A method for improving oil displacement efficiency in tight oil reservoirs

Abstract

This invention belongs to the technical field of indoor simulation experiments for improving oil and gas recovery, and relates to a method for improving the oil displacement efficiency of tight oil reservoirs. In this invention, water-saturated carbon dioxide is injected into the tight oil reservoir. Relying on the mass transfer between the water-saturated carbon dioxide and the crude oil in the tight oil reservoir, the water-saturated carbon dioxide undergoes a phase change, causing some of the carbon dioxide in the water-saturated carbon dioxide to dissolve into the crude oil. At this time, the water phase in the water-saturated carbon dioxide is supersaturated and precipitates out, which increases the water saturation in the tight pore throat, resulting in a decrease in the relative permeability of the water-saturated carbon dioxide and an increase in seepage resistance. For water-wetted pores, the supersaturated precipitated water phase will be adsorbed on the pore wall surface, resulting in a decrease in the effective flow area and an increase in seepage resistance. Finally, the subsequent water-saturated carbon dioxide is forced to enter other smaller pores and channels, increasing the swept volume of water-saturated carbon dioxide in the tight oil reservoir, thereby achieving the purpose of increasing CO2 storage and oil displacement efficiency.

Description

Technical Field

[0001] This invention belongs to the technical field of indoor simulation experiments for improving oil and gas recovery, and specifically relates to a method for improving the oil displacement efficiency of tight oil reservoirs. Background Technology

[0002] For reservoirs lacking natural energy, injecting external fluids is typically used to supplement energy and drive oil recovery, thereby improving development efficiency. Gas injection is highly adaptable and widely applicable, making it one of the effective methods for supplementing energy in tight oil reservoirs and enhancing oil recovery. Currently, gas injection projects are on the rise worldwide, with CO2 enhanced oil recovery (CO2-EOR) projects accounting for over 60%. However, the low viscosity and density of CO2, along with reservoir heterogeneity, easily lead to gravity overlap and gas channeling problems, limiting the enhanced recovery and storage efficiency of CO2. Various processes have been developed to improve CO2 oil recovery efficiency, such as CO2 huff and puff, carbonated water injection (CWI), water-CO2 co-injection (SWAG), water-CO2 alternating injection (WAG), and CO2 foam injection. However, these technologies require complex process optimization and have limitations in application scenarios. For example, SWAG and WAG have very limited effectiveness in heterogeneous reservoirs, and CO2 foam has a short effective period in high-temperature reservoirs. On the other hand, the proportion of the aqueous phase in CWI, SWAG, or WAG technologies remains high. The injection of large amounts of water restricts the contact between CO2 and crude oil, creating a "water lock," which hinders the full realization of CO2's effects on increasing fluid elasticity and miscible displacement. Furthermore, because injected water occupies part of the pore space, CO2 storage efficiency is significantly reduced. Addressing the unique characteristics of tight oil reservoirs and the shortcomings of existing CO2 flooding technologies, this invention proposes a method to improve the oil displacement efficiency of tight oil reservoirs, solving the problems of poor injection characteristics, low oil displacement efficiency, and low CO2 storage efficiency encountered in the application of existing energy replenishment and enhanced oil recovery technologies for tight oil reservoirs. Summary of the Invention

[0003] In view of the shortcomings of the prior art, the purpose of this invention is to provide a method to improve the oil displacement efficiency of tight oil reservoirs, effectively solving the problems of poor injection performance, low oil displacement efficiency and low CO2 storage efficiency encountered in the application of existing energy replenishment and enhanced oil recovery technologies for tight oil reservoirs.

[0004] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0005] This invention provides a method for improving the CO2 flooding efficiency of tight oil reservoirs, comprising the following steps:

[0006] Water-saturated carbon dioxide is injected into tight oil reservoirs, and the breakthrough time of CO2 is monitored. By adjusting the injection rate, subsequent water-saturated carbon dioxide is forced to be directed towards unaffected micropores.

[0007] Preferably, when the monitored CO2 breakthrough time is earlier than the expected breakthrough time, the pressure value of water-saturated carbon dioxide injected into the target tight oil reservoir is adjusted by reducing the injection rate. In this case, the pressure value of water-saturated carbon dioxide injected into the target tight oil reservoir is always less than the upper pressure limit. The steps to obtain the upper pressure limit are as follows:

[0008] Acquire temperature, pressure, and crude oil composition data of the target tight oil reservoir, and configure crude oil simulation oil based on the crude oil composition data;

[0009] Preparation of water-saturated carbon dioxide under the temperature and pressure conditions of the target tight oil reservoir;

[0010] The amount of water phase precipitation after the reaction of water-saturated carbon dioxide with crude oil was obtained through a simulated mass transfer experiment.

