# The Visual and Quantitative Study of the Microoccurrence of Irreducible Water at the Pore and Throat System in a Low-Permeability Sandstone Reservoir by Using Microcomputerized Tomography

Authors: Xiaoyu Gu; Chunsheng Pu; Hai Huang; Nasir Khan; Jing Liu; Yanlong He; Cheng Jing; Feifei Huang; Aojiang Qi
Journal: Geofluids
 (2018)

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## Abstract

The microflow equipment monitored with micro X-ray computerized tomography (CT) is employed to investigate the microoccurrence of the irreducible water in a low-permeability sandstone core. By means of image segmentation and the three-dimensional (3D) image reconstruction technique, the visual microdistribution characteristics of irreducible water in two-dimensional (2D) slices and the 3D pore-throat system are quantitatively evaluated. Some interesting findings are list as below. Firstly, due to the variant micro geometric structures of the pore-throat systems, specific core slices showed significantly different irreducible water saturation even though these slices had same areal porosity. Secondly, due to the influence of capillary trapping and the existence of oil-wetting clay (main chlorite), the irreducible water saturation in the throat system (64%) is much larger than that in the pore system (36%). Furthermore, the wetting phase (irreducible water) did not spread all over the surface of the pore-throat network which caused a much more complicated oil-water two-phase interface. Thirdly, in micro scale, the main irreducible water occurrence mode in the pore system is much different from that in the throat system. In the pore system, the irreducible water principally existed in the corner of the pores which are linked through a water film. While in the throat system, the irreducible water occurrence is dominated by the water film. However, 25.5% of the throats are blocked by the irreducible water which cut off the crude oil drainage channels.


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## Body

## 1. Introduction

With the increasing worldwide demand for crude oil, low-permeability reservoirs are considered to overcome the energy deficit in developing countries [1, 2]. Theory and practice have proved that water flooding is the most economical technology to replenish formation energy and enhance oil recovery (EOR) in low-permeability reservoirs [3]. During the water flooding, many geological factors determine the efficiency of water drives, such as rock permeability, pore-throat structure, irreducible water saturation, and crude oil viscosity [4–6]. Among these factors, the microdistribution of irreducible water confined in the pore-throat system is one of the critical factors affecting the ultimate recovery [7]. Because of pore geometry structure, the microdistribution of irreducible water in porous media is very complicated. [8]. According to the results of Kewen Li and Roland N. Horne, the irreducible water saturation has a considerable effect on the spontaneous imbibition recovery. Higher irreducible water saturation is always correlated to the lower recovery tendency in low permeable rock [9]. This phenomenon is also verified by Wang et al. [10]. Therefore, how to obtain the quantitative visualized information about the irreducible water microdistribution is necessary to investigate the mechanisms of irreducible water on microflow behaviors in low permeable porous media. For this purpose, specific techniques are currently employed which will be discussed in the below paragraphs.The two-dimensional (2D) etching glass model and core displacement monitored by nuclear magnetic resonance (NMR) are the main qualitative and quantitative approaches, respectively, to look into the fluid occurrence and its distribution in porous media [11–14]. The principal advantage of the former experimental method is to visualize the water displacement. Specifically, the wettability difference between the natural porous surface and modeled interface poorly reflects the real microdisplacement in natural core samples. Also, the 2D etching glass model cannot be perfectly related to the natural 3D core sample. The above discrepancies have constrained the application of the qualitative approach. As far as core displacement based on NMR is concerned, it is a currently reliable method to determine the distribution of fluids in porous media [15–19]. However, 3D visualization of the natural core sample could not be perfectly developed. As a result, it is essential to look for such a novel experimental method which can perfectly encompass the quantitative microdistribution of irreducible water in 3D porous media along with the acquisition of high-resolution visualizations.The recent improvement in CT scanning resolution from micron to nanoscale makes it possible for the researchers to investigate pore scale and contained fluid in the low permeable core samples [20–23]. Due to high-precision 3D visualization and quantitative characterization, this technology overcomes the shortcomings of conventional experiment methods [24]. Nevertheless, owing to a limitation in the microflow device, CT scanning technology was frequently applied to analyze the micro pore-throat structure in dry core samples [25–27]. In contrast, this technology was seldom used in research of fluid microdistribution characteristics in low permeable sandstone. The considerable research was conducted to analyze water or gas flooding to investigate the evolution law of residual oil in high-permeability core samples using the CT scanning technique [28–30]. However, to the best of our knowledge, scarce literature is available that has perfectly visualized and quantified the microoccurrence of irreducible water in the pore and throat system [31]. Besides, owing to the huge difference between pore and throat geometries, particularly in low-permeability porous media, the microoccurrence of irreducible water is very complicated. Consequently, it led to lack of in-depth understanding of the microinfluence mechanisms of irreducible water on oil-water two-phase flow behaviors.The key aim of this study is to investigate the irreducible water microoccurrence which was confined in the pore and throat system in low permeable sandstone, taking the Chang2 low-permeability reservoir sandstone of Ordos Basin in northwest China as a case study. Afterwards, the microflow equipment monitored by micro X-ray computerized tomography (CT) is employed to investigate the microoccurrence of the irreducible water in the pore-throat system. Subsequently, the visible information of the pore-throat structure and irreducible water in the pore-throat network is acquired with 1.5 microns per pixel. Besides, the microdistribution characteristics of irreducible water in two-dimensional (2D) core slices and 3D pore-throat network are also quantitatively evaluated using the image segmentation and three-dimensional (3D) image reconstruction technique. As a consequence, our work would provide reliable experimental evidence for recognizing the micro hydrodynamic mechanisms which play a pivotal role in enhancing water-flooding displacement effectiveness.


