# Characteristics of Seepage Barriers in Fluvial Reservoirs of Meandering Rivers and Their Impacts on Water Injection Development: A Case Study of Neogene Guantao Formation in Block M, Gudao Oilfield

Authors: Ruhao Liu; Yu Sun; Xinrui Wang; Baiquan Yan; Haitao Yu
Journal: Geofluids
 (2022)

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## Abstract

To deepen our understanding of reservoir heterogeneity, seepage barriers in the study area were divided into interlayer, intralayer, and planar levels based on their spatial distribution characteristics and then investigated by taking the third member of the Neogene Guantao Formation in the block M of the Gudao oilfield as an example. Based on their genesis characteristics, the interlayer seepage barriers were divided into the overbank sand-overbank sand type, overbank sand-channel type, isolated channel superposition type, and channel shallow-cut type. The planar seepage barriers were categorized into the channel boundary type, abandoned channel type, channel-overbank sand type, and floodplain mudstone type. The intralayer seepage barriers were classified into the mudstone type and physical property type. Classifying multiple levels of different types of seepage barriers led to the refinement of their spatial characteristics. The strength of seepage barriers was characterized using the “seepage barrier coefficient” and “reservoir quality coefficient,” and the small-valued coefficients of the interlayer and intralayer seepage barriers indicated that these seepage barriers are characterized by large thickness, high mudstone content, poor physical properties, and weak seepage ability. The strength of the planar seepage barriers was dependent on the sedimentary facies types and channel stages, and the difference between planar seepage barriers was characterized using the reservoir quality coefficient. The seepage barriers were described based on the multilevel classification of seepage barrier categories, the intralayer seepage barriers with a lateral accretion pattern in the meandering river point bars were identified and described, and the distribution characteristics of seepage barriers were summarized at multiple levels. The method of “hierarchical analysis, skeleton construction, and category fitting” was used to establish 3D models of different levels of seepage barriers, and a modeling method based on the characteristics of intralayer seepage barriers under the constraint of architecture pattern was used to model intralayer seepage barriers with a lateral accretion pattern. In the monitoring of water injection profiles, it was found that the extent of blockage achieved by seepage barriers affects the water injection volume and thus controls the fluid transport pattern. Due to the development characteristics of the seepage barrier in the formation, there are some differences in injection production efficiency under different well pattern matching modes. The research on the distribution characteristics of different levels of seepage barrier categories provides a reliable geological basis for improving the injection-production relationship.


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## Body

## 1. Introduction

Under the current economic and technological conditions, the average oil and gas recovery rate in China is only about 30%. Nearly 70% of the oil and gas remains underground, a large part of which is retained in the subsurface as movable macroscopic remaining oil due to the heterogeneity within the reservoir, especially the impact of the reservoir architecture, which results in seepage barriers and seepage difference within the reservoir. In particular, seepage barriers are an important factor controlling the pattern of fluid movement. Water injection development without a clear knowledge of the spatial distribution of seepage barriers is prone to inefficient water injection cycles, thereby leading to inefficient extraction.At present, the classification of seepage barriers is rather general [1–6]. Some researchers classify seepage barriers into mudstone barriers, cementation zone barriers, and closed fault barriers [7–10]. With the study of reservoir architecture and the development of oilfields generally entering the middle and late stages, it is important to carry out a more detailed classification and characterization of seepage barriers in order to further enhance the tapping of remaining oil. After more than 40 years of injection production development in the block M of Gudao oilfield, there are still problems such as unclear sand body contact relationship and uncertain injection production relationship. At present, a high-density well pattern with an average well spacing of less than 200 m has been formed, which provides a data basis for the fine research of seepage barriers. In this work, the hierarchical classification and characterization of seepage barriers, as well as the impacts of different types of seepage barriers on water injection development, were systematically studied with Ng35 as the target layer to provide a geological basis for the improvement of the injection-production relationship and tapping of remaining oil in the study area.


## 2. Seepage Barriers in Fluvial Reservoirs of Meandering Rivers

Seepage barriers are rock masses that block fluid seepage in a reservoir system [11–13]. They have a complex genesis, either formed by changes in river hydrodynamic conditions during deposition or differences in sediment lithology after deposition, or by the action of diagenetic cementation bands. Seepage barriers typically correspond to different levels of architecture interfaces and have a certain level of hierarchy. For research purposes, seepage barriers in the study area were classified into interlayer, plane, and intralayer seepage barriers based on their spatial distribution characteristics.


### 2.1. Interlayer Seepage Barriers

Interlayer seepage barriers are mainly formed in a sedimentary environment with weak hydrodynamic conditions. They are dominated by mudstone deposits in the floodplain but can also be attributed to sandy mudstones due to insufficient channel undercutting. Changes in the source supply to accommodating space ratio affect the contact relationship and superposition styles between sand bodies. In turn, these directly affect the pattern and development characteristics of interlayer seepage barriers and thus affect the connectivity level of the reservoir. Based on the vertical superposition relationship between different sand bodies in the study area, the interlayer seepage barriers were classified into four categories, namely, (I) the overbank sand-overbank sand type with strong blockage, (II) the channel-overbank sand type, (III) the isolated channel superposition type, and (IV) the channel shallow-cut type. In addition to the above four categories of interlayer seepage barriers, there were also isolated sand body types (isolated overbank sand and isolated channel type) and the channel deep-cut type formed by channel undercutting (Figure1).Figure 1
Types of interlayer seepage barriers.


### 2.2. Planar Seepage Barriers

Planar seepage barriers are mainly the muddy sediments of the floodplain that accompany the joint formation of complex lateral shear erosion superposition relationships in the plane between channel sand bodies and other genetic sand bodies, during the process of “formation⟶development⟶lateral erosion⟶abandonment.” The scale of these seepage barriers is affected by the characteristics of the sand assemblages, based on which the planar seepage barriers were divided into four categories: channel boundary type, abandoned channel type, channel overbank sand type, and floodplain mudstone type (Figure2). A channel boundary type seepage barrier is formed when the distributary channel sand bodies of the same period in the plane are diverted laterally to connect and erode during travel, leading to the connection of the near upper part of the plane of the two stages of channel sand bodies while the lower part is the floodplain mudstone. The abandoned channel type planar seepage barrier has a blocked upper part and a connected lower part of the sand body, formed by the filling of the original channel with silt as a result of the river breaking through the bend and taking a straight forward course during the flood period. The channel-overbank sand type is a weakly connected planar seepage barrier formed by intrusions present above and below the overbank sand bodies, which develop laterally between the channels by floodplain mudstone due to the poor physical properties and thin thickness of the overbank sand bodies. The floodplain mudstone type is a planar unconnected seepage barrier because only floodplain mudstone develops between the two-stage channel sand bodies.Figure 2
Types of planar seepage barriers.


### 2.3. Intralayer Seepage Barriers

Intralayer seepage barriers are mainly unstable, relatively low-permeability, or nonpermeable layers dispersed in sand bodies. Intralayer seepage barriers formed under different circumstances have different blocking abilities. On this basis, intralayer seepage barriers were divided into the mudstone type and the physical property type. The mudstone intralayer seepage barrier is mainly formed by the deposition of suspended fine-grained materials due to the weakening of hydrodynamics, with the lithology dominated by mudstone and silty mudstone, and the physical characteristics are similar to those of floodplain mudstone, but with a small thickness. The physical property of the intralayer seepage barrier is mainly formed by diagenetic cementation or sedimentation, with the lithology dominated by muddy siltstone, with a small thickness, and some permeability (Figure3).Figure 3
Types of intralayer seepage barriers.