[0011] The amount of water phase precipitation measured in the simulated oil experiment was input into the reservoir numerical model to obtain the incremental seepage resistance of water-saturated carbon dioxide in the pore throat.

[0012] Based on the obtained seepage resistance increment and the porosity, permeability and pore size distribution data of the target reservoir, the formation pressure rise value of water-saturated carbon dioxide injection into the target tight reservoir is calculated, and this formation pressure rise value is taken as the upper limit of the pressure of water-saturated carbon dioxide injection into the target tight reservoir.

[0013] Preferably, the preparation steps of the water-saturated carbon dioxide are as follows:

[0014] By measuring the absorbance values ​​of dissolved 0.1 mol% to 0.5 mol% carbon dioxide under temperature and pressure conditions in the target tight oil reservoir, a standard curve of absorbance-water solubility for the CO2-water system was established.

[0015] Then, an aqueous phase is pulsedly injected into liquid CO2, and the absorption peak area of ​​the OH bond in CO2 is monitored online. The obtained OH bond absorption peak area is compared with the absorbance-water solubility standard curve to calculate the solubility of water in carbon dioxide. After repeated pulsed injections of the aqueous phase into liquid CO2 until the increase in the calculated solubility of water in carbon dioxide is <2%, water-saturated carbon dioxide is prepared.

[0016] Preferably, the apparatus for preparing water-saturated carbon dioxide includes:

[0017] A visual PVT device includes a housing, an exhaust pipe at the top of the housing with an exhaust valve, a pressure probe for monitoring the internal pressure of the housing and an ATR probe for monitoring the water phase signal in carbon dioxide at the top inside the housing, and a micro needle with a hydrophobic surface at the bottom inside the housing.

[0018] The ATR-FTIR online spectral testing platform is connected to the ATR probe via optical fiber and is used to analyze the water phase solubility of CO2 in the shell in real time.

[0019] The first storage unit is filled with carbon dioxide. Its top is connected to the bottom inlet of the casing via a first pipeline. The bottom of the first storage unit is connected to a horizontal flow pump. A first one-way valve is provided on the first pipeline.

[0020] The second storage unit is filled with water, and its top is connected to the inlet of a microneedle via a second pipeline. The bottom of the second storage unit is connected to a microfluidic platform, and a second one-way valve is provided on the second pipeline.

[0021] Preferably, the housing is provided with an observation window and an LED light source alignment window for transmitting LED light source, the observation window being used to capture the state of the aqueous droplet at the tip of the microneedle using a high-resolution camera.

[0022] Preferably, a constant temperature chamber is provided outside the housing, the first storage unit, and the second storage unit.

[0023] Preferably, the steps of the method for preparing water-saturated carbon dioxide using the apparatus are as follows:

[0024] The carbon dioxide filled in the first storage unit is delivered into the shell using a horizontal flow pump until the pressure monitored by the pressure probe reaches the target tight reservoir pressure. The amount of carbon dioxide injected is recorded, and the FTIR spectrum obtained at this time is used as the baseline.

[0025] Aqueous phase was repeatedly injected in a pulsed manner using a microfluidic platform via a microneedle. After each injection, the area of ​​the OH peak inside the shell was monitored using an ATR probe and an ATR-FTIR online spectral testing platform until the signal stabilized. The area of ​​the OH bond absorption peak obtained from the spectrum was compared with the absorbance-water solubility standard curve to calculate the solubility of water in carbon dioxide. After repeated injections, water-saturated carbon dioxide was prepared when the formation of water droplets or water films on the needle no longer decreased or disappeared, or when the increase in the calculated solubility of water in carbon dioxide was less than 2%.

[0026] Preferably, before the aqueous phase is injected multiple times in a pulsed manner using a microfluidic platform via a microneedle, a co-solvent is dissolved in the water of the second storage unit to enhance the precipitation of the aqueous phase. The co-solvent is one or more of a fluorinated surfactant or a short-chain alcohol.

[0027] Preferably, the steps for obtaining the amount of aqueous phase precipitation after the reaction of water-saturated carbon dioxide and crude oil simulated oil through a simulated mass transfer experiment are as follows:

[0028] The first storage unit is filled with simulated oil with the same composition as crude oil, and the second storage unit is filled with water-saturated carbon dioxide. A set volume of simulated oil is injected into the housing using a horizontal flow pump. The exhaust valve is opened, and water-saturated carbon dioxide is injected into the housing through a microfluidic platform using a micro-needle. After the air in the housing is expelled, the exhaust valve is closed. Water-saturated carbon dioxide is injected into the housing until the pressure of water-saturated carbon dioxide in the housing and the second storage unit is the same and reaches the initial pressure for preparing water-saturated carbon dioxide. The amount of water-saturated carbon dioxide injected is recorded. The peak area of ​​OH bonds is continuously monitored using an ATR probe and an ATR-FTIR online spectral testing platform until the peak surface signal of OH bonds stabilizes. At this point, the mass transfer between the simulated oil and water-saturated carbon dioxide reaches equilibrium.