## 2. Experiments

### 2.1. Rock Sample and Fluids

According to the China National Petroleum Corporation’s reservoir quality grading standards (SY/T6285-2011), the Triassic Yanchang Chang2 formation in Ordos Basin is declared as a typical low-porosity and low-permeability reservoir [32]. The oil and core samples were obtained from the Chang2 reservoir in Ordos Basin, northwest China. The parameters of the fluid and rock samples used in this experiment are as follows. Simulated oil had 6.4 mPa·s dynamic viscosity at 25°C temperature. Simulated formation brine was prepared by dissolving potassium iodide (KI). The salinity of formation water is 70,000–90,000 mg/L in the given formation. Therefore, 8 wt. % (KI) brine solution was injected for the reason to exactly simulate the formation water. In this way, the density difference between oil and brine (KI) was enhanced in comparison to distilled water. Furthermore, it could help to differentiate between the oil and water phase in the CT image. Consequently, contained fluids, such as water and oil, could easily be distinguished by image segmentation step. The sandstone core sample that was used in this study has a 5 mm diameter and 10 mm length as shown in Figure 1. The gas permeability and porosity are 4.25 × 10−3 μm2 and 15.3%, respectively.Figure 1
The CT scanning image of the core sample.


### 2.2. Experimental Apparatus

A micron CT scanning equipment and microdisplacement device are shown in Figure2. The experimental parameters of the microdisplacement system and CT scanning are all controlled by a computer. Other instruments used in this research goal are a HAAKE RS600 Rheometer (Thermo Scientific, Germany), a DV-III Brookfield viscometer (Brookfield Company, America), and a 510 model micrometer CT scanner (Zeiss, Germany).Figure 2
(a) The image of a microflow equipment monitored by micro X-ray computerized tomography. (b) The schematic illustration of the microflow equipment. Note: the pumps and valve arrangement is used to control fluid flow to the flow cell within the CT enclosure.
(a)
(b)


### 2.3. Experimental Procedure

Before acquiring a high-resolution image, the CT scanning equipment is configured with 50 KeV operational voltage, 1.5 s exposure time span, and 1.5 microns per pixel scanning precision. Besides, the slice thickness is 1.5μm which contains 1024 × 1024 pixels. These parameters can meet the requirement of pore size recognition above the micron level. The exact experimental procedure is as follows:
(1)
The system consisting of a CT scanner and flow cell system is assembled as shown in Figure2. After that, a selected core sample is fixed in the core holder.(2)
Vacuum (0.1 MPa) the core sample under a confining pressure of 0.1 MPa for 10 hours. Scan the core by CT to obtain the characteristics of pore-throat structures (pore volume as well) of the core.(3)
Close the vacuum valve. Then, saturate the core with brine (5 PV) at a low speed of 0.002 mL/min. The confining pressure was held at least 0.5 MPa higher than the injection pressure. Then the sample was kept for 72 hours.(4)
Saturate the core sample with oil (5 PV) by constant flowrate of 0.002 mL/min at room temperature (25°C). In this way, the oil was driven through the core to generate the irreducible water in the core. After keeping the core sample at room temperature (25°C) for 72 hours, CT scanning is conducted to obtain the microdistribution of the irreducible water contained in the pore-throat network.


## 2.1. Rock Sample and Fluids

According to the China National Petroleum Corporation’s reservoir quality grading standards (SY/T6285-2011), the Triassic Yanchang Chang2 formation in Ordos Basin is declared as a typical low-porosity and low-permeability reservoir [32]. The oil and core samples were obtained from the Chang2 reservoir in Ordos Basin, northwest China. The parameters of the fluid and rock samples used in this experiment are as follows. Simulated oil had 6.4 mPa·s dynamic viscosity at 25°C temperature. Simulated formation brine was prepared by dissolving potassium iodide (KI). The salinity of formation water is 70,000–90,000 mg/L in the given formation. Therefore, 8 wt. % (KI) brine solution was injected for the reason to exactly simulate the formation water. In this way, the density difference between oil and brine (KI) was enhanced in comparison to distilled water. Furthermore, it could help to differentiate between the oil and water phase in the CT image. Consequently, contained fluids, such as water and oil, could easily be distinguished by image segmentation step. The sandstone core sample that was used in this study has a 5 mm diameter and 10 mm length as shown in Figure 1. The gas permeability and porosity are 4.25 × 10−3 μm2 and 15.3%, respectively.Figure 1
The CT scanning image of the core sample.


