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Article

Enhanced Oil Recovery Mechanism Mediated by Reduced Miscibility Pressure Using Hydrocarbon-Degrading Bacteria During CO2 Flooding in Tight Oil Reservoirs

by
Chengjun Wang
1,2,*,
Xinxin Li
1,
Juan Xia
1,
Jun Ni
3,
Weibo Wang
3,
Ge Jin
1 and
Kai Cui
4,*
1
School of Chemistry and Chemical Engineering, Xi’an Shiyou University, Xi’an 710065, China
2
Shaanxi Key Laboratory of Carbon Dioxide Sequestration and Enhanced Oil Recovery, Xi’an 710075, China
3
Research Institute of Shaanxi Yanchang Petroleum (Group) Co., Ltd., Xi’an 710075, China
4
School of Chemical Engineering and Technology, Xi’an Jiaotong University, Xi’an 710049, China
*
Authors to whom correspondence should be addressed.
Energies 2025, 18(5), 1123; https://doi.org/10.3390/en18051123
Submission received: 11 January 2025 / Revised: 15 February 2025 / Accepted: 19 February 2025 / Published: 25 February 2025
(This article belongs to the Special Issue Sustainable Energy Solutions Through Microbial Enhanced Oil Recovery)
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Abstract

:
CO2 flooding technology for tight oil reservoirs not only effectively addresses the challenge of low recovery rates, but also facilitates geological CO2 sequestration, thereby achieving the dual objective of enhanced CO2 utilization and secure storage. However, in the development of continental sedimentary tight oil reservoirs, the high content of heavy hydrocarbons in crude oil leads to an elevated minimum miscibility pressure (MMP) between crude oil and CO2, thereby limiting the process to non-miscible flooding. Conventional physical and chemical methods, although effective in reducing MMP, are often associated with high costs, environmental concerns, and limited efficacy. To address these challenges, we propose a novel approach utilizing petroleum hydrocarbon-degrading bacteria (PHDB) to biodegrade heavy hydrocarbons in crude oil. This method alters the composition of crude oil, thereby lowering the MMP during CO2 flooding, facilitating the transition from non-miscible to miscible flooding, and enhancing oil recovery. Results demonstrated that, after 7 days of cultivation, the selected PHDB achieved a degradation efficiency of 56.4% in crude oil, significantly reducing the heavy hydrocarbon content. The relative content of light-saturated hydrocarbons increased by 15.6%, and the carbon atom molar percentage in crude oil decreased from C8 to C6. Following the biodegradation process, the MMP of the lightened crude oil was reduced by 20.9%. Core flood experiments indicated that CO2 flooding enhanced by PHDB improved oil recovery by 17.7% compared to conventional CO2 flooding. This research provides a novel technical approach for the green and cost-effective development of tight oil reservoirs with CO2 immiscible flooding.

