The phosphorus source can be phosphates. The target trace element solution can contain various metal ions required for microbial growth, such as magnesium, calcium, iron, and zinc ions. These components are formulated into solutions to ensure good dispersibility in the reservoir and rapid absorption by microorganisms.

[0032] In the second phase, a nutrient maintenance fluid slug is injected into the reservoir. This injection can occur immediately after the activation slug or after the activation slug has been injected and acted upon for a period of time. The injection method can utilize conventional water injection wells or specialized injection equipment. The purpose of this slug is to provide continuous nutrient support for microorganisms in the deep and lateral regions of the reservoir. The main component of this nutrient maintenance fluid includes a second carbon source. In addition to the second carbon source, the nutrient maintenance fluid may also contain water as a solvent, as well as other auxiliary components, such as dispersants to maintain the dispersion stability of the solid carbon source. The second carbon source is designed as a nanoscale solid carbon source with slow-release properties. Various materials with slow-release properties can be selected for this solid carbon source, such as modified cellulose derivatives, starch derivatives, polylactic acid microspheres, or chitosan microparticles. These solid carbon sources are characterized by their nanoscale particle size and ability to slowly degrade in the reservoir environment, gradually releasing small molecule organic matter that can be utilized by microorganisms.

[0033] This nutrient-maintaining fluid slug can enter different permeable layers based on the velocity difference in the reservoir, providing nutrients to the microorganisms both deep and laterally within the reservoir. This encourages the microorganisms to produce oil-displacing metabolites, increasing the amount of crude oil displaced. Specifically, due to the heterogeneity of the reservoir formation, layers with different permeabilities have different fluid velocities. Nanoscale solid carbon source particles possess a certain degree of suspension stability in the fluid, allowing them to enter and remain in layers with relatively low permeability. Over time, these solid carbon sources are slowly degraded by microbial enzymes, continuously releasing nutrients, thus achieving a long-term, uniform nutrient supply to the microorganisms in the deep and lateral regions of the reservoir. Consequently, the microorganisms can continuously produce oil-displacing metabolites such as biosurfactants, organic acids, and gases. These metabolites can reduce oil-water interfacial tension, improve rock wettability, increase formation energy, and ultimately improve crude oil displacement efficiency and recovery rate.

[0034] This application provides a dual-carbon-source microbial activation oil recovery method. It involves injecting two plugs containing different types of carbon sources in stages. First, water-soluble carbon sources activate near-wellbore microorganisms, providing a rapid start-up effect. Then, a nutrient maintenance fluid composed of nanoscale solid carbon sources with slow-release properties utilizes the velocity differences between different permeable layers in the reservoir to achieve continuous nutrient supply to microorganisms at different locations both vertically and horizontally within the reservoir. The nanoscale solid carbon sources can penetrate and remain in deeper reservoirs, enabling in-situ fermentation in the deep reservoir and activating areas inaccessible to traditional liquid nutrients. Their slow-release properties ensure a long-term nutrient supply to deep reservoir microorganisms, extending the microbial action cycle from weeks to months, effectively overcoming the limitations of limited activation range and short effective period in existing technologies. This achieves both temporal and long-term nutrient supply, solving the problems of premature nutrient consumption, difficulty in transporting to deep reservoirs, and inability to adapt to reservoir heterogeneity in existing single-carbon-source systems. It has the advantages of expanding the microbial action range, maintaining continuous production of oil displacement metabolites, extending the action cycle, and improving oil recovery. Furthermore, the "start-up-maintenance" dual-carbon-source sequential injection strategy better aligns with the growth and metabolism patterns of microorganisms, more stably guiding them to produce highly efficient oil-displacing metabolites, thereby increasing crude oil utilization and avoiding metabolic pathway deviations that may occur with a single carbon source, thus improving the stability of oil displacement efficiency. Moreover, this oil displacement method exhibits good adaptability to different reservoir environments and native microbial communities, and the solid carbon source poses a low risk of formation damage.

[0035] In an optional embodiment, this application further proposes that the volume ratio of the activation solution to the nutrient maintenance solution be 1:2.