## 2.2. Experimental Apparatus

A micron CT scanning equipment and microdisplacement device are shown in Figure2. The experimental parameters of the microdisplacement system and CT scanning are all controlled by a computer. Other instruments used in this research goal are a HAAKE RS600 Rheometer (Thermo Scientific, Germany), a DV-III Brookfield viscometer (Brookfield Company, America), and a 510 model micrometer CT scanner (Zeiss, Germany).Figure 2
(a) The image of a microflow equipment monitored by micro X-ray computerized tomography. (b) The schematic illustration of the microflow equipment. Note: the pumps and valve arrangement is used to control fluid flow to the flow cell within the CT enclosure.
(a)
(b)


## 2.3. Experimental Procedure

Before acquiring a high-resolution image, the CT scanning equipment is configured with 50 KeV operational voltage, 1.5 s exposure time span, and 1.5 microns per pixel scanning precision. Besides, the slice thickness is 1.5μm which contains 1024 × 1024 pixels. These parameters can meet the requirement of pore size recognition above the micron level. The exact experimental procedure is as follows:
(1)
The system consisting of a CT scanner and flow cell system is assembled as shown in Figure2. After that, a selected core sample is fixed in the core holder.(2)
Vacuum (0.1 MPa) the core sample under a confining pressure of 0.1 MPa for 10 hours. Scan the core by CT to obtain the characteristics of pore-throat structures (pore volume as well) of the core.(3)
Close the vacuum valve. Then, saturate the core with brine (5 PV) at a low speed of 0.002 mL/min. The confining pressure was held at least 0.5 MPa higher than the injection pressure. Then the sample was kept for 72 hours.(4)
Saturate the core sample with oil (5 PV) by constant flowrate of 0.002 mL/min at room temperature (25°C). In this way, the oil was driven through the core to generate the irreducible water in the core. After keeping the core sample at room temperature (25°C) for 72 hours, CT scanning is conducted to obtain the microdistribution of the irreducible water contained in the pore-throat network.


## 3. Image Processing and Analysis

### 3.1. Image Segmentation

Image segmentation is the fundamental part for the distinction of rock particles, the oil phase, and the water phase in the porous media. Median filtration method based on a nonlinear signal processing was used to minimize the noise in the scanning images. More specifically, the filtering mechanism is detailed in Andrew’s dissertation [33]. The images before and after median filtration are shown in Figure 3.Figure 3
Images filtration (a) before and (b) after.
(a)
(b)By following the abovementioned procedure, the core sample would contain three kinds of substances, such as rock particles, water, and oil. It is visualized on the basis of different X-ray propagations in the abovementioned phases which led to the variant scanning gray peak of each phase. The corresponding peak intensity for rock particles, water, and oil is 18,700, 10,400, and 11,700, respectively. Therefore, it could be distinguished from image segmentation. As a result, the acquired image segmentation outcomes are shown in Figure4.Figure 4
Images segmentation results: (a) intensity of gas and grain; (b) intensity of oil and brine; (c) pore-throat network and rock matrix; (d) oil and irreducible water. Note: yellow = pore-throat; gray = rock particles; blue = irreducible water; red = oil.
(a)
(b)
(c)
(d)


### 3.2. Image 3D Reconstruction

Two-dimensional image segmentation of core sample slices was obtained. Then, the distribution of pore-throat, oil, and water in all 2D slice images was acquired. In the next step, the ORS 3D software was used to add all the 2D images to become a 3D dataset. The 3D reconstruction principle is based on the marching cube algorithm [34]. After that, the visualized 3D distribution of the pore-throat, oil, and water in the core sample was obtained as shown in Figure 5 and Table 1.Figure 5
3D reconstruction results. Note: (a) distribution of the pore-throat system in the core sample; (b) distribution of irreducible water; (c) distribution of simulated oil.
(a)
(b)
(c)Table 1
Segmentation result of the core sample containing irreducible water.
Pore and throat (volume/μm3)
Irreducible water (volume/μm3)
Oil volume (μm3)
Irreducible water saturation (%)
5.80 × 107
2.14 × 107
3.67 × 107
36.94


### 3.3. Quantitative Analysis Method

#### 3.3.1. Calculation of Surface Area and Volume of the Data Cube

Pixels are the basic unit of the building dataset, and these pixel dots of irreducible water and rock particles were obtained after image segmentation. In the next step, the characteristic parameters, for instance, the surface area and the volume of irreducible water and the pore-throat in the core samples, can be calculated using the following equations.(1)Vi=Nv×VP,where Vi is the volume of pixel dots (μm3), Nv is the total number of the pixel dots of the same phase (dimensionless), and VP is the volume of a single pixel dot which is 3.375 μm3 in this experiment.
(2)Si=Ns×Sp,where Si is the surface area of a given phase (μm2), Ns is the number of pixel dots of a given phase (dimensionless), and Sp is the surface area of a single pixel dot which is 9.0 μm2 in this experiment.


#### 3.3.2. Microoccurrence of Irreducible Water in the Pore-Throat Network

Due to the micro-occurrence of irreducible water in pore-throat network is very complicated. Hence, different parameters, such as the shape factor and the irreducible water saturation in a given single pore-throat, were used to categorize the complex heterogeneous structures of the given irreducible water block.(3)Gi=6πVwSw1.5,where Gi is the shape factor of the irreducible water block (dimensionless), Vw is the volume (μm3), and Sw is the surface area of the irreducible water block (μm2). The shape factor represents the degree of sphericity of an object. In other terms, the lower the Gi value, the more would be the shape irregularity of the given irreducible water blocks and, subsequently, the higher would be the surface roughness. In contrast, the greater the Gi value, the more smooth the surface would be and the more likely the shape would be spherical.Saturation of irreducible water (SSwi′) shows the amount of irreducible water in a given pore or throat. This parameter can be obtained using (4). In general, the higher the irreducible water saturation (SSwi′), the lower would be the flowability in a given channel.
(4)SSwi′=VwVp‐t×100%,where SSwi′ is the saturation of irreducible water in either the pore or the throat (%), Vw is the irreducible water volume confined in a given pore or throat (μm3), and Vp−t is the pore or throat volume (μm3).


