In order to improve the research and evaluation of residual oil in high water cut oil fields, based on extensive literature research, this article summarizes the concept and influencing factors of residual oil, and summarizes the research methods and applicable conditions of residual oil from three aspects: micro distribution, macro distribution, and saturation quantitative analysis. The distribution characteristics of residual oil in water drive reservoirs, heavy oil reservoirs, and chemical drive reservoirs are summarized, and the current difficulties and development trends in residual oil research are further proposed. The results show that the influencing factors of residual oil mainly include geological structure, sedimentary microfacies, reservoir heterogeneity and well network density, well network mode, completeness of injection production system, production performance, etc; The research methods for residual oil include experimental analysis methods, numerical simulation methods, and field testing methods. The research objectives and applicable conditions of each method are different, and the test results reflect the distribution of residual oil saturation at different positions and scales; The overall distribution of residual oil in high water bearing oil fields shows a highly dispersed and relatively enriched characteristic, and the micro distribution of residual oil presents various forms of continuous and discontinuous phases; The development trends of residual oil research include but are not limited to the following five aspects: the construction of ultra large physical models, the integration of multi-scale high-resolution imaging systems, the improvement of numerical simulation methods considering time-varying and discontinuous phase nonlinear seepage of different displacement media and physical properties, the comprehensive application of multi-disciplinary and multi method mine testing, and the widespread application of big data artificial intelligence.

The main oil fields in eastern China have entered the stage of medium, high, and ultra-high water cut development, and some oil fields have also undergone chemical flooding development. The stable production of crude oil mainly relies on measures such as water control and oil stabilization based on flow field regulation, and comprehensive treatment to tap the potential. After primary and secondary oil recovery, 60% to 70% of the remaining oil remains underground. Some old oil fields have undergone tertiary oil recovery, but the recovery rate is also difficult to exceed 50%. These remaining oils have great potential for increasing recoverable reserves and improving recovery rates.The distribution of remaining oil is influenced by various factors such as geological conditions and oil displacement processes, and its distribution pattern is extremely complex. The research on residual oil in China began during the "Sixth Five-Year Plan" period. After the 1970s, oilfield workers began to preliminarily study the movement law of underground oil and water in oilfield development. They found that the low oil and gas recovery rate was due to a considerable amount of residual oil remaining in areas with poor physical properties, making it difficult for oil and water to circulate and extract. After decades of research, China's understanding of the formation and distribution of residual oil has become more mature. The research methods for residual oil can be summarized into three categories: one-dimensional longitudinal, two-dimensional planar, and three-dimensional spatial. By corresponding to different scales of oil reservoirs, the research on residual oil can be divided into macro scale, large-scale, small-scale, and micro scale. These different scales of residual oil research are interrelated and rely on different main methods. At present, there are many research methods for residual oil, mainly including development geology methods, core observation description and analysis testing methods, logging interpretation methods, four-dimensional seismic methods, reservoir numerical simulation methods, dynamic monitoring analysis methods, and reservoir engineering methods. Each method has its own characteristics and limitations. How to select a reasonable research method for residual oil based on specific reservoir conditions and effectively improve the evaluation effect of residual oil in high water cut oil fields has always been a key issue of concern in oilfield development. Therefore, this article systematically reviews the research methods and understanding of residual oil, summarizes and proposes several development trends in residual oil research, aiming to provide reference for related research.

1. Concept and Influencing Factors of Residual Oil

1.1 Concept of Residual Oil

The exploration of remaining oil began in the 1930s. In 1956, American geologist HUBBERT first proposed the concept of "remaining oil", which refers to the remaining unexplored oil reserves after the extraction of petroleum resources. In the 1960s and 1970s, the rise in oil prices and changes in supply and demand triggered people's attention to remaining oil resources. In 1975, the United States established the Remaining Oil Saturation Committee to conduct specialized research on remaining oil. In the 1980s and 1990s, the distribution of remaining oil and techniques for improving oil recovery rates attracted widespread attention from scholars both domestically and internationally. The concept of remaining oil has gradually evolved into an "unexplored reservoir", which refers to petroleum resources that have not yet been exploited and utilized. Entering the 21st century, with the development of new technologies, research on remaining oil has been improved accordingly. From the perspective of extraction, residual oil refers to the crude oil that can be extracted through measures such as deepening understanding of geological bodies and improving the level of extraction technology. From the perspective of residual resources, residual oil refers to the crude oil that remains in the oil layer after a certain degree of extraction. At present, the concept of the latter is widely adopted in research by technology personnel.