[0029] The remaining water phase saturation in the upper part of the shell was calculated using the test results from the ATR-FTIR online spectral testing platform, and the amount of water phase precipitated after the phase change between water-saturated carbon dioxide and simulated oil mass transfer under this state was obtained.

[0030] Preferably, the simulated oil is a mixture of white oil, gum, and asphaltene added to n-decane. Under the premise of ensuring that the pressure, temperature, amount of simulated oil injected, and amount of water-saturated carbon dioxide are the same in each experiment, the ratio of white oil, gum, and asphaltene added to n-decane is adjusted to form simulated oils with different components. By recording the amount of water precipitated after the simulated oils with different components reach equilibrium with water-saturated carbon dioxide, the influence of crude oil components on the precipitation law of water-saturated carbon dioxide aqueous phase is analyzed.

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

[0032] This invention injects water-saturated carbon dioxide (wsCO2) into tight oil reservoirs. Relying on the mass transfer between the water-saturated carbon dioxide (wsCO2) and the crude oil in the tight oil reservoir, a phase transition occurs in the water-saturated carbon dioxide (wsCO2). This causes some of the carbon dioxide in the water-saturated carbon dioxide (wsCO2) to dissolve into the crude oil. At this point, the aqueous phase in the water-saturated carbon dioxide (wsCO2) becomes supersaturated and precipitates out, increasing the saturation of the aqueous phase in the tight pore throats. This leads to a decrease in the relative permeability of the water-saturated carbon dioxide (wsCO2) and an increase in seepage resistance. For water-wetted pores, the supersaturated aqueous phase will be adsorbed on the pore wall surface, resulting in a reduction in the effective flow area and an increase in seepage resistance. Ultimately, the subsequent water-saturated carbon dioxide (wsCO2) is forced to enter other smaller pores and channels, increasing the swept volume of water-saturated carbon dioxide (wsCO2) in the tight oil reservoir, thereby increasing CO2 storage and oil displacement efficiency.

[0033] This invention also provides a phase transition law research device that can accurately measure the saturated solubility of water in CO2 under specific reservoir temperatures and pressures, as well as the amount of water phase precipitation after interaction with crude oil. This is inherently linked to improving oil displacement efficiency and CO2 storage efficiency. The device utilizes ATR-FTIR online spectroscopy to monitor the water phase solubility in CO2 (OH bond peak area) in real time, combined with visualized needle droplet observation, to precisely guide the preparation of water-saturated carbon dioxide (wsCO2) with optimal water phase saturation. Simultaneously, this phase transition law research device quantifies the amount of water phase precipitation after mass transfer between wsCO2 and crude oil by simulating oil component regulation (colloid / asphaltite ratio), predicting the seepage resistance intensity caused by the adsorption of precipitated water on the water-wetted pore throat wall, forcing subsequent wsCO2 to redirect to unaffected micropores, thereby expanding the CO2 sweep volume and improving oil displacement efficiency. This process allows for the quantifiable control of the wsCO2 "on-demand water precipitation-dynamic plugging-sweep expansion" mechanism, aiming to alleviate problems such as low oil displacement efficiency caused by the heterogeneity of tight oil reservoirs. Furthermore, this phase transition law research device fills the gap in the field of basic parameter research devices for improving reservoir oil displacement efficiency with water-saturated carbon dioxide (wsCO2). The experimental device has a simple structure, reasonable design, convenient installation and operation, reliable performance, and accurate test results, making it highly practical. Through this phase transition law research device, the relationship between water-saturated carbon dioxide (wsCO2) and the precipitated water phase can be obtained, which can be used to guide the preparation and injection process of water-saturated carbon dioxide under specific reservoir temperature and pressure conditions, providing a theoretical basis for improving reservoir oil displacement efficiency and increasing CO2 storage efficiency. Attached Figure Description

[0034] Figure 1 A hypothetical diagram illustrating the mechanism by which mass transfer between crude oil and wsCO2 in a water-wetted pore increases seepage resistance, where (a) represents the process before mass transfer and (b) represents the process after mass transfer.