## 2.1. Interlayer Seepage Barriers

Interlayer seepage barriers are mainly formed in a sedimentary environment with weak hydrodynamic conditions. They are dominated by mudstone deposits in the floodplain but can also be attributed to sandy mudstones due to insufficient channel undercutting. Changes in the source supply to accommodating space ratio affect the contact relationship and superposition styles between sand bodies. In turn, these directly affect the pattern and development characteristics of interlayer seepage barriers and thus affect the connectivity level of the reservoir. Based on the vertical superposition relationship between different sand bodies in the study area, the interlayer seepage barriers were classified into four categories, namely, (I) the overbank sand-overbank sand type with strong blockage, (II) the channel-overbank sand type, (III) the isolated channel superposition type, and (IV) the channel shallow-cut type. In addition to the above four categories of interlayer seepage barriers, there were also isolated sand body types (isolated overbank sand and isolated channel type) and the channel deep-cut type formed by channel undercutting (Figure1).Figure 1
Types of interlayer seepage barriers.


## 2.2. Planar Seepage Barriers

Planar seepage barriers are mainly the muddy sediments of the floodplain that accompany the joint formation of complex lateral shear erosion superposition relationships in the plane between channel sand bodies and other genetic sand bodies, during the process of “formation⟶development⟶lateral erosion⟶abandonment.” The scale of these seepage barriers is affected by the characteristics of the sand assemblages, based on which the planar seepage barriers were divided into four categories: channel boundary type, abandoned channel type, channel overbank sand type, and floodplain mudstone type (Figure2). A channel boundary type seepage barrier is formed when the distributary channel sand bodies of the same period in the plane are diverted laterally to connect and erode during travel, leading to the connection of the near upper part of the plane of the two stages of channel sand bodies while the lower part is the floodplain mudstone. The abandoned channel type planar seepage barrier has a blocked upper part and a connected lower part of the sand body, formed by the filling of the original channel with silt as a result of the river breaking through the bend and taking a straight forward course during the flood period. The channel-overbank sand type is a weakly connected planar seepage barrier formed by intrusions present above and below the overbank sand bodies, which develop laterally between the channels by floodplain mudstone due to the poor physical properties and thin thickness of the overbank sand bodies. The floodplain mudstone type is a planar unconnected seepage barrier because only floodplain mudstone develops between the two-stage channel sand bodies.Figure 2
Types of planar seepage barriers.


## 2.3. Intralayer Seepage Barriers

Intralayer seepage barriers are mainly unstable, relatively low-permeability, or nonpermeable layers dispersed in sand bodies. Intralayer seepage barriers formed under different circumstances have different blocking abilities. On this basis, intralayer seepage barriers were divided into the mudstone type and the physical property type. The mudstone intralayer seepage barrier is mainly formed by the deposition of suspended fine-grained materials due to the weakening of hydrodynamics, with the lithology dominated by mudstone and silty mudstone, and the physical characteristics are similar to those of floodplain mudstone, but with a small thickness. The physical property of the intralayer seepage barrier is mainly formed by diagenetic cementation or sedimentation, with the lithology dominated by muddy siltstone, with a small thickness, and some permeability (Figure3).Figure 3
Types of intralayer seepage barriers.


## 3. Characterization and Modeling of Seepage Barriers in Fluvial Reservoirs

### 3.1. Characterization of Seepage Barrier Parameters

To further characterize the seepage barriers based on the classification of the different levels of seepage barriers presented in the previous section, the seepage barriers were quantitatively characterized with respect to the physical properties and thickness of the different types of seepage barriers.


#### 3.1.1. Characterization of Interlayer Seepage Barrier Parameters

Depending on the vertical assemblages and contact relationship of sand bodies, certain differences in the scale and physical characteristics of different types of interlayer seepage barriers were noted during their classification. In this study, the reservoir quality coefficient (RQI) was referred to and adjusted in order to characterize seepage barriers to some extent. In essence, a seepage barrier can impede fluid movement because it is denser than the sand body and has extremely low Seepage characteristics, thus preventing fluid from passing through. Also, the larger the thickness of the interlayer seepage barrier, the larger the scale of the barrier, and thus, the more likely it is to achieve a stable separation of the sand body. Therefore, the seepage barrier index (SBI) was established by adding the thickness factor to the RQI in order to characterize the blocking ability of different types of seepage barriers.(1)SBI=0.01πHKϕ=0.0314HKϕ,where SBI is the seepage barrier index, H is the seepage barrier thickness (m), K is the seepage barrier permeability (md), and Φ is the seepage barrier porosity (%).To a certain extent, the SBI comprehensively reflects both the physical property and thickness characteristics of the seepage barrier. The SBI is small when the seepage barrier has poor physical characteristics and a large thickness, and the SBI is large when the seepage barrier has relatively good physical characteristics and a small thickness.The relationship between the SBI and the permeability of interlayer seepage barriers is plotted in Figure4(a). The overbank sand-overbank sand type is mainly formed by the general overbank during two successive extreme flood periods, with a lithology dominated by floodplain mudstone, a large thickness, poor overall physical properties, and an SBI distributed within the range of 0.001–0.003. The channel-overbank sand type is characterized by a second deposition developed after the initial deposition and has a floodplain mudstone thickness slightly smaller than that of the overbank sand-overbank sand type, poor physical properties, and an SBI distributed within the range of 0.001–0.0045. The isolated channel superposition type has a complete sedimentary sequence for each of the upper and lower channels of different stages, the thickness is controlled by the thickness between different stages of channels, the physical properties are poor, and the SBI is distributed within the range of 0.0015–0.014. The channel shallow-cut type is formed by the insufficient undercutting of the upper and lower channels of different stages, with a small thickness, a lithology barrier varying from silty mudstone to mudstone due to the weak scouring and erosion at the top, and an SBI distributed within the range of 0.008–0.083. Among other types, the isolated sand body types (isolated channel and isolated overbank sand) are sand bodies wrapped in extremely thick floodplain mudstone, and the channel deep-cut type is superposed by the undercutting of channels of multiple stages, resulting in excellent connectivity between the upper and lower layers. Based on this classification of interlayer seepage barriers, the independence of sand bodies in various parts of the plane can be sufficiently described (Table 1).Figure 4
Scatter cross plot of SBI and permeability. (a) Relationship between SBI and permeability of interlayer seepage barriers. (b) Relationship between SBI and permeability of intralayer seepage barriers.Table 1
Parameters of interlayer and intralayer seepage barriers.
TypePorosity (%)Permeability (10−3 μm2)Mudstone content (%)Thickness (m)SBI (10−2)Interlayer seepage barriersOverbank sand-overbank sand0.1–12.10.1–0.881–1001.5–8.10.1–0.3Overbank sand-channel0.1–12.40.1–0.881–1001.2–7.20.1–0.45Isolated channel superposition0.1–13.40.2–1.179–1001–5.20.15–1.4Channel shallow-cut0.1–17.40.2–3.235–800.2–1.90.8–8.3Intralayer seepage barriersMudstone7–20.71.2–3429–670.2–1.22–15Physical property17.4–27.29.5–21418–300.1–0.39–70


#### 3.1.2. Characterization of Intralayer Seepage Barrier Parameters

The intralayer seepage barriers are discriminated using the concept of the seepage barrier coefficient (Figure4(b)). The mudstone type intralayer seepage barriers have a lithology dominated by silty mudstone and mudstone, poor overall physical properties, a thickness of 0.2–1.2 m, and an SBI distributed within the range of 0.02–0.15, thus showing a strong blocking effect. The physical property type intralayer seepage barriers have an overall small thickness, physical properties inferior to those of the reservoir, a low mudstone content, an SBI distributed within the range of 0.09–0.7, and some connectivity (Table 1).


