Study on Flow Unit of Heavy Oil Bottom Water Reservoir with Over-Limited Thickness in Offshore Oilfield

The upper Ming section of L oilfield is a typical offshore heavy oil bottom-water reservoir with thick fluvial layers. All horizontal wells are developed by natural energy. Due to the few drilling holes and influence by the resolution of seismic data, it is difficult to describe reservoirs with thickness over 20 meters. In this paper, seismic resonance amplitude inversion technology is introduced to restore the real response of thick reservoirs and interbeds by drilling and drilling verification, and the geological bodies with different thickness are displayed by frequency division RGB three primary colors. Flow units of heavy oil reservoirs with bottom water are divided according to the three major factors of interlayer, lithologic internal boundary and water-oil thickness ratio which have the greatest influence on horizontal well development, thick sand bodies are divided into 10 different flow units in three levels, each unit is separated from each other, and the reservoir structure, water-cut characteristics and water-flooding characteristics are different. The reliability of the research is improved by using the dynamic data of horizontal wells and newly drilled passing wells, which provides a basis for tapping the potential of heavy oil reservoirs with bottom water.


Introduction
Since Hearn C.L. put forward the concept of flow unit in 1984, [1] [2], many scholars have expanded the concept by studying reservoir characterization and reservoir evaluation. Relevant studies [3] consider that under the condition of similar reservoir rock properties and physical properties, the characteristics of reservoir structure, fluid properties and remaining oil distribution are similar, and there is no geological unit with fault and lithology affecting oil-water flow. It is called "flow unit". The study of flow unit is the key to fine reservoir description and the main goal of development and adjustment. The concept of flow unit and the formation and development of its research methods provide an effective means for understanding reservoir heterogeneity. The study of flow units is a further deepening of the study of reservoir configuration, which combines the distribution of reservoir configuration, the division of flow units and the distribution of remaining oil. From the above definition, we can see that this concept can solve the problem of subdivision of thick reservoir and existing small reservoir well according to different scales, which is of great significance for tapping potential and detailed description of oilfield. At present, there are two main methods for dividing flow units at home and abroad: 1) Physical property dividing method. This method chooses drilling parameters such as porosity, permeability, particle size, shale content and so on, and carries out quantitative dividing of flow units between wells by cluster analysis and random simulation, which requires very high sample points. 2) The second method is lithofacies analysis. On the basis of cycle division and correlation, the reservoir sedimentary thickness and lithologic boundary are discussed, which can be divided into four scales: large, medium, small and micro. According to the degree of core research, it can be precise to micro scale. Because the following studies on fluvial oilfields in domestic offshore oilfields are typical high porosity and permeability oilfields with few drilling wells and high core cost, lithofacies analysis method should be adopted mainly according to commonly used seismic data, which is limited by the resolution of seismic data from thick reservoirs. At present, domestic research scale is too large and uncertain. Therefore, starting from the original seismic data, this paper dissects the frequency. Data, frequency division method is used to study the distribution characteristics of this seemingly "sand-filled" reservoir, which is rarely reported at home and abroad.
Offshore oilfields mainly consist of thick-bed heavy oil and bottom water reservoirs of fluvial facies. The reservoir thickness generally exceeds 20 m. Because of multi-stage channel overlapping, it is difficult to precisely split the drilling wells. Many years of production practice has proved that the distribution of residual oil in this pan-connected body is very complex, and it is necessary to carry out flow unit research to improve the development effect. At present, scholars at home and abroad have done a lot of research on the formation mechanism and control factors of flow units from different perspectives. Many research methods have been put forward by analyzing a large number of drilling data. The development cost of offshore oilfields is high and most of them are horizontal wells. Taking L oilfield as an example, starting from the original earthquake, this paper discusses the flow unit research method of combining frequency division method with bottom water reservoir attributes under the condition of few wells in offshore oilfields. The main contribution of this paper is to break through the limitation of large-scale and uncertain original seismic data, to dissect deep frequency data, to study the distribution characteristics of residual oil in water cut rising and passing wells according to the development practice of all horizontal wells in the oilfield, to carry out the study of flow units under the condition of large offshore well spacing, and to open up a new way for tapping potential remaining oilfields or even comprehensive adjustment in the later period. However, due to the different characteristics of the original seismic data in different oilfields, this paper still has further improvement. Later, we can analyze the applicability and differences of this method in different oilfields, and fundamentally put forward suggestions for seismic re-acquisition, but this method can be popularized and applied in ultra-thick layers [4] [5].