1. Introduction

Tight oil reservoirs represent one of the world’s critical hydrocarbon resources, characterized by challenges such as difficult matrix mobilization, pronounced reservoir heterogeneity, fracture-water breakthrough, and low recovery rates [1,2]. As conventional oil fields enter their middle to late stages of development, the exploration of new oil and gas fields has become increasingly challenging. Consequently, tight oil reservoirs have emerged as a vital alternative resource for China’s petroleum industry [3]. Therefore, achieving efficient, environmentally friendly, and safe exploitation of tight oil and gas reservoirs has become a key research focus for both the scientific community and the petroleum industry [4]. Given that CO2 possesses characteristics such as enhancing crude oil expansion, reducing the interfacial tension between oil and water [5], lowering crude oil viscosity, and improving oil displacement efficiency [6], CO2 flooding technology has emerged as an effective method to improve recovery rates in these reservoirs.
The CO2 flooding process can be categorized into miscible and immiscible flooding. Under miscible conditions, CO2 and crude oil can mix in any proportion, significantly reducing the oil–water interfacial tension (to as low as 0.1–1.0 mN/m), and thereby achieving higher oil displacement efficiency [7]. Chen et al. [8] have demonstrated that CO2 miscible flooding is more effective in displacing residual oil from blind-end pores. However, during the miscible flooding process, the solubility of CO2 in pore-scale crude oil and its extraction effect on light hydrocarbon components are significantly higher than in immiscible flooding [9]. This leads to increased asphaltene precipitation from crude oil, causing substantial reservoir damage and a marked reduction in permeability, which poses significant challenges for enhancing development outcomes in subsequent operations [10,11,12].
Currently, the majority of tight oil reservoirs in China are continental sedimentary formations characterized by a relatively high content of heavy components in crude oil [13]. This results in an elevated MMP between crude oil and carbon dioxide, limiting the current practice to non-miscible CO2 flooding, which adversely affects the efficiency of CO2-enhanced oil recovery (CO2-EOR) [14]. Previous studies have demonstrated that the primary factors influencing MMP include crude oil composition (such as the molecular weight of C5+, volatile components, intermediate components, etc.), reservoir temperature, and the composition of injected gas [15]. Rathmell et al. found that MMP is closely associated with the volatile and intermediate components in crude oil [16]. Alston et al. [17] also reported that significant variations in the content of these components can substantially influence MMP.
The author’s previous research indicates that using MMP as the objective function, studying its sensitivity to the mole fractions of volatile and intermediate components, reservoir temperature, and MWC5+, reveals that MWC5+ exhibits the highest sensitivity in light crude oil. MMP increases with higher levels of volatile components, MWC5+, and reservoir temperature, while it decreases with increasing intermediate component content [18]. Consequently, a higher proportion of light hydrocarbons in crude oil facilitates easier miscibility between CO2 and crude oil, indicating that the extent of mass transfer between CO2 and crude oil components directly impacts miscibility.
In recent years, extensive research has been conducted both domestically and internationally to address the issue of high MMP [19,20]. Various methods have been explored, including the miscible solvent method [21], supercritical CO2 microemulsion technique [22], gas-soluble surfactant approach [23], and chemical agent treatment [23]. However, each method has its limitations, and the miscible solvent method requires substantial amounts of hydrocarbon gas, leading to high operational costs [21]. While the supercritical CO2 microemulsion technique can reduce MMP to some extent, the reduction is limited [22]. For conventional CO2 flooding, MMP typically ranges from 20 to 35 MPa. In light oil reservoirs, the MMP of supercritical CO2 microemulsion can be reduced to 2 to 5 MPa, achieving a reduction of 10% to 20%; for heavy oil reservoirs or high-temperature oil reservoirs, the MMP can be reduced to 5 to 10 MPa, with a reduction of 20% to 40%. Gas-soluble surfactants can lower the interfacial tension between oil and gas, thereby reducing MMP, but they are not environmentally friendly and have limited effectiveness [23]. The chemical agent method faces challenges such as high reagent costs, limited efficacy, and potential for secondary environmental pollution and damage to the reservoir structure [23]. Therefore, developing green and cost-effective non-miscible CO2 flooding to reduce MMP is a critical issue that must be urgently addressed for advancements in integrated CO2-EOR and storage technology in tight oil reservoirs.
As natural biogeochemical reactors, oil reservoirs host diverse microbial communities with distinct functional roles [24]. Particularly, petroleum hydrocarbon-degrading bacteria (PHDB), which utilize petroleum hydrocarbons as their sole carbon source, play a pivotal role in the initial stages of microbial-enhanced oil recovery [25]. Research has demonstrated that PHDB, including Pseudomonas and Bacillus species, along with denitrifying bacteria, are prevalent in oil reservoir environments and have been shown to significantly enhance oil recovery rates [26,27]. These microorganisms can effectively degrade n-alkanes and cycloalkanes of varying carbon chain lengths in the absence of alternative carbon sources [28]. Crude oil, a complex mixture comprising thousands of chemical compounds such as n-alkanes, cycloalkanes, monoaromatic hydrocarbons, polycyclic aromatic hydrocarbons, and heterocyclic compounds, serves as a carbon source for various types of microorganisms during biodegradation [29]. During this process, crude oil is degraded through the synergistic action of microorganisms and their metabolic byproducts, leading to increased sweep efficiency and enhanced oil recovery [30]. While numerous studies have examined the relationship between PHDB and crude oil biodegradation in the context of improved oil recovery [31,32], few works have explored how PHDB influences crude oil properties during CO2 flooding, thereby altering the miscibility pressure between crude oil and CO2 and impacting the effectiveness of CO2 flooding for enhanced oil recovery.
In this study, we utilized highly efficient PHDB isolated from water produced via CO2 flooding in reservoirs to systematically investigate their degradation efficiency for crude oil across various time scales. Based on these findings, we evaluated the impact characteristics of PHDB on the MMP between CO2 and crude oil, and analyzed changes in crude oil composition during the MMP reduction process mediated by PHDB. Finally, through core physical simulation experiments, we assessed the mechanisms and efficacy of PHDB in enhancing oil recovery by reducing MMP during CO2 flooding. This study demonstrated that PHDB in oil reservoirs can effectively reduce the MMP of CO2 non-miscible flooding by degrading the heavy components of crude oil, thereby enhancing oil recovery. Building on the utilization of microbial resources and the modification of crude oil properties, this research proposes a green, efficient, and safe technology that employs petroleum hydrocarbon-degrading functional bacteria to enhance CO2 flooding for improved oil recovery. The objective is to address the low efficiency of CO2 non-miscible flooding in tight oil reservoirs in China.