#### 3.1.3. Characterization of Planar Seepage Barrier Parameters

Planar seepage barriers are mainly floodplain mudstones accompanying other genetic sand body assemblages in the channel migration process. Considering that the blocking effect of planar seepage barriers is closely related to the connectivity of the reservoir, which in turn is limited by both physical property and thickness characteristics, to a certain extent RQIs can reflect the physical properties of the reservoir, and the reservoir can be further evaluated by also considering the thickness of sand bodies. Accordingly, the RQI is modified by multiplying the reservoir thickness to enhance the characterization:(2)RQI=0.01πHSKϕ=0.0314HSKϕ,where RQI is the reservoir quality index, Hs is the reservoir thickness (m), K is the average reservoir permeability (md), and Φ is the average reservoir porosity (%).The relationship between RQI and permeability is plotted (Figure5). The study area is judged to be in stage 5 of the channel, and sand bodies are spliced and assembled in the plane. There is a difference in the energy supply of different channel stages. Therefore, the RQI can provide some supporting evidence for the description of the boundaries of different channel stages. The abandoned channels are affected by the floodplain mudstone at the top, and hence, their RQI is lower than that of the distribution area of channel sand bodies and smaller than 2. The overbank sand bodies have poor physical properties, generally a small thickness, and an RQI is only higher than that of floodplain mudstone with a value less than 1. The floodplain mudstone has extremely poor physical properties, and as a result, its RQI is generally close to 0.1. The RQI is not only able to clarify the difference in the quality of different microfacies reservoirs but can also describe the contact sites of different stages of channels based on the similarity of RQIs.Figure 5
RQI distribution of different microfacies types.


### 3.2. Distribution Characteristics of Different Types of Seepage Barriers

#### 3.2.1. Distribution Characteristics of Interlayer Seepage Barriers

After clarifying the characteristics of different types of interlayer seepage barriers reflected by the SBI, the planar distribution of the upper and lower interlayer seepage barriers of the target layer was also described by further fusing and superposing the sedimentary facies distribution characteristics of different layers (Figure6). The upper interlayer seepage barrier of the Ng35 layer is limited by the small development scale of the channel in the Ng34 layer. The upper interlayer seepage barrier is dominated by isolated sand bodies and floodplain mudstone, which are of strong seepage barrier categories, and supplemented by a small number of channel shallow-cut types, showing a poor overall connectivity between upper and lower sand bodies. Due to the large scale of sand bodies in the northern channel of the lower Ng42 layer, the lower interlayer seepage barriers formed as the channel shallow-cut type at multiple sites, as the channel deep-cut type with good upper and lower connectivity at localized sites, and mostly as the isolated sand body type and floodplain mudstone type with strong connectivity in the southern part.Figure 6
Distribution characteristics of interlayer seepage barriers.


#### 3.2.2. Distribution Characteristics of Planar Seepage Barriers

After the classification and characterization of planar seepage barriers, the main distribution locations of planar seepage barriers in the target layer of the study area were visually clarified by further considering the planar distribution of sedimentary microfacies (Figure7). The main channel of the first stage (channel 2) from the northwest source was cut and eroded during the oscillatory migration of the other two stages of channels (channel 1 and channel 3) to form a large-scale composite channel and two-channel boundary type seepage barriers. Meanwhile, the main seepage barriers in this stage of the channel are formed within the plane of the abandoned channel that formed within channel 2, while mostly disconnected and weakly connected seepage barriers of the floodplain mudstone type and the overbank sand type are formed at the outer side of the southern channel (channel 5).Figure 7
Intensity diagram of the injection-production relationship under planar seepage barriers.


#### 3.2.3. Distribution Characteristics of Intralayer Seepage Barriers in Point Bars

A point bar is a sand-rich zone of sand bodies of the complex meandering belt in a meandering river, corresponding to the architectural unit defined by the level 4 interface proposed by Miall. The internal structure of the point bar is the most complex among all of the genetic sand bodies in the meandering river, consisting of several lateral accretions, between which intralayer seepage barriers with a lateral accretion pattern that obliquely intersects the bedding plane are developed. To investigate the development characteristics of different types of intralayer seepage barriers inside the point bar, the internal architecture of point-bar sand bodies was examined. The empirical equation proposed by Leeder [14] on the relationship between the bankfull depth, bankfull width, and width of a single lateral accretion in combination with the pair-wells method was used to determine that the dip angle of seepage barriers with a lateral accretion pattern in the study area is small (approximately 3°–10°). The relational expression proposed by Schumm [15] was employed to determine that the average width of the intercalation of the seepage barriers with a lateral accretion pattern in the sand bodies of the meandering channel at this location is less than 77 m. Based on the comprehensive classification of intralayer seepage barriers in the earlier sections of the article, the categories of intralayer seepage barriers with a lateral accretion pattern in the point-bar sand bodies of this layer were described.


