A method to improve the recovery rate of medium- to low-maturity shale oil reservoirs

Abstract

This invention provides a method for improving the recovery rate of medium- to low-maturity shale oil reservoirs, relating to the field of shale oil development technology. This invention utilizes the oxidation reaction between biomass oil and reservoir organic matter to release heat, while simultaneously combining supercritical CO2 and air injection technology to significantly increase the effective range of supercritical CO2 in the reservoir, thereby improving the recovery rate.

Description

Technical Field

[0001] This invention relates to the field of shale oil development technology, and in particular to a method for improving the recovery rate of medium- to low-maturity shale oil reservoirs. Background Technology

[0002] China's shale oil is mainly composed of medium- to low-maturity shale oil, which generally has the following characteristics: (1) poor reservoir properties, with extremely low reservoir porosity and permeability, where the porosity is generally 2% to 6% and the permeability is generally 0.01 × 10⁻⁶. -3 ~100×10 -3 μm 2 (2) Low proportion of movable oil, for example, the proportion of movable oil in the Shahe Formation shale reservoir of the Shahe Group in the Jiyang Depression is 5.8% to 18%; (3) High viscosity, for example, the viscosity range of sweet spot shale oil in the Jimsar Depression at 50℃ is 94.20 to 407.08 mPa·s; (4) Deep burial depth, usually between 3000 and 5000 m. Therefore, conventional development methods such as water drive and chemical drive are difficult to apply to this type of reservoir. At present, medium- to low-maturity shale reservoirs are mainly developed effectively through large-scale hydraulic fracturing. However, the production of shale reservoirs decreases rapidly after hydraulic fracturing. For example, the monthly decline rate of the Ji 251-H well during the self-flowing period is 10.1%, the annual decline rate is 70.2%, and the recovery rate is low, usually below 10%. Therefore, it is necessary to find a method for efficient development of shale reservoirs after fracturing.

[0003] Recent domestic and international studies have shown that supercritical CO2 can extract not only free light and medium-grade oil components from shale reservoirs, but also some crude oil in an adsorbed-miscible state. Therefore, supercritical CO2 technology is considered to have broad application prospects in enhancing oil recovery in medium- to low-maturity shale reservoirs. Publication number CN108661614A discloses a supercritical CO2 huff-and-puff method for tight oil reservoirs. This method improves recovery by injecting a mixture of preheated chemical fluid and supercritical CO2 into the reservoir for well-steaming development. However, shale reservoirs are typically buried deep (3000–5000 m), and the preheated chemical fluid and supercritical CO2 mixture suffer significant heat loss during injection, resulting in poor development performance. Publication number CN109779582A discloses a method for in-situ extraction of hydrocarbon compounds from oil shale through downhole heating. This method first involves hydraulically fracturing the reservoir between the heating well and the production well, followed by lowering a downhole heater and injecting carbon dioxide at a certain pressure. The injected carbon dioxide is heated by the downhole heater to a supercritical state, thereby extracting shale oil. However, this method has the following problems: 1) The injected CO2 requires continuous heating, resulting in huge energy consumption; 2) Supercritical CO2 has extremely high solubility in water, causing severe corrosion to the wellbore and the downhole heater. In addition, supercritical CO2 exhibits carburizing behavior in a high-temperature and high-pressure environment, which further accelerates material corrosion; 3) The effective range of supercritical CO2 is limited, often only acting on the main seepage channels formed after hydraulic fracturing.

[0004] Therefore, it is necessary to find a method that can generate supercritical CO2 in situ and increase its range of action in reservoirs, so as to reduce energy consumption and improve recovery rate. Summary of the Invention

[0005] To address the aforementioned problems, this invention provides a method for improving the recovery rate of medium- to low-maturity shale oil reservoirs. This method utilizes the oxidation reaction between biomass oil and reservoir organic matter to release heat, while simultaneously combining supercritical CO2 and air injection technology. This significantly increases the effective range of supercritical CO2 within the reservoir, thereby enhancing the recovery rate. The method for improving the recovery rate of medium- to low-maturity shale oil reservoirs includes the following steps:

[0006] S1. Extract biomass oil from biomass feedstock, mix the biomass oil with fracturing fluid and inject it into the formation;

