Shale oil adsorption state-free state targeted conversion method based on pore wall surface in-situ electrochemical modification

Abstract

The invention belongs to the technical field of oil and gas exploitation, and particularly relates to a shale oil adsorption state-free state targeted conversion method based on pore wall surface in-situ electrochemical modification. According to the method, a pulse electric field is utilized to drive a special targeting modification liquid to enter shale nano pores, an electrochemical polarization reaction is induced on the kerogen / mineral surface (pore wall surface), and the original oleophylic (Oil-wet) pore wall is forcibly modified into hydrophilic / aerophilic (Water / Gas-wet), so that the interfacial adsorption energy is greatly reduced, and the technical effects of stripping adsorbed oil and converting the adsorbed oil into flowable free oil are achieved. Compared with the prior art, the treatment efficiency is greatly improved, the chemical properties of kerogen / mineral wall surfaces can be permanently changed, and secondary adsorption of crude oil is avoided.

Description

Technical Field

[0001] This invention belongs to the field of oil and gas extraction technology, and particularly relates to a targeted conversion method for adsorbed-free shale oil based on in-situ electrochemical modification of pore walls. Background Technology

[0002] In shale oil reservoirs, 40%-60% of crude oil exists in an adsorbed state on the surface of kerogen or between clay mineral layers. During shale oil development, only by first desorbing the adsorbed oil and converting it into free oil can the free oil be further extracted using methods such as fracture networks, thereby improving oil recovery. Therefore, converting adsorbed oil in shale into free oil is a key approach to improving shale oil recovery. However, shale generally exhibits nanopores, resulting in high capillary pressure. Existing conventional water / gas drive techniques can only target large-pore fluids and cannot reach micro- and nano-pores, thus limiting the effectiveness of converting adsorbed oil into free oil.

[0003] Against the backdrop of global efforts to address climate change and advance the "carbon peaking and carbon neutrality" strategy, carbon dioxide capture, utilization, and storage (CCUS) technology has gained significant attention in the oil and gas industry due to its dual benefits of enhancing oil and gas recovery while achieving geological sequestration of greenhouse gases. Injecting captured CO2 into reservoirs to drive crude oil production is considered a crucial pathway for the oil and gas industry to achieve a low-carbon transformation. Shale oil reservoirs, characterized by strong heterogeneity, well-developed nanopores, and large specific surface areas, possess both significant potential for CO2 enrichment and CO2 sequestration. Therefore, research on integrated CO2 enrichment and sequestration technologies for shale oil reservoirs has substantial strategic and practical significance.

[0004] However, CO2 injection into shale oil reservoirs faces a significant scientific challenge: Shale oil reservoirs consist of both organic matter (80-90% composed of kerogen, possessing molecular structural units, primarily composed of C, H, O, and small amounts of S and N) and inorganic matter (quartz, clay minerals, etc.). The occurrence state of crude oil differs between kerogen and inorganic matter; in inorganic matter, it is predominantly in a free state, while in kerogen, it is predominantly in a free and adsorbed state. This is because kerogen is considered hydrophobic (oleophilic), while inorganic matter is hydrophilic (oleophobic). Compared to inorganic matter, the nanopore walls of kerogen exhibit a stronger interaction with crude oil, with a large amount of crude oil (29.59%-50.71%) being directionally adsorbed near the kerogen pore walls, forming a "solid-like" layer. During CO2 injection, crude oil on the kerogen wall surface is difficult to be stripped away by CO2 competition, and CO2 is also difficult to be adsorbed on the wall surface. This significantly affects the CO2 flooding and storage efficiency, thus limiting the large-scale application of CO2 flooding and storage technology in shale oil reservoirs. Studies have found that the strong interaction energy between crude oil and the kerogen wall (-5508.29 kJ / mol) is much higher than that between crude oil and kaolinite (-1865.30 kJ / mol) and crude oil and quartz (-403.60 kJ / mol). Under the same treatment time, the degree of crude oil stripping from the kerogen wall by CO2 is only 26.60%, which is far lower than that of kaolinite (34.71%) and quartz (55.29%). Furthermore, considering that crude oil accounts for a significant proportion of the total crude oil content in shale kerogen reservoirs (15%-45%), this is a significant challenge. Therefore, compared to oil displacement and storage in inorganic materials, the oil displacement and storage problems in kerogen are key to maximizing CO2 displacement and storage efficiency in shale oil reservoirs. Thus, enhancing the competitive adsorption behavior of CO2 and crude oil on the kerogen wall helps improve oil displacement and storage effects, and is an important frontier direction in the efficient development of shale oil reservoirs and CCUS research. Related research has significant potential application value for the large-scale implementation of shale oil reservoir oil displacement and storage technology in the field, and can thus serve the national "dual carbon" goal. In view of this, this project will focus on this frontier research direction, conducting basic research on how to enhance CO2 adsorption and weaken crude oil adsorption.