### 3.3. 3D Modeling of Seepage Barriers

To finely characterize the spatial position categories of different levels of seepage barriers using a 3D geological model, the method of “hierarchical analysis, skeleton construction, and category fitting” was applied.“Hierarchical analysis” involved building a seepage barrier model at each of the interlayer, intralayer, and planar levels. “Skeleton construction” involved the establishment of a skeleton model of the seepage barrier based on parameters of the development position and thickness scale of the seepage barrier at the interlayer, intralayer, and planar levels. “Category fitting” involved using deterministic modeling and a sequential Gaussian indicator simulation on the basis of the skeleton model of the seepage barrier to characterize different levels of seepage barrier categories.Due to the complex cutting and superposition of the point-bar sand bodies in meandering rivers, it is always technically difficult to accurately describe and characterize the 3D features of the internal architecture of the point bar [15–21]. Therefore, this work applied a method for building the intralayer seepage barrier of the point bar based on the characteristics of intralayer seepage barriers under the constraint of architecture pattern during the skeleton construction. This method using petrel modeling software assigns the meaning of a thin layer to the intralayer seepage barrier with a lateral accretion pattern, which can not only describe the form of lateral accretion but can also finely characterize the feature information (e.g., type, physical properties, and thickness) of seepage barriers with a lateral accretion pattern:(1)
Combined with the anatomical structure of the point dam sand body in the study area, the spatial position and combination relationship of the internal building structure of the point dam, as well as the plane distribution position of the lateral interlayer, the density, and thickness of the lateral interlayer and other relevant parameters are obtained. The next work is carried out according to these parameters (Figure8(a)).(2)
According to the internal structure level of the sand body, it is given the characteristics of the layer above the well (Figure8(b)). From top to bottom, it is the top of the sand body layer and the top and bottom of the lateral accumulation seepage barrier at different stages. The connected layer top at the bottom of the sand body and the layer at the bottom of the sand body are determined according to the lateral accumulation form. The spatial position of the internal structural characteristics of point-dam sand body in the meandering river is divided into different levels, and the structural characteristics of the same level are classified, combined, and fixed, so as to realize the structural modeling of point-dam sand body under the constraint of the multilayer structure.(3)
According to the spatial distribution characteristics that the plane of lateral seepage barrier presents crescent shape and the vertical is inclined, the spatial form of lateral seepage barrier is divided into vertical and plane levels for research, respectively (Figure8(c)).(4)
According to the characteristic parameters of the lateral accretion interlayer dissected by the sand body configuration above, and combined with the above well position characteristics of each level of lateral accretion interlayer in step B, the vertical relationship curve of each lateral accretion interlayer is fitted. Together with the plane relationship fitting in step D, it forms the spatial relationship curve of the lateral interlayer, so that each lateral interlayer has a characteristic spatial fitting relationship. For the lateral interlayer that cannot be corrected on the well due to the well pattern density, the average comprehensive calculation is carried out through the spatial fitting relationship of the two adjacent lateral interlayer. Furthermore, the spatial characterization of lateral interlayer under fine anatomy of sand body configuration is realized (Figure8(d)).(5)
According to the spatial distribution characteristics of each seepage barrier, it is divided into vertical and plane combination modes (Figure8(c)).(6)
In combination with the relationship of various levels within the sand body established in the above steps, from low to high, it is sand body bottom, sand body bottom connecting layer top, N lateral seepage barrier layer bottom, and N lateral seepage barrier layer top. According to the sequence of No. 1 lateral seepage barrier layer bottom, No. 1 lateral seepage barrier layer top, sand body layer top, combined with the horizon modeling method of truncation method, the spatial stack and curved surface packaging are carried out, so as to realize the establishment of the structural framework under the internal structural characteristics of point-dam sand body (Figure8(f)).(7)
On the premise of establishing the structural framework in step 6, lithologic filling is carried out for the structural framework of different levels, so as to realize the three-dimensional characterization of the reservoir under the control of meandering river sand body configuration (Figure8(g)).Figure 8
Comprehensive distribution pattern of seepage barriers.The 3D characterization of interlayer, intralayer, and planar seepage barriers was realized using the method of “hierarchical analysis, skeleton construction, and category fitting” (Figure9). This method can accurately characterize the scale and category of each level of seepage barriers, which provide a more solid database for subsequent numerical simulations and other studies. A comprehensive distribution pattern diagram of seepage barriers was established (Figure 10) based on the classification, parameter characterization, and 3D geological modeling of different levels of seepage barriers.Figure 9
Water injection profiles in different periods under the control of interlayer seepage barriers.Figure 10
Water injection profiles in different periods under the control of intralayer barrier systems.
(a)(b)(c)(d)(e)(f)


## 3.1. Characterization of Seepage Barrier Parameters

To further characterize the seepage barriers based on the classification of the different levels of seepage barriers presented in the previous section, the seepage barriers were quantitatively characterized with respect to the physical properties and thickness of the different types of seepage barriers.


### 3.1.1. Characterization of Interlayer Seepage Barrier Parameters

Depending on the vertical assemblages and contact relationship of sand bodies, certain differences in the scale and physical characteristics of different types of interlayer seepage barriers were noted during their classification. In this study, the reservoir quality coefficient (RQI) was referred to and adjusted in order to characterize seepage barriers to some extent. In essence, a seepage barrier can impede fluid movement because it is denser than the sand body and has extremely low Seepage characteristics, thus preventing fluid from passing through. Also, the larger the thickness of the interlayer seepage barrier, the larger the scale of the barrier, and thus, the more likely it is to achieve a stable separation of the sand body. Therefore, the seepage barrier index (SBI) was established by adding the thickness factor to the RQI in order to characterize the blocking ability of different types of seepage barriers.(1)SBI=0.01πHKϕ=0.0314HKϕ,where SBI is the seepage barrier index, H is the seepage barrier thickness (m), K is the seepage barrier permeability (md), and Φ is the seepage barrier porosity (%).To a certain extent, the SBI comprehensively reflects both the physical property and thickness characteristics of the seepage barrier. The SBI is small when the seepage barrier has poor physical characteristics and a large thickness, and the SBI is large when the seepage barrier has relatively good physical characteristics and a small thickness.The relationship between the SBI and the permeability of interlayer seepage barriers is plotted in Figure4(a). The overbank sand-overbank sand type is mainly formed by the general overbank during two successive extreme flood periods, with a lithology dominated by floodplain mudstone, a large thickness, poor overall physical properties, and an SBI distributed within the range of 0.001–0.003. The channel-overbank sand type is characterized by a second deposition developed after the initial deposition and has a floodplain mudstone thickness slightly smaller than that of the overbank sand-overbank sand type, poor physical properties, and an SBI distributed within the range of 0.001–0.0045. The isolated channel superposition type has a complete sedimentary sequence for each of the upper and lower channels of different stages, the thickness is controlled by the thickness between different stages of channels, the physical properties are poor, and the SBI is distributed within the range of 0.0015–0.014. The channel shallow-cut type is formed by the insufficient undercutting of the upper and lower channels of different stages, with a small thickness, a lithology barrier varying from silty mudstone to mudstone due to the weak scouring and erosion at the top, and an SBI distributed within the range of 0.008–0.083. Among other types, the isolated sand body types (isolated channel and isolated overbank sand) are sand bodies wrapped in extremely thick floodplain mudstone, and the channel deep-cut type is superposed by the undercutting of channels of multiple stages, resulting in excellent connectivity between the upper and lower layers. Based on this classification of interlayer seepage barriers, the independence of sand bodies in various parts of the plane can be sufficiently described (Table 1).Figure 4
Scatter cross plot of SBI and permeability. (a) Relationship between SBI and permeability of interlayer seepage barriers. (b) Relationship between SBI and permeability of intralayer seepage barriers.Table 1
Parameters of interlayer and intralayer seepage barriers.
TypePorosity (%)Permeability (10−3 μm2)Mudstone content (%)Thickness (m)SBI (10−2)Interlayer seepage barriersOverbank sand-overbank sand0.1–12.10.1–0.881–1001.5–8.10.1–0.3Overbank sand-channel0.1–12.40.1–0.881–1001.2–7.20.1–0.45Isolated channel superposition0.1–13.40.2–1.179–1001–5.20.15–1.4Channel shallow-cut0.1–17.40.2–3.235–800.2–1.90.8–8.3Intralayer seepage barriersMudstone7–20.71.2–3429–670.2–1.22–15Physical property17.4–27.29.5–21418–300.1–0.39–70


### 3.1.2. Characterization of Intralayer Seepage Barrier Parameters

The intralayer seepage barriers are discriminated using the concept of the seepage barrier coefficient (Figure4(b)). The mudstone type intralayer seepage barriers have a lithology dominated by silty mudstone and mudstone, poor overall physical properties, a thickness of 0.2–1.2 m, and an SBI distributed within the range of 0.02–0.15, thus showing a strong blocking effect. The physical property type intralayer seepage barriers have an overall small thickness, physical properties inferior to those of the reservoir, a low mudstone content, an SBI distributed within the range of 0.09–0.7, and some connectivity (Table 1).