[0007] Injected air can react with reservoir organic matter under reservoir conditions, releasing heat. However, medium- to low-maturity shale reservoirs are highly heterogeneous, rich in various rock minerals, and contain bound water in their pores. During the heating process, the temperature rises slowly and unevenly, making it difficult to establish a stable oxidative heat front. Previous methods typically involved injecting combustion-promoting agents into the reservoir, but this approach has the following problems when applied to shale oil reservoirs: 1) Shale oil reservoirs are deeply buried, making injection difficult; 2) Combustion-promoting agents, such as calcium peroxide, ammonium persulfate, and calcium ammonium nitrate, are expensive. To overcome these problems, this invention uses biomass oil as a combustion-promoting agent, which not only overcomes the aforementioned problems but also has advantages such as high volatile content, low ignition point, energy saving, and environmental friendliness.

[0008] S2. Inject air into the formation, and stop injecting air when the formation pressure reaches the predetermined pressure.

[0009] S3. Perform well-blocking operation. Well-blocking allows air and solid organic matter such as kerogen and asphalt in the hydraulically fractured area to oxidize or even burn under the catalysis of biomass oil, releasing heat. The generated oxidation heat energy enhances the connectivity between the fracture and the matrix and improves the reservoir conductivity.

[0010] S4. When the oxygen concentration in the produced gas is below 5%, the well should be started for production. When the oxygen concentration in the produced fluid is below 5%, it indicates that the oxygen in the reservoir air has been basically consumed and the oxygen content in the huff and puff development exceeds the warning value of 5%. Therefore, the well should be started for production when the oxygen content in the produced gas is below 5% to reduce production risk.

[0011] S5. During production, monitor formation pressure. When the formation pressure drops to 0.5 to 0.8 times the critical formation fracture pressure, shut down the production well and switch to CO2 injection. The thermal effect generated after air injection will cause the reservoir pressure to rise rapidly in a short period of time. If CO2 is directly injected, the requirements for the surface gas injection equipment are high. In addition, the safe gas injection pressure specified by the oilfield operation is 25.5 MPa. At this time, there is a great safety hazard in gas injection.

[0012] S6. After injecting 0.05 to 0.1 PV CO2, perform well shut-in operation, where PV is the pore volume;

[0013] Well shut-in allows CO2 to form supercritical CO2 in situ under the action of oxidative heat. This supercritical CO2 then enters the shale matrix through the dominant fracture-matrix channels created by the oxidative heat, eroding matrix particles and promoting the formation of microfractures within the matrix. This further expands the thermally induced fracture network structure formed by the oxidative heat. During this process, supercritical CO2 can extract light and medium-weight components present in the newly formed fracture-matrix structure, enhancing air injection and improving oil recovery. After air injection, free and adsorbed oil in the matrix decreases in viscosity under oxidative heat. However, the high critical thermal fracturing temperature of some rocks limits the extent to which oxidatively induced fractures extend into the matrix, making it difficult for modified oil to flow out. Supercritical CO2 possesses unique properties such as low viscosity, high diffusivity, and zero surface tension, making it relatively easy to diffuse into the tight shale matrix. Supercritical CO2 formed in situ under the action of oxidative heat enters the shale matrix through the dominant fracture-matrix channel formed by the action of oxidative heat, erodes the matrix particles, promotes the formation of microcracks in the matrix, enhances the communication between the matrix and the oxidative heat-induced fractures, extracts this part of the modified oil, and improves the recovery degree of air injection.

[0014] S7. Well opening for depletion development, setting up a gas-liquid separation device at the production well to separate the produced fluid, and storing the separated CO2 in a surface storage device.

[0015] S8. When the formation temperature is above 150℃, inject the CO2 stored on the surface into the formation and repeat steps S5 to S7 to continue supercritical CO2 huff and puff oil production until the formation temperature is below 150℃, then switch to air injection for a new round of production.

[0016] When the temperature is above 150℃, the solubility of the N2 and CO2 mixture with crude oil gradually increases under different pressures. Considering that the injected CO2 may be miscible with the flue gas produced by the combustion of organic matter, thereby increasing the extraction capacity of crude oil in the matrix in the adsorbed-miscible state, CO2 is continued to be injected when the formation temperature is above 150℃, which promotes the generation of more supercritical CO2 and flue gas mixture to enhance the extraction degree of liquid hydrocarbons in the reservoir.