[0035] Figure 2 This diagram illustrates the mechanism of seepage control through mass transfer between crude oil and wsCO2 in a water-wetted branched pore structure. (a) represents the heterogeneous displacement process where CO2 selectively enters the large pores; (a1) CO2 enters the branched pore structure with pore size differences before entering; (a2) CO2 selectively enters the large pores with lower seepage resistance; (a3) ​​CO2 rushes along the large pores, while crude oil is retained in the small pores. (b) The phase change of wsCO2 leads to a thickening of the water film in the large pores, increasing seepage resistance, causing wsCO2 to redirect and enter the smaller pores; (b1) wsCO2 enters the branched pore structure with pore size differences before entering; (b2) wsCO2 selectively enters the large pores with lower seepage resistance; (b3) after mass transfer occurs between wsCO2 and crude oil.

[0036] Figure 3 A flowchart of the experimental setup for studying the phase transition law of wsCO2 provided by this invention;

[0037] Figure 4 A diagram to visualize the internal structure of the PVT device.

[0038] Explanation of reference numerals in the attached figures

[0039] 1. Visualized PVT device; 2. LED light source; 3. Exhaust valve; 4. Pressure probe; 5. First storage unit; 6. Second storage unit; 7. Microfluidic platform; 8. Advection pump; 9. High-resolution camera; 10. ATR-FTIR online spectral testing platform; 11. Temperature chamber; 101. Miniature needle; 102. ATR probe; 103. Water-saturated carbon dioxide; 104. Simulated oil. Detailed Implementation

[0040] The specific embodiments of the present invention are described in detail below, but it should be understood that the scope of protection of the present invention is not limited to the specific embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention. Unless otherwise specified, the experimental methods described in the embodiments of the present invention are conventional methods.

[0041] Based on the premise that water-saturated carbon dioxide undergoes a phase transition with crude oil in the reservoir and has a seepage regulation effect, the inventors proposed a device and experimental method for studying the phase transition law of water-saturated carbon dioxide.

[0042] This invention provides a method for improving the CO2 flooding efficiency of tight oil reservoirs, comprising the following steps:

[0043] Water-saturated carbon dioxide is injected into tight oil reservoirs, and the breakthrough time of CO2 is monitored. By adjusting the injection rate, subsequent water-saturated carbon dioxide is forced to be directed towards unaffected micropores.

[0044] Figure 1 A diagram illustrating the mechanism by which mass transfer between crude oil and wsCO2 in water-wetted channels increases seepage resistance. Figure 1 (a) is a diagram of a water-wetted pore structure before mass transfer, and (b) is a diagram of mass transfer between crude oil and wsCO2 in a water-wetted pore structure. The comparison is significant. Figure 1 (a) and Figure 1 (b) indicates that after crude oil and wsCO2 undergo mass transfer in a water-wetted pore, the water film on the pore wall will thicken.

[0045] Figure 2 This diagram illustrates a hypothetical mechanism by which mass transfer between crude oil and wsCO2 occurs in a water-wetted branched channel, resulting in seepage control. Figure 2(a) is a diagram of the heterogeneous displacement process where CO2 selectively enters large pores, where (a1) is the diagram before CO2 enters the branched pores with pore size differences, (a2) is the diagram of CO2 selectively entering the large pores with lower flow resistance, and (a3) ​​is the diagram of CO2 rushing into the small pores along the large pores and the crude oil being retained; (b) is the process where the phase change of wsCO2 causes the water film in the large pores to thicken, the flow resistance to increase, and wsCO2 to turn into the smaller pores, where (b1) is the diagram before wsCO2 enters the branched pores with pore size differences, (b2) is the diagram of wsCO2 selectively entering the large pores with lower flow resistance, and (b3) is the diagram after mass transfer between wsCO2 and crude oil. Figure 2 A comparison of (a1), (a2), and (a3) ​​with (b1), (b2), and (b3) reveals that crude oil and wsCO2 undergo mass transfer in the water-wetted branched channels. The water film on the channel wall of the water-wetted branched channels thickens, increasing the seepage resistance. Subsequently, wsCO2 is diverted into the smaller channels of the water-wetted branched channels, where the crude oil is effectively displaced.

[0046] Specifically, when the CO2 breakthrough time is earlier than the expected breakthrough time, the pressure value of water-saturated carbon dioxide injected into the target tight oil reservoir is adjusted by reducing the injection rate. At this time, the pressure value of water-saturated carbon dioxide injected into the target tight oil reservoir is always lower than the upper pressure limit. The steps to obtain the upper pressure limit are as follows:

[0047] Acquire temperature, pressure, and crude oil composition data of the target tight oil reservoir, and configure crude oil simulation oil based on the crude oil composition data;

[0048] Preparation of water-saturated carbon dioxide under the temperature and pressure conditions of the target tight oil reservoir;

[0049] The amount of water phase precipitation after the reaction of water-saturated carbon dioxide with crude oil was obtained through a simulated mass transfer experiment.

