3.Distribution Characteristics of Remaining Oil

Due to factors such as reservoir heterogeneity, water injection development in China results in uneven water advance in both horizontal and vertical directions, complex oil-water relationships, and a spatially intertwined state. The distribution of remaining oil is highly scattered and relatively enriched. According to the different geological characteristics, reservoir characteristics, and development methods of high water bearing reservoirs in China, the distribution of remaining oil also has its own characteristics. Let's take Shengli Oilfield as an example to discuss.

3.1 Water Driven Reservoirs

The integrated oil reservoirs of Shengli Oilfield are distributed in four oilfields: Shengtuo, Gudao, Gudong, and Chengdong. Currently, the comprehensive water content is 95.7%, and the overall development stage is in the late stage of ultra-high water content. During the period from the Ninth Five Year Plan to the Eleventh Five Year Plan, the understanding of macro residual oil has undergone a transformation from "severe water flooding and highly dispersed" to "overall dispersion and local concentration", and then to "widespread distribution and differential occurrence". The core well indicates that the average remaining oil saturation of the integrated reservoir has decreased, the mainstream line consumes high water, and the remaining oil saturation has decreased to near the remaining oil. The difference in remaining oil saturation between the weak and strong flooding sections has gradually increased. From the perspective of the occurrence state of residual oil at the micro level, water flooding mainly displaces the contiguous residual oil in the pores, and as the water content increases, the dispersibility of the residual oil increases. From the perspective of micro residual oil storage space, under high injection rate conditions, residual oil is mainly distributed in small pores. The geological reserves of the fault block oil reservoir are 16.1×108 tons, mainly distributed in oil fields such as Dongxin, Linpan, and Xianhe, with a comprehensive water content of 93.2%. After entering the ultra-high water cut stage, the scale of remaining oil enrichment becomes smaller and smaller. Due to the blocking effect of the fault seepage barrier, the closer the fault is, the smaller the displacement pressure gradient, making it difficult for crude oil to flow within 50m from the fault (100m from the oil well), and the remaining oil is enriched near the fault. Under the action of hydrodynamics and buoyancy, high dip fault block oil reservoirs during the ultra-high water cut period are dispersed, and residual oil undergoes re migration and differential accumulation along the upward dip direction of the oil layer, forming secondary enrichment zones of residual oil in favorable trap areas at high structural positions.

Overall, with the continuous deepening of development, the dynamic connectivity and spatial streamline distribution of water drive old oilfield reservoirs are complex, and the remaining oil saturation is generally low. The statistical results of remaining oil at different streamline positions show that the distribution of remaining oil is significantly affected by the well network, and the remaining oil is relatively enriched in non mainstream lines and oil well drainage; The degree of utilization in non main interlayer areas is relatively low, and the remaining oil saturation is high; Uneven utilization within thick layers, with relatively abundant remaining oil at the top of the positive rhythm.

3.2 Chemical Flooding Reservoirs

The chemical flooding reservoir in Shengli Oilfield has the characteristics of high formation temperature, high salinity of formation water, high viscosity of crude oil, and strong reservoir heterogeneity. It is mainly distributed in oilfields such as Gudao, Gudong, and Shengtuo, covering a geological reserve of 5.8×108 tons and a cumulative oil production of 7226×104 tons. After polymer flooding, the recovery rate of the reservoir is high, the remaining oil is more dispersed, and the heterogeneity of the reservoir is more severe.

After polymer flooding, the sealed core of the reservoir with ultra-high water content in the later stage showed that the remaining oil showed a characteristic of "widespread distribution and local enrichment". From a planar perspective, the remaining oil saturation is relatively high at the positions of oil well intervals, non mainstream lines, and fault edges. From a vertical perspective, the sedimentary rhythm affects the inner layer, and the water flooding degree at the top of the rhythmic layer is relatively low, with remaining oil enriched. The micro quantitative analysis results of residual oil after polymer flooding show that the micro residual oil can be divided into contiguous, porous, columnar, blind end, membranous, and patchy types. After polymer flooding, the remaining oil saturation of continuous and porous types decreased significantly, by 28% and 3.5% respectively.