[0017] S9. After each subsequent round of air injection, monitor the gas production volume of the production well. If the gas production volume is less than 0.12 times the injected air volume, stop air injection and proceed with depletion-type development.

[0018] Furthermore, the biomass raw material is one or more of corn stalks, wheat stalks, and peanut stalks.

[0019] Preferably, the biomass raw material is wheat straw.

[0020] Furthermore, the pyrolysis temperature in the pyrolysis method is 500℃, the reaction time is 15min, and the gas produced by pyrolysis is condensed into a condensation system to obtain biomass oil, wherein the condensation temperature is -10℃.

[0021] Furthermore, the biomass can increase the overall combustibility index of kerogen to 10. -9 Within.

[0022] Furthermore, the calculation method for the comprehensive combustion index SN is as follows:

[0023]

[0024] Where: (dw / dt)max - maximum combustion rate (peak value of DTG curve), % / min; (dw / dt)mean - average combustion rate (T i and T h Average inter-combustion rate), % / min; T h - Combustion temperature (the temperature at which a combustible material loses 98% of its weight); T i - Ignition temperature.

[0025] Furthermore, the biomass can reduce the initial temperature of kerogen combustion by more than 40°C.

[0026] Furthermore, the predetermined pressure is 1 to 1.5 times the original formation pressure.

[0027] This invention uses biomass as a catalytic combustion agent, which causes less damage to the reservoir compared to using nano-metal dispersants. Furthermore, biomass has low ash content, resulting in less dust emissions during combustion, thus helping to reduce suspended particulate matter emissions. Compared to liquid hydrocarbons, the combustion produces less CO2 and NO... x With low SO2 content, it helps reduce the difficulty and cost of exhaust gas treatment, improves environmental impact, and is a sustainable option.

[0028] This invention combines air injection technology with supercritical CO2 extraction technology. After air injection, the solid organic matter in the hydraulic fracturing zone is oxidized or even burned under the catalysis of biomass oil, releasing heat. The oxidative heat energy enhances the connectivity between the fractures and the matrix. Subsequently, the injected CO2 forms supercritical CO2 in situ under the action of oxidative heat energy. The supercritical CO2 enters the shale matrix through the dominant fracture-matrix channels formed after the action of oxidative heat energy, eroding the matrix particles and promoting the formation of fractures of different scales in the matrix. This further expands the thermally induced fracture network structure formed after the action of oxidative heat energy, producing a synergistic effect and expanding the connectivity of the underground fracture network structure.

[0029] This invention utilizes supercritical CO2, which diffuses through a newly formed complex fracture network structure to areas unaffected by oxidative heat, extracting crude oil from kerogen and micropores. Furthermore, supercritical CO2 is miscible with flue gas generated from the combustion of organic matter, enhancing the extraction capacity for liquid hydrocarbons in the reservoir, both in adsorbed and miscible states, thereby significantly improving oil recovery. Simultaneously, during development, CO2 is separated from the produced fluid and reused, reducing CO2 emissions while enhancing shale oil reservoir recovery.

[0030] This invention combines supercritical CO2 with air injection technology. Injected air reacts with organic matter in the reservoir through oxidation, generating heat and increasing energy to connect the reservoir fracture-matrix structure. Subsequently, CO2 gas is injected into the reservoir, where it forms supercritical CO2 in situ under the influence of oxidation heat. This supercritical CO2 enters the shale matrix through the dominant fracture-matrix channels formed by the oxidation heat, eroding matrix particles and promoting the formation of fractures of different scales within the matrix. This further expands the thermally induced fracture network structure formed by the oxidation heat. During this process, supercritical CO2 can extract light and medium-weight components present in the newly formed fracture-matrix structure, enhancing air injection and improving oil recovery. Furthermore, supercritical CO2 is miscible with flue gas generated from the combustion of organic matter, enhancing its extraction capability for liquid hydrocarbons existing in adsorbed and miscible states in the reservoir.