[0005] The key to solving the above problems lies in regulating the wettability of the kerogen pore walls, that is, weakening their oleophilicity and enhancing their hydrophilicity or aerophilicity. The wall wettability affects the interaction between kerogen and crude oil, and thus affects the competitive adsorption behavior of CO2-crude oil. The stronger the oleophilicity of the kerogen wall, the stronger the interaction between the wall and crude oil, making it more difficult for CO2 to overcome this interaction force to displace the crude oil. Conversely, the weaker the oleophilicity (the stronger the hydrophilicity) of the kerogen wall, the weaker the interaction force between the wall and crude oil, making it easier for CO2 to overcome the force to displace the crude oil, thereby enhancing the competitive adsorption effect of CO2 and crude oil. Hydrogenation / methylation treatment of oxygen atoms on the quartz surface to simulate different wettability (hydrophilic / oleophilic) walls revealed that, at the same time, the competitive adsorption effect of CO2 on crude oil on oleophilic walls (30.50%) was much lower than that on hydrophilic walls (55.29%). Therefore, it is evident that by regulating the wettability of the kerogen pore wall towards hydrophilicity, and thereby selectively enhancing the competitive adsorption effect of CO2 and crude oil, it has significant engineering application implications for maximizing the CO2 oil displacement efficiency and storage efficiency of shale oil reservoirs.

[0006] Controlling reservoir wettability is one of the most classic problems in the oil and gas field, and many control methods (altering rock or fluid properties) have emerged, such as adding surfactants and nanofluids, all of which have achieved good results. However, kerogen is too oleophilic, and methods such as surfactants and nanofluids have limited effects on displacing shale oil and cannot be targeted. Directly borrowing previous wettability modification methods is difficult to match with the unique structure of kerogen for targeted control. Therefore, how to control the wettability of kerogen walls is an urgent problem to be solved in shale oil reservoir displacement and storage technology engineering. Summary of the Invention

[0007] Based on the above analysis, the technical solution of this application discloses a targeted conversion method for adsorbed-free shale oil based on in-situ electrochemical modification of pore walls, comprising the following steps:

[0008] S1. Inject targeted modified fluid into the target shale oil reservoir through injection wells;

[0009] S2. Shut down the well and apply a low-frequency, asymmetric pulsed electric field to the formation;

[0010] S3. Monitor the conversion ratio of adsorbed oil to free oil;

[0011] S4. Once the conversion ratio described in S3 reaches the target threshold, stop the electric field and open the well to produce free oil.

[0012] Furthermore, the targeted modification liquid described in S1 includes a pre-solution composed of an electrolyte and a wall-targeting oxidant.

[0013] Furthermore, the total solute mass fraction in the pretreatment solution is 2%-5%, and the mass ratio of added electrolyte to wall-targeting oxidant is (5-10):1.

[0014] Furthermore, the electrolyte is selected from highly conductive salts; the wall-targeting oxidant is selected from any one or any combination of hydrogen peroxide, potassium persulfate, and potassium hypochlorite.

[0015] Furthermore, the electric field frequency of the low-frequency, asymmetric pulsed electric field described in S2 is determined based on the relaxation time of the double electric layer at the pore interface of the target shale oil reservoir.

[0016] Furthermore, the low-frequency, asymmetric pulsed electric field described in S2 is applied periodically; the electric field waveform is a sawtooth wave or a rectangular wave; and the electric field strength is 1V / cm - 5V / cm.

[0017] Furthermore, the periodic application specifically refers to a cyclical process of energizing for 24 hours and then resting for 12 hours.

[0018] Furthermore, step S3 involves monitoring the conversion ratio using nuclear magnetic resonance (NMR) technology.