### 3.1.3. Characterization of Planar Seepage Barrier Parameters

Planar seepage barriers are mainly floodplain mudstones accompanying other genetic sand body assemblages in the channel migration process. Considering that the blocking effect of planar seepage barriers is closely related to the connectivity of the reservoir, which in turn is limited by both physical property and thickness characteristics, to a certain extent RQIs can reflect the physical properties of the reservoir, and the reservoir can be further evaluated by also considering the thickness of sand bodies. Accordingly, the RQI is modified by multiplying the reservoir thickness to enhance the characterization:(2)RQI=0.01πHSKϕ=0.0314HSKϕ,where RQI is the reservoir quality index, Hs is the reservoir thickness (m), K is the average reservoir permeability (md), and Φ is the average reservoir porosity (%).The relationship between RQI and permeability is plotted (Figure5). The study area is judged to be in stage 5 of the channel, and sand bodies are spliced and assembled in the plane. There is a difference in the energy supply of different channel stages. Therefore, the RQI can provide some supporting evidence for the description of the boundaries of different channel stages. The abandoned channels are affected by the floodplain mudstone at the top, and hence, their RQI is lower than that of the distribution area of channel sand bodies and smaller than 2. The overbank sand bodies have poor physical properties, generally a small thickness, and an RQI is only higher than that of floodplain mudstone with a value less than 1. The floodplain mudstone has extremely poor physical properties, and as a result, its RQI is generally close to 0.1. The RQI is not only able to clarify the difference in the quality of different microfacies reservoirs but can also describe the contact sites of different stages of channels based on the similarity of RQIs.Figure 5
RQI distribution of different microfacies types.


## 3.1.1. Characterization of Interlayer Seepage Barrier Parameters

Depending on the vertical assemblages and contact relationship of sand bodies, certain differences in the scale and physical characteristics of different types of interlayer seepage barriers were noted during their classification. In this study, the reservoir quality coefficient (RQI) was referred to and adjusted in order to characterize seepage barriers to some extent. In essence, a seepage barrier can impede fluid movement because it is denser than the sand body and has extremely low Seepage characteristics, thus preventing fluid from passing through. Also, the larger the thickness of the interlayer seepage barrier, the larger the scale of the barrier, and thus, the more likely it is to achieve a stable separation of the sand body. Therefore, the seepage barrier index (SBI) was established by adding the thickness factor to the RQI in order to characterize the blocking ability of different types of seepage barriers.(1)SBI=0.01πHKϕ=0.0314HKϕ,where SBI is the seepage barrier index, H is the seepage barrier thickness (m), K is the seepage barrier permeability (md), and Φ is the seepage barrier porosity (%).To a certain extent, the SBI comprehensively reflects both the physical property and thickness characteristics of the seepage barrier. The SBI is small when the seepage barrier has poor physical characteristics and a large thickness, and the SBI is large when the seepage barrier has relatively good physical characteristics and a small thickness.The relationship between the SBI and the permeability of interlayer seepage barriers is plotted in Figure4(a). The overbank sand-overbank sand type is mainly formed by the general overbank during two successive extreme flood periods, with a lithology dominated by floodplain mudstone, a large thickness, poor overall physical properties, and an SBI distributed within the range of 0.001–0.003. The channel-overbank sand type is characterized by a second deposition developed after the initial deposition and has a floodplain mudstone thickness slightly smaller than that of the overbank sand-overbank sand type, poor physical properties, and an SBI distributed within the range of 0.001–0.0045. The isolated channel superposition type has a complete sedimentary sequence for each of the upper and lower channels of different stages, the thickness is controlled by the thickness between different stages of channels, the physical properties are poor, and the SBI is distributed within the range of 0.0015–0.014. The channel shallow-cut type is formed by the insufficient undercutting of the upper and lower channels of different stages, with a small thickness, a lithology barrier varying from silty mudstone to mudstone due to the weak scouring and erosion at the top, and an SBI distributed within the range of 0.008–0.083. Among other types, the isolated sand body types (isolated channel and isolated overbank sand) are sand bodies wrapped in extremely thick floodplain mudstone, and the channel deep-cut type is superposed by the undercutting of channels of multiple stages, resulting in excellent connectivity between the upper and lower layers. Based on this classification of interlayer seepage barriers, the independence of sand bodies in various parts of the plane can be sufficiently described (Table 1).Figure 4
Scatter cross plot of SBI and permeability. (a) Relationship between SBI and permeability of interlayer seepage barriers. (b) Relationship between SBI and permeability of intralayer seepage barriers.Table 1
Parameters of interlayer and intralayer seepage barriers.
TypePorosity (%)Permeability (10−3 μm2)Mudstone content (%)Thickness (m)SBI (10−2)Interlayer seepage barriersOverbank sand-overbank sand0.1–12.10.1–0.881–1001.5–8.10.1–0.3Overbank sand-channel0.1–12.40.1–0.881–1001.2–7.20.1–0.45Isolated channel superposition0.1–13.40.2–1.179–1001–5.20.15–1.4Channel shallow-cut0.1–17.40.2–3.235–800.2–1.90.8–8.3Intralayer seepage barriersMudstone7–20.71.2–3429–670.2–1.22–15Physical property17.4–27.29.5–21418–300.1–0.39–70


## 3.1.2. Characterization of Intralayer Seepage Barrier Parameters

The intralayer seepage barriers are discriminated using the concept of the seepage barrier coefficient (Figure4(b)). The mudstone type intralayer seepage barriers have a lithology dominated by silty mudstone and mudstone, poor overall physical properties, a thickness of 0.2–1.2 m, and an SBI distributed within the range of 0.02–0.15, thus showing a strong blocking effect. The physical property type intralayer seepage barriers have an overall small thickness, physical properties inferior to those of the reservoir, a low mudstone content, an SBI distributed within the range of 0.09–0.7, and some connectivity (Table 1).


## 3.1.3. Characterization of Planar Seepage Barrier Parameters

Planar seepage barriers are mainly floodplain mudstones accompanying other genetic sand body assemblages in the channel migration process. Considering that the blocking effect of planar seepage barriers is closely related to the connectivity of the reservoir, which in turn is limited by both physical property and thickness characteristics, to a certain extent RQIs can reflect the physical properties of the reservoir, and the reservoir can be further evaluated by also considering the thickness of sand bodies. Accordingly, the RQI is modified by multiplying the reservoir thickness to enhance the characterization:(2)RQI=0.01πHSKϕ=0.0314HSKϕ,where RQI is the reservoir quality index, Hs is the reservoir thickness (m), K is the average reservoir permeability (md), and Φ is the average reservoir porosity (%).The relationship between RQI and permeability is plotted (Figure5). The study area is judged to be in stage 5 of the channel, and sand bodies are spliced and assembled in the plane. There is a difference in the energy supply of different channel stages. Therefore, the RQI can provide some supporting evidence for the description of the boundaries of different channel stages. The abandoned channels are affected by the floodplain mudstone at the top, and hence, their RQI is lower than that of the distribution area of channel sand bodies and smaller than 2. The overbank sand bodies have poor physical properties, generally a small thickness, and an RQI is only higher than that of floodplain mudstone with a value less than 1. The floodplain mudstone has extremely poor physical properties, and as a result, its RQI is generally close to 0.1. The RQI is not only able to clarify the difference in the quality of different microfacies reservoirs but can also describe the contact sites of different stages of channels based on the similarity of RQIs.Figure 5
RQI distribution of different microfacies types.