[0036] The activation fluid and nutrient maintenance fluid are mixed at a volume ratio of 1:2. This specific ratio is designed to optimize the activation and subsequent maintenance of microorganisms in the reservoir. The activation fluid is primarily used for the rapid activation of microorganisms in the near-wellbore area, while the nutrient maintenance fluid is responsible for the continuous nutrient supply to the depth and lateral areas. This ratio ensures sufficient initial stimulation during the activation phase, while providing a larger amount of nutrients during the maintenance phase to support the long-term growth and metabolic activities of the microorganisms. This can be achieved by pre-calculating the total volume of the two fluids and then preparing and injecting them in batches at a 1:2 ratio; or by using an independent injection pump to monitor and adjust the injection flow rates of the two fluids in real time, maintaining the flow rate ratio at 1:2 at all times.

[0037] This application's scheme achieves refined management of reservoir microbial nutrient supply by setting the volume ratio of activating fluid to nutrient maintenance fluid at 1:2. The activating fluid injected in the first stage primarily serves to rapidly activate dormant microorganisms in the near-wellbore zone, laying the foundation for subsequent oil displacement activities. The nutrient maintenance fluid injected in the second stage, with a volume twice that of the activating fluid, allows for the delivery of more nanoscale solid carbon sources to the depth and lateral areas of the reservoir. This ratio fully considers the differences in nutrient requirements of microorganisms at different stages and the difficulty of nutrient replenishment in deep reservoir areas. The appropriate injection of the activating fluid ensures the initial activation effect, while the larger volume of nutrient maintenance fluid guarantees that microorganisms receive continuous and sufficient nutrition across a wider reservoir area, thereby promoting their long-term stable production of oil displacement metabolites. This synergistic effect avoids a decline in microbial activity due to insufficient or excessive nutrient supply, optimizes the microbial growth environment, and thus improves overall oil displacement efficiency.

[0038] In an optional embodiment, this application further proposes that the water-soluble organic compound be ethanol.

[0039] Ethanol is a common organic compound with good water solubility. As a carbon source for microorganisms, ethanol can be rapidly absorbed and metabolized by a variety of microorganisms, providing energy and carbon skeletons for their growth and reproduction. Its molecular structure is relatively simple, making it easy to diffuse in aqueous solutions, thus enabling it to quickly reach the microbial community near the well site and promote its rapid activation.

[0040] This application's scheme achieves rapid and effective activation of microorganisms in the near-wellbore zone of the reservoir by using ethanol as a water-soluble organic compound in the activation fluid. Ethanol, as a small-molecule water-soluble organic compound, can rapidly dissolve and diffuse into the pore space of the near-wellbore zone after injection into the reservoir. Microorganisms can efficiently utilize ethanol as a carbon source for their growth and reproduction, thus rapidly proliferating and increasing their activity in the first phase. This rapid activation lays the foundation for the utilization of nanoscale solid carbon sources in the nutrient maintenance fluid sluice injected in the subsequent second phase, ensuring a continuous supply of nutrients and stable production of oil displacement metabolites for microorganisms in the deep and lateral areas of the reservoir. The rapid availability of ethanol effectively overcomes the problem of low microbial activation efficiency caused by inappropriate carbon source selection, providing a strong starting force for the entire oil displacement process.

[0041] In an optional embodiment, this application further proposes that the weight ratio of the first carbon source, nitrogen source, phosphorus source and target trace element solution in the activation solution is 2-4:0.2-0.4:0.04-0.06:0.008-0.012.

[0042] Specifically, the first carbon source provides energy and the carbon skeleton, while the nitrogen and phosphorus sources support the synthesis and energy metabolism of macromolecules such as proteins and nucleic acids, respectively. The target trace element solution serves as a cofactor for enzymes, ensuring the smooth progress of various physiological activities of microorganisms.