[0031] Compared with the prior art, the beneficial technical effects of the present invention are as follows:

[0032] (1) This invention reduces exhaust gas and dust emissions and reduces damage to reservoirs by using biomass oil;

[0033] (2) The supercritical CO2 of this invention can be miscible with the flue gas generated by the combustion of organic matter, which greatly improves the recovery rate. The CO2 in the produced liquid is separated and reused, which reduces the CO2 emissions while enhancing the recovery rate of shale oil reservoirs.

[0034] (3) This invention combines air injection technology with supercritical CO2 extraction technology to produce a synergistic effect and expand the connectivity of the underground network structure. Attached Figure Description

[0035] The present invention will be further described below with reference to the accompanying drawings.

[0036] Figure 1 The weight loss characteristic curves of kerogen after the addition of different biomass;

[0037] Figure 2 The weight loss rate curves of kerogen after the addition of different biomass;

[0038] Figure 3 A diagram of the experimental setup for the thermal effects of porous media provided in an embodiment of the present invention;

[0039] Figure 4 The reaction temperature change curves during the combustion of kerogen after the addition of different biomass;

[0040] Figure 5 A diagram of an experimental apparatus for supercritical CO2 huff and puff, air huff and puff, and air huff and puff combined with supercritical CO2 to enhance oil recovery provided in an embodiment of the present invention;

[0041] Figure 6 The embodiments of the present invention provide different purging cycles of supercritical CO2 huff and puff, air huff and puff, and air huff and puff combined with supercritical CO2 huff and puff to improve the recovery rate.

[0042] Figure 7 Microscopic SEM image (15 μm) of shale after supercritical CO2 huff and puff;

[0043] Figure 8 Microscopic SEM image (8 μm) of shale after supercritical CO2 huff and puff;

[0044] Figure 9 Microscopic SEM image (15 μm) of shale after air inhalation;

[0045] Figure 10 Microscopic SEM image (8 μm) of shale after air inhalation and exhalation;

[0046] Figure 11 Microscopic SEM image (15 μm) of shale after supercritical CO2 inhalation assisted by air inhalation;

[0047] Figure 12 Microscopic SEM image (8 μm) of shale after supercritical CO2 inhalation assisted by air inhalation. Detailed Implementation

[0048] The technical solution provided by the present invention will be further described below with reference to the embodiments.

[0049] Example 1

[0050] (1) Screening of biomass oil

[0051] Thermogravimetric analysis (TG) and a self-developed porous media reaction heat effect monitoring device were used to evaluate the catalytic combustion performance of biomass. Common agricultural wastes, including corn stalks, wheat stalks, and peanut stalks, were selected. Biomass and kerogen were mixed evenly at a 1:1 ratio, placed in a crucible, and then placed in the TG instrument. The air flow rate was set at 50 mL / min, the heating rate at 10 °C / min, and the experimental temperature range was 40–800 °C. The results are shown in [Figure number missing]. Figure 1 and Figure 2As shown, the comprehensive combustion coefficients of the three samples were calculated, and it can be seen that wheat straw has the most significant catalytic effect on kerogen combustion.

[0052] sample <![CDATA[T i / ℃]]> <![CDATA[T h / ℃]]> <![CDATA[W max / (% / min)]]> <![CDATA[W mean (% / min)]]> <![CDATA[S / (min -2 ·K -3 )]]> corn stalks 425 638 0.31 0.23 <![CDATA[6.19×10 -10 ]]> wheat straw 381 620 0.296 0.226 <![CDATA[5.77×10 -9 ]]> peanut stalks 428 677 0.282 0.188 <![CDATA[4.27×10 -10 ]]>

[0053] The catalytic effect of biomass on the exothermic combustion of kerogen was systematically evaluated using a self-developed porous media reaction heat effect monitoring device (CN201810822811.8).

