Surfactant technology for improved hydrocarbon recovery in unconventional liquid reservoirs: a systematic literature review

An unconventional liquid reservoir (ULR) is a type of underground geological deposit containing liquid hydrocarbons, such as crude oil, natural gas liquids, or condensate, not found in traditional oil and gas reservoirs. These formations have distinct properties that make oil and gas production more difficult than in typical reservoirs. Surfactants can be added to the fracking fluid to aid in the release of hydrocarbons. By reducing the amount of water and chemicals used in the process, surfactants can assist to reduce some of the negative environmental implications associated with fracking. In the current study, a systematic literature review was used to analyze and identify existing literature on surfactant technology in unconventional liquid reservoirs from the previous five years. 12 papers out of the 500 papers collected showed that studies had been performed and proved that surfactants can potentially increase the recovery of unconventional liquid reservoirs. The mechanism behind the positive outcome was concluded to be the alteration of wettability of reservoir rock and interfacial tension. Based on the conducted review, evaluation of the environmental impact from the use of surfactant and assessment of economic feasibility of surfactant technology in ULR could be the future research topics.


Introduction
An unconventional liquid reservoir refers to a type of underground formation that contains liquid hydrocarbons, such as oil or natural gas liquids, but has unique characteristics and properties that differ from conventional reservoirs.Unconventional reservoirs often have low permeability, which means that the flow of liquids through the rock formations is limited [1].They require advanced extraction techniques, such as hydraulic fracturing and horizontal drilling, to recover the hydrocarbons effectively.Examples of unconventional liquid reservoirs include shale oil reservoirs, tight oil formations, and certain types of oil sands deposits.
Surfactant Enhanced Oil Recovery (EOR) is a technique used in the oil industry to improve the extraction of oil from reservoirs.EOR refers to a set of methods employed to increase the amount of oil that can be recovered from an oil field beyond what can be achieved through primary and secondary recovery methods.Surfactants, also known as surface-active agents, are chemical compounds that have the ability to lower the surface tension between two substances, such as oil and water.In surfactant EOR, surfactants are injected into the reservoir to alter the interfacial tension between the oil and reservoir rock, improving the displacement and flow of oil [2].Surfactant EOR can work in several ways.One method is to enhance the wettability of the reservoir rock, making it more water-wet and facilitating the displacement of oil.Another approach is to form microemulsions or nanoemulsions in the reservoir, which can solubilize and mobilize trapped oil, allowing it to be produced more efficiently.Surfactant EOR is often used in combination with other techniques, such as polymer flooding or alkaline flooding, to further enhance oil recovery [3].The specific surfactants and formulation used depend on the characteristics of the reservoir and the type of oil being produced.Surfactants can significantly impact capillary pressure in porous media by altering the interfacial tension between immiscible fluids.When surfactants are introduced into a system containing two immiscible fluids, such as oil and water, they can modify the interfacial tension at the fluid-fluid interface.This modification can lead to changes in the capillary pressure behavior and affect the displacement of fluids in the porous medium.
Capillary pressure refers to the pressure difference across the interface between two immiscible fluids (such as oil and water) in a porous medium, such as a rock formation [4].It is influenced by the properties of the fluids and the characteristics of the pore structure.A few ways in which surfactants can influence capillary pressure such as, interfacial tension reduction: surfactants can lower the interfacial tension between oil and water, reducing the capillary pressure required to displace one fluid phase with another.This reduction in interfacial tension can enhance the efficiency of fluid displacement and improve oil recovery by facilitating the movement of oil through the porous medium.Wettability alteration: surfactants can modify the wettability of the rock surface, which affects the contact angle between the fluids and the solid matrix.By changing the wetting properties, surfactants can influence the capillary pressure by altering the distribution and configuration of the fluid phases in the porous medium.Emulsification and mobilization: surfactants can promote the formation of emulsions or microemulsions in the system.These emulsions can solubilize and mobilize trapped oil or other immiscible fluids, reducing the capillary pressure and allowing for improved fluid displacement and recovery.
By modifying interfacial tension, wettability, and emulsion formation, surfactants can impact the capillary pressure behavior in porous media [5].The specific effects depend on the type of surfactant, concentration, and the characteristics of the fluids and porous medium involved.Understanding these interactions is crucial for optimizing surfactant applications in various processes, including enhanced oil recovery (EOR) and other subsurface fluid displacement operations.
The current study focuses on reviewing the improved hydrocarbon recovery in unconventional liquid reservoirs using surfactant technology.To the best of our knowledge, there has yet to be published a paper of a systematic literature review (SLR) using the Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) method on surfactant technology in improving hydrocarbon recovery in unconventional liquid reservoirs.SLR is a review of a clearly defined subject that use systematic and explicit procedures to locate, select, and critically appraise relevant research, as well as gather and analyze data from the included studies [6]- [8].SLR was chosen because it summarizes and synthesizes available literature on a study topic or field.The lack of published SLR using the PRISMA method that specifically examine the application of surfactant technology in improving hydrocarbon recovery in unconventional liquid reservoirs is a barrier to gaining a thorough understanding of how much surfactant technology contributes to oil recovery in such reservoirs.Furthermore, this knowledge gap inhibits a fuller understanding of the underlying mechanisms responsible for surfactant technology's favorable effects.
In this review, we conduct a systematic review on the contribution of surfactant technology to bringing effect during oil production stage of unconventional liquid reservoirs, observing the methodology of examining the relationship between the use of surfactant and the mechanism behind the positive effect on oil recovery from previous publications collected over the last five years, i.e., 2018 to 2022, studying the limitations, and offering suggestions.This review focuses on a major research question of How has the surfactant technology been studied in unconventional liquid reservoirs over five years?The goal of this review is to determine the current state of understanding the surfactant technology as potential contributing option to improving oil recovery in an unconventional liquid reservoir.