[0050] The amount of water phase precipitation measured in the simulated oil experiment was input into the reservoir numerical model to obtain the incremental seepage resistance of water-saturated carbon dioxide in the pore throat.

[0051] Based on the obtained seepage resistance increment and the porosity, permeability and pore size distribution data of the target reservoir, the formation pressure rise value of water-saturated carbon dioxide injection into the target tight reservoir is calculated, and this formation pressure rise value is taken as the upper limit of the pressure of water-saturated carbon dioxide injection into the target tight reservoir.

[0052] Specifically, the preparation steps of the water-saturated carbon dioxide are as follows:

[0053] By measuring the absorbance values ​​of dissolved 0.1 mol% to 0.5 mol% carbon dioxide under temperature and pressure conditions in the target tight oil reservoir, a standard curve of absorbance-water solubility for the CO2-water system was established.

[0054] Then, an aqueous phase is pulsedly injected into liquid CO2, and the absorption peak area of ​​the OH bond in CO2 is monitored online. The obtained OH bond absorption peak area is compared with the absorbance-water solubility standard curve to calculate the solubility of water in carbon dioxide. After repeated pulsed injections of the aqueous phase into liquid CO2 until the increase in the calculated solubility of water in carbon dioxide is <2%, water-saturated carbon dioxide is prepared.

[0055] like Figures 3-4 As shown, the apparatus for preparing water-saturated carbon dioxide includes:

[0056] The visual PVT device 1 includes a housing, an exhaust pipe at the top of the housing, an exhaust valve 3 on the exhaust pipe, a pressure probe 4 for monitoring the internal pressure of the housing and an ATR probe 102 for monitoring the water phase signal in carbon dioxide at the top inside the housing, and a micro needle 101 with a hydrophobic surface treatment at the bottom inside the housing.

[0057] The ATR-FTIR online spectral testing platform 10 is connected to the ATR probe 102 via optical fiber and is used to analyze the water phase solubility in CO2 inside the shell in real time.

[0058] The first storage unit 5 is filled with carbon dioxide. Its top is connected to the bottom inlet of the shell through a first pipeline. The bottom of the first storage unit 5 is connected to the horizontal flow pump 8. A first one-way valve is provided on the first pipeline.

[0059] The second storage unit 6 is filled with water, and its top is connected to the inlet of the microneedle 101 through a second pipeline. The bottom of the second storage unit 6 is connected to the microfluidic platform 7, and a second one-way valve is provided on the second pipeline.

[0060] Specifically, the housing is provided with an observation window and an LED light source alignment window for transmitting LED light source 2. The observation window is used to capture the state of the aqueous droplet at the tip of the micro needle 101 by a high-resolution camera 9.

[0061] Specifically, a constant temperature chamber 11 is also provided outside the housing, the first storage unit 5, and the second storage unit 6.

[0062] Specifically, the steps for preparing water-saturated carbon dioxide using the aforementioned apparatus are as follows:

[0063] A standard curve of absorbance-water solubility for the CO2-water system was established by measuring the absorbance values ​​of 0.1 mol% to 0.5 mol% carbon dioxide dissolved under the temperature and pressure conditions of the target tight oil reservoir.

[0064] Carbon dioxide is filled into the first storage unit 5. The carbon dioxide filled into the first storage unit 5 is sent into the housing by the horizontal flow pump 8 until the pressure monitored by the pressure probe 4 reaches the target tight reservoir pressure. The amount of carbon dioxide injected is recorded, and the FTIR spectrum obtained at this time is used as the baseline.

[0065] Water is filled into the second storage unit 6, and the aqueous phase is injected multiple times in a pulsed manner through the microfluidic platform 7 using the micro-needle 101. After each injection, the area of ​​the OH peak inside the shell is monitored using the ATR probe 102 and the ATR-FTIR online spectral testing platform 10 until the signal stabilizes. The area of ​​the OH bond absorption peak obtained from the spectrum is compared with the absorbance-water solubility standard curve to calculate the solubility of water in carbon dioxide. After multiple injections, when the formation of water droplets or water film on the needle no longer decreases or disappears, or until the increase in the calculated solubility of water in carbon dioxide is <2%, water-saturated carbon dioxide is prepared.

[0066] Specifically, before the aqueous phase is injected multiple times in a pulsed manner by the microfluidic platform 7 using the microneedle 101, a co-solvent is dissolved in the water of the second storage unit 6 to enhance the precipitation of the aqueous phase. The co-solvent is one or more of a fluorinated surfactant or a short-chain alcohol.