## 3.1. Image Segmentation

Image segmentation is the fundamental part for the distinction of rock particles, the oil phase, and the water phase in the porous media. Median filtration method based on a nonlinear signal processing was used to minimize the noise in the scanning images. More specifically, the filtering mechanism is detailed in Andrew’s dissertation [33]. The images before and after median filtration are shown in Figure 3.Figure 3
Images filtration (a) before and (b) after.
(a)
(b)By following the abovementioned procedure, the core sample would contain three kinds of substances, such as rock particles, water, and oil. It is visualized on the basis of different X-ray propagations in the abovementioned phases which led to the variant scanning gray peak of each phase. The corresponding peak intensity for rock particles, water, and oil is 18,700, 10,400, and 11,700, respectively. Therefore, it could be distinguished from image segmentation. As a result, the acquired image segmentation outcomes are shown in Figure4.Figure 4
Images segmentation results: (a) intensity of gas and grain; (b) intensity of oil and brine; (c) pore-throat network and rock matrix; (d) oil and irreducible water. Note: yellow = pore-throat; gray = rock particles; blue = irreducible water; red = oil.
(a)
(b)
(c)
(d)


## 3.2. Image 3D Reconstruction

Two-dimensional image segmentation of core sample slices was obtained. Then, the distribution of pore-throat, oil, and water in all 2D slice images was acquired. In the next step, the ORS 3D software was used to add all the 2D images to become a 3D dataset. The 3D reconstruction principle is based on the marching cube algorithm [34]. After that, the visualized 3D distribution of the pore-throat, oil, and water in the core sample was obtained as shown in Figure 5 and Table 1.Figure 5
3D reconstruction results. Note: (a) distribution of the pore-throat system in the core sample; (b) distribution of irreducible water; (c) distribution of simulated oil.
(a)
(b)
(c)Table 1
Segmentation result of the core sample containing irreducible water.
Pore and throat (volume/μm3)
Irreducible water (volume/μm3)
Oil volume (μm3)
Irreducible water saturation (%)
5.80 × 107
2.14 × 107
3.67 × 107
36.94


## 3.3. Quantitative Analysis Method

### 3.3.1. Calculation of Surface Area and Volume of the Data Cube

Pixels are the basic unit of the building dataset, and these pixel dots of irreducible water and rock particles were obtained after image segmentation. In the next step, the characteristic parameters, for instance, the surface area and the volume of irreducible water and the pore-throat in the core samples, can be calculated using the following equations.(1)Vi=Nv×VP,where Vi is the volume of pixel dots (μm3), Nv is the total number of the pixel dots of the same phase (dimensionless), and VP is the volume of a single pixel dot which is 3.375 μm3 in this experiment.
(2)Si=Ns×Sp,where Si is the surface area of a given phase (μm2), Ns is the number of pixel dots of a given phase (dimensionless), and Sp is the surface area of a single pixel dot which is 9.0 μm2 in this experiment.


### 3.3.2. Microoccurrence of Irreducible Water in the Pore-Throat Network

Due to the micro-occurrence of irreducible water in pore-throat network is very complicated. Hence, different parameters, such as the shape factor and the irreducible water saturation in a given single pore-throat, were used to categorize the complex heterogeneous structures of the given irreducible water block.(3)Gi=6πVwSw1.5,where Gi is the shape factor of the irreducible water block (dimensionless), Vw is the volume (μm3), and Sw is the surface area of the irreducible water block (μm2). The shape factor represents the degree of sphericity of an object. In other terms, the lower the Gi value, the more would be the shape irregularity of the given irreducible water blocks and, subsequently, the higher would be the surface roughness. In contrast, the greater the Gi value, the more smooth the surface would be and the more likely the shape would be spherical.Saturation of irreducible water (SSwi′) shows the amount of irreducible water in a given pore or throat. This parameter can be obtained using (4). In general, the higher the irreducible water saturation (SSwi′), the lower would be the flowability in a given channel.
(4)SSwi′=VwVp‐t×100%,where SSwi′ is the saturation of irreducible water in either the pore or the throat (%), Vw is the irreducible water volume confined in a given pore or throat (μm3), and Vp−t is the pore or throat volume (μm3).


## 3.3.1. Calculation of Surface Area and Volume of the Data Cube

Pixels are the basic unit of the building dataset, and these pixel dots of irreducible water and rock particles were obtained after image segmentation. In the next step, the characteristic parameters, for instance, the surface area and the volume of irreducible water and the pore-throat in the core samples, can be calculated using the following equations.(1)Vi=Nv×VP,where Vi is the volume of pixel dots (μm3), Nv is the total number of the pixel dots of the same phase (dimensionless), and VP is the volume of a single pixel dot which is 3.375 μm3 in this experiment.
(2)Si=Ns×Sp,where Si is the surface area of a given phase (μm2), Ns is the number of pixel dots of a given phase (dimensionless), and Sp is the surface area of a single pixel dot which is 9.0 μm2 in this experiment.