[0043] As a specific implementation method, the above-mentioned technical means can be implemented with reference to the following example. When preparing the activation solution, the first carbon source can be 3 parts, the nitrogen source 0.3 parts, the phosphorus source 0.05 parts, the target trace element solution 0.01 parts, and the remainder being simulated reservoir formation water. The first carbon source can be ethanol, the nitrogen source can be ammonium nitrate, the phosphorus source can be dipotassium hydrogen phosphate, and the target trace element solution can contain magnesium ions, calcium ions, and ferrous ions. Specifically, the target trace element solution can be a mixed aqueous solution containing magnesium ions, calcium ions, and ferrous ions; wherein the weight ratio of magnesium ions, calcium ions, and ferrous ions in the target trace element solution can be 3:5:2. After these components are mixed evenly with water, an activation solution is formed and injected into the reservoir using a slug injection method.

[0044] By employing the aforementioned technical solution, the weight ratio of the first carbon source, nitrogen source, phosphorus source, and target trace element solution in the activation fluid was precisely controlled. This optimized activation fluid ratio can efficiently activate endogenous microorganisms in the near-wellbore zone, enabling them to rapidly proliferate and reach a highly active state. Once effectively activated, the microorganisms can more effectively utilize the second carbon source in the subsequently injected nutrient maintenance fluid, thereby continuously producing oil-displacing metabolites in both the depth and lateral regions of the reservoir, ultimately achieving a more significant crude oil displacement effect. This precise nutrient ratio is crucial to the success of the entire dual-carbon-source microbial activation oil displacement method, laying a solid foundation for subsequent deep microbial activity.

[0045] In an optional embodiment, this application further proposes that the nitrogen source be NH4NO3 and the phosphorus source be K2HPO4.

[0046] NH4NO3 provides nitrogen in both ammonium and nitrate forms, enabling microorganisms to effectively obtain nitrogen nutrition under different redox conditions, thus supporting their vigorous growth in the reservoir environment. K2HPO4, as a phosphorus source, not only provides essential phosphorus for microorganisms but also acts as a buffer salt, helping to maintain pH stability in the local reservoir environment and creating suitable conditions for microbial enzyme activity and metabolic processes. The target trace element solution contains Mg²⁺. + Ca² + Fe² + Ions are essential trace elements for the growth and metabolism of microorganisms. These specific forms of nutrients have good water solubility and biocompatibility, ensuring that microorganisms can efficiently absorb and utilize them after injection into the reservoir, thereby promoting their rapid proliferation and metabolic activity.

[0047] The scheme of this application involves NH4NO3, K2HPO4, and Mg²⁺. + Ca² + Fe² + The target trace element solution, as a specific nutrient component in the activating fluid, ensures that microorganisms can obtain a balanced and efficient nutrient supply in the near-wellbore zone of the reservoir. These specific forms of nitrogen, phosphorus, and trace elements can dissolve rapidly and be absorbed and utilized by microorganisms after being injected into the reservoir, thereby effectively activating the growth and metabolism of microorganisms.

[0048] In an optional embodiment, this application further proposes that the nanoscale solid carbon source be carboxylated cellulose nanocrystals (CNCs).

[0049] Carboxylated cellulose nanocrystals are a type of nanoscale cellulose material obtained through chemical modification (usually by introducing carboxyl groups). They are typically prepared from natural cellulose through acid hydrolysis and mechanical treatment, exhibiting high crystallinity, high specific surface area, good biocompatibility, and biodegradability. Carboxylation imparts a negative surface charge, enhancing their dispersion stability and hydrophilicity in aqueous solutions. As a nanoscale solid carbon source, carboxylated cellulose nanocrystals provide continuous carbon nutrition for microorganisms in oil reservoirs. Their nanoscale size allows them to effectively penetrate the micropores of the reservoir, while carboxylation modification facilitates stable dispersion in the aqueous phase, reducing sedimentation and aggregation, thus ensuring the carbon source can be transported to the deep and lateral regions of the reservoir. Simultaneously, their biodegradability ensures the slow release of carbon, providing long-term nutrition for microorganisms. The preparation of carboxylated cellulose nanocrystals involves pretreating natural cellulose (such as wood pulp, cotton, and agricultural waste) with acid hydrolysis to obtain cellulose nanocrystals, followed by carboxylation modification through methods such as TEMPO oxidation and chloroacetic acid esterification. Alternatively, commercially available carboxylated cellulose nanocrystal products can be selected, or their performance can be optimized by adjusting parameters such as the degree of carboxylation and the size distribution of nanocrystals, depending on specific needs.