[0067] Specifically, the steps for obtaining the amount of aqueous phase precipitation after the reaction of water-saturated carbon dioxide and simulated crude oil through a simulated mass transfer experiment are as follows:

[0068] Simulated oil 104 with the same composition as crude oil is loaded into the first storage unit 5, and water-saturated carbon dioxide is loaded into the second storage unit 6. A set volume of simulated oil is injected into the housing using a horizontal flow pump 8. The exhaust valve 3 is opened, and water-saturated carbon dioxide is injected into the housing through a micro-needle 101 using a microfluidic platform 7. After the air in the housing is expelled, the exhaust valve 3 is closed. Water-saturated carbon dioxide 103 is injected into the housing until the pressure of water-saturated carbon dioxide in the housing and the second storage unit 6 is the same and reaches the initial pressure for preparing water-saturated carbon dioxide 103. The amount of water-saturated carbon dioxide 103 injected is recorded. The peak area of ​​OH bonds is continuously monitored using an ATR probe 102 and an ATR-FTIR online spectral testing platform 10 until the peak surface signal of OH bonds is stable. At this time, the mass transfer between simulated oil 104 and water-saturated carbon dioxide 103 reaches equilibrium.

[0069] The remaining water phase saturation in the upper part of the shell was calculated using the test results from the ATR-FTIR online spectral testing platform 10, and the amount of water phase precipitation after the mass transfer phase change between water-saturated carbon dioxide 103 and simulated oil 104 under this state was obtained.

[0070] Specifically, simulated oil 104 is a mixture of n-decane with added white oil, gum, and asphaltenes. Under the premise of ensuring that the pressure, temperature, injected amount of simulated oil 104, and water-saturated carbon dioxide content 104 are the same in each experiment, the proportions of white oil, gum, and asphaltenes added to the n-decane are adjusted to form simulated oil 104 with different compositions. The amount of water precipitated after the simulated oil 104 with different compositions reaches equilibrium with water-saturated carbon dioxide 103 is recorded to analyze the influence of crude oil components on the precipitation law of the aqueous phase of water-saturated carbon dioxide.

[0071] The following specific examples further illustrate this:

[0072] Example 1

[0073] The steps for preparing water-saturated carbon dioxide using the phase transition law research device are as follows:

[0074] Step 1: In a PVT apparatus under the temperature and pressure conditions of the target oil reservoir, measure the CO2 absorbance values ​​of three groups of dissolved water phases at proportions of 0.1 mol%, 0.2 mol%, and 0.5 mol%, and plot the "absorbance-water solubility" standard curve.

[0075] Step 2: Inject anhydrous CO2 into the PVT device at this pressure to reach the set reservoir pressure, record the CO2 injection amount, and record the FTIR spectrum at this time as the baseline.

[0076] Step 3: Using a microfluidic platform, inject the aqueous phase into the PVT chamber in a pulsed manner through a needle. Each injection is 0.005 mL, and injections are repeated every 30 minutes. After each injection, monitor the peak area of ​​the OH bonds using ATR-FTIR. The signal stabilizes within 10-20 minutes. The solubility of water in CO2 is determined by the OH bond absorption peak (3400 cm⁻¹) obtained from the spectrum. -1 The concentration was calculated using a standard curve of spectral absorbance versus concentration, based on the area. When the formation of water droplets or films on the microneedle 3 no longer decreases or disappears, or when the increase in water solubility in CO2 after three consecutive injections is less than 2%, it indicates that the dissolution of water in CO2 has reached saturation, and the injection of the water phase is stopped. The water phase solubility at this point is the saturated solubility of the water phase in CO2 under the set conditions.

[0077] Repeat steps 2 and 3 above to systematically study the effects of temperature, pressure, and aqueous phase mineralization on the saturated solubility of water in CO2 by controlling for single factors. Specifically, the pressure was kept constant, the temperature was adjusted, and the underwater saturated solubility was recorded at different temperatures; the temperature was kept constant, the pressure was changed, and the effect of pressure on saturated solubility was analyzed; different concentrations of inorganic salts were added, and the inhibitory effect of mineralization on saturated solubility was measured. A regression equation was established to study the effects of the ternary variables of temperature, pressure, and mineralization on saturated solubility. By comparing the correlation coefficients, the main controlling factors of temperature, pressure, and mineralization on saturation were identified.

[0078] Example 2

[0079] The difference between this embodiment and Embodiment 1 is that one or more co-solvents (fluorinated surfactants or short-chain alcohol co-solvents) are dissolved in the aqueous phase injected into the PVT chamber via pulse injection. By changing the type and concentration of the co-solvent, the influence of its effect on the saturation of the aqueous phase in CO2 is clarified, and the type and amount of co-solvent that best enhances the saturation of the aqueous phase in CO2 are selected, thus providing a guarantee for enhancing the seepage control effect of water-saturated carbon dioxide in the reservoir.