2. Materials and Methods

2.1. Materials

The PHDB strain utilized in this study was isolated from water produced by the CO2 flooding reservoir at Huaziping, Xingzi Valley, Yanchang, China (Yan’an, China). The properties of the produced water are detailed in the Supplementary Materials (Table S1). Based on 16S rRNA gene analysis, the PHDB strain exhibits a sequence similarity of 98–99% to Bacillus subtilis. The CO2 employed in the experiments was procured from Xi’an Tenglong Chemical Co., Ltd. (Xi’an, China), with a purity of 99.9%. The morphological characteristics of the PHDB strain are shown in Figure S1a. Both the crude oil and the PHDB strain were sourced from the same oil well. The crude oil has a density of 0.835 g/cm3 and a viscosity of 14.8 mPa·s at 45 °C. Additionally, the experimental water used for preparing the reaction medium is produced water that has undergone sterilization at 121 °C for 15 min.

2.2. Efficiency of PHDB Degradation of Crude Oil

The activation of PHDB was conducted using LB medium (peptone 10.0 g/L, yeast extract 5.0 g/L, and NaCl 10.0 g/L) for 40 h. The activation duration was determined based on the growth activity of the PHDB strain, as measured by the OD600 value (Figure S1b). The reaction medium used in the experiment was an improved inorganic salt medium, composed of KH2PO4 2.0 g/L, Na2HPO4 1.5 g/L, NH4Cl 2.0 g/L, MgSO4·7H2O 0.5 g/L, NaNO3 4.0 g/L, and 1.0 mL of trace element solution [24]. The composition of the trace elements is provided in Table S2. Based on our previous studies [24,33] and the degradation efficiency of crude oil by different inoculation concentrations of PHDB, the optimal inoculation concentration of activated PHDB in the fresh medium was determined to be 7%. To ensure the accuracy of the experimental results, the culture medium and environment for the PHDB were kept consistent across all batches of experiments. The test results of the degradation efficiency of crude oil by PHDB at different inoculation concentrations are shown in Table S3. Additionally, the experimental conditions were designed to simulate the reservoir environment for CO2 flooding at Huaziping, with a temperature of 45 °C, pressure of 20 MPa, and pH of 7.5.
To determine the optimal degradation efficiency of PHDB, the experiment evaluated the crude oil degradation rate under different time conditions. To minimize sampling and testing errors, five parallel experiments were conducted, involving one control group and four treatment groups that performed degradation reactions on days 3, 5, 7, and 10. The experimental conditions for the control group were identical to those of the treatment groups, except that no petroleum hydrocarbon-degrading bacterial liquid was added to the control group.
The experimental procedure was as follows: 279 mL of fresh culture medium and 21 mL of activated bacterial suspension were added to a 500 mL high-temperature and high-pressure reactor, along with 50 mL of crude oil. CO2 was introduced into the reactor to simulate the CO2 flooding reservoir environment. The reactor temperature was set to 45 °C, pressure to 20 MPa, and magnetic stirring speed to 150 rpm, after which the experiment commenced. At the designated time points, the reaction was halted, and crude oil samples were collected for degradation rate analysis. To ensure reliable data, all treatments were conducted in duplicate. The experimental schematic diagram and equipment layout are detailed in Figure S2 of the Supplementary Materials.

2.3. Impact of PHDB on MMP

Under the optimal time point conditions for PHDB degradation of crude oil, the variation characteristics of the MMP between crude oil and CO2 after PHDB action were investigated. The experimental procedures for the interaction between PHDB and crude oil are consistent with those described in Section 2.2. Following the experiment, the crude oil from the reactor was collected for miscibility pressure testing and compositional analysis. The MMP of CO2 and crude oil was determined according to SY/T 6573-2016 “Experimental Determination Method of Minimum Miscible Pressure-Capillary Method” [34]. During the MMP test, the volume of produced oil was recorded to calculate the CO2 oil displacement efficiency. The experimental conditions were set at a temperature of 45 °C and pressures ranging from 2 to 20 MPa. Through the analysis of the linear relationship between CO2 oil displacement efficiency and pressure, the MMP between CO2 and crude oil was determined.