Methodology
This work built an SLR based on the contribution of surfactant technology in improving oil recovery in unconventional liquid reservoirs.This study was designed in accordance with the PRISMA checklist standards.It is critical to establish the research questions that will be addressed in this study, as this will ensure the clarity of the research purpose.The data mining source was described, as well as the requirements for further analysis.

Search Strategy
The search method was carried out by accessing the Google Scholar database.The usage of unconventional reservoir as a keyword narrowed the search to the study area.To support the goal of this study, both review and research publications were used.Google Scholar was chosen to gather the articles because it has shown to be a readily accessible and regularly updated source.The data was collected from 2018 to 2022.
To determine if a research article or review article matched the inclusion criteria, a list of article titles was compiled from the Google Scholar database using the applied keyword and transferred to Mendeley.The inclusion criteria began by reducing the publication year from 2018 to 2022 in order to determine the present state of understanding on the topic and limiting the number of search to be 500 publications.Following that, the title was screened based on the source title.Only publications from journal and proceeding sources were taken into account.

Relevance and quality appraisal
Mendeley was used to finish the title search from the Google Scholar database referencing the keyword and data collection.One reviewer scanned the titles to determine the title's compatibility to the journal and proceeding source criteria, which are published from 2018 to 2022.The articles were thoroughly evaluated by one reviewer.The quality assessment tool looked at the essential backdrop and objective, sample selection, methodology, results interpretation, and main discoveries.The criteria for selecting a paper for synthesis include the availability of surfactant technology use in unconventional liquid reservoir and a clear definition of unconventional liquid reservoir.

Data extraction and analysis
Using a data collection table, the inclusion and quality criteria derived the date from the gathered articles.The table contains references, methodology, summary, and key findings (Tables 1).To describe the strength of the evidence surrounding the mechanism of surfactant technology in unconventional liquid reservoir context, a descriptive synthesis of the results and tabular data system were utilized.