## 3.3.2. Microoccurrence of Irreducible Water in the Pore-Throat Network

Due to the micro-occurrence of irreducible water in pore-throat network is very complicated. Hence, different parameters, such as the shape factor and the irreducible water saturation in a given single pore-throat, were used to categorize the complex heterogeneous structures of the given irreducible water block.(3)Gi=6πVwSw1.5,where Gi is the shape factor of the irreducible water block (dimensionless), Vw is the volume (μm3), and Sw is the surface area of the irreducible water block (μm2). The shape factor represents the degree of sphericity of an object. In other terms, the lower the Gi value, the more would be the shape irregularity of the given irreducible water blocks and, subsequently, the higher would be the surface roughness. In contrast, the greater the Gi value, the more smooth the surface would be and the more likely the shape would be spherical.Saturation of irreducible water (SSwi′) shows the amount of irreducible water in a given pore or throat. This parameter can be obtained using (4). In general, the higher the irreducible water saturation (SSwi′), the lower would be the flowability in a given channel.
(4)SSwi′=VwVp‐t×100%,where SSwi′ is the saturation of irreducible water in either the pore or the throat (%), Vw is the irreducible water volume confined in a given pore or throat (μm3), and Vp−t is the pore or throat volume (μm3).


## 4. Results and Discussion

This section of the current study mainly emphasizes on three aspects: irreducible water distribution characteristics in 500 2D slices, 3D pore-throat network, and microoccurrence modes in the pore and throat system.


### 4.1. Distribution Characteristics of Irreducible Water in 2D Slices

In this section, irreducible water distribution and its exact content in 500 2D core slices are evaluated by areal porosity and irreducible water that is contained in the pore-throat space (after image segmentation). Meanwhile, the irreducible water saturation of each slice is mathematically calculated using (5), where the areal porosity and irreducible water porosity can be figured out in Figure 6(a).
(5)Sswi=ϕpϕswi×100%,where Sswi is the irreducible water saturation for a given 2D scanning slice (%), ϕp is the areal porosity of a 2D slice (%), and ϕswi is the areal porosity of irreducible water in a given 2D slice (%).Figure 6
2D distribution characteristics of irreducible water in core slices: (a) distribution of areal porosity and irreducible water porosity in 2D core slices; (b) distribution of irreducible water saturation of a 2D slice; (c) frequency histogram of irreducible water distribution in the core sample; (d) relationship between areal porosity and irreducible water saturation.
(a)
(b)
(c)
(d)Irreducible water saturation distribution of all 2D slices is explicitly represented in Figure6(b) that illustrates the irreducible water saturation alteration in the flow direction. For the evaluated irreducible water saturation in all 2D core slices, the maximum irreducible water saturation is measured to be 50.69% (slice number: 226), while the minimum irreducible water saturation remains at 26.59% (slice number: 343). At the same time, the average value of irreducible water saturation is 36.94%. Additionally, three typical areas along with the alteration of the irreducible water saturation in the flow direction, including the peak, medium, and low regions are identified in Figure 6(b). The average saturation is up to 48.08% in the high-value zone in the continuous region (slice numbers: 213–232). In the peak-value area, average irreducible water saturation is 36.42% in the continuous part (slice numbers: 152–171). As far as the low-value part is concerned, average irreducible water saturation is 29.04% in the continuous part (slice numbers: 333–352).Figure6(c) shows the frequency histogram of irreducible water saturation distribution of all slices in the core sample. It indicates that the law of irreducible water saturation (core slices) belongs to a normal distribution, where the peak-value region of irreducible water saturation ranges from 35% to 40%, while the irreducible water saturation value in either the high-value part (over 40%) or the low-value region (below 30%) shares lesser proportions with the corresponding frequency value of 6.8% and 5.6%.A reasonable relationship between irreducible water saturation and areal porosity can be shown in Figure6(d). It indicates that (i) an inverse relation is observed between irreducible water saturation and porosity. Moreover, the line became steeper at higher areal porosity. (ii) A significant difference in irreducible water saturation existed among the slices even at the same areal porosity.To investigate the reason for the phenomenon of the same (similar) areal porosity with a significant difference in 2D slice irreducible water saturation, three typical slices were selected as shown in Figure7.Figure 7
Distribution of irreducible water saturation of a given slice with the same areal porosity (void space): (a)Sswi=29.57% (slice number: 362); (b) Sswi=36.37% (slice number: 425); (c) Sswi=50.69% (slice number: 226). Note: gray = rock matrix; white = pyrite; blue = irreducible water; red = oil.
(a)
(b)
(c)Despite the same porosity of the three slices (16.1%~16.4%), the corresponding irreducible water saturation is measured to be 29.57%, 36.37%, and 50.69% as shown in Figure7. Due to the capillary force, rock surface wettability, and pore geometry, the displacing oil mainly existed in the large-size pore and connected pore-throat. The variant irreducible water saturation in the case of identical porosity of three slices is due to the difference in pore-throat geometry. In the case of the higher pore radius, fewer pore corners, and lesser narrow throats, lower irreducible water saturation would exist in 2D slices. The distribution characteristics of 2D-slice irreducible water saturation are shown in Figure 8. It can be inferred that the irreducible water is mainly trapped by capillary force. In addition, the in-depth analysis helps to classify irreducible water into three main types:
(i)
Water trapped in pendular rings in complex angular pore geometry beside surface roughness (stationary fluid part).(ii)
Water trapped in dead ends of the pore-network (pore corner).(iii)
Water film adherence in wider throats, while narrow throats filled with irreducible water.Figure 8
Schematic of irreducible water distribution in the pore-throat system.