2.4. Core Displacement Tests to Evaluate the EOR

The core samples used in this study were artificial sandstone cores, primarily composed of quartz sand minerals. The physical properties of the experimental cores are summarized in Table S3. Prior to the core displacement tests, cores #1, #2, and #3 were saturated with sterilized crude oil to simulate reservoir conditions. To replicate the characteristics of Huaziping’s tight oil reservoir, the oil saturation levels of the aged core plugs were maintained at approximately 50–60%. In the CO2 flooding experiment, core #1 served as the control group and underwent only microbial-enhanced oil recovery using 7.0% PHDB. Meanwhile, core #2 was subjected to CO2 flooding without PHDB, while core #3 underwent microbial composite CO2 flooding with 7.0% PHDB for oil recovery. The temperature and pressure were controlled at 45 °C and 20 MPa, respectively. The detailed experimental design is outlined in Table 1. The schematic diagram of the experimental setup and process for core displacement tests is provided in Figure S3 of the Supplementary Materials.
Water displacement fluid: The produced water was subjected to sterilization to eliminate microorganisms and subsequently utilized as the water displacement fluid.
PHDB displacement fluids: A total of 5.0 mL of the activated PHDB was inoculated into 95 mL of the screening medium for enrichment at 45 °C. When the concentration of PHDB reached 105–106 cells/mL, the bacteria solution was incorporated into sterilized, produced water at a ratio of 7.0% (v/v) to formulate the PHDB displacement fluids.
The specific operation steps of the core displacement test are as follows: (1) The aging core #1 was inserted into the primary waterflooding experimental group with an oil displacement rate set at 0.25 mL/min. Injection continued for 4 to 5 h until the oil production became negligible. The volumes of produced oil and water were measured using an oil–water separation meter, with measurement intervals of 0.1 pore volume (PV). Additionally, aged cores #2 and #3 were, respectively, placed into the CO2 flooding experimental group, following the same operational procedures as those in the microbial flooding experiment. (2) PHDB flooding fluid was injected into cores #1 and #3 using the alternating slug method, with an injection volume of 0.5 PV throughout, for 0.5 to 1.0 h. For the CO2 flooding control group (core #2), the flooding fluid consisted of sterilized produced water. It should be particularly noted that, unlike conventional chemical displacement fluids, an excessive injection volume of microbial displacement fluid may lead to blockage of the core’s tight pores. Furthermore, based on the core’s pore volume, injecting a 0.5 PV microbial slug is not low, as validated by our previous research [33]. (3) The ends of the core holder were closed and then incubated at 45 °C for 7 days. (4) When the incubation ended, waterflooding (for core #1) or CO2 flooding (for cores #2 and #3) resumed immediately until the cumulative injection volume reached 2.0 PV. The experiment was then terminated. After the displacement tests concluded, the volumes of produced oil and water were measured using a calibrated oil–water separator. The oil recovery efficiency (η1, %) was calculated by the following formulas:
η 1 , % = V 1 V 0 × 100 %
where V1 (mL) was the cumulative oil output after displacement; V0 (mL) was the saturated oil in the core before displacement.

2.5. Analysis Methods and Calculations

PHDB concentration measurement: Take 10 mL of the working fluid sample and measure its optical density at a wavelength of 600 nm using a UV-Vis spectrophotometer (Model 6B-3000A, Jiangsu Shenggao Hua Environmental Protection Technology Co., Ltd., Changzhou, China).
Analysis of crude oil composition: To characterize the biodegradation features of crude oil by PHDB in experiments aimed at reducing miscible pressure, residual oil extracted from the samples using a mixture of n-hexane and dichloromethane (1:1, v/v) was analyzed by gas chromatography–mass spectrometry (GC-MS) following the procedure described by Wang et al. [35]. The analysis was conducted on an Agilent 7890-5975C (Agilent Technologies Inc., Palo Alto, CA, USA) gas chromatograph equipped with an elastic quartz capillary column (60 m × 0.25 mm × 0.25 μm) coated with HP-5MS (modified 5% phenyl 95% methyl polysiloxane). Helium (99.99%) served as the carrier gas. The column temperature program involved holding the initial temperature at 50 °C for 1 min, followed by heating at 20 °C/min to 120 °C, then at 4 °C/min to 250 °C, and finally at 3 °C/min to 310 °C, with the final temperature held constant for 30 min. D50-nC30 was used as the internal standard for GC-MS analysis of alkanes to estimate the extent of crude oil biodegradation.
Surface tension measurement: Surface tension was measured using a JK99B tensiometer (Zhongchen, Shanghai, China). The working fluid was centrifuged at 8000 rpm for 10 min, and then the surface tension of the supernatant was determined using the tensiometer at room temperature.
Interfacial tension measurement: Interfacial tension was measured by Gemini HRnano rotating drop interfacial tensiometer (Malvern Instruments, Malvern, UK). The working fluid was centrifuged at 8000 rpm for 10 min, and then the interfacial tension of the supernatant was determined using the interfacial tensiometer at room temperature.
Degradation rate of crude oil: The crude oil after biodegradation was separated by repetitive extraction using an equal volume of chloroform. Subsequently, the solvent was evaporated under controlled conditions, and the residual crude oil was carefully weighed [34]. The degradation rate (η2, %) of crude oil was calculated by the following equation:
η 2 , % = m 0 − m 1 m 0 × 100 %
where m0 (g) was the initial weight of crude oil; m1 (g) was the final weight of crude oil.