## 3.2. Distribution Characteristics of Different Types of Seepage Barriers

### 3.2.1. Distribution Characteristics of Interlayer Seepage Barriers

After clarifying the characteristics of different types of interlayer seepage barriers reflected by the SBI, the planar distribution of the upper and lower interlayer seepage barriers of the target layer was also described by further fusing and superposing the sedimentary facies distribution characteristics of different layers (Figure6). The upper interlayer seepage barrier of the Ng35 layer is limited by the small development scale of the channel in the Ng34 layer. The upper interlayer seepage barrier is dominated by isolated sand bodies and floodplain mudstone, which are of strong seepage barrier categories, and supplemented by a small number of channel shallow-cut types, showing a poor overall connectivity between upper and lower sand bodies. Due to the large scale of sand bodies in the northern channel of the lower Ng42 layer, the lower interlayer seepage barriers formed as the channel shallow-cut type at multiple sites, as the channel deep-cut type with good upper and lower connectivity at localized sites, and mostly as the isolated sand body type and floodplain mudstone type with strong connectivity in the southern part.Figure 6
Distribution characteristics of interlayer seepage barriers.


### 3.2.2. Distribution Characteristics of Planar Seepage Barriers

After the classification and characterization of planar seepage barriers, the main distribution locations of planar seepage barriers in the target layer of the study area were visually clarified by further considering the planar distribution of sedimentary microfacies (Figure7). The main channel of the first stage (channel 2) from the northwest source was cut and eroded during the oscillatory migration of the other two stages of channels (channel 1 and channel 3) to form a large-scale composite channel and two-channel boundary type seepage barriers. Meanwhile, the main seepage barriers in this stage of the channel are formed within the plane of the abandoned channel that formed within channel 2, while mostly disconnected and weakly connected seepage barriers of the floodplain mudstone type and the overbank sand type are formed at the outer side of the southern channel (channel 5).Figure 7
Intensity diagram of the injection-production relationship under planar seepage barriers.


### 3.2.3. Distribution Characteristics of Intralayer Seepage Barriers in Point Bars

A point bar is a sand-rich zone of sand bodies of the complex meandering belt in a meandering river, corresponding to the architectural unit defined by the level 4 interface proposed by Miall. The internal structure of the point bar is the most complex among all of the genetic sand bodies in the meandering river, consisting of several lateral accretions, between which intralayer seepage barriers with a lateral accretion pattern that obliquely intersects the bedding plane are developed. To investigate the development characteristics of different types of intralayer seepage barriers inside the point bar, the internal architecture of point-bar sand bodies was examined. The empirical equation proposed by Leeder [14] on the relationship between the bankfull depth, bankfull width, and width of a single lateral accretion in combination with the pair-wells method was used to determine that the dip angle of seepage barriers with a lateral accretion pattern in the study area is small (approximately 3°–10°). The relational expression proposed by Schumm [15] was employed to determine that the average width of the intercalation of the seepage barriers with a lateral accretion pattern in the sand bodies of the meandering channel at this location is less than 77 m. Based on the comprehensive classification of intralayer seepage barriers in the earlier sections of the article, the categories of intralayer seepage barriers with a lateral accretion pattern in the point-bar sand bodies of this layer were described.


## 3.2.1. Distribution Characteristics of Interlayer Seepage Barriers

After clarifying the characteristics of different types of interlayer seepage barriers reflected by the SBI, the planar distribution of the upper and lower interlayer seepage barriers of the target layer was also described by further fusing and superposing the sedimentary facies distribution characteristics of different layers (Figure6). The upper interlayer seepage barrier of the Ng35 layer is limited by the small development scale of the channel in the Ng34 layer. The upper interlayer seepage barrier is dominated by isolated sand bodies and floodplain mudstone, which are of strong seepage barrier categories, and supplemented by a small number of channel shallow-cut types, showing a poor overall connectivity between upper and lower sand bodies. Due to the large scale of sand bodies in the northern channel of the lower Ng42 layer, the lower interlayer seepage barriers formed as the channel shallow-cut type at multiple sites, as the channel deep-cut type with good upper and lower connectivity at localized sites, and mostly as the isolated sand body type and floodplain mudstone type with strong connectivity in the southern part.Figure 6
Distribution characteristics of interlayer seepage barriers.


## 3.2.2. Distribution Characteristics of Planar Seepage Barriers

After the classification and characterization of planar seepage barriers, the main distribution locations of planar seepage barriers in the target layer of the study area were visually clarified by further considering the planar distribution of sedimentary microfacies (Figure7). The main channel of the first stage (channel 2) from the northwest source was cut and eroded during the oscillatory migration of the other two stages of channels (channel 1 and channel 3) to form a large-scale composite channel and two-channel boundary type seepage barriers. Meanwhile, the main seepage barriers in this stage of the channel are formed within the plane of the abandoned channel that formed within channel 2, while mostly disconnected and weakly connected seepage barriers of the floodplain mudstone type and the overbank sand type are formed at the outer side of the southern channel (channel 5).Figure 7
Intensity diagram of the injection-production relationship under planar seepage barriers.


## 3.2.3. Distribution Characteristics of Intralayer Seepage Barriers in Point Bars

A point bar is a sand-rich zone of sand bodies of the complex meandering belt in a meandering river, corresponding to the architectural unit defined by the level 4 interface proposed by Miall. The internal structure of the point bar is the most complex among all of the genetic sand bodies in the meandering river, consisting of several lateral accretions, between which intralayer seepage barriers with a lateral accretion pattern that obliquely intersects the bedding plane are developed. To investigate the development characteristics of different types of intralayer seepage barriers inside the point bar, the internal architecture of point-bar sand bodies was examined. The empirical equation proposed by Leeder [14] on the relationship between the bankfull depth, bankfull width, and width of a single lateral accretion in combination with the pair-wells method was used to determine that the dip angle of seepage barriers with a lateral accretion pattern in the study area is small (approximately 3°–10°). The relational expression proposed by Schumm [15] was employed to determine that the average width of the intercalation of the seepage barriers with a lateral accretion pattern in the sand bodies of the meandering channel at this location is less than 77 m. Based on the comprehensive classification of intralayer seepage barriers in the earlier sections of the article, the categories of intralayer seepage barriers with a lateral accretion pattern in the point-bar sand bodies of this layer were described.