### 4.2. Distribution Characteristics of Irreducible Water in the 3D Pore-Throat Network

The pore-throat network governs the liquid flow in low-permeability porous media. However, distinct distribution characteristics of irreducible water exist in the pore-throat network. The pore and throat system recognition was based on the 3D reconstruction technology. The maximal inscribed sphere algorithm was used to identify the certain throat part in varying diameter channels. Along with the connected channel, the shortest distance portion was categorized as the throat. While the connected spherical-shaped body is recognized as the pore. The principle of pore and throat recognition is shown in Figure9. The details of pore and throat classification methods and principles are reported in Denney and Yang et al. [35, 36]. The information of overall 3D distribution characteristics of irreducible water is extracted by identification of 3D coordinates of the irreducible water pixels existing in the pore and throat system as shown in Figure 10.Figure 9
Pore and throat recognition principle.Figure 10
3D structure: (a) pore system; (b) throat system. Note:length×width×height=750×750×750μm.
(a)
(b)According to the 3D reconstruction results based on CT scanning, the volumes of the pore system and throat system are determined to be 5.63 × 107 μm3 and 1.69 × 106 μm3, respectively. Moreover, the radius distribution, in either case, followed a normal distribution (Figure 10), where the peak-value regions of the pore and throat system are found in the 25–30 μm and 2–2.5 μm ranges, respectively (Figure 11). This fundamental information provides a better platform for less proportion of irreducible water saturation in the pore system than the throat system (Figure 12). The exact irreducible water saturation proportions are figured out to be (36%) in the pore system which is far less than in the throat system (64%) (Figure 13). This low percentage of irreducible water saturation in the pore system could be attributed to the difference in radius. It is a well-understood fact that permeability of a porous matrix is chiefly relying on the throat radius. However, irreducible water occupied the throat system either in the form of water film or being completely filled with it. Consequently, the effective throat radius would further be decreased in the flooding fluid. This obtained knowledge provided a rationalized answer to the high water-flooding pressure and low hydrocarbon recovery in a low-permeability reservoir. Characteristic parameters of irreducible water, including numbers and volume that exist in the pore and throat system with different sizes, are quantitatively studied in the upcoming part of the current section. In this connection, distribution of frequency number and irreducible water volume in different sizes of the pore and throat system is determined by following the same calculation method which is discussed in Section 3.3. As a consequence, the results are elucidated in Figures 14 and 15.Figure 11
(a) Pore; (b) throat radius distribution of the core sample.
(a)
(b)Figure 12
3D distribution of irreducible water: (a) pore system; (b) throat system.
(a)
(b)Figure 13
Histogram of irreducible water void space in the pore and throat system.Figure 14
(a) Volume; (b) frequency number distribution of irreducible water blocks with different sizes in the throat system.
(a)
(b)Figure 15
(a) Volume; (b) frequency number distribution of irreducible water blocks with different sizes in the pore system.
(a)
(b)Figure14 relates frequency number and volume number to irreducible water saturation in the throat system. It is shown that both characteristic parameters sharply decrease upon an increase in irreducible water saturation in the range of volume less than the average throat volume. Furthermore, this phenomenon can further be interpreted by throat radius determination. As it is widely accepted that the throat volume is proportional to the throat radius which subsequently provides a relatively larger flow area. Consequently, the exerted capillary force in a comparatively larger flow area would decrease, and it further resulted in the reduction in the throat’s capability to trap irreducible water during the process of oil displacement.Figure15 shows that the distribution frequency of the number of irreducible water in the pore system and throat system followed a similar trend, but the volume of irreducible water in the pore system and throat system varies slightly (Figures 14(a) and 15(a)). We can see that with the increase in the number of irreducible water blocks, the trend of the volume frequency distribution of irreducible water with different sizes first descended and then increased. This dramatic turning point (Figure 15(a)) in the trend happened when the volume of irreducible water was over 105 μm3. This obscure behaviour is explained as; since there are 13 irreducible water blocks in pore system which has volume over 105 μm3, these 13 irreducible water blocks were extracted. On this basis, the 13 pore spaces were retrieved that contained the corresponding 13 irreducible water blocks. In next step, SSwi′ was calculated. Consequently, results showed that SSwi′ fell in the 80–98% range. It can be inferred that this dramatic turning is due to poor sweeping of irreducible water during oil displacement.