3. Results and Discussion

3.1. Efficiency of Crude Oil Degradation by PHDB

Petroleum hydrocarbon-degrading bacteria can reduce the MMP between crude oil and CO2 by altering the chemical properties of crude oil, particularly by lightening the heavy components [18]. Consequently, the biodegradation efficiency of crude oil is a critical factor influencing the MMP in the CO2 flooding process. As illustrated in Figure 1, the concentration of PHDB in the working fluid gradually increased with prolonged reaction time between microorganisms and crude oil, leading to improved bio-degradation efficiency. On the 7th day, the OD600 value of PHDB reached 1.45, and the biodegradation efficiency of crude oil approached its maximum value, approximately 50.4% higher than that of the control group. This indicates that PHDB can effectively reproduce using crude oil as a growth carbon source in a CO2 environment, achieving over 55.8% degradation efficiency within 7 days. According to a study by Li et al. [36], when crude oil biodegradation efficiency exceeds 50%, microorganisms have already utilized heavy components in crude oil (such as aromatic compounds, resins, and asphaltenes) as growth carbon sources. Furthermore, extending the reaction time to the 10th day resulted in negligible changes in crude oil degradation efficiency (56.4%). Therefore, for subsequent evaluation experiments on the influence of PHDB on MMP, 7 days was selected as the optimal reaction time point.

3.2. Evaluation of MMP Reduction by PHDB

Under optimal reaction time conditions, we evaluated the MMP between crude oil and CO2 before and after treatment with PHDB (Figure 2). According to the principle of MMP testing for crude oil and CO2, the crude oil recovery rate exhibits a linear increase with rising reaction pressure. Upon reaching the MMP, a turning point occurs, after which the recovery rate increases linearly again as pressure continues to rise [37]. In Figure 2a, when the reaction pressure rises to 14.65 MPa, the MMP value for undegraded crude oil and CO2 is observed. In contrast, for crude oil degraded by PHDB, the MMP was achieved at a lower reaction pressure of 11.58 MPa (Figure 2b). Experimental results demonstrate that the degradation of heavy components in crude oil (see Figure 3) by PHDB leads to a significant reduction in MMP by 20.9%. This finding strongly indicates that activating indigenous PHDB in tight oil reservoirs can effectively reduce the miscibility pressure in non-miscible zones during CO2 flooding, thereby enhancing the overall recovery rate. Furthermore, as shown in Figure 1, with the increased in the concentration of PHDB, the biodegradation efficiency of crude oil gradually improved, indicating a negative correlation between the concentration of PHDB and the MMP value.

