Identification of Interlayer in Strong Bottom Water Reservoir and Its Influence on Development Effect ()
1. Introduction
Interlayer is one of the key factors causing reservoir heterogeneity, and it is also a necessary research content in the fine characterization of the reservoir. The development of oil field and the formation and distribution of remaining oil in the reservoir are affected by many factors, such as the type of interlayer, vertical and horizontal distribution. Therefore, the research on the identification and characteristics of interlayer in the research area has a very important guiding role and practical value for the next exploitation of the oilfield (Liu & Cheng, 2013; Sun et al., 2020).
At present, most scholars generally agree that the interlayer is an impermeable rock layer with an area greater than half of the flow unit and small thickness restrictions (up to tens of meters thick and tens of centimeters thin), which can effectively block the free movement of fluid between different sand bodies. At the same time, generally speaking, the partition distribution is relatively stable and has good continuity. Interlayer refers to the impermeable rock stratum in the same sand body. Compared with the interlayer, the interlayer area is smaller, generally not more than half of the area of the flow unit, and the thickness is significantly smaller than the interlayer. It ranges from a few centimeters to dozens of centimeters, with poor continuity and unstable distribution. Its blocking effect on the fluid is limited, but it still has a significant impact on the fluid flow in the same sand body. However, when studying the interlayer and interlayer, people usually call the rock strata with low or zero permeability distributed in the reservoir as interlayer (Zheng et al., 2015; Zhang et al., 2011; Qu et al., 2013).
The G oil formation of X oilfield is a northeast trending drape anticline trap, clamped by faults on the East and west sides, and the structure is consistent with the basement morphology. The trap area is large, the trap amplitude is high, and it is relatively complete. Its top stratum is thin and its edge is thick, which is a typical drape anticline structure.
The stratum thickness of oil formation G is 220 - 450 m. On the profile, there is a large set of sandy conglomerate mixed with mudstone. The sand layer is developed, which is characterized by extremely thick sandstone and sandy gravel mixed with thin mudstone. The stratum thickness changes greatly on the plane. The buried depth of G oil formation is 1333 - 1650 m, which is composed of multiple sets of thick sandy conglomerate. An unstable sandstone strip (1 sublayer) is developed in the mudstone at the top of oil formation g, which is limited in distribution and only developed at the West high point. The oil layer is 2.2 - 7.5 m thick with an average thickness of 4.2 m. The main oil layer (2 sublayer) is developed in a set of glutenite with a thickness of nearly 80 m in the upper part of the oil formation. The reservoir is stably distributed. The thickness of the oil layer is 2.7 - 28.3 m, with an average thickness of 19.2 m.
Through the observation and research of core of coring well, it is the most direct (visual observation of core) and accurate (identification thickness is as small as a few centimeters) to identify the interlayer. At the same time, it can clearly see the sedimentary rhythm of the reservoir and the occurrence of the interlayer, but this method is limited by the data of coring well.
The genetic types of interlayers in fluvial reservoirs can be divided into two categories: those formed by sedimentation and those formed by diagenesis. For the shallow and middle sedimentary reservoirs in Bohai oilfield, because of the weak diagenesis, the interlayer is mainly formed by sedimentation. According to the influence mechanism of interlayer, it can be divided into argillaceous interlayer, calcareous interlayer and physical interlayer. From the perspective of development effect, the argillaceous interlayer has a great impact on the oilfield. The calcareous interlayer has little impact on the bottom water reservoir because it is cemented at the bottom, top and near the argillaceous blocks in the reservoir. The physical intercalation is mainly composed of silty mudstone and argillaceous fine and siltstone (Huang et al., 2018; Zhang et al., 2017). This kind of intercalation has certain porosity and permeability conditions, but does not reach the lower limit of physical properties of effective thickness, and its distribution is unstable; In addition, it also includes medium fine gravel, sandstone and disordered conglomerate physical intercalation supported by complex foundation. This kind of intercalation has high shale content, poor physical properties and strong micro heterogeneity. It is mainly residual sediment distributed in the main channel and braided channel, and is formed at the edge or flank of the channel. In view of the retarding effect of argillaceous intercalation and conglomerate intercalation on the rising rate of water cut in bottom water reservoir, it is taken as the key research.