## 3.3. 3D Modeling of Seepage Barriers

To finely characterize the spatial position categories of different levels of seepage barriers using a 3D geological model, the method of “hierarchical analysis, skeleton construction, and category fitting” was applied.“Hierarchical analysis” involved building a seepage barrier model at each of the interlayer, intralayer, and planar levels. “Skeleton construction” involved the establishment of a skeleton model of the seepage barrier based on parameters of the development position and thickness scale of the seepage barrier at the interlayer, intralayer, and planar levels. “Category fitting” involved using deterministic modeling and a sequential Gaussian indicator simulation on the basis of the skeleton model of the seepage barrier to characterize different levels of seepage barrier categories.Due to the complex cutting and superposition of the point-bar sand bodies in meandering rivers, it is always technically difficult to accurately describe and characterize the 3D features of the internal architecture of the point bar [15–21]. Therefore, this work applied a method for building the intralayer seepage barrier of the point bar based on the characteristics of intralayer seepage barriers under the constraint of architecture pattern during the skeleton construction. This method using petrel modeling software assigns the meaning of a thin layer to the intralayer seepage barrier with a lateral accretion pattern, which can not only describe the form of lateral accretion but can also finely characterize the feature information (e.g., type, physical properties, and thickness) of seepage barriers with a lateral accretion pattern:(1)
Combined with the anatomical structure of the point dam sand body in the study area, the spatial position and combination relationship of the internal building structure of the point dam, as well as the plane distribution position of the lateral interlayer, the density, and thickness of the lateral interlayer and other relevant parameters are obtained. The next work is carried out according to these parameters (Figure8(a)).(2)
According to the internal structure level of the sand body, it is given the characteristics of the layer above the well (Figure8(b)). From top to bottom, it is the top of the sand body layer and the top and bottom of the lateral accumulation seepage barrier at different stages. The connected layer top at the bottom of the sand body and the layer at the bottom of the sand body are determined according to the lateral accumulation form. The spatial position of the internal structural characteristics of point-dam sand body in the meandering river is divided into different levels, and the structural characteristics of the same level are classified, combined, and fixed, so as to realize the structural modeling of point-dam sand body under the constraint of the multilayer structure.(3)
According to the spatial distribution characteristics that the plane of lateral seepage barrier presents crescent shape and the vertical is inclined, the spatial form of lateral seepage barrier is divided into vertical and plane levels for research, respectively (Figure8(c)).(4)
According to the characteristic parameters of the lateral accretion interlayer dissected by the sand body configuration above, and combined with the above well position characteristics of each level of lateral accretion interlayer in step B, the vertical relationship curve of each lateral accretion interlayer is fitted. Together with the plane relationship fitting in step D, it forms the spatial relationship curve of the lateral interlayer, so that each lateral interlayer has a characteristic spatial fitting relationship. For the lateral interlayer that cannot be corrected on the well due to the well pattern density, the average comprehensive calculation is carried out through the spatial fitting relationship of the two adjacent lateral interlayer. Furthermore, the spatial characterization of lateral interlayer under fine anatomy of sand body configuration is realized (Figure8(d)).(5)
According to the spatial distribution characteristics of each seepage barrier, it is divided into vertical and plane combination modes (Figure8(c)).(6)
In combination with the relationship of various levels within the sand body established in the above steps, from low to high, it is sand body bottom, sand body bottom connecting layer top, N lateral seepage barrier layer bottom, and N lateral seepage barrier layer top. According to the sequence of No. 1 lateral seepage barrier layer bottom, No. 1 lateral seepage barrier layer top, sand body layer top, combined with the horizon modeling method of truncation method, the spatial stack and curved surface packaging are carried out, so as to realize the establishment of the structural framework under the internal structural characteristics of point-dam sand body (Figure8(f)).(7)
On the premise of establishing the structural framework in step 6, lithologic filling is carried out for the structural framework of different levels, so as to realize the three-dimensional characterization of the reservoir under the control of meandering river sand body configuration (Figure8(g)).Figure 8
Comprehensive distribution pattern of seepage barriers.The 3D characterization of interlayer, intralayer, and planar seepage barriers was realized using the method of “hierarchical analysis, skeleton construction, and category fitting” (Figure9). This method can accurately characterize the scale and category of each level of seepage barriers, which provide a more solid database for subsequent numerical simulations and other studies. A comprehensive distribution pattern diagram of seepage barriers was established (Figure 10) based on the classification, parameter characterization, and 3D geological modeling of different levels of seepage barriers.Figure 9
Water injection profiles in different periods under the control of interlayer seepage barriers.Figure 10
Water injection profiles in different periods under the control of intralayer barrier systems.
(a)(b)(c)(d)(e)(f)


## 4. Impact of Seepage Barriers on Water Injection Development

Since seepage barriers are characterized by blocking fluid transport, and the complexity of seepage mechanism [22–26], which makes them the main geological factor controlling the complex water-flooding process in thick oil layers, the clarification of the distribution characteristics and distribution categories of different types of seepage barriers provides theoretical support for the elucidation of the injection-production relationship.


### 4.1. Impact of Interlayer Seepage Barriers on Water Injection Development

The main production layer in the study area is typically characterized by large reservoir thickness, wide distribution, good connectivity, and reservoir concentration. Therefore, it is often developed as a whole set of reservoir units. The existence of interlayer seepage barriers between reservoir units can easily lead to interlayer interference in the water injection development process, resulting in incompatible injection-production relations and low injection-production efficiency. The strength of seepage barriers at different positions can be determined intuitively based on the classification and planar description of interlayer seepage barriers presented in earlier sections of this article. A comparison of the continuous tracer flow injection profiles in different periods between layers (Figure11) indicated that the water injection characteristics of different layers are well independent under the stable control of isolated channel superposition type interlayer seepage barriers. The existence of different types of seepage barriers leads to different water absorption capacities in different layers in the process of water injection development. As can be seen from the water injection profiles of the well in different periods, water injection in the Ng35, Ng44, and Ng43 layers showed some unevenness during the initial injection, and the water injection in the Ng43 layer was improved after water injection in the Ng44 layer was stopped in 2007.Figure 11
Water injection profiles in different periods under the control of interlayer seepage barriers.


### 4.2. Impact of Intralayer Seepage Barriers on Water Injection Development

Intralayer seepage barriers are often thin mudstone and relatively small in scale, so there is a certain hydrodynamic connection between the upper and lower parts of the oil layer, which is generally manifested as the downward channeling of water injection. As can be seen from the water injection profile of the well at different moments in time (Figure12), multiple sets of intralayer seepage barriers have developed in Ng35. The initial water injection profile shows a multistep distribution due to the presence of intralayer seepage barriers. In the later stage of water injection, the water injection volume in the lower part increases due to gravity, whereas the overall water injection decreases rapidly since the physical property type seepage barriers have strong connectivity. Although the mudstone type intralayer seepage barriers do not ensure the independence of sand bodies the way strong interlayer seepage barriers do, they still demonstrate a slow decrease of water injection and have a certain blocking effect.Figure 12
3D characterization of intralayer seepage barriers with a lateral accretion pattern.


### 4.3. Impact of Planar Seepage Barriers on Water Injection Development

A planar graph of the injection-production relationship of this layer was established based on the information of the water injection wells and oil wells currently in operation in the study area. According to the type of planar seepage barriers, the intensity of the dominant injection-production relationship under planar seepage barriers was described by means of arrows indicating the injection-production relationship (Figure7). Due to the semiconnectivity of the abandoned channel type planar seepage barriers, the injection-production effect around the abandoned channel is relatively poor, which is the main reason for the imperfect injection-production relationship under the planar seepage barriers in the study area. Meanwhile, there are some semiconnected areas caused by the channel boundary type planar seepage barriers. The description of the planar seepage barriers provides guidance for the subsequent improvement of the injection-production relationship.Compound flooding solution was injected into well block 42 in May 1997 and took effect in December 1998. Alkali and polymer were found in well 52, and the concentration of alkali and polymer increased sharply. The interference well test results show that the contributions of four injection wells to 42 central wells vary greatly, of which well 52 has the largest contribution and well 32 has no injection contribution (Table2). According to the sand body configuration characteristics, the contribution difference of the four wells is analyzed (Figure 13). (1) The lateral accretion interlayer between well 52 and well 42 is weakly developed, the fluid flow channel is large, and the water injection wave and volume are large, so the polymer effect is fast. (2) The water injection between well 41 and well 42 is in the reverse inclined lateral accretion interlayer direction, the fluid flow channel is distributed along the lateral accretion interlayer from the bottom connecting channel, and the water injection sweep range is large. (3) The water injection between well 31 and well 42 is in the direction of inclined lateral accretion interlayer, the fluid flow channel is distributed along the lateral accretion interlayer and converges to the bottom of the sand body, and the water injection only spreads to the bottom. (4) The abandoned river channel is separated, and the sand body connectivity is very poor.Table 2
Interference well test interpretation results in well block 42.
Well415231Total flow coefficient1657.15657.1964.3Mobility (10−3μm2·(MPa·s)−1)88.85397.156Permeability (10−3 μm2)9221480792Total energy storage coefficient (10−3 MPa−1)1.4295.3049.544Specific reservoir coefficient (10−3 Pa−1)0.0770.3730.055Pressure coefficient (cm2·s−1)1.1541.0651.018Figure 13
Comprehensive 3D characterization of multilevel seepage barriers.Therefore, the structural characteristics inside the sand body greatly affect the distribution of fluid flow channels. This feature will make some inherent dominant channels of the reservoir ineffective due to the configuration characteristics (such as the middle and upper part of the well in ②). At the same time, the characteristics of lateral interlayer increase the flow at the bottom of the sand body, thus forcing the formation of dominant flow channels at the bottom of the sand body under the configuration characteristics.


