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. 2026 Feb 23;11(9):14279–14286. doi: 10.1021/acsomega.5c03705

Performance Evaluation of Enhanced Recovery Efficiency for Oxygen-Reducing Air Huff and Puff in Tight Reservoirs

Tao Liu †,‡,*, Weiming Liu ∥, Xing Huang §, Yong Fu †,‡, Jiayou Chen ⊥
PMCID: PMC12980262  PMID: 41835564

Abstract

This study investigates the dynamic evolution and efficacy of oxygen-reducing air huff-and-puff (HnP) for enhanced oil recovery in tight reservoirs, employing a novel large-scale two-dimensional plate model (30 × 30 × 3.5 cm) and a single-dimensional elongated core to simulate oxygen-depletion processes across 13 HnP cycles. Through systematic experimental trials, we examined key operational factorswell shut-in duration and production pressure differentialsto quantify recovery efficiency. Results reveal that oil displacement is primarily driven by pressure-difference-induced fluid dynamics (63.04% of total recovery), with diffusion-mass transfer as a secondary mechanism (36.96%), highlighting minimal light hydrocarbon extraction and limited crude oil fractionation capacity. Crucially, HnP performance is governed by shut-in duration and pressure differentials, with a single cycle categorized into three distinct stages: single-phase gas flowback, free gas drive, and declining productivity. The free gas drive stage dominates oil production, evidenced by the gas-to-oil ratio, oil rate, and bottom-hole pressure variations, underscoring its critical role in optimizing recovery strategies for tight reservoirs.

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1. Introduction

Crude oil with a high viscosity has incrementally risen to prominence as a pivotal asset in the domain of petroleum prospecting and production within the Chinese territory. − Due to its unique physical properties, such as permeability below 0.1 mDa, the production from these reservoirs tends to experience rapid decline, resulting in suboptimal recovery rates during the exploitation and development phases. − As natural energy resources are depleted, the primary challenge in enhancing the development efficiency of tight reservoirs lies in the methods employed for replenishing formation energy and continuing secondary exploitation. Numerous on-site experiments have demonstrated that the efficacy of waterflooding during the huff-and-puff process is significantly lower compared to gas injection techniques under similar conditions. , Consequently, the application of gas-based energy enhancement agents to bolster the energy supply framework within compact reservoirs has piqued the interest of researchers both domestically and internationally. Common gaseous energy supplement media include CO2, CH4, oxygen-reducing air, and nitrogen. − While CO2 huff-and-puff techniques offer certain advantages in energy supplement and production enhancement in tight reservoirs, several challenges remain. − For example, the strong extraction of CO2 during the process leads to the accumulation of dense fractions within the crude oil, which obstructs the flow pathways. Additionally, field applications require specialized gathering, transportation, and processing equipment, resulting in high operational costs. The high-pressure CO2 gas will cause strong corrosion to the downhole pipeline after encountering water in the well, so it is greatly limited in the field application process. , In contrast, oxygen-reducing air, as an environmentally friendly energy supplement, has gained increasing attention due to its advantages. Composed primarily of nitrogen, a neutral gas, oxygen-depleted air avoids the pipeline corrosion issues associated with CO2. Moreover, its use does not require specialized gathering and transportation equipment, offering clear cost advantages. −

Zolghadr and colleagues (2013) investigated the impact of various factors such as temperature, atmospheric pressure, and crude oil thickness on the efficacy of nitrogen (N2) injection using laboratory-based physical modeling tests. In 2018, Ma’s research team conducted N2 huff-and-puff tests on compact cores from the Lucaogou Formation, determining that the optimal number of cycles for the N2 huff-and-puff process was three. Li and colleagues (2022), , drawing from confined indoor huff-and-puff tests focusing on core samples, concluded that CO2 plays a critical role in reducing fluid resistance and mitigating expansion during tight oil and gas exploitation, whereas N2 primarily contributes to increasing reservoir elastic energy. In low-viscosity environments, the huff-and-puff action of N2 demonstrates an enhanced performance. At present, the primary method for simulating the dynamics of oxygen-depleted air in huff-and-puff operations within confined reservoirs is the use of one-dimensional cylindrical core models. − However, due to the limitations of model size, accurately simulating the complex seepage behavior of oxygen-depleted air in tight reservoirs remains challenging. − Several studies have focused on large-scale experimental trials involving significant inhalation and exhalation dynamics. − Nevertheless, a comprehensive investigation into the fluctuation patterns of dynamic parameters and the underlying mechanisms of oil recovery in the oxygen-depletion air huff-and-puff technique remains insufficient.