[0050] This application's solution, by specifically defining the nanoscale solid carbon source as carboxylated nanocellulose crystals, effectively solves the problems of poor transport and dispersion and uncontrollable carbon source release of ordinary solid carbon sources in deep and lateral reservoir regions. Carboxylated nanocellulose crystals, with their nanoscale size, good water dispersibility, and biodegradability, ensure that the carbon source can be uniformly and stably transported to all areas of the reservoir, achieving slow and continuous release. This greatly enhances the activity of microorganisms in deep and lateral reservoir regions and their ability to continuously produce oil-dispatch metabolites, thereby significantly improving the efficiency of microbial enhanced oil recovery and oil recovery rate.

[0051] In an optional embodiment, this application further proposes that the diameter of the carboxylated cellulose nanocrystals be 50 nm to 150 nm.

[0052] The diameter of carboxylated cellulose nanocrystals is one of the key physical parameters affecting the transport behavior of nanoscale solid carbon sources in porous media. It can influence the crystals' ability to pass through reservoir pore throats, their dispersion stability in fluids, and their contact efficiency with microorganisms. This diameter can be controlled in various ways. For example, in the preparation of carboxylated cellulose nanocrystals, precise control of the pretreatment conditions of the cellulose raw material, such as the temperature, time, and acid concentration of acid hydrolysis or enzymatic hydrolysis, can yield cellulose nanocrystal precursors with a specific diameter range. Furthermore, subsequent mechanical treatments, such as high-pressure homogenization, ultrasonic treatment, or microfluidization, combined with size fractionation techniques such as centrifugation or membrane filtration, can be used to precisely screen and obtain carboxylated cellulose nanocrystals with diameters within the desired range.

[0053] The above technical solution precisely limits the diameter of nanoscale solid carbon sources to 50nm-150nm, effectively solving the problems of insufficient transport efficiency and stability of nanoscale solid carbon sources in oil reservoirs. These carboxylated nanocellulose crystals with a specific diameter significantly improve their permeability in the complex pore structure of oil reservoirs, reduce the risk of blockage due to excessive size, and optimize their dispersion stability in fluids, ensuring that the carbon source can effectively reach the depth and lateral regions of the oil reservoir with the nutrient maintenance fluid slug. This allows microorganisms to obtain a continuous and extensive nutrient supply, thereby being activated and producing oil-displacing metabolites over a wider range, ultimately significantly improving the oil displacement effect and oil recovery rate of the reservoir.

[0054] In an optional embodiment, this application further proposes that the length of the carboxylated cellulose nanocrystals be 200 nm - 500 nm.

[0055] The length of the carboxylated cellulose nanocrystals refers to the dimension of the nanomaterial along its main axis. This length range is designed to optimize its migration ability in porous media and its contact efficiency with microorganisms. For example, the length can be controlled between 200 nm and 300 nm to ensure good permeability in micropores while providing sufficient surface area for microbial attachment and degradation. Alternatively, the length can be controlled between 300 nm and 500 nm, which helps it form a more stable dispersion system in specific permeable layers and prolongs its residence time in deep reservoirs, thereby more fully releasing the carbon source.

[0056] By precisely controlling the length of carboxylated cellulose nanocrystals within a specific range of 200 nm to 500 nm using the aforementioned technical solution, the problem of insufficient transport efficiency and microbial utilization efficiency of nanoscale solid carbon sources in deep oil reservoirs is effectively solved. This optimized length allows solid carbon sources in the nutrient maintenance fluid to have better permeability and lower clogging risk in porous media, ensuring their smooth arrival in deep oil reservoir areas. Simultaneously, this length range also ensures that microorganisms can efficiently attach to and degrade these solid carbon sources, thereby continuously providing sufficient nutrients for deep-seated microorganisms. Therefore, this solution can significantly improve the activity of deep-seated microorganisms and the production of oil displacement metabolites, ultimately achieving more efficient and thorough oil displacement and improving oil recovery.