[0054] Experimental flowchart as follows Figure 3 As shown, the specific experimental steps were as follows: 1) Insert a thermocouple from the injection end of the reactor, and sequentially fill the outlet end with a mixture of quartz sand, silica gel particles, kerogen, and biomass (mixed evenly in a 1:1 ratio), and silica gel particles. The remaining space in the reactor was filled with ordinary sand. 2) Insert a thermocouple at the kerogen and biomass filling point to test the temperature at the reaction sample location. 3) Heat the reactor using a heater. The air injection rate was set to 0.3 L / min, the heating rate to 10 °C / min, and the experimental temperature range to 40–500 °C. Results are shown below. Figure 4 It was found that wheat straw exhibited a strong catalytic effect. After adding wheat straw, the initial combustion temperature of kerogen decreased by 50℃; the reaction temperature corresponding to the combustion peak increased by 85℃. These results indicate that after adding wheat straw, kerogen can start burning at a lower temperature and the heat release during combustion is significantly increased.

[0055] (2) Supercritical CO2 swallowing experiment

[0056] like Figure 5 As shown, the experimental setup includes a high-temperature and high-pressure combustion experimental apparatus, a constant-temperature oven, a gas compressor, an intermediate container, a booster pump, and a gas-liquid separation device.

[0057] The natural fractured cores used in the experiment were pretreated (oil washing and drying). The dry weight, length, and diameter of the natural fractured cores were measured, and a ZYB-Ⅲ type high-vacuum molecular pump was used for saturation oil operation. Then, the wet weight of the cores was measured, and the saturated oil volume was calculated. The natural fractured cores were placed in the high-temperature and high-pressure reaction device, and the igniter of the high-temperature and high-pressure reaction device was turned off. The oven temperature was set to 90℃, the CO2 storage tank booster pump pressure was set to 10MPa, and the back pressure was set to 10MPa. CO2 was injected into the high-temperature and high-pressure combustion experimental device at an injection rate of 0.5mL / min, injecting 0.5 times the pore volume of CO2. The well was shut in for 6 hours. After shutting in, the outlet end was opened for production. The experiment was stopped when the pressure dropped to 10MPa, and the recovery rate was recorded. A new round of huff and puff was then carried out, for a total of 5 rounds.

[0058] (3) Air intake and output experiment

[0059] like Figure 5 As shown, the experimental setup includes a high-temperature and high-pressure combustion experimental apparatus, a constant-temperature oven, a gas compressor, an intermediate container, a booster pump, and a gas-liquid separation device.

[0060] The biomass oil selected in (1) was injected into the core using an ISCO pump at an injection rate of 1 mL / min. After injecting 0.1 times the pore volume of biomass oil, the injection end was closed. An air injection huff and puff experiment was conducted. Air was injected at a rate of 0.5 mL / min, and 0.5 times the pore volume of air was injected. The igniter was turned on (ignition temperature set at 400℃), and the temperature at the injection end was monitored in real time. When a stable combustion chamber (temperature > 350℃) was formed near the injection end, the well was shut in for 6 hours. After the shut-in was completed, the outlet end was opened for production, and the huff and puff recovery rate was recorded. A new round of huff and puff was then carried out, for a total of 5 rounds.

[0061] (4) Experiment on air injection combined with supercritical CO2 injection

[0062] like Figure 5 As shown, the experimental setup includes a high-temperature and high-pressure combustion experimental apparatus, a constant-temperature oven, a gas compressor, an intermediate container, a booster pump, and a gas-liquid separation device.

[0063] The natural fracture cores used in the experiment were pretreated (washed and dried). The dry weight, length, and diameter of the natural fracture cores used in the experiment were measured, and saturated oil operation was performed using a ZYB-Ⅲ type high vacuum molecular pump. Then the wet weight of the cores was measured, and the amount of saturated oil was calculated. The natural fracture cores were placed in the high temperature and high pressure reaction device. The oven temperature was set to 90℃, the CO2 tank booster pump was turned off, and the back pressure was set to 10MPa. The biomass oil selected in (1) was injected into the cores using an ISCO pump. The injection rate was set to 1mL / min. After injecting 0.1 times the pore volume of biomass oil, the injection end was closed. An air injection and purging experiment was carried out. Air was injected at a rate of 0.5mL / min. Air was injected at 0.5 times the pore volume. The igniter was turned on (ignition temperature was set to 400℃). The temperature at the injection end was monitored in real time. When a stable combustion chamber (temperature > 350℃) was formed near the injection end, the well was shut down for 6 hours. After the well was shut-in time was reached, production was initiated by opening the outlet. The pressure of the experimental instruments was monitored. When the pressure dropped to approximately 8 MPa, the outlet was closed, and CO2 was injected to confirm the ability to generate supercritical CO2 in situ. The CO2 injection rate was set to 0.5 mL / min, injecting 0.5 times the pore volume of CO2, and the well was shut-in for 6 hours. The outlet was then opened, and the experiment was stopped when the pressure dropped to 10 MPa. The recovery rate was recorded. A new round of injection was then conducted, for a total of 5 rounds.