### 4.3. Microoccurrence of Irreducible Water in the Pore and Throat System

The 3D structure of irreducible water distribution in pore space is complicated due to the combined effect of porous media heterogeneity and the oil-water displacement process. Meanwhile, by 3D visualization, the irreducible water microoccurrence in pore and throat systems is categorized into each mode (Figures16 and 17), which are further explained below:
(a)
Irreducible water in the pore system:(i)
Conglobation shape: this kind of pores has comparatively small room and has a complicated structure. Moreover, they are produced due to dissolution at different stages and are referred to secondary pores. Referring to irreducible water presence, it occupied a small volume and its flow is governed by the bottle-necking structure at certain positions as shown in (Figure16(a)).(ii)
Cluster shape: the cluster shape exists in a primary pore with the large compartment. However, due to surface roughness and complex pore geometry, the irreducible water present in the corner is linked to the adhered water film at other parts of the pore space (Figure16(b)). This shape is more complicated than the water film but simpler than the conglobation shape.(iii)
Water film: this type of irreducible water appears as a water film which sticks to the relatively smooth surface of regularly shaped pores (Figure16(c)). However, it does not occupy all the rock surfaces.(b)
Irreducible water in the throat system:(i)
Curved piece-necking shape: this kind of throat structure featured with bottle-necking and curved segments besides microeffective seepage radius and large surface area. Consequently, it almost helps to fill the compartment entirely (Figure17(a)).(ii)
Slug-filling shape: irreducible water locates precisely at the neck in the slug-filling shape. As a result, it blocks the path for the displacing oil. At the same time, it breaks the continuous oil phase which leads to high water drive pressure. However, upon entering into the wide segment of the same throat, the capillary force suddenly disappeared which tends to cease the spontaneous imbibition phenomenon (Figure17(b)).(iii)
Water-film shape: we deal with that kind of irreducible shape, where it sticks to the surface throat in the form of a film, the so-called water film. Meanwhile, it reduced the effective radius for seepage. The channel radius and its complex geometry, the shape of irreducible water along with surface polarity of the pore-throat, are the controlling factors in the 3D distribution of irreducible water. Irreducible water sticks to the surface of the throat as the shape of the water film. Subsequently, the small seepage space is becoming much narrower (Figure16(c)).Figure 16
Different modes of irreducible water microoccurrence in the pore system: (a) conglobation pattern (ineffective oil reservoir space); (b) cluster pattern (partially effective oil reservoir space); (c) water-film pattern (effective oil reservoir space). Note: the yellow area represents a given pore or throat surface and the blue area represents irreducible water.
(a)
(b)
(c)Figure 17
Different modes of irreducible water microoccurrence in the throat system: (a) curved piece-necking pattern (complete blockage, ineffective percolation channel); (b) slug-filling (partially blockage, ineffective percolation channel); (c) water-film pattern (partial blockage, effective percolation channel). Note: the yellow area represents a given pore or throat surface and the blue area represents irreducible water.
(a)
(b)
(c)Figures16(b), 16(c), and 17(c) show that the wetting phase (irreducible water) did not spread all over the surface. It is considered as the “mixed” wetting characteristics of the rock surface. Since the sandstone core sample is water-wetting in nature.But due to the presence of certain clay, for instance, chlorite, oil may wet certain parts of the entire surface. XRD of the core sample has been conducted which showed that chlorite content contributed around 17.5% of the core sample as shown in Table 2. In a nutshell, the oil-water interface in natural sandstone porous media is complex due to the influence of capillary trapping and the existence of oil-wetting clay. This phenomenon is in agreement with Trantham and Clampitt and Feng et al. [37, 38].Table 2
XRD results of the core sample.
Clay minerals
Quartz
Plagioclase
Potash feldspar
Calcite
Illite
Chlorite
Wt. %
28.9
34.5
17.3
0.3
1.5
17.5Irreducible water in the pore and throat system can be studied by 3D visualization technique. In this context, irreducible water distribution can be classified according to specific essential parameters, such as shape factors and irreducible water saturation. The standard classification is illustrated in Table3.Table 3
Classification of microcharacteristics of irreducible water in the pore and throat system.
Unit
Irreducible water patterns
Shape factor ranges
Range of saturation of irreducible water in a single pore or throat
Pore system
Conglobation
G < 0.1
90 <SSwi′ < 100
Cluster
0.1 <G < 0.2
30 <SSwi′ < 90
Water film
0.2 <G < 0.4
10 <SSwi′ < 30
Throat system
Curved piece-necking
G < 0.2
90 <SSwi′ < 100
Slug-filling
0.4 <G < 0.7
30 <SSwi′ < 90
Water film
0.2 <G < 0.4
10 <SSwi′ < 30Figure18(a) shows the irreducible water frequency proportions in three kinds of irreducible water microoccurrence. In detail, the respective proportions in conglobation shape, cluster shape, and water film shape are 14.2%, 68.4%, and 17.4%, respectively. Figure 18(b) depicts the volume frequency of irreducible water microoccurrence in the throat system which is the curved piece-necking shape (16.9%), slug-filling shape (8.6%), and water-film shape (74.5%). Therefore, the main irreducible water microoccurrence in pores and throats is in cluster and water-film shapes.Figure 18
Histogram of irreducible water modes distribution: (a) pore system; (b) throat system.
(a)
(b)At the same time, from the results of the irreducible water microoccurrence, the water-film shape irreducible water is found to be unflowable in the pore-throat network. It is mainly attributed to the attractive force between the water film and the rock surface. As far as the other two kinds of irreducible water patterns, for instance, conglobation shape and cluster shape, are concerned, irreducible water should be confined by geological structure. It should be noted that these latter types of irreducible water could be fluidized using advanced technology, such as hydraulic fracturing.Compared with irreducible water contained in the pore system, the microoccurrence of irreducible water in the throat system has severely influenced the oil-water two-phase flow behavior. In the process of spontaneous imbibition, due to the capillary force, water is imbibed along narrow channels and crude oil is driven out from the pores through a comparatively larger size channel. As the saturation of irreducible water in the throat system is much higher than that in the pore system, the amount of imbibed water in the throat system will significantly be reduced. Additionally, some of the channels for crude oil are plugged due to the presence of irreducible slug water (curved piece-necking and slug-filling) in the throat system. Therefore, an inverse correlation between the saturation of irreducible water and spontaneous imbibition recovery could be concluded from this study. Last but not least, the role of lower rock permeability always linked with higher irreducible water saturation could also be explained in the above micromechanism.