[0057] In an optional embodiment, this application further proposes that the nutrient maintenance solution also includes a dispersing stabilizer and water; the weight percentage of the second carbon source is 0.6%-1.0%, the weight percentage of the dispersing stabilizer is 0.08%-0.12%, and the balance is water.

[0058] The dispersant stabilizer is a substance that can adsorb onto the surface of solid particles and prevent particle aggregation and sedimentation through steric hindrance or electrostatic repulsion, thereby maintaining uniform dispersion of particles in a liquid medium. In this application, the role of the dispersant stabilizer is to ensure the long-term stable dispersion of nanoscale solid carbon sources in the nutrient maintenance fluid, preventing aggregation and sedimentation before or during reservoir injection, thus ensuring that the solid carbon sources can effectively enter the deep and lateral regions of the reservoir with the nutrient maintenance fluid slug. The dispersant stabilizer can be an organic polymer, such as polyethylene glycol, polyacrylamide and its derivatives; or it can be a surfactant, such as nonionic surfactants like polyoxyethylene ethers and fatty acid esters, or ionic surfactants like sodium dodecylbenzenesulfonate. Setting the weight percentage of the second carbon source in the range of 0.6%-1.0% aims to provide sufficient carbon source to meet the long-term nutritional needs of microorganisms in the deep and lateral regions of the reservoir, while avoiding excessively high concentrations that lead to excessive solution viscosity, injection difficulties or carbon source waste, and avoiding excessively low concentrations that lead to insufficient nutrient supply, affecting microbial activity and the production efficiency of oil displacement metabolites. The weight percentage of the dispersant stabilizer is set within the range of 0.08%-0.12% to provide optimal dispersion, effectively inhibiting the aggregation and sedimentation of nanoscale solid carbon sources and ensuring their stability and transportability in the nutrient maintenance fluid. This concentration range ensures effective dispersion while avoiding increased costs or adverse effects on the reservoir environment due to excessive use of the dispersant stabilizer. Water, as the main carrier and solvent of the nutrient maintenance fluid, dissolves or disperses other components and acts as a fluid medium to transport nutrients to the deep reservoir. As a balance, water ensures the nutrient maintenance fluid has suitable viscosity and flowability, facilitating injection into the reservoir and providing the necessary aquatic environment for microorganisms.

[0059] As a specific implementation method, the nutrient maintenance solution can be composed of the following components: carboxylated cellulose nanocrystals as a second carbon source, with a weight percentage of 0.8%; polyoxyethylene (20) sorbitan monooleate as a dispersant and stabilizer, with a weight percentage of 0.1%; and the remainder being water. During preparation, the carboxylated cellulose nanocrystals can be mixed with water, and then polyoxyethylene (20) sorbitan monooleate can be added. The mixture is thoroughly mixed using high-speed shearing or ultrasonic dispersion to ensure that the carboxylated cellulose nanocrystals form a stable dispersion system in the aqueous solution.

[0060] Through the above technical solution, the dispersion stability of nanoscale solid carbon sources in the nutrient maintenance solution is significantly improved, effectively avoiding their aggregation and sedimentation before and during reservoir injection. This allows the second carbon source to penetrate more uniformly and deeply into the longitudinal and lateral regions of the reservoir, ensuring a continuous and sufficient nutrient supply to the reservoir's microorganisms. Consequently, microorganisms can be more effectively activated and their oil-producing metabolic activity can be maintained, thereby significantly improving the production efficiency of oil displacement metabolites and ultimately increasing the amount of crude oil displaced from the reservoir.

[0061] In an optional embodiment, this application further proposes that the dispersion stabilizer be polyoxyethylene (20) sorbitan monooleate.

[0062] Polyoxyethylene (20) sorbitan monooleate is a nonionic surfactant whose molecular structure contains hydrophilic polyoxyethylene chains and lipophilic sorbitan monooleate groups. This amphiphilic structure allows it to adsorb onto the surface of nanoscale solid carbon sources, reducing the surface tension at the solid-liquid interface and forming a stable adsorption film around the particles, thereby providing a steric hindrance effect and effectively preventing particle aggregation. Due to its good biocompatibility and stability in aqueous solutions, this substance is often used in the biological and pharmaceutical fields.