3.3 Heavy Oil Reservoirs

The proven geological reserves of the heavy oil reservoir in Shengli Oilfield are 6.99×108 tons, mainly distributed in the eastern Danjiasi, Le'an, and western Chunfeng oilfields. In 2022, the geological reserves will be utilized by 7× 108tons. The development method of deep heavy oil reservoirs in the eastern part of Shengli Oilfield is mainly steam stimulation, and it has entered the stage of high cycle stimulation. Due to the thin thickness, large well spacing, and strong heterogeneity of heavy oil in Shengli Oilfield, conventional steam flooding faces challenges such as narrow steam bands, wide hot water bandwidth, and imbalanced displacement. The remaining oil exhibits a distribution pattern of overall enrichment and strip water flooding.

Since 2011, research and analysis on sealed core wells in heavy oil reservoirs have shown that the remaining oil saturation is higher in the middle and later stages of huff and puff. On the plane, the further away from the huff and puff well, the higher the remaining oil saturation. Vertically, the remaining oil in the lower layer of the interlayer is relatively enriched. Due to the influence of steam overlap, the steam absorption in each small layer is uneven, and there is a significant difference in interlayer utilization. The upper small layer has high steam absorption intensity and good development effect, while the lower small layer has poor utilization and residual oil enrichment. Under the influence of rhythmicity, the remaining oil in the upper part of the positive rhythmic reservoir is enriched.

4. Difficulties and Development Trends in Remaining Oil Research

The study of residual oil, as one of the most important aspects of fine reservoir description, involves a wide range of aspects. Taking Shengli Oilfield as an example, summarize the difficulties in residual oil research and propose several ideas on the development trend of residual oil research.

4.1 Difficulties in Residual Oil Research

From the development practice of Shengli Oilfield, it can be seen that in the later stage of high water content in integrated reservoirs, high water consumption zones are developed and inefficient water circulation is severe; The distribution of remaining oil in fault block reservoirs varies greatly and it is difficult to effectively utilize them; Deep and thin layer ultra heavy oil reservoirs face difficulties in steam injection and significant heat loss. After polymer flooding, the dynamic heterogeneity of the reservoir is stronger, and the remaining oil is more dispersed. 

From the current research methods for residual oil, on the one hand, there are many methods for residual oil research, each with its own advantages and disadvantages. It is difficult to select suitable methods to characterize residual oil based on research objectives and basic data mastered; On the other hand, current research methods for residual oil are difficult to fully apply to increasingly complex reservoir development conditions. Taking indoor physical simulation as an example. Firstly, the directly obtained core data cannot reflect the entire reservoir; Secondly, there are many uncertainties in the indirectly obtained data, making it difficult for conventional indoor experimental simulations to understand the evolution of residual oil and flow fields. Therefore, it is difficult to improve the accuracy of residual oil description and fully utilize the advantages of various research methods.

4.2 Development Trends of Remaining Oil Research

Based on literature research and the development practice of Shengli Oilfield, this article believes that with the advancement of technology, the future development trend of residual oil research includes but is not limited to the following five aspects:

(1).Construction of ultra large scale physical models. For the study of remaining oil in reservoirs, large-scale physical simulation methods have unparalleled advantages in considering factors such as well network and spacing in the reservoir. By moving underground reservoirs to the surface, constructing large-scale physical models, reproducing the oilfield development process, directly obtaining information such as physical properties, saturation, and pressure, and recognizing the spatial distribution characteristics and movable potential of remaining oil, it is of great significance for the sustained and efficient development of ultra-high water cut oil fields.

(2).Integration of multi-scale high-resolution imaging systems. High resolution imaging techniques such as scanning electron microscopy, laser confocal microscopy, and CT scanning have achieved significant results in the construction of digital cores and the microscopic study of remaining oil. However, due to the contradiction between resolution and observation field, relying on a single method can obtain the distribution characteristics of residual oil at the same scale, but the understanding of the characteristics of residual oil at continuous scales such as micrometers, millimeters, and centimeters is unclear. By using data fusion technology to achieve continuous scale analysis of remaining oil in the same rock core, it is expected to achieve multi-scale upgrade and leap in understanding of remaining oil at a microscopic level.