## 4.1. Distribution Characteristics of Irreducible Water in 2D Slices

In this section, irreducible water distribution and its exact content in 500 2D core slices are evaluated by areal porosity and irreducible water that is contained in the pore-throat space (after image segmentation). Meanwhile, the irreducible water saturation of each slice is mathematically calculated using (5), where the areal porosity and irreducible water porosity can be figured out in Figure 6(a).
(5)Sswi=ϕpϕswi×100%,where Sswi is the irreducible water saturation for a given 2D scanning slice (%), ϕp is the areal porosity of a 2D slice (%), and ϕswi is the areal porosity of irreducible water in a given 2D slice (%).Figure 6
2D distribution characteristics of irreducible water in core slices: (a) distribution of areal porosity and irreducible water porosity in 2D core slices; (b) distribution of irreducible water saturation of a 2D slice; (c) frequency histogram of irreducible water distribution in the core sample; (d) relationship between areal porosity and irreducible water saturation.
(a)
(b)
(c)
(d)Irreducible water saturation distribution of all 2D slices is explicitly represented in Figure6(b) that illustrates the irreducible water saturation alteration in the flow direction. For the evaluated irreducible water saturation in all 2D core slices, the maximum irreducible water saturation is measured to be 50.69% (slice number: 226), while the minimum irreducible water saturation remains at 26.59% (slice number: 343). At the same time, the average value of irreducible water saturation is 36.94%. Additionally, three typical areas along with the alteration of the irreducible water saturation in the flow direction, including the peak, medium, and low regions are identified in Figure 6(b). The average saturation is up to 48.08% in the high-value zone in the continuous region (slice numbers: 213–232). In the peak-value area, average irreducible water saturation is 36.42% in the continuous part (slice numbers: 152–171). As far as the low-value part is concerned, average irreducible water saturation is 29.04% in the continuous part (slice numbers: 333–352).Figure6(c) shows the frequency histogram of irreducible water saturation distribution of all slices in the core sample. It indicates that the law of irreducible water saturation (core slices) belongs to a normal distribution, where the peak-value region of irreducible water saturation ranges from 35% to 40%, while the irreducible water saturation value in either the high-value part (over 40%) or the low-value region (below 30%) shares lesser proportions with the corresponding frequency value of 6.8% and 5.6%.A reasonable relationship between irreducible water saturation and areal porosity can be shown in Figure6(d). It indicates that (i) an inverse relation is observed between irreducible water saturation and porosity. Moreover, the line became steeper at higher areal porosity. (ii) A significant difference in irreducible water saturation existed among the slices even at the same areal porosity.To investigate the reason for the phenomenon of the same (similar) areal porosity with a significant difference in 2D slice irreducible water saturation, three typical slices were selected as shown in Figure7.Figure 7
Distribution of irreducible water saturation of a given slice with the same areal porosity (void space): (a)Sswi=29.57% (slice number: 362); (b) Sswi=36.37% (slice number: 425); (c) Sswi=50.69% (slice number: 226). Note: gray = rock matrix; white = pyrite; blue = irreducible water; red = oil.
(a)
(b)
(c)Despite the same porosity of the three slices (16.1%~16.4%), the corresponding irreducible water saturation is measured to be 29.57%, 36.37%, and 50.69% as shown in Figure7. Due to the capillary force, rock surface wettability, and pore geometry, the displacing oil mainly existed in the large-size pore and connected pore-throat. The variant irreducible water saturation in the case of identical porosity of three slices is due to the difference in pore-throat geometry. In the case of the higher pore radius, fewer pore corners, and lesser narrow throats, lower irreducible water saturation would exist in 2D slices. The distribution characteristics of 2D-slice irreducible water saturation are shown in Figure 8. It can be inferred that the irreducible water is mainly trapped by capillary force. In addition, the in-depth analysis helps to classify irreducible water into three main types:
(i)
Water trapped in pendular rings in complex angular pore geometry beside surface roughness (stationary fluid part).(ii)
Water trapped in dead ends of the pore-network (pore corner).(iii)
Water film adherence in wider throats, while narrow throats filled with irreducible water.Figure 8
Schematic of irreducible water distribution in the pore-throat system.


## 4.2. Distribution Characteristics of Irreducible Water in the 3D Pore-Throat Network

The pore-throat network governs the liquid flow in low-permeability porous media. However, distinct distribution characteristics of irreducible water exist in the pore-throat network. The pore and throat