3.3. Change in Crude Oil Composition by PHDB

The physical properties of crude oil are primarily governed by its four basic components: saturated hydrocarbons, aromatics, resins, and asphaltenes [38]. The degradation characteristics and efficiency of these components by PHDB in oil reservoirs play a crucial role in reducing the MMP between crude oil and CO2 during CO2 flooding. In Figure 3a, after PHDB treatment, the aromatic and resin components in crude oil exhibit significant reductions, while the asphaltene content also decreases. This indicates that the selected PHDB can effectively degrade heavy components in crude oil, with their relative mass contents decreasing by 2.7%, 3.8%, and 0.97%, respectively. Concurrently, the relative mass content of saturated hydrocarbons increases by 8.4%, suggesting that PHDB generates light-saturated hydrocarbons during the degradation process, leading to an increase of 15.6% in light hydrocarbon content. Typically, oil reservoir microorganisms preferentially utilize light hydrocarbons when using crude oil as a carbon source for growth [34]. The observed increase in light hydrocarbons in this experiment further confirms the ability of PHDB to convert heavy components into light-saturated hydrocarbons. This transformation facilitates the reduction in the MMP between crude oil and CO2.
The hydrocarbons in the saturated hydrocarbon fraction of crude oil primarily consist of normal alkanes, branched alkanes, and cycloalkanes [38]. Among these, normal alkanes are particularly susceptible to degradation by PHDB. In Figure 3b, normal alkanes with carbon numbers ranging from C11 to C35 constitute 71.78% of the total saturated hydrocarbon fraction in crude oil, with the highest relative abundance being observed for those with carbon numbers between C16 and C30. Compared to the control group, after PHDB treatment, the relative content of normal alkanes significantly decreased to 64.71%. This indicates that during the initial growth phase of microorganisms, PHDB preferentially utilizes saturated hydrocarbons as a carbon source, leading to their effective degradation.
Additionally, cycloalkanes and their homologs also occupy a significant proportion within the saturated hydrocarbon fraction, but exhibit greater resistance to degradation compared to normal alkanes [39]. After PHDB treatment, the relative content of cycloalkane compounds decreased (Figure 3c), indicating that these substances were also biodegraded and transformed. Aromatic hydrocarbons are crucial components of crude oil, and their composition is closely related to the physical properties of crude oil. In Figure 3d, microbial treatment resulted in changes in the relative abundance of benzene compounds, exhibiting a similar trend to the degradation of saturated hydrocarbons, where components with fewer or no branches were preferentially degraded. Specifically, compounds such as 3-methylbiphenyl, 3′,3′-dimethylbiphenyl, and 2-methyl-dibenzofuran showed significant degradation. In summary, the changes in the relative content of different components in crude oil before and after biodegradation demonstrate that PHDB alters the physical properties of crude oil by degrading heavy components and generating lighter ones. This process reduces the MMP between crude oil and CO2, thereby enhancing the efficiency of miscible flooding.
By comparing the changes in the molar percentage of carbon atoms in crude oil before and after PHDB treatment, the impact of PHDB on crude oil lightening can be more clearly demonstrated (Figure 4). Before PHDB treatment, the main peak of the molar percentage of carbon atoms in crude oil was at C8. Following PHDB action, this peak shifted to C6, with a significant increase in peak intensity. This indicates that PHDB action increases the proportion of lower-carbon-number components in crude oil, promoting its lightening. Consequently, this leads to a reduction in crude oil viscosity (from 14.8 mPa·s to 13.6 mPa·s) and density (from 0.835 g/cm3 to 0.806 g/cm3), enhancing the solubility and diffusivity of CO2 in crude oil [18], and thereby improving enhanced oil recovery through CO2 non-miscible flooding.

3.4. EOR by PHDB-Enhanced CO2 Flooding

Core flooding experiments were conducted to evaluate the oil recovery ability of PHDB-enhanced CO2 flooding. The EOR results are shown in Figure 5; during the primary CO2 flooding (cores #1 and #3) or water flooding (core #2) stage, the crude oil displacement efficiency in the rock cores was approximately 25–26%. Subsequently, different displacement media were injected into rock core #1 (7% PHDB), core #2 (without PHDB), and core #3 (7% PHDB). As the amount of PV injected increased, both PHDB flooding and PHDB-enhanced CO2 flooding demonstrated significantly higher crude oil displacement efficiencies compared to the control group using CO2 flooding alone. The similar displacement efficiencies observed for PHDB flooding and PHDB-enhanced CO2 flooding can be attributed to the insufficient time available for PHDB to degrade heavy components in the crude oil, thereby reducing the MMP between crude oil and CO2. Therefore, following the injection of displacement media, a 7-day “shut-in” period was implemented for the rock cores. After this “shut-in” period, a secondary displacement was conducted. At this point, the crude oil displacement efficiencies for CO2 flooding, PHDB flooding, and PHDB-enhanced CO2 flooding were 30.1%, 43.3%, and 47.8%, respectively. Compared with CO2 flooding, PHDB-enhanced CO2 flooding increased the recovery rate by 17.7%, while PHDB flooding increased it by 13.2%. The experimental results indicate that during CO2 flooding, PHDB can alter the physical properties of crude oil, reduce the MMP between CO2 and crude oil, and facilitate the transition from non-miscible to miscible zones, thereby effectively enhancing oil recovery.