Study characteristics
Figure 1 shows the outcomes of the article search and selection.The article search yielded 500 distinct titles for screening for relevancy, and each abstract was then chosen for inclusion.12 publication met the inclusion criteria, and the full manuscripts were collected for review.The absence of surfactant technology use and discussions were the key reasons for exclusion in each research.All 12 articles chosen were descriptive and focused on the surfactant technology topics in ULR.6 titles provided experimental study approach on assessing the feasibility of surfactant technology in ULR. 4 investigations were carried out both in the laboratory using measurement apparatus, and by using numerical simulation.A review of the surfactant's application on unconventional resources was conducted in a study.The remaining study examined Surfactant-Assisted Spontaneous Imbibition using a numerical approach.

Results and discussions
The majority of the research concluded that surfactant enhances oil recovery from ULR by altering the wettability and IFT.It was supported by the laboratory experiment data of using various crude oil samples and surfactant types.The wettability alteration results were proportional to the promotion of imbibition, thus oil expel caused by surfactant.The summary of the surfactant technology in ULR can be seen in Table 1.
The articles describe the potential of surfactants in enhancing oil recovery from ULR. SASI is a widely believed process that triggers imbibition through wettability alteration and interfacial tension reduction.The use of surfactants in fracturing fluids can also promote imbibition and enhance well performance.CWI and ACWI are promising EOR techniques that incorporate the benefit from water and gas flooding.To develop optimal surfactant formulations, ability to shift rock surface wettability, favorable phase behavior in reservoir fluids, and minimal chemical losses on reservoir minerals should be appraised.Customizations can be optimized based on specific field criteria.Reservoir performance affected by clay particles can be mitigated by the use of gemini surfactants in fracturing fluids.A study investigates the potential of using surfactants and CO2 huff-n-puff to optimize oil recovery in ultralow-permeability Wolfcamp and Eagle Ford shale cores.A study on the effect of surfactant molecular structure on wettability alteration and spontaneous imbibition in enhanced oil recovery also being discussed.Separate investigation is carried out through experimental and numerical simulation approach on the effect of wettability alteration and IFT reduction on the performance of surfactant-based EOR in unconventional oil reservoirs.Studies that compares the performances of different injection methods, including carbonated water and surfactant injection, in a tight oil reservoir are also examined.The utilization of SASI along with gas injection has been validated as an effective approach to enhance production in ULR.
Upscaling the data from laboratory of SASI for completion purposes and gas injection plus SASI for EOR are conducted by numerical modeling.
The two key processes for SASI in increasing production are wettability and IFT modification.The level of enhancement is controlled by surfactant concentration, soak times, cycle duration, injection pressure, and cumulative gas injection.[17] Formation of an osmotic pressure gradient between the microfractures and the matrix pores when the salinity of the water is lower than resident brine, allowing water to enter and driving oil out.One such method is the use of surfactants to alter the wettability of the rock surface and reduce the interfacial tension between the oil and water phases.This alteration in the properties of the reservoir fluids increases oil recovery rates.The success of surfactant-assisted EOR depends on the selection of the appropriate surfactant for the specific reservoir conditions, including temperature, salinity, and lithology.Surfactants can be added to stimulation fluids during hydraulic fracturing to promote imbibition by moderate IFT reduction and wettability alteration.CO2 huff-n-puff is another EOR technique that can improve oil recovery in ULRs, and the combination of CO2 and surfactants can enhance production [9], [16].Gas flooding limited EOR capability due to gas breakthrough, while water flooding shows low displacement efficiency, so CWI and ACWI are favorable methods that combine the benefit of water and gas flooding.
The selection of surfactants depends on the reservoir type, such as sandstone or carbonate reservoirs, and surfactants that possess larger IFT reduction, wettability alteration, and asphaltene inhibition capabilities are preferred.Novel surfactants, such as the synthesized gemini surfactant, have been used to provide clay stability in unconventional tight sandstone rock fracturing [14].Developing optimal surfactant formulations requires the ability to shift the rock surface wettability to a more waterwet, favorable phase behavior in reservoir fluids, and minimal chemical losses on reservoir minerals.Additionally, specific control variables, such as surfactant concentration and injection pressure, can be customized for different field applications.The combination of surfactants and gas injection, along with SASI, can significantly improve production from ULRs.Overall, understanding the role of surfactants in EOR is critical to maximizing oil recovery rates in unconventional reservoirs.
The studies cover various experiments and numerical simulations conducted to optimize oil recovery in ULR through EOR techniques.The use of hybrid EOR technology, SASI and CO2 huff-npuff, improved oil recovery in ULR [16].The discussions focus on understanding the effect of surfactant molecular structure on wettability alteration, retention of surfactant on the rock surface, and the IFT reduction.A systematic workflow for surfactant choice according to optimize wettability change.The results showed that surfactants having long tails could better change the wettability of rock to more water-wet and reduce the IFT [1].Nonionic surfactants generally result in better efficiency than ionic surfactants [5].Investigation of the effect of wettability alteration and IFT reduction on the efficiency of surfactant-based EOR in UORs was performed.The studies suggest that obtaining a water-wet status is critical for oil recovery improvement in tight reservoirs and shale, and the effects of IFT reduction could be detrimental to the recovery when the wettability is changed to water-wet due to a lower capillary force.Lastly, the numerical and experimental approaches on a case study in Changqing Oilfield, China of carbonated water and surfactant injection based on a case study in was presented.The study shows that ACWI attained the highest incremental oil, and the EOS/H model is appropriate to relate phase behavior of carbonated water and oil system [11].
Overall, the studies demonstrate the importance of understanding the molecular structure of surfactants and the effects of various EOR techniques on oil recovery in ULR.The significance of experimental and numerical simulations in evaluating the performance of different EOR techniques and selecting the appropriate one for optimal oil recovery are also emphasized.One possible research gap in the presented study is the lack of information on the environmental impact of using surfactants for enhanced oil recovery in unconventional liquid reservoirs.While the articles mention the benefits of using surfactants, such as improving oil recovery and maximizing well performance, it does not discuss the potential negative effects on the environment, such as increased pollution and harm to aquatic life.Therefore, future research could focus on evaluating the environmental impact of surfactant use in unconventional oil reservoirs and developing strategies to mitigate these impacts.Moreover, the assessment of economic feasibility on the incorporation of surfactant in improving oil recovery in ULR can be added in the future study.