[0080] Example 3

[0081] The steps for studying the water evolution law of water-saturated carbon dioxide using a phase transition law research device are as follows:

[0082] Step 1: Add white oil, gum and asphalt to n-decane to make a simulated oil with specific components (the same as crude oil components), and load it into the first storage unit 5.

[0083] Step 2: Pre-set the temperature of the constant temperature chamber, inject a set volume of simulated oil 104 into the reactor (below the needle outlet), open the exhaust valve 3, and slowly inject water-saturated carbon dioxide 103 into the PVT device through the micro-needle 101 using the microfluidic platform 7. After purging the air from the PVT device, close the exhaust valve; continue injecting water-saturated carbon dioxide until the initial pressure of the water-saturated carbon dioxide initially prepared in the PVT device is the same as that in the second storage unit 6, and record the amount of wsCO2 injected. Continuously monitor using ATR-FTIR until the OH peak signal stabilizes, indicating that the mass transfer between the simulated oil and wsCO2 has reached equilibrium.

[0084] Step 3: Calculate the saturation of the remaining aqueous phase in the upper CO2 of the PVT cavity using the ART-FTIR test results, and obtain the amount of aqueous phase precipitated after the phase change of wsCO2 under this state.

[0085] Repeat steps 1 to 3, ensuring consistent pressure and temperature conditions, and identical amounts of injected simulated oil and wsCO2 in each experiment. Different proportions of white oil, gum, and asphaltenes were incorporated into n-decane to form simulated oils with varying compositions. The influence of crude oil components on the precipitation pattern of the aqueous phase of wsCO2 was analyzed by recording the amount of water released after the simulated oil and wsCO2 reached equilibrium.

[0086] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, if these modifications and variations fall within the scope of the claims of this invention and their equivalents, this invention also intends to include these modifications and variations.

Claims

1. A method for improving the oil displacement efficiency of tight oil reservoirs, characterized in that, Includes the following steps: Water-saturated carbon dioxide is injected into tight oil reservoirs, and the breakthrough time of CO2 is monitored. By adjusting the injection rate, subsequent water-saturated carbon dioxide is forced to be directed to unaffected micropores. When the CO2 breakthrough time is earlier than the expected breakthrough time, the pressure value of water-saturated carbon dioxide injected into the target tight oil reservoir is adjusted by reducing the injection rate. In this case, the pressure value of water-saturated carbon dioxide injected into the target tight oil reservoir is always lower than the upper pressure limit. The steps to obtain the upper pressure limit are as follows: Acquire temperature, pressure, and crude oil composition data of the target tight oil reservoir, and configure crude oil simulation oil based on the crude oil composition data; Preparation of water-saturated carbon dioxide under the temperature and pressure conditions of the target tight oil reservoir; The amount of water phase precipitation after the reaction of water-saturated carbon dioxide with crude oil was obtained through a simulated mass transfer experiment. The amount of water phase precipitation measured in the simulated oil experiment was input into the reservoir numerical model to obtain the incremental seepage resistance of water-saturated carbon dioxide in the pore throat. Based on the obtained seepage resistance increment and the porosity, permeability and pore size distribution data of the target reservoir, the formation pressure rise value of water-saturated carbon dioxide injection into the target tight reservoir is calculated, and this formation pressure rise value is taken as the upper limit of the pressure of water-saturated carbon dioxide injection into the target tight reservoir.

2. The method for improving oil displacement efficiency in tight oil reservoirs according to claim 1, characterized in that, The steps for preparing the water-saturated carbon dioxide are as follows: By measuring the absorbance values ​​of dissolved 0.1 mol% to 0.5 mol% carbon dioxide under temperature and pressure conditions in the target tight oil reservoir, a standard curve of absorbance-water solubility for the CO2-water system was established. Then, an aqueous phase is pulsedly injected into liquid CO2, and the absorption peak area of ​​the OH bond in CO2 is monitored online. The obtained OH bond absorption peak area is compared with the absorbance-water solubility standard curve to calculate the solubility of water in carbon dioxide. After repeated pulsed injections of the aqueous phase into liquid CO2 until the increase in the calculated solubility of water in carbon dioxide is <2%, water-saturated carbon dioxide is prepared.