1.2 Factors Affecting the Formation of Residual Oil

The formation of remaining oil is influenced by a combination of geological and development conditions. Among them, geological conditions include geological structure, sedimentary microfacies, and reservoir heterogeneity. The influence of geological structures on the formation of residual oil can be divided into two categories: first, large enclosed faults, which are usually the boundaries of oil reservoirs. Due to the influence of injected water waves, residual oil is often enriched in the high parts and corners of the underlying oil layers under the faults; The second is low sequence faults, which are not discovered between injection and production wells and can block fluid flow locally, affecting the oil recovery effect and causing residual oil enrichment near the low sequence faults (Figure 1). Sedimentary microfacies are the main influencing factors of oil-water movement on a plane, and their control over remaining oil is mainly reflected in the external geometric shape of the sand body, the differences in physical properties between different microfacies, and the extension direction and distribution pattern of the sand body. The vertical heterogeneity of the reservoir controls the swept volume within a single sand layer and the degree of interlayer contradictions, resulting in residual oil often existing in intervals with strong heterogeneity and poor physical properties. The heterogeneity on the reservoir plane affects the formation of channeling ways, resulting in uneven distribution of remaining oil on the plane, enriched at the edges of the same phase zone or in areas with poor physical properties.

The development conditions that affect the distribution of remaining oil include well network density, well network mode, completeness of injection production system, production performance, etc. The most important aspect is the completeness of the injection production system and its interrelationship with geological factors. The higher the density of the well network, the higher the water drive sweep coefficient, and the fewer enriched areas of remaining oil. The linear well network is influenced by the heterogeneity of the reservoir in the direction of injection wells, and the remaining oil may be enriched between two injection wells. The remaining oil in the four point area well network may be enriched in the pressure balance zone between injection wells. The water drive sweep efficiency of the seven point and five point well networks is better than that of the inverse nine point well network. Unstable sand body distribution or low well network control may lead to incomplete injection production systems, resulting in more complex distribution characteristics of remaining oil. During the displacement process of high water bearing reservoirs, high displacement speed and high oil-water viscosity ratio can induce an increase in fingering degree, resulting in the enrichment of remaining oil. Overall, the geological and development conditions that affect the formation of residual oil interact and influence each other, increasing the difficulty of understanding residual oil.

2.Research Methods for Residual Oil

The basic content of residual oil research mainly includes the distribution characteristics of residual oil and quantitative analysis of residual oil saturation. Sort out the main research methods corresponding to different research contents from three aspects: micro distribution, macro distribution, and quantitative analysis of saturation of remaining oil.

2.1 Research Methods for Microscopic Distribution of Remaining Oil

The study of micro distribution of remaining oil is aimed at the distribution characteristics of remaining oil at the pore scale after the displacement process is completed. At present, the main research methods include traditional optical methods, physical simulation methods, high-resolution imaging technology, and nuclear magnetic resonance imaging technology.

2.1.1 Traditional Optical Methods

The traditional optical method is the earliest experimental technique to study the microscopic distribution of remaining oil, and the research objects mainly include oil bearing thin sections, real core models, and microscopic simulation models. Based on different research objects, this article summarizes the main technical characteristics and applicable conditions of traditional optical methods (Table 1). The advantages of traditional optical methods lie in the convenience of making experimental models and relatively low costs. This technology is still the main means of studying the microscopic distribution of remaining oil. The disadvantage is that it cannot fully reflect the three-dimensional spatial characteristics of residual oil distribution. The future development trends include high-resolution imaging technology, the introduction of new optical sensors, and the combination of various analytical methods.

2.1.2 Physical Simulation Methods

The physical simulation method is based on the characteristics of oil reservoirs, using the principle of similarity to reduce the research object to the level of indoor experiments, in order to simulate the displacement process, predict production capacity, evaluate the effect of stimulation measures, and so on. Zhou Fengjun et al. designed physical simulation parameters based on the principle of similarity and studied the impact of different rhythmic formations on the distribution pattern and production performance of residual oil in early polymer flooding. Xu Bing quantitatively analyzed the distribution of remaining oil in heterogeneous models based on the resistivity method. The advantage of physical simulation methods is that they can simulate the remaining oil behavior under real reservoir structure and conditions in the laboratory by adjusting experimental parameters and conditions, with strong controllability. The obtained data and results can intuitively reflect the flow of remaining oil. The disadvantage lies in the relatively high cost and long time cycle of physical simulation methods. Due to limitations in experimental conditions, there are limitations in simulating certain special oil reservoir situations. The development trends of physical simulation methods mainly include the improvement of simulation techniques at different scales, the development of simulation techniques under high temperature and pressure conditions, the integration of on-site experiments and simulations, and the application of data processing and machine learning technologies.