Conclusions
Several conclusions can be drawn regarding the use of surfactants and EOR techniques in ULR:  CWI and ACWI are promising EOR methods that combine the advantages of water and gas flooding, and the combination of surfactants and gas injection can significantly improve production from ULRs. Surfactants with larger interfacial tension reduction, wettability alteration, and asphaltene inhibition capabilities are preferred for sandstone or carbonate reservoirs. Changing the rock surface wettability to a water-wet state, showing favorable phase behavior in reservoir fluids, and resulting minimal chemical losses on reservoir minerals must be considered when developing optimal surfactant formulations. The molecular structure of surfactants plays a significant role in wettability alteration and interfacial tension reduction, and longer-tailed nonionic surfactants generally show better performance than ionic surfactants. Obtaining a water-wet state is critical for higher oil recoveries in shale and tight reservoirs. Experimental and numerical simulations are critical in evaluating the performance of different EOR techniques and selecting the appropriate one for optimal oil recovery. Hybrid EOR technology, such as the combination of CO2 huff-n-puff and SASI, can improve oil recovery of ULR significantly.

Figure 1 .
Figure 1.The article selection strategy

Table 1 .
Summary of surfactant technology studies in unconventional liquid reservoirs DiscussionsThe oil industry continues to seek more efficient ways to extract oil from ULR through EOR methods.