This study employs a large-scale 2D physical model integrated with an advanced high-temperature and high-pressure experimental apparatus to systematically develop a comprehensive physical simulation methodology for the oxygen-depleted air huff-and-puff process. The entire cycle of oxygen-depletion air injection, including injection (inhalation) and production (exhalation) stages, is replicated by using this indoor physical modeling approach. Key factors influencing the effectiveness of the huff-and-puff process are identified as critical to understanding the mechanisms of energy replenishment and enhanced oil recovery during each cycle. This research contributes to a more in-depth understanding of oxygen-depleted air injection behavior in tight reservoirs, thereby improving insight into the underlying mechanisms of energy restoration and hydrocarbon production enhancement in such complex geological systems.

2. Materials and Methods

2.1. Experimental Material Design

2.1.1. One-Dimensional Core Material

A linear cylindrical core was employed as the primary element for conducting oxygen-reduction tests under alternating air intake and exhaust conditions, simulating various environmental scenarios. This approach facilitated an in-depth investigation into the mechanism of energy supplementation and production enhancement during the oxygen-depleted air huff-and-puff process. Distinctive cylindrical samples exhibiting varied physical characteristics were employed; each sample had a diameter of 2.5 cm and a length of 29.8 cm. The permeability of the first cylindrical core was recorded at 0.17 × 10–3 μm2, while the permeability of the second core measured 0.9 × 10–3 μm2.

2.1.2. Two-Dimensional Core Material

The model used in this laboratory experiment is a physical model of a tight reservoir made from natural cores in the target area. The specific production steps are as follows: a) By employing an indigenous rock outcrop sample to construct a tangible prototype, the model size was 30 cm × 30 cm × 3.5 cm. b) A horizontal well running through cores 1 # and 2 # is drilled on the side of the model (Figure ). Additionally, monitoring wells were strategically positioned across the model’s surface to track pressure variations in different regions. The structural layout and detailed specifications can be observed in Figure and are listed in Table .

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Recovery degree of different pressurization methods.

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Two-dimensional physical model design and physical map.

1. Core Model Parameters Table.
model scale (cm) horizontal well length (cm) permeability (mD) porosity (%) Primitive oil-bearing saturability (%)
30 × 30 × 3.5 28 0.18 8.94 60.41
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2.2. Experimental Methods Design

All experimental temperatures were 96.3 °C, and the external confining pressure of the two-dimensional model was 20 MPa. The test-run lubricant exhibited a viscosity of 2.93 MPa under the specified testing parameters. The fluid utilized in the trial mimicked the properties of reservoir water, possessing a salinity level of 5000 mg/L and a viscosity reading of 0.85 MPa under the established experimental settings. The experimental gas was oxygen containing 1% oxygen-reducing air.

2.2.1. One-Dimensional Core Experimental Method Design

The device is shown in Figure . A core holder equipped with an intermediate pressure measuring point was used to conduct the experiment. The experimental steps are as follows:

  • (a)

    The core sample was securely placed within the clamp; subsequently, it was subjected to a vacuum extraction process utilizing the model 117 core vacuum apparatus.

  • (b)

    Following the completion of the vacuum process, the core was saturated with water and oil to establish an initial oil saturation environment, which was then maintained for 72 h.

  • (c)

    For the first core sample, an oxygen-reduction technique was employed to gradually increase the pressure to 16 MPa. Once equilibrium was achieved, the injection end was sealed. In the case of the second core sample, crude oil was injected at a stable pressure of 16 MPa to enhance system performance. After pressure stabilization, oxygen-reducing air was introduced to displace crude oil from the dead volume at the injection end, ensuring stable, pressure-free contact between the gas medium and the core surface.

  • (d)

    Upon well shutdown, the back pressure was adjusted to atmospheric pressure, marking the initiation of the experiment. The trial was concluded once no liquid was detected at the discharge point.

  • (e)

    After the experiment, samples were collected. Gas chromatography analysis was conducted to examine the compositional transformations of crude oil components during the oxygen-reducing air huff-and-puff process.

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Flowchart of the one-dimensional core huff-and-puff experiment.