## 4.1. Impact of Interlayer Seepage Barriers on Water Injection Development

The main production layer in the study area is typically characterized by large reservoir thickness, wide distribution, good connectivity, and reservoir concentration. Therefore, it is often developed as a whole set of reservoir units. The existence of interlayer seepage barriers between reservoir units can easily lead to interlayer interference in the water injection development process, resulting in incompatible injection-production relations and low injection-production efficiency. The strength of seepage barriers at different positions can be determined intuitively based on the classification and planar description of interlayer seepage barriers presented in earlier sections of this article. A comparison of the continuous tracer flow injection profiles in different periods between layers (Figure11) indicated that the water injection characteristics of different layers are well independent under the stable control of isolated channel superposition type interlayer seepage barriers. The existence of different types of seepage barriers leads to different water absorption capacities in different layers in the process of water injection development. As can be seen from the water injection profiles of the well in different periods, water injection in the Ng35, Ng44, and Ng43 layers showed some unevenness during the initial injection, and the water injection in the Ng43 layer was improved after water injection in the Ng44 layer was stopped in 2007.Figure 11
Water injection profiles in different periods under the control of interlayer seepage barriers.


## 4.2. Impact of Intralayer Seepage Barriers on Water Injection Development

Intralayer seepage barriers are often thin mudstone and relatively small in scale, so there is a certain hydrodynamic connection between the upper and lower parts of the oil layer, which is generally manifested as the downward channeling of water injection. As can be seen from the water injection profile of the well at different moments in time (Figure12), multiple sets of intralayer seepage barriers have developed in Ng35. The initial water injection profile shows a multistep distribution due to the presence of intralayer seepage barriers. In the later stage of water injection, the water injection volume in the lower part increases due to gravity, whereas the overall water injection decreases rapidly since the physical property type seepage barriers have strong connectivity. Although the mudstone type intralayer seepage barriers do not ensure the independence of sand bodies the way strong interlayer seepage barriers do, they still demonstrate a slow decrease of water injection and have a certain blocking effect.Figure 12
3D characterization of intralayer seepage barriers with a lateral accretion pattern.


## 4.3. Impact of Planar Seepage Barriers on Water Injection Development

A planar graph of the injection-production relationship of this layer was established based on the information of the water injection wells and oil wells currently in operation in the study area. According to the type of planar seepage barriers, the intensity of the dominant injection-production relationship under planar seepage barriers was described by means of arrows indicating the injection-production relationship (Figure7). Due to the semiconnectivity of the abandoned channel type planar seepage barriers, the injection-production effect around the abandoned channel is relatively poor, which is the main reason for the imperfect injection-production relationship under the planar seepage barriers in the study area. Meanwhile, there are some semiconnected areas caused by the channel boundary type planar seepage barriers. The description of the planar seepage barriers provides guidance for the subsequent improvement of the injection-production relationship.Compound flooding solution was injected into well block 42 in May 1997 and took effect in December 1998. Alkali and polymer were found in well 52, and the concentration of alkali and polymer increased sharply. The interference well test results show that the contributions of four injection wells to 42 central wells vary greatly, of which well 52 has the largest contribution and well 32 has no injection contribution (Table2). According to the sand body configuration characteristics, the contribution difference of the four wells is analyzed (Figure 13). (1) The lateral accretion interlayer between well 52 and well 42 is weakly developed, the fluid flow channel is large, and the water injection wave and volume are large, so the polymer effect is fast. (2) The water injection between well 41 and well 42 is in the reverse inclined lateral accretion interlayer direction, the fluid flow channel is distributed along the lateral accretion interlayer from the bottom connecting channel, and the water injection sweep range is large. (3) The water injection between well 31 and well 42 is in the direction of inclined lateral accretion interlayer, the fluid flow channel is distributed along the lateral accretion interlayer and converges to the bottom of the sand body, and the water injection only spreads to the bottom. (4) The abandoned river channel is separated, and the sand body connectivity is very poor.Table 2
Interference well test interpretation results in well block 42.
Well415231Total flow coefficient1657.15657.1964.3Mobility (10−3μm2·(MPa·s)−1)88.85397.156Permeability (10−3 μm2)9221480792Total energy storage coefficient (10−3 MPa−1)1.4295.3049.544Specific reservoir coefficient (10−3 Pa−1)0.0770.3730.055Pressure coefficient (cm2·s−1)1.1541.0651.018Figure 13
Comprehensive 3D characterization of multilevel seepage barriers.Therefore, the structural characteristics inside the sand body greatly affect the distribution of fluid flow channels. This feature will make some inherent dominant channels of the reservoir ineffective due to the configuration characteristics (such as the middle and upper part of the well in ②). At the same time, the characteristics of lateral interlayer increase the flow at the bottom of the sand body, thus forcing the formation of dominant flow channels at the bottom of the sand body under the configuration characteristics.


## 5. Conclusions

(1)
To meet the ongoing needs of oilfield development, a method was proposed in this study for classifying seepage barriers in meandering river reservoirs. For research purposes, the seepage barriers were divided into interlayer, intralayer, and planar levels. The interlayer seepage barriers were divided into overbank sand-overbank sand, overbank sand-channel, isolated channel superposition, and channel shallow-cut types. The planar seepage barriers were divided into channel boundary, abandoned channel, channel overbank sand, and floodplain mudstone types. The intralayer seepage barriers were divided into mudstone and physical property types.(2)
Different seepage barriers were characterized parametrically using the seepage barrier coefficient and reservoir quality coefficient.(3)
The 3D characterization of seepage barriers was realized using the method of “hierarchical analysis, skeleton construction, and category fitting.” In addition, a method for establishing the intralayer seepage barriers in a point bar based on the intralayer seepage barrier characteristics under the constraint of an architectural pattern was proposed and used to describe intralayer seepage barriers with a lateral accretion pattern in the point bar of the meandering river.(4)
The blocking ability of different types of seepage barriers was verified using water injection profiles and the interpretation of data for water-flooded zones, providing guidance for the subsequent well pattern transformation and tapping of remaining oil as the next step.



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*Source: 8782179-2022-04-15.xml*