(3).Consider numerical simulation improvement methods for different displacement media and driving modes. With the continuous deepening of understanding of underground reservoir structure and fluids, conventional numerical simulation techniques are difficult to accurately describe the dynamic changes of complex reservoirs. Therefore, multi-scale and multiphase flow simulation has become a development trend. With the development of big data and artificial intelligence technology, data-driven simulation methods will gradually be applied to residual oil research. By analyzing and modeling a large amount of observational data, it is possible to better understand the distribution and migration patterns of remaining oil in reservoirs.

(4).The comprehensive application of multidisciplinary and multi method mine testing. With the development of high water content reservoirs, conventional reservoirs have become more complex, and a single logging technique is no longer sufficient to meet the needs of reservoir development. It is necessary to combine open hole logging technology with cased well logging technology, and combine logging knowledge with inter well seismic technology to achieve the transformation of "point" information into "body" information. Similarly, the development of tracer technology in the future also needs to be combined with technologies such as nuclear magnetic resonance and artificial intelligence to improve the efficiency and accuracy of reservoir interpretation.

(5).The widespread application of big data and artificial intelligence. Due to the characteristics of multiple wells, large amount of basic data, and heavy workload of information analysis in oil reservoirs that have been developed for many years, traditional methods for studying the distribution of remaining oil have limitations in terms of data volume, computational power, and analysis methods. The emergence of artificial intelligence and big data has provided new solutions for residual oil research. The accumulated various types of data have laid a solid foundation for deep discrimination, prediction, and application of machine learning technology. The application of big data and artificial intelligence technology will be a new trend in residual oil research, and it will also make deep analysis of geological data possible.

5.Conclusions

(1) The formation of remaining oil is influenced by both geological and development conditions. Geological conditions such as geological structure, sedimentary microfacies, and reservoir heterogeneity are the internal factors for the formation of residual oil in reservoirs, while development conditions such as well network density, well network mode, completeness of injection production systems, and production performance are the external factors for the formation of residual oil. The interaction between the two increases the difficulty of understanding remaining oil.

(2) The research methods for quantitative analysis of residual oil distribution characteristics and saturation include traditional optical methods, physical simulation methods, high-resolution imaging techniques, nuclear magnetic resonance imaging techniques, numerical simulation methods, four-dimensional seismic techniques, logging techniques, and tracer techniques. Different methods reflect the remaining oil characteristics at different locations and scales. Core analysis and logging techniques reflect the saturation characteristics of remaining oil near the wellbore. The tracer technology reflects the average remaining oil saturation characteristics of the permeability zone of the reservoir. Numerical simulation technology provides macroscopic distribution characteristics of remaining oil at different development stages and reservoir locations.

(3) 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 remaining oil in the water drive reservoir is relatively enriched in non mainstream lines and oil wells on the remaining oil plane; The remaining oil saturation of non main interlayer is high; The remaining oil at the top of the positive rhythm layer is relatively enriched. On the plane of chemical flooding reservoirs, non mainstream lines and residual oil from oil wells are relatively enriched; Vertically, the remaining oil at the top of the positive rhythm is relatively enriched. The further away from the stimulation well on the plane of a heavy oil reservoir, the higher the remaining oil saturation; The remaining oil in the lower layer of the interlayer is relatively enriched; The remaining oil in the upper part of the positive rhythm layer is relatively enriched.

(4) The development trend of residual oil evaluation technology is mainly reflected in the sustained development of large-scale physical simulation technology for residual oil research. The microscale analysis of residual oil requires the use of CT technology, nuclear magnetic resonance imaging technology, and pore network methods, and the future development trend is towards the integration of multi-scale and high-precision testing methods. The collaborative work of multiple logging methods is a development direction for solving the diverse types, complex lithology, and difficult evaluation of remaining oil in high water cut oil reservoirs. The application of big data and artificial intelligence technology will be a new trend in residual oil research. The combination of machine learning and improved numerical simulation technology can effectively improve the accuracy and efficiency of residual oil simulation, which is the main direction for future development.