2.2.2. Two-Dimensional Core Experimental Method Design

Figure illustrates the layout of a dual-axis physical simulation apparatus designed for high-temperature and high-pressure experiments. The system comprises a high-temperature and high-pressure testing facility, a pressure measurement assembly, a thermal regulation unit, an oil-gas–water separation metering device, an ISCO pump, and other essential components. The experimental enclosures are capable of withstanding pressures of up to 30 MPa. By integrating a pressure monitoring system and a temperature control system, real-time observation of pressure variations at different points within the model was achieved, enabling the accurate replication of extreme reservoir conditions in a controlled indoor environment.

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Flowchart of the two-dimensional physical model high-temperature and high-pressure experimental system.

Employing an expansive, two-dimensional, level panel as a physical prototype, a controlled experiment involving an oxygen-depletion cycle of inhalation and exhalation was carried out to investigate the influence of the duration of well closure and the differential in production pressure on the oil recovery rate from the reservoir. The length of the well was set to 0.25, 4, 6, 12, 24, and 48 h. The production pressure difference was set to 4, 6, 8, 10, and 12 MPa.

The experimental procedure was conducted using the following steps:

  • (a)

    Establish the original oil saturation of the formation. The flat-panel model was maintained in a high-pressure environment (96.3 °C, 20 MPa) throughout the experiment. A continuous vacuum extraction process was performed for 3 days to eliminate residual air, ensuring complete saturation. The vacuum process was deemed complete once all pressure measuring points reached a vacuum state. Subsequently, the displacement pump was used to perform 7.8 MPa high-pressure water flooding at the model’s horizontal well (1#, 2#). The process continued for 96 h until complete water saturation was achieved. The permeability of the model was then determined by evaluating the discrepancy between the volume of injected and expelled water. Following water saturation, oil saturation was established using a procedure similar to water injection, with liquid injection through horizontal wells and production from distal wells. After complete oil displacement at the production end, the entire system was restored to the initial reservoir pressure of 7.8 MPa. The initial oil saturation of the flat-panel model was then determined and allowed to stabilize for 72 h. The experimental setup was carefully designed to eliminate interference from dead volumes.

  • (b)

    Injection of oxygen-reducing air to supplement formation energy. Oxygen-reducing air was injected into the injection–production well to replenish the formation energy. Upon achieving the designated design pressure, the injection ceased and the well was sealed off. Throughout the entire operation, the fluctuation in pressure at every location within the model was tracked continuously.

  • (c)

    Open well production. After well saturation, the pressure relief valve at the outlet of the injection-production system was adjusted to 7.8 MPa. The production well was then opened for operation. During the development process, the pressure changes at each point of the model were monitored, and the produced fluid volume and gas volume were measured. The experiment was terminated once no further fluid production was observed at the outlet.

  • (d)

    Resaturation. A back pressure of 7.8 MPa was maintained, while oil injection displacement was performed at a steady pressure of 15.8 MPa through three remote wells (wells 12, 13, and 14). Injection continued until no gas production was detected at the outlet. When liquid production at the outlet ceased, the second saturation process was deemed complete.

3. Results and Discussion

In this article, a one-dimensional core test is employed to elucidate the primary control mechanisms governing oxygen-reduced air huff-and-puff development. Subsequently, two-dimensional core test results were analyzed to investigate the dynamic development characteristics of the oxygen-reduced air injection in tight reservoirs.

3.1. Study on the Replacement Mechanism of Oxygen-Reducing Air Huff and Puff

As shown in Figure , the final degree of recovery for the different pressurization methods was quite different. The gas injection pressurization method achieved a recovery factor of 5.98%, whereas the oil injection pressurization method resulted in a recovery factor of only 2.21%. The gas injection pressurization method incorporated two mechanisms: differential pressure-driven flow and gas diffusion, whereas the oil injection pressurization method relied solely on gas diffusion. By subtraction of the recovery contribution of gas injection pressurization from that of oil injection pressurization, the recovery factor attributable to differential pressure-driven flow was determined to be 3.77%, accounting for 63.04% of the total recovery. In contrast, the recovery factor attributed to pure diffusion displacement was 2.21%, representing 36.96% of the total recovery. These findings indicate that differential pressure-driven flow is the primary mechanism for enhancing production during the oxygen-reduced air huff-and-puff process, while molecular diffusion serves as a secondary mechanism. Unlike CO2, which effectively extracts lighter fractions from crude oil, oxygen-enriched air demonstrated a comparatively weaker ability to mobilize these light components.