The Survey of Oilfield
The upper Ming member of L oilfield is a typical meandering river shallow water delta deposit in the sea. The long-term base level decline has resulted in the formation of overlapping foliated thick sand bodies [6]. The average thickness of

The Method of Research and Expected Results
Early drilling and drilling proved that the reservoir thickness ranged from 8 to

Internal Structure of Thick Sand Layer
The thickness of Um797 sandstone layer varies in different planes. The average sand body in the west is more than 25 m. Affected by the resolution, there are two or even three sets of axes, some of which are similar to the response of interlayer. A11, A28H and A9H drilled-through reservoirs show a complete set of thick layers. The 90-degree phase-shift data show the illusion of interlayer. In the actual research process, it is found that the corresponding relationship between different frequencies and thickness is obvious [8]. Under the background of 2100 m/s seismic velocity, 25 m thick reservoirs correspond to 20 Hz frequency, while 8 m single channel sand body corresponds to 70 Hz. From these frequency bodies, most of them are selected. The neural network inversion under well matching constraints can obtain a better combination of frequency-division data sets, which are in good agreement with well and seismic data. By using this combination of frequency-division data sets to invert harmonic amplitude, seismic data bodies whose thickness can be characterized by reflection thickness can be obtained in the whole area. This method can be used to characterize the profile of ultra-thick reservoir. Compared with 90-degree phase-shift profile, applying this set of data to the characterization of interlayer in thick sand layer can restore the true response characteristics of interlayer in thick sand layer. Figure  3 is a comparison of the interlayer range depicted by 90-degree phase-shift data and inversion data of harmonic amplitude modulation. After eliminating the false response, the interlayer [9] which has great influence on development is obviously narrowed. The production performance of horizontal wells can be well verified. Taking A67H1/A72H and A78H in areas with low well control degree as examples, the water cut of the three wells breaks through 90% in less than one month, compared with well control. The A63H/A68H region with high degree

Plane Distribution Characteristics of Thick Sand Layer
Conventional amplitude attributes are difficult to characterize reservoirs with excessive thickness, and the whole reservoir is full of sand. In practical research, it is found that on the basis of the data processing of the resonant amplitude, the multi-attribute fusion display based on the frequency division RGB color model can distinguish different thicknesses [10]. The specific operation method is to extract attributes of different frequencies from seismic data, and then generate slices of three attributes for RGB mixed-color display. Three-dimensional seismic data volume is transformed into four-dimensional data volume by frequency division and back to three-dimensional data volume by fusion display, but the boundary between different lithologies is highlighted, and the shape of each lithologic combination geological body is distinguished by color, which is the best imaging method of time-frequency analysis at present. This RGB fusion display method has a good effect on highlighting the energy approximation feature area of each frequency division attribute, and can highlight the commonness and weaken the difference. For the recognition of geological bodies, if the energy characteristics are strong in different frequency bands after frequency division, the geological bodies on the color data volume after RGB mode fusion will show obvious differences with surrounding rock in near white characteristics. As shown in Figure 4, using this method, the data are improved obviously. Using RGB attribute fusion attribute slices, the plane contact relationship between geologic bodies with different thickness of Um797 sand body can be clearly seen.

Analysis and Classification of Influencing Factors of Flow Units
According

The Effect of Development Practice
Five horizontal wells are deployed in the interlayer and thin bottom water area. The average low water cut period lasts for one and a half years when the water avoidance height is obviously reduced. A50H2 is a newly drilled adjustment well, located in the thin bottom water area, about 160 m away from the old well, and the water cut is still below 80% in 5 months when the water avoidance height is only 8.4 m, and the oil production is nearly 100 square/day. That is, the situation of high water cut. Considering that horizontal wells in thin bottom water area have dual functions of water ridge and lateral oil displacement, A33H well is selected to make the greatest liquid extraction attempt in this offshore oilfield. The effect is obvious. The liquid volume is increased from 1700 to 2700, the oil production is increased from 45 to 125 square per day, and the water cut is reduced by 1.4%.

Conclusions
In this paper, the resonance amplitude modulation technique is applied to restore the true response characteristics of thick sand body. On this basis, the frequency division RGB fusion technique is applied to identify reservoir ranges with different thickness. On this basis, the geophysical prospecting method of thick bottom water reservoir-sedimentary genesis-flow unit division method is established by using reservoir boundary, interlayer and water-oil thickness ratio.
It is under the condition of high cost drilling in offshore oilfields. Through this research result, adjusting wells can be precisely implemented in thin bottom water or interlayer area, and large-scale fluid extraction can be carried out, which can effectively reduce the development risk. This research result has strong practicability and can be further promoted in similar oilfields. It should be pointed out that this technique has some limitations, such as that the flow units caused by the smaller 3-level and 4-level interfaces in the thick sand body need to be identified by more drilling and even core data, and the deep excavation frequency data are also needed to realize the smaller-scale flow unit division. With the large-scale adjustment of offshore oilfields in the later stage, more abundant flow units can be obtained. Static data are further excavated.