2. Interlayer Identification
The interlayer identification is mainly based on core identification and logging identification, and the electrical discrimination standards (GR, den and CNC) are summarized. Argillaceous intercalation is characterized by high GR, high density and low resistivity (Liu et al., 2009; Luo et al., 2019; Wang et al., 2018).
After the transformation of GR, den and CNC neutron logging curves, it can be seen from the well connection profile that the curve section with both yellow and gray is the mud gravel interlayer section (Figure 1).
The statistical criteria for different electrical properties of argillaceous interlayer, physical interlayer and argillaceous gravel interlayer are as follows (Table 1). From the table, it can be seen that the electrical gamma of argillaceous interlayer and argillaceous gravel interlayer accounts for more than 0.6, the electrical
![]()
Figure 1. Identification features of well connection profile.
density accounts for more than 0.6, and the electrical neutron accounts for 0.2 - 0.7.
The main criterion for sandstone and mudstone is resistance, and sandstone shows high resistance of 1 - 95 Ω∙m. Mudstone mainly has low resistivity and high gamma value, and the resistivity is less than 2 Ω∙m. Gamma value is greater than 80; The logging response characteristics of mud gravel interlayer are “three high and one low”: high GR, high density, high resistivity, low neutron, and the low neutron value is 14% - 20% (Table 2).
3. Influence of Interlayer on Development Effect
3.1. With Interlayer
For the production well of the first layer with interlayer, the average water cut reaches 90% in 2 years of operation. Due to the slow rise of water content, the average initial water content is 34% (Table 3). The water cut curve is mainly “s” type, and the curve “inflection point” appears at 80%.
The average water cut of the second production well with interlayer reaches 90% in 2 years of operation (Table 4). The water cut curve is mainly of “s” type, and the curve “inflection point” appears at 90%.
3.2. No Interlayer
Without interlayer, the bottom water can quickly form water cone, and the average initial water content is 81.8% (Table 5).
![]()
Table 1. Statistical table of electrical discrimination standards for different interlayer.
![]()
Table 2. Interlayer identification standard of NG III oil formation in X Oilfield.
![]()
Table 3. Production statistics of the 1st sublayer with interlayer.
![]()
Table 4. Production statistics of the 2nd sublayer with interlayer.
![]()
Table 5. Statistics of production without interlayer.
4. Summary
1) The interlayer of sandstone reservoir is mudstone, which can be identified by core and logging. The argillaceous interlayer has the characteristics of high GR, high density and low resistivity. The weight of electric gamma of argillaceous intercalation and mud gravel intercalation is greater than 0.6, the weight of electric density is greater than 0.6, and the weight of electric neutron is between 0.2 - 0.7. The main criterion for sandstone and mudstone is resistance, and sandstone shows high resistance of 1 - 95 Ω∙m. Mudstone mainly has low resistivity and high gamma value, and the resistivity is less than 2 Ω∙m. Gamma value is greater than 80; The response characteristics of mud gravel interbed logging are “three high and one low”: high GR, high density, high resistivity, low neutron, and the low neutron value is 14% - 20%.
2) The interlayer of strong bottom water reservoir has a great influence on production performance. The water content of the non barrier interlayer rises rapidly, with an average annual water content of 90% after 14 months of operation. The bottom water can quickly form a water cone, and the average initial water content is 81.8%. When there is a layer of interlayer, the average water cut of the production well is 90% after it is put into operation for 2 years. The water cut curve is mainly of near “s” type, and the curve “inflection point” appears at 90%. When there are two layers of interlayer, due to the slow rise rate of water content, the average initial water content is 34%, and the water content curve is mainly “s” type, and the curve “inflection point” appears at 80%.