As shown in Figure , the dominant mechanisms of gaseous media in this process include diffusion waves, driven by molecular diffusion, and flow waves, governed by pressure differentials. Based on the one-dimensional core huff-and-puff experiments, crude oil displacement during the production phase of the oxygen-reduced air huff-and-puff process is predominantly characterized by a flushing action, which accounts for 63.04% of the total recovery, while diffusion accounts for 36.96%. Additionally, the oxygen-reducing atmosphere had a negligible effect on the extraction of lighter hydrocarbon fractions from crude oil.

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Gas chromatographic analysis of crude oil before and after huff and puff.

When using oxygen-reducing air as an energy-supplementing medium, no significant deposits of dense elements were found within the narrow passageways of compact formations. The removal of volatile substances facilitated by the energy-enhancing medium contributed to a reduction in reservoir seepage capacity. Therefore, oxygen reduction was more suitable for tight reservoirs with narrow pore throat structures and complex crude oil components.

3.2. Investigation into the Dynamic Characteristics and Influencing Factors of Oxygen-Enhanced Air Huff-and-Puff Production in Tight Reservoirs

3.2.1. Braised Well Stage

As shown in Figure , the pressure at the lower perforation declined during the injection balance phase but eventually stabilized. After 400 min, no further pressure changes were observed, indicating that the fluid distribution within the model had reached equilibrium.

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Bottom hole flowing pressure change curve during the well-squeezing period.

3.2.2. Development Phase

Illustrated in Figure , during the oxygen-reduced air injection process, cumulative hydrocarbon production (at standard atmospheric conditions) exhibited a characteristic trend. Initially, the bottom hole flowing pressure declined, followed by a continued reduction. The variations in both gas and oil production rates mirrored this behavior, initially decreasing rapidly and then declining at a more gradual pace.

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Dynamic production curve of oxygen-reducing air huff-and-puff development.

In order to examine the rate of oil output and the gas-to-oil proportion throughout the production phase (refer to Figure ), the sequential progression of the oxygen-depletion air strategy was segmented into a trio of distinct phases:

  • (a)

    Single-phase gas flowback stage (0–1 min): During this brief phase, the predominant source of oxygen-depleting air is attributed to the compressed gas present within the borehole. Due to the influence of a pressure differential, there was an expedited discharge. Consequently, the gas-to-oil ratio at the production outlet was extremely high, and the produced fluid consisted predominantly of a single-phase gas flow. This led to a rapid decline in bottom hole flow pressure.

  • (b)

    Free gas drive stage (1–25 min): During gas production, significant oil production was observed. The gas-to-oil ratio and oil yield remained relatively stable. In contrast to the fluctuations in the gas-to-liquid ratio, there was a noticeable delay in the responsiveness of the oil output rate. The overall performance was consistent with the characteristics of free gas flooding in previous research results. − During the energy supplementation phase, the majority of the injected oxygen-reducing air was compressed into tiny bubbles within the pore matrix, while a small fraction dissolved in the crude oil. As pressure decreased, the compressed bubbles expanded, leading to the reverse displacement and mobilization of crude oil by the expanding oxygen-reducing air.

  • (c)

    Productivity slowing stage (25–45 min): At this stage, productivity gradually decreased due to the depletion of free gas energy in the matrix. Nevertheless, the minimal quantity of gas present in the unrefined oil caused it to expand, which led to a suboptimal dissolved gas flooding impact. Consequently, there was only a marginal uptick in the oil extraction yield.

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Dynamic parameter curves of oxygen-reducing air huff and puff at different times.

3.2.3. Effect of Huff-and-Puff Cycles on Recovery

As depicted in Figure , the recovery factor during successive huff-and-puff cycles followed a declining trend. A total of 10 cycles yielded a cumulative recovery factor of 19.8%. The huff-and-puff process in tight reservoirs was categorized into distinct phases:

  • (a)

    Initial production phase (Cycles 1–4)

  • During this phase, a substantial amount of residual oil remained within the reservoir matrix. The injected oxygen-reducing gas effectively mobilized this oil during the injection and production cycles. The efficiency of each huff-and-puff cycle remained relatively stable, with recovery factors ranging from 4.3% to 4.0%. The oil-to-gas displacement efficiency was high, and the utilization rate of the injected gaseous energy supplement was optimal.

  • (c)

    Declining productivity phase (Cycles 5–8)

  • As the number of cycles increased, the majority of mobile residual oil was extracted, leading to a decline in single-cycle recovery efficiency from a maximum of 4.3% to 0.6%. The gas/liquid replacement efficiency dropped sharply from about 0.35 to less than 0.05, and a large amount of injected gas was invalidly discharged.