3.5. Discussion

Owing to the heterogeneity of reservoir geological structures, water flooding can only recover 20–30% of the oil reserves in tight reservoirs [2]. Supercritical CO2, characterized by its exceptional solubility (with a diffusion coefficient nearly 100 times that of liquids) [40,41], can effectively dissolve into crude oil, altering its physical properties and flow behavior, thereby enhancing the recovery efficiency of residual crude oil in tight reservoirs [42]. However, during actual CO2 flooding operations, both miscible and immiscible regions coexist within the reservoir. In the miscible zone, at specific formation temperatures (57.22 °C) and pressures (17.5 MPa) [43], CO2 exists in a fluid state and fully mixes with crude oil, reducing interfacial tension and capillary pressure to zero. This significantly diminishes the trapping effect on crude oil, thereby improving displacement efficiency [44]. Conversely, as CO2 advances deeper into the reservoir under high-temperature and high-pressure conditions, it primarily extracts lighter components from the crude oil. As the proportion of heavier components increases, an immiscible region gradually forms, leading to reduced CO2 solubility in crude oil and ultimately impacting well recovery rates [45,46].
Given the limitations of conventional physicochemical methods in reducing the MMP between crude oil and CO2, this study explores the approach of lightening heavy components in crude oil. By integrating the biodegradation mechanism of PHDB within reservoirs [36], we investigate the use of these bacteria to transform heavy components into lighter ones through metabolic activities, resulting in a 15.6% increase in light-saturated hydrocarbons. This process alters the physical properties of crude oil during CO2 flooding, enhancing CO2 solubility and facilitating the transition from immiscible to miscible regions. Experimental results demonstrate that the application of PHDB can significantly reduce MMP by 20.9% and improve oil recovery rates by 17.7%, surpassing the performance of conventional CO2 flooding methods. In addition, after PHDB treatment, the surface tension of the working fluid decreased from 68.5 ± 0.5 mN/m to 43.2 ± 1.0 mN/m, and the interfacial tension decreased from 1.8 ± 0.2 mN/m to 0.13 ± 0.05 mN/m. These findings strongly support the feasibility of this technology for future CO2 flooding applications in tight reservoirs.
Furthermore, the technology of using PHDB to reduce the MMP during enhanced CO2 flooding for oil recovery is currently in the laboratory research stage. Future research and field applications face several challenges: (1) The experimental studies achieved a high crude oil biodegradation rate (up to 56.4%) and a significant reduction in MMP under conditions of high microbial biomass in the reaction medium. However, ensuring a high concentration of PHDB in actual reservoir environments remains a significant challenge. (2) Geological conditions in different reservoirs (such as temperature, salinity, and pressure) vary widely, which may limit the survival and effectiveness of PHDB in extreme environments, raising concerns about their adaptability to various reservoir conditions. (3) While the overall cost of this technology is relatively low, the initial investment required for bacterial cultivation, screening, and on-site injection can be substantial. Reducing the costs associated with strain cultivation and maintenance is another critical challenge for the large-scale application of this technology.

4. Conclusions

Currently, conventional physicochemical methods exhibit limited efficacy in reducing the MMP between crude oil and CO2 during CO2 flooding. These methods also present significant challenges, including potential damage to the reservoir’s geological structure, high costs associated with chemical agents, and the risk of secondary environmental pollution. This study proposes an environmentally friendly, efficient, and safe approach, by leveraging reservoir microbial resources, specifically PHDB, to alter the physicochemical properties of crude oil, thereby reducing MMP. Experimental results indicated that the selected PHDB achieved a biodegradation efficiency exceeding 50%. Under the influence of these bacteria, heavy components in crude oil, such as aromatics and asphaltenes, are significantly degraded, while the proportion of light-saturated hydrocarbons increases by 15.6%. The resultant lightening of crude oil components enhances CO2 solubility and diffusion within the oil, leading to a reduction in MMP from an initial value of 14.65 to 11.58 MPa. Furthermore, this reduction in MMP contributes to a 17.77% increase in crude oil recovery rates. Therefore, this study demonstrates the feasibility and potential of enhancing CO2 flooding technology through the application of PHDB, highlighting broad prospects for practical implementation.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/en18051123/s1: Figure S1. Morphological characteristics (a) and growth activity (b) of PHDB. Figure S2. The experimental schematic diagram (a) and equipment layout (b) of the interaction between petroleum hydrocarbon-degrading bacteria and crude oil. Figure S3. Schematic diagram of the core displacement tests. (1) flooding pump; (2) flooding me-dium vessel; (3) crude oil in the vessel; (4) CO2 in the vessel; (5) microbial flooding fluid in the ves-sel; (6) constant-temperature oven; (7) flooding pressure gauge; (8) peripheral pressure gauge; (9) core holder; (10) pressure transducer; (11) exit pressure gauge; (12) measuring cylinder; (13) man-ual metering pump; (14) hydrating device. Table S1. The properties of the produced water. Table S2. Composition of trace element solution. Table S3. The degradation efficiency of crude oil by PHDB at different inoculation concentrations. Table S4. The physical properties of the experimental cores.

Author Contributions

Conceptualization, C.W., W.W. and K.C.; Methodology, X.L., J.N. and G.J.; Formal analysis, X.L.; Investigation, C.W., J.X. and G.J.; Resources, W.W.; Data curation, J.X. and G.J.; Writing—original draft, C.W.; Writing—review & editing, K.C.; Project administration, J.N.; Funding acquisition, J.N. and W.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by National Natural Science Foundation of China (No. 52204043) and Natural Science Foundation of Shaanxi (No. 2023-JC-YB-330). And the APC was funded by Natural Science Foundation of Shaanxi (No. 2023-JC-YB-330).