[0019] Beneficial Effects: This application differs from existing technologies that modify the viscosity of adsorbed crude oil. It utilizes a pulsed electric field to drive a specially formulated "targeted modification liquid" into the nanopores of shale, inducing electrochemical polarization reactions on the kerogen / mineral surface (pore walls). This forcibly modifies the originally oleophilic (Oil-wet) pore walls into hydrophilic / gas-wet pores, significantly reducing the interfacial adsorption energy and achieving the technical effect of stripping adsorbed oil and converting it into flowable free oil. Compared to existing technologies, this application significantly improves processing efficiency and permanently alters the chemical properties of the kerogen / mineral walls, preventing secondary adsorption of crude oil. Attached Figure Description

[0020] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the description of the embodiments of the present invention will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0021] Figure 1 This is a flow chart of the existing resonant electrochemical wetting reversal process;

[0022] Figure 2 This is the process flow diagram of this application. Detailed Implementation

[0023] To make the technical problems solved, the technical solutions, and the beneficial effects of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

[0024] like Figure 1 The diagram shows a prior art method for converting adsorbed oil into free oil using a resonant electrochemical wetting reversal process. In this method, the pulsed electric field is applied intermittently with an irregular frequency. Furthermore, both the modified liquid and the electric field act on the adsorbed oil itself. On the one hand, the amount of adsorbed oil that can be dissociated is limited. On the other hand, it uses a physical displacement effect on the adsorbed oil. Once the electric field stops, the dissociated free oil may be adsorbed back, limiting the conversion rate and recovery rate.

[0025] like Figure 2 The diagram shown is a flow chart of the in-situ electrochemical wetting reversal process used in this application, which includes the following steps:

[0026] S1. Inject targeted modified fluid into the target shale oil reservoir through injection wells;

[0027] The total solute mass fraction in the pretreatment solution is 2% - 5% to balance conductivity and protection of the rock skeleton.

[0028] In this embodiment, the purpose of injecting the targeted modification liquid is to provide the necessary oxygen source for the occurrence of the electrochemical oxidation reaction. The mass ratio of the electrolyte to the wall-targeted oxidant is (5-10):1. The electrolyte is selected from any one or a combination of highly conductive salts such as sodium chloride (NaCl), sodium sulfate (Na2SO4), potassium nitrate (KNO3), sodium bicarbonate (NaHCO3), magnesium sulfate (MgSO4), and potassium chloride (KCl). It is used to maintain the formation conductivity and provide the double-layer compression effect. KCl is preferred, as it is more conducive to inhibiting the expansion of shale clay. The wall-targeted oxidant is selected from any one or a combination of hydrogen peroxide (H2O2), potassium persulfate (K2S2O8), sodium hypochlorite (NaClO), and potassium permanganate (KMnO4). It is used to perform in-situ electrochemical modification (increasing the content of hydroxyl and carboxyl groups) on the hydrophobic long chains on the kerogen surface under the induction of an electric field. These oxidants can accurately generate highly active free radicals at the pore wall interface under the induction of a pulsed electric field.

[0029] The targeted modification liquid described in this embodiment has high conductivity and low surface tension, exhibiting extremely strong interfacial activity. It can penetrate between the oil film and the rock like a "wedge," thereby acting on the kerogen wall. The electrolyte is rich in oxygen, providing an oxygen source for electrochemical modification.

[0030] S2. Shut down the well and apply a low-frequency, asymmetric pulsed electric field to the formation;

[0031] In this embodiment, the low-frequency, asymmetric pulsed electric field is applied periodically; the electric field waveform is a sawtooth wave or a rectangular wave; the electric field strength is 1V / cm - 5V / cm; the periodic application specifically involves a cyclic process of energizing for 24 hours and then resting for 12 hours.

[0032] In this embodiment, under the action of a pulsed electric field, on the one hand, the electric double layer is continuously compressed / stretched by the electro-osmosis effect of the pulsed electric field, loosening the oil film; on the other hand, under the action of an electroosmotic pump, water molecules are forcibly squeezed into the "oil-rock interface" by the electro-osmosis flow of the pulsed electric field, forming a nano-water film. This allows the modifying liquid acting on the kerogen wall to undergo a weak electrochemical oxidation reaction at the pore wall interface, oxidizing the nonpolar groups (CH) on the kerogen surface into polar groups (-COOH, -OH). This transforms the surface from "lipophilic" to "hydrophilic," thus completing the chemical modification of the kerogen surface and fundamentally eliminating the physical basis for oil adsorption and retention. Even after the electric field is removed, the modified kerogen wall surface remains hydrophilic. Therefore, the technical solution of this application, by reconstructing the chemical bonds on the kerogen surface (converting CH bonds into -COOH bonds, etc.), achieves an irreversible reversal of wettability from lipophilic to hydrophilic, improving oil washing efficiency and recovery rate.