  • (d)

    Low-yield phase (Cycles 9–10)

  • During this stage, the efficiency of each huff-and-puff cycle stabilized but remained low. The overall recovery factor was limited, and the liquid-to-gas displacement efficiency remained below 0.05.

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Recovery curve of the rounds of oxygen-reducing air huff and puff.

3.2.4. Effects of Squeezing Time on Productivity

Figure illustrates the influence of the soaking time on recovery. Prolonging the shut-in period initially enhanced recovery; however, beyond 400 min, no further increase in bottom-hole pressure was observed. This indicates that gas diffusion and pressure-driven displacement had reached their maximum efficiency threshold. Thus, extending the shut-in time beyond this limit was not beneficial.

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Relationship between recovery and shut-in time.

3.2.5. Effects of Pressure Differential on Recovery Efficiency

Illustrated in Figure , increasing the operational pressure differential resulted in a steady rise in the total recovery factor from 2.8% to 4.6%. When the pressure differential exceeded 6 MPa, recovery efficiency improved significantly due to the expansion of trapped gas within the formation. Enhancing the pressure differential during the huff-and-puff process can therefore effectively boost oil recovery rates.

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Relationship between production pressure difference and degree of recovery.

Overall, experimental findings suggest that the number of huff-and-puff cycles and the pressure differential during injection and production phases are critical factors influencing the oil recovery efficiency. Once the shut-in time exceeds 400 min, the oxygen-reducing air huff-and-puff process reaches its operational limit, making further time extensions redundant.

4. Conclusions

  • (a)

    Analysis of the oxygen-reducing air huff-and-puff process in tight reservoirs, considering various influencing factors, revealed that both recovery efficiency and oil–gas displacement effectiveness per cycle declined progressively as the number of cycles increased. Notably, the efficacy of the huffing and puffing technique diminished significantly upon surpassing four cycles of repetition. Additionally, increasing the operational pressure differential led to an improvement in oil recovery, and extending the soaking duration further enhanced recovery efficiency. However, once the soaking time exceeded five hours, the incremental benefit plateaued, indicating a saturation threshold beyond which further extension of soaking time provided no additional advantage.

  • (b)

    Combined with the dynamic production curve of oxygen-reducing air huff and puff, a single round of oxygen-reducing air huff and puff was divided into three stages: (1) oxygen-reducing air backflow, (2) expansion liquid recovery, and (3) production capacity reduction. Among these, the gas expansion-driven liquid recovery stage played the most critical role in enhancing oil recovery.

  • (c)

    The primary displacement mechanism in the oxygen-reducing air huff-and-puff process was differential pressure-driven flow, accounting for 63.04% of the total recovery, while gas molecular diffusion contributed 36.96%. Compared to CO2, which has a strong capacity to extract lighter hydrocarbons from crude oil, the impact of oxygen-enriched air on these same light elements within the crude was relatively modest.

  • (d)

    Following the energy replenishment, the pressure dynamics within the reservoir exhibited a distinct trend: an initial sharp decline in bottom-hole pressure during the early stages of soaking, followed by a more gradual decrease. Once the soaking duration exceeded 400 min, bottom-hole pressure stabilized, showing no further fluctuations. In contrast to the 7% reduction in pressure observed during CO2 immersion in prior research, the decline in the oxygen-lowering atmosphere was merely 2%. Therefore, compared with CO2, oxygen-reducing air has more advantages in supplementing formation energy, it is less efficient in promoting crude oil expansion and increasing fluidity. Therefore, the selection of an appropriate energy-supplementing medium should be optimized based on specific reservoir conditions.

Acknowledgments

The work was supported by the Shaanxi Provincial Natural Science Basic Research Program (2025JC-YBQN-772), the Shaanxi Provincial Department of Education special scientific research project (No.24JK0604), Project of Basic Research of Natural Science of Shaanxi Province (2025JC-YBQN-437), the Special Scientific Research Program Project of Shaanxi Provincial Department of Education (No.24JK0603), the National Science Foundation of China (No. 52574046), Shaanxi Province Outstanding Young Scientists Fund (No. 2025JC-JCQN-028), and Shaanxi Province Key R&D Program (No. 2024SF-ZDCYL-05-04).

The authors declare no competing financial interest.

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