[0063] The above technical solution, using polyoxyethylene (20) sorbitan monooleate as a dispersant and stabilizer, effectively improves the dispersion stability and anti-settling ability of nanoscale solid carbon sources in the nutrient maintenance solution. This specific dispersant and stabilizer has excellent surface activity and biocompatibility, and can maintain the uniform dispersion of solid carbon sources in aqueous solution for a long time, avoiding agglomeration or pore throat blockage during reservoir injection. This ensures that nanoscale solid carbon sources can be smoothly transported to the depth and lateral areas of the reservoir, providing a continuous nutrient supply for deep microorganisms, thereby significantly improving the efficiency and coverage of microbial enhanced oil recovery, and ultimately increasing the oil recovery rate.

[0064] The following experimental results illustrate the effectiveness of the dual-carbon-source microbial activation oil displacement method of this application.

[0065] The activation solution in this experiment consists of the following raw materials: 3 parts of primary carbon source (ethanol), 0.3 parts of nitrogen source (NH4NO3), 0.05 parts of phosphorus source (K2HPO4), and trace element solution (containing Mg²⁺). + Ca² + Fe² + (etc.) 0.01 parts), the remainder is the formation water of the simulated target oil reservoir.

[0066] The nutrient maintenance solution in this experiment is composed of the following ingredients: The second carbon source (carboxylated nanocellulose crystals, 50-150 nm in diameter and 200-500 nm in length) is 0.8%, the dispersant and stabilizer (Tween-80) is 0.1%, and the balance is simulated formation water of the target oil reservoir.

[0067] Example 1 In sterile serum bottles, the activating solution and nutrient maintenance solution were mixed with reservoir produced water (containing native microorganisms) at a volume ratio of 1:2 and anaerobically cultured at reservoir temperature (60℃). Samples were taken periodically, and after centrifugation, the ability of surfactants in the supernatant to reduce oil-water interfacial tension was measured. A long-term (90-day) batch culture experiment was set up, with periodic supplementation of micronutrients (N, P) but no carbon source. Surfactant yield and the duration of interfacial tension maintenance were monitored. The experimental results are shown in Table 1.

[0068] Oil displacement experiment: The core was vacuum-saturated with formation water, and the pore volume was measured. Crude oil was then saturated at a rate of 0.5 mL / min and aged at 60℃ for 4 weeks to establish the initial reservoir wettability. Waterflooding was then performed with formation water at a rate of 0.2 mL / min until the outlet water cut reached over 98%, and the waterflood recovery rate was recorded. At this point, the core contained residual oil. Activating fluid (0.05 PV) and nutrient maintenance fluid (0.10 PV) were injected sequentially. The core was placed in a 60℃ constant temperature chamber to simulate the formation shut-in reaction for 30 days. Formation water displacement continued until the outlet fluid water cut again reached 98%, and the final recovery rate was recorded. The experimental results are shown in Table 2. Table 1 - Variation of oil-water interfacial tension over time in Scheme 1

[0069] Table 2 - Oil Displacement Effect of Scheme 1

[0070] Comparative Example 1 In sterile serum bottles, the activating solution was mixed with reservoir produced water (containing native microorganisms) and anaerobically cultured at reservoir temperature (60℃). Samples were taken periodically, and after centrifugation, the ability of surfactants in the supernatant to reduce oil-water interfacial tension was measured. A long-term (90-day) batch culture experiment was set up, with periodic supplementation of micronutrients (N, P) but no carbon source. Surfactant yield and the duration of interfacial tension were monitored. The experimental results are shown in Table 3.

[0071] Oil displacement experiment: The core was vacuum-saturated with formation water, and the pore volume was measured. Crude oil was then saturated at a rate of 0.5 mL / min and aged at 60℃ for 4 weeks to establish the original reservoir wettability. Waterflooding was then performed with formation water at a rate of 0.2 mL / min until the outlet water cut reached over 98%, and the waterflood recovery rate was recorded. At this point, the core contained residual oil. Activating fluid of 0.05 PV was injected, and the core was placed in a 60℃ constant temperature chamber to simulate a formation shut-in reaction for 30 days. Formation water displacement continued until the outlet fluid water cut again reached 98%, and the final recovery rate was recorded. The experimental results are shown in Table 2, and the oil displacement experiment results are shown in Table 4.