3. The method for improving oil displacement efficiency in tight oil reservoirs according to claim 2, characterized in that, The apparatus for preparing water-saturated carbon dioxide includes: A visual PVT device includes a housing, an exhaust pipe at the top of the housing with an exhaust valve, a pressure probe for monitoring the internal pressure of the housing and an ATR probe for monitoring the water phase signal in carbon dioxide at the top inside the housing, and a micro needle at the bottom inside the housing along the vertical direction. The ATR-FTIR online spectral testing platform is connected to the ATR probe via optical fiber and is used to analyze the water phase solubility of CO2 in the shell in real time. The first storage unit is filled with carbon dioxide. Its top is connected to the bottom inlet of the casing via a first pipeline. The bottom of the first storage unit is connected to a horizontal flow pump. A first one-way valve is provided on the first pipeline. The second storage unit is filled with water, and its top is connected to the inlet of a microneedle via a second pipeline. The bottom of the second storage unit is connected to a microfluidic platform, and a second one-way valve is provided on the second pipeline.

4. The method for improving oil displacement efficiency in tight oil reservoirs according to claim 3, characterized in that, The housing is provided with an observation window and an LED light source alignment window for transmitting LED light source. The observation window is used to capture the state of the aqueous droplet at the tip of the micro needle using a high-resolution camera.

5. The method for improving oil displacement efficiency in tight oil reservoirs according to claim 3, characterized in that, The outer casing, the first storage unit, and the second storage unit are also equipped with a constant temperature chamber.

6. The method for improving oil displacement efficiency in tight oil reservoirs according to claim 3, characterized in that, The steps for preparing water-saturated carbon dioxide using the aforementioned apparatus are as follows: The carbon dioxide filled in the first storage unit is delivered into the shell using a horizontal flow pump until the pressure monitored by the pressure probe reaches the target tight reservoir pressure. The amount of carbon dioxide injected is recorded, and the FTIR spectrum obtained at this time is used as the baseline. Aqueous phase was injected multiple times in a pulsed manner using a microfluidic platform via a microneedle. After each injection, the area of ​​the OH peak inside the shell was monitored using an ATR probe and an ATR-FTIR online spectral testing platform until the signal stabilized. The solubility of water in carbon dioxide was calculated by comparing the OH bond absorption peak area obtained from the spectrum with the absorbance-water solubility standard curve. After repeated injections, water-saturated carbon dioxide is prepared when the formation of water droplets or films on the needle tip no longer decreases or disappears, or until the calculated increase in the solubility of water in carbon dioxide is less than 2%.

7. The method for improving oil displacement efficiency in tight oil reservoirs according to claim 6, characterized in that, Before injecting the aqueous phase into the second storage unit in a pulsed manner using a microfluidic platform, a co-solvent is dissolved in the water to enhance the precipitation of the aqueous phase. The co-solvent is one or more of a fluorinated surfactant or a short-chain alcohol.

8. The method for improving oil displacement efficiency in tight oil reservoirs according to claim 3, characterized in that, The steps for obtaining the amount of aqueous phase precipitation after the reaction of water-saturated carbon dioxide with simulated crude oil through a simulated mass transfer experiment are as follows: The first storage unit is filled with simulated oil with the same composition as crude oil, and the second storage unit is filled with water-saturated carbon dioxide. A set volume of simulated oil is injected into the housing using a horizontal flow pump. The exhaust valve is opened, and water-saturated carbon dioxide is injected into the housing through a microfluidic platform using a micro-needle. After the air in the housing is expelled, the exhaust valve is closed. Water-saturated carbon dioxide is injected into the housing until the pressure of water-saturated carbon dioxide in the housing and the second storage unit is the same and reaches the initial pressure for preparing water-saturated carbon dioxide. The amount of water-saturated carbon dioxide injected is recorded. The peak area of ​​OH bonds is continuously monitored using an ATR probe and an ATR-FTIR online spectral testing platform until the peak surface signal of OH bonds stabilizes. At this point, the mass transfer between the simulated oil and water-saturated carbon dioxide reaches equilibrium. The remaining water phase saturation in the upper part of the shell was calculated using the test results from the ATR-FTIR online spectral testing platform, and the amount of water phase precipitated after the phase change between water-saturated carbon dioxide and simulated oil mass transfer was obtained.

9. The method for improving the oil displacement efficiency of tight oil reservoirs according to claim 1 or 8, characterized in that, The simulated oil is made by mixing n-decane with white oil, gum, and asphaltene. Under the premise of ensuring that the pressure, temperature, injected simulated oil, and water-saturated carbon dioxide are the same in each experiment, the proportion of white oil, gum, and asphaltene mixed into n-decane is adjusted to form simulated oils with different components. By recording the amount of water precipitated after the simulated oils with different components reach equilibrium with water-saturated carbon dioxide, the influence of crude oil components on the precipitation law of water-saturated carbon dioxide aqueous phase is analyzed.

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