[0033] It should be noted that the pulsed electric field application method and parameters determined in this application are obtained by measuring the relaxation time of the double electric layer at the pore interface and matching the resonant frequency. Only at this frequency can the electric field force be superimposed to form a huge "electroosmotic separation pressure," thereby achieving the effect of separating the oil film from the wall surface. In other words, the pulsed electric field frequency is not a fixed value when facing different shale oil reservoirs, but is precisely matched based on the relaxation time of the double electric layer at the pore interface of the target shale reservoir.

[0034] Furthermore, the chemical change of the wall surface achieved by this application (from lipophilic to hydrophilic) is permanent. It utilizes a Kolbe-type electrolysis reaction to allow the hydrophilic functional groups in the electrolyte to covalently bond with the kerogen surface, thereby permanently changing the lipophilic chemical properties of the wall surface and eliminating secondary adsorption after the adsorbed oil dissociates.

[0035] The technical solutions and effects of this application will be described in detail below through specific embodiments.

[0036] Example 1: A method for targeted conversion of adsorbed to free state in shale oil based on in-situ electrochemical modification of pore walls.

[0037] Experimental sample: Shale cores (25mm in diameter and 50mm in length) from the same block with similar physical properties were selected. After washing and drying, they were vacuum saturated to simulate crude oil. In this specific embodiment, it was determined through specific testing that the frequency of the pulsed electric field was set to the characteristic frequency of 200Hz, which can achieve "resonance" between the electric field and the double-layer polarization process.

[0038] Preparation of targeted modification solution (S1): Potassium chloride (KCl) and potassium persulfate (K2S2O8) were mixed at a mass ratio of 10:1. The mixture was dissolved in deionized water to prepare an aqueous solution with a total solute mass fraction of 3.5%, and ultrasonic vibration was used for 30 min to ensure uniform mixing.

[0039] Pre-filling liquid injection (S1): The modified liquid is injected into the holder at a constant flow rate of 0.05 mL / min, with an injection volume of approximately 2.0 times the pore volume (PV), so that the modified liquid can fully reach the nanopores.

[0040] Electric field loading and modification (S2): Parameter settings: Turn on the pulse power supply, set the electric field waveform to an asymmetric sawtooth wave, the field strength to 3.0 V / cm (corresponding to the downhole field strength), and the frequency to the pre-calculated resonant frequency of 200 Hz. Cyclic processing: Perform 5 cycles of "powering on for 24 hours + resting for 12 hours".

[0041] In-situ monitoring and transformation assessment (S3): Relaxation spectra of cores were acquired using a nuclear magnetic resonance spectrometer at every cycle. Data analysis: The changes in the peak area of ​​0.1–3 ms (adsorbed oil) were observed, and the proportion of transformation to the 10–100 ms (free oil) peak was recorded.

[0042] Production evaluation (S4): When NMR monitoring shows that the conversion rate exceeds 80%, the electric field is stopped and the total amount of free oil extracted is recorded.

[0043] It is understood that, specifically in this embodiment, 80% is used as the target threshold for the conversion ratio; however, those skilled in the art can set different target thresholds as needed for different shale oil reservoirs and different conversion purposes.

[0044] Comparative Example 1: A targeted conversion method for adsorbed-to-free shale oil based on in-situ electrochemical modification of pore walls.

[0045] The implementation steps are the same as in Example 1, except that the electric field frequency of 200Hz is replaced with 0 Hz (DC electric field, Experiment 1) and 100,000 Hz (high-frequency electric field, Experiment 2), and the electric field strength of 3V / cm at the 200Hz electric field frequency is replaced with 0.5V / cm (Experiment 3) and 10V / cm (Experiment 4), respectively. The changes in the T2 spectrum at the non-resonant frequency are recorded to prove that simple physical thrust (DC) or excessively fast oscillation (high frequency) cannot effectively squeeze into the nano-water film and induce chemical reactions.

[0046] The results are shown in Table 1.

[0047] Table 1

[0048] Group Electric field frequency (Hz) Electric field strength (V / cm) Peeling pressure Pe (kPa) Adsorbed oil conversion rate (%) in conclusion Example 1 200 (resonance frequency) 3 45.2 82.40% Optimal effect: Electroosmotic separation pressure superposition Experimental group 1 0 (DC electric field) 3 5.8 12.50% It only generates constant thrust and cannot be stripped away. Experimental group 2 100,000 (high frequency) 3 0.2 3.10% The frequency is too high, and the double layer cannot respond. Experimental group 3 200 (resonance frequency) 0.5 4.1 8.60% Field strength below critical stripping threshold Experimental group 4 200 (resonance frequency) 10 severe polarization heating 24.00% This leads to rock burning and chemical side reactions.