[0064] Depend on Figure 6The results show that the recovery rates of the first round of supercritical CO2 huff and puff, air huff and puff, and air huff and puff assisted supercritical CO2 huff and puff were 22.4%, 22.8%, and 26.8%, respectively; the recovery rates of the second round were 14.6%, 15.8%, and 16.8%; the recovery rates of the third round were 6.8%, 8.2%, and 10.2%; the recovery rates of the fourth round were 0.2%, 2.4%, and 5.4%; and the recovery rates of the fifth round were 0%, 1.6%, and 2.9%. It can be seen that after the fourth round of supercritical CO2 huff and puff, virtually no crude oil is recovered. The recovery rate of air huff and puff after the fourth round was 1.6%. However, air huff and puff assisted supercritical CO2 still maintained a 5.4% recovery rate after four rounds of huff and puff. Therefore, it is believed that utilizing oxidation heat energy to generate supercritical CO2 in situ has a good effect on improving oil recovery. Subsequently, the microstructure of the shale after supercritical CO2 huff and puff, air huff and puff, and air huff and puff assisted supercritical CO2 huff and puff was observed using scanning electron microscopy. Figures 7-12 It can be seen that after supercritical CO2 huff and puff, the degree of fracturing in the shale is limited, with some areas remaining smooth and without obvious extended fractures. After air huff and puff, obvious fracture structures appear on the shale surface. Furthermore, after air huff and puff assisted supercritical CO2 huff and puff, shale minerals fragment, and the surface appears "molten" with obvious fracture structures. In conclusion, utilizing oxidative heat to generate supercritical CO2 in situ can enhance the connectivity of reservoir microfractures and improve oil recovery.

[0065] This document uses specific examples to illustrate the principles and implementation methods of the present invention. The descriptions of the above embodiments are only for the purpose of helping to understand the method and core ideas of the present invention. Furthermore, those skilled in the art will recognize that, based on the ideas of the present invention, there will be changes in the specific implementation methods and application scope. Therefore, the content of this specification should not be construed as a limitation of the present invention.

Claims

1. A method of enhancing recovery of a mid- to low-maturity shale reservoir, characterized by, Includes the following steps: S1. Biomass oil is extracted from biomass feedstock using pyrolysis, and the biomass oil is mixed with fracturing fluid and injected into the formation; the biomass can reduce the initial temperature of kerogen combustion by more than 40°C. The biomass raw material is one or more of corn stalks, wheat stalks, and peanut stalks; S2. Inject air into the formation, and stop injecting air when the formation pressure reaches the predetermined pressure. S3. Perform well-sealing operation; S4. Start production when the oxygen concentration in the produced gas is below 5%; S5. During production, monitor formation pressure. When the formation pressure drops to 0.5 to 0.8 times the critical formation fracture pressure, shut down the production well and inject CO2. S6. After injecting 0.05~0.1 PV CO2, perform a well-clogging operation to promote the in-situ formation of supercritical CO2 under the action of oxidation heat energy. S7. Well opening for depletion development, setting up a gas-liquid separation device at the production well to separate the produced fluid, and storing the separated CO2 in a surface storage device. S8. When the formation temperature is above 150℃, inject the CO2 stored on the surface into the formation and repeat steps S5 to S7 to continue supercritical CO2 huff and puff oil production until the formation temperature is below 150℃, then switch to air injection for a new round of production. S9. After each subsequent round of air injection, monitor the gas volume produced by the production well. If the ratio of the produced gas volume to the injected air volume is less than 0.12, stop air injection and proceed with depletion-type development.

2. The method for improving the recovery rate of medium- to low-maturity shale oil reservoirs according to claim 1, characterized in that, The predetermined pressure is 1 to 1.5 times the original formation pressure.

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