[0072] Table 3 - Changes in oil-water interfacial tension over time in Scheme 2

[0073] Table 4 - Oil Displacement Effect of Scheme 2

[0074] As can be seen from Tables 1 and 3, when both activating fluid and nutrient maintenance fluid are added to the reservoir produced water, the oil-water interfacial tension does not change much over time; however, when only activating fluid is added to the reservoir produced water, the oil-water interfacial tension changes significantly over time. This indicates that the addition of nutrient maintenance fluid can effectively activate microorganisms, promote their continuous synthesis of biosurfactants and polymers, and effectively reduce the oil-water interfacial tension.

[0075] As can be seen from Tables 2 and 4, the oil recovery rate increased by 14.8% after core injection of activation fluid, and by 31.8% after core injection of both activation fluid and nutrient maintenance fluid. This indicates that the addition of nutrient maintenance fluid greatly improved the oil washing efficiency.

[0076] It should be understood that the various forms of processes shown above can be used to rearrange, add, or delete steps. For example, the steps described in this application can be executed in parallel, sequentially, or in different orders, as long as the desired result of the technical solution disclosed in this application can be achieved, and this is not limited herein.

[0077] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this application, "a plurality of" means two or more, unless otherwise explicitly specified.

[0078] The above description is merely a specific embodiment of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A method for oil displacement based on dual-carbon-source microbial activation, characterized in that, include; In the first phase, an activation fluid slug is injected into the reservoir. The activation fluid includes a first carbon source, a nitrogen source, a phosphorus source, a target trace element solution, and water. The first carbon source includes water-soluble organic matter to provide nutrients to the microorganisms in the near-wellbore zone of the reservoir. The target trace element solution contains Mg²⁺ as the trace element. + Ca² + Fe² + ; In the second time period, a nutrient maintenance fluid slug is injected into the reservoir; the nutrient maintenance fluid includes a second carbon source, which includes a nanoscale solid carbon source with slow-release properties; the nutrient maintenance fluid slug can enter different permeable layers based on the flow rate difference of different permeable layers in the reservoir, so as to provide nutrients to the microorganisms in the depth and lateral of the reservoir, promote the microorganisms to produce oil displacement metabolites, and increase the crude oil displaced from the reservoir.

2. The oil displacement method based on dual-carbon-source microbial activation according to claim 1, characterized in that, The volume ratio of the activating solution to the nutrient maintenance solution is 1:

2.

3. The oil displacement method based on dual-carbon-source microbial activation according to claim 1, characterized in that, The water-soluble organic compound is ethanol.

4. The oil displacement method based on dual-carbon-source microbial activation according to claim 1, characterized in that, The weight ratio of the first carbon source, nitrogen source, phosphorus source and target trace element solution in the activation solution is 2-4:0.2-0.4:0.04-0.06:0.008-0.

012.

5. The oil displacement method based on dual-carbon-source microbial activation according to claim 1, characterized in that, The nitrogen source is NH4NO3, and the phosphorus source is K2HPO4.

6. The oil displacement method based on dual-carbon-source microbial activation according to claim 1, characterized in that, The nanoscale solid carbon source is carboxylated nanocellulose crystals.

7. The oil displacement method based on dual-carbon-source microbial activation according to claim 6, characterized in that, The diameter of the carboxylated cellulose nanocrystals is 50 nm - 150 nm.

8. The oil displacement method based on dual-carbon-source microbial activation according to claim 6, characterized in that, The length of the carboxylated cellulose nanocrystals is 200 nm - 500 nm.

9. The oil displacement method based on dual-carbon-source microbial activation according to claim 1, characterized in that, The nutrient maintenance solution also includes a dispersing stabilizer and water; The second carbon source has a weight percentage of 0.6%-1.0%, the dispersant stabilizer has a weight percentage of 0.08%-0.12%, and the balance is water.

10. The oil displacement method based on dual-carbon-source microbial activation according to claim 9, characterized in that, The dispersion stabilizer is polyoxyethylene (20) sorbitan monooleate.

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