[0049] This experiment demonstrates that only a specific frequency electric field matching the relaxation time of the double layer can produce the "resonance stripping effect".

[0050] Comparative Example 2: A targeted conversion method for adsorbed-to-free shale oil based on in-situ electrochemical modification of pore walls.

[0051] The implementation steps are the same as in Example 1, except that the K2S2O8 + NaCl oxidation system is replaced with traditional surfactants (0.2% SDS solution, Example 1), 3.5% KCl solution (Example 2), and formation water (Example 3), respectively, and the conversion ratio is recorded after S3.

[0052] The NMR was scanned again 48 hours after the experiment was stopped. The focus was on whether the oil droplets were re-adsorbed back onto the wall surface (T2 short peak rebound) to demonstrate that physically covering surfactants cannot prevent secondary adsorption, while the covalent bond modification of this application is permanent.

[0053] The comparative analysis of varying intensities (verifying the necessity of the "electric field threshold") involved adjusting the field strength to 0.5 V / cm (low field strength) or 10 V / cm (high field strength). Observation points were used to demonstrate that insufficient field strength could overcome interfacial energy, or that excessively high field strength could lead to formation heat loss and electrochemical side reactions, thus determining the optimal range for industrial application. The results are shown in Table 2.

[0054] Table 2

[0055] System Name Main ingredients Mechanism of action Contact angle after treatment (°) Secondary adsorption rate (%) Example 1 <![CDATA[K2S2O8 + NaCl]]> Electrochemical in-situ oxidation 15.2 (Strongly hydrophilic) < 2% Comparative Example 1 Traditional surfactants (SDS) Physical coverage / tension reduction 65.4 (weakly hydrophilic) 48.60% Comparative Example 2 Nanoparticle modified liquid Physically occupying pores 82.1 (weakly oleophilic) 62.10% Comparative Example 3 Simulated formation water none 135.0 (Strong affinity for oil) -

[0056] Parallel experiments of the above embodiments and comparative examples revealed that when the resonant frequency and targeted oxidation modification liquid specified in this application are used, the hydrophilicity of the shale pore walls undergoes an irreversible transformation. Specifically, the secondary adsorption rate of the embodiment group after the electric field is stopped is less than 2%, while the secondary adsorption rate of the comparative example group using traditional surfactants is as high as 45% or more. This fully demonstrates that the electrochemical in-situ modification described in this application has a significant targeted conversion effect.

[0057] The above-described embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention, and should all be included within the protection scope of the present invention.

Claims

1. A targeted conversion method for adsorbed-to-free shale oil based on in-situ electrochemical modification of pore walls, characterized in that, Includes the following steps: S1. Inject targeted modified fluid into the target shale oil reservoir through injection wells; S2. Shut down the well and apply a low-frequency, asymmetric pulsed electric field to the formation; S3. Monitor the conversion ratio of adsorbed oil to free oil; S4. Once the conversion ratio described in S3 reaches the target threshold, stop the electric field and open the well to produce free oil.

2. The conversion method according to claim 1, characterized in that, The targeted modification liquid described in S1 includes a pre-solution composed of an electrolyte and a wall-targeting oxidant.

3. The conversion method according to claim 2, characterized in that, The total solute mass fraction in the pretreatment solution is 2%-5%, and the mass ratio of added electrolyte to wall-targeting oxidant is (5-10):

1.

4. The conversion method according to claim 2, characterized in that, The electrolyte is selected from highly conductive salts; the wall-targeting oxidant is selected from any one or any combination of hydrogen peroxide, potassium persulfate, and potassium hypochlorite.

5. The conversion method according to claim 1, characterized in that, The electric field frequency of the low-frequency, asymmetric pulsed electric field described in S2 is determined based on the relaxation time of the double electric layer at the pore interface of the target shale oil reservoir.

6. The conversion method according to claim 5, characterized in that, The low-frequency, asymmetric pulsed electric field described in S2 is applied periodically; the electric field waveform is a sawtooth wave or a rectangular wave; the electric field strength is 1V / cm - 5V / cm.

7. The conversion method according to claim 5, characterized in that, The periodic application specifically refers to a cycle of energizing for 24 hours and then resting for 12 hours.

8. The conversion method according to claim 1, characterized in that, Step S3 involves monitoring the conversion ratio using nuclear magnetic resonance (NMR) technology.

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