Data Availability Statement

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (No. 52204043), the Natural Science Foundation of Shaanxi (No. 2023-JC-YB-330), the Open Foundation of Shaanxi Key Laboratory of Carbon Dioxide Sequestration and Enhanced Oil Recovery, and the China Postdoctoral Science Foundation General Project (No. 2024M762609).

Conflicts of Interest

Authors Jun Ni and Weibo Wang were employed by the company Research Institute of Shaanxi Yanchang Petroleum (Group) Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Abbreviations

PHDBPetroleum hydrocarbon-degrading bacteria
MMPMinimum miscibility pressure
EOREnhanced oil recovery
GC-MSGas chromatography-mass spectrometry
PVPore volume

References

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Figure 1. The variation characteristics of the growth concentration of PHDB (a) and their efficiency in biodegrading crude oil (b) under different time point conditions.
Figure 1. The variation characteristics of the growth concentration of PHDB (a) and their efficiency in biodegrading crude oil (b) under different time point conditions.
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Figure 2. Variation characteristics of the MMP of crude oil and CO2 before (a) and after (b) PHDB action.
Figure 2. Variation characteristics of the MMP of crude oil and CO2 before (a) and after (b) PHDB action.
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Figure 3. The characteristics of crude oil composition changed during the pressure drop of the miscible system of crude oil and CO2 mediated by PHDB. (a) Change in crude oil composition; (b) change in normal alkanes; (c) change in cyclohexane, and (d) change in aromatics.
Figure 3. The characteristics of crude oil composition changed during the pressure drop of the miscible system of crude oil and CO2 mediated by PHDB. (a) Change in crude oil composition; (b) change in normal alkanes; (c) change in cyclohexane, and (d) change in aromatics.
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Figure 4. The characteristics of the molar percentage of carbon atoms changed in crude oil before and after the action of PHDB.
Figure 4. The characteristics of the molar percentage of carbon atoms changed in crude oil before and after the action of PHDB.
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Figure 5. Evaluation of EOR using PHDB to reduce the MMP between CO2 and crude oil in non-miscible CO2 flooding.
Figure 5. Evaluation of EOR using PHDB to reduce the MMP between CO2 and crude oil in non-miscible CO2 flooding.
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Table 1. Experimental design of core displacement tests.
Table 1. Experimental design of core displacement tests.
Core NumberPretreatmentInjection MediumDisplacement MediumDisplacement TestsObjectives
#1Crude oilWater flooding
(7% PHDB)		WaterPHDB floodingExperimental control
#2Crude oilWater flooding
(without PHDB)		CO2CO2 floodingCO2 flooding control
#3Crude oilWater flooding
(7% PHDB)		CO2PHDB + CO2 floodingEOR of PHDB enhanced CO2 flooding
Note: To ensure the accuracy of the data in this study, the test data of cores #1, #2, and #3 adopted the average value of three sets of parallel experiments.
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MDPI and ACS Style

Wang, C.; Li, X.; Xia, J.; Ni, J.; Wang, W.; Jin, G.; Cui, K. Enhanced Oil Recovery Mechanism Mediated by Reduced Miscibility Pressure Using Hydrocarbon-Degrading Bacteria During CO2 Flooding in Tight Oil Reservoirs. Energies 2025, 18, 1123. https://doi.org/10.3390/en18051123

AMA Style

Wang C, Li X, Xia J, Ni J, Wang W, Jin G, Cui K. Enhanced Oil Recovery Mechanism Mediated by Reduced Miscibility Pressure Using Hydrocarbon-Degrading Bacteria During CO2 Flooding in Tight Oil Reservoirs. Energies. 2025; 18(5):1123. https://doi.org/10.3390/en18051123

Chicago/Turabian Style

Wang, Chengjun, Xinxin Li, Juan Xia, Jun Ni, Weibo Wang, Ge Jin, and Kai Cui. 2025. "Enhanced Oil Recovery Mechanism Mediated by Reduced Miscibility Pressure Using Hydrocarbon-Degrading Bacteria During CO2 Flooding in Tight Oil Reservoirs" Energies 18, no. 5: 1123. https://doi.org/10.3390/en18051123

APA Style

Wang, C., Li, X., Xia, J., Ni, J., Wang, W., Jin, G., & Cui, K. (2025). Enhanced Oil Recovery Mechanism Mediated by Reduced Miscibility Pressure Using Hydrocarbon-Degrading Bacteria During CO2 Flooding in Tight Oil Reservoirs. Energies, 18(5), 1123. https://doi.org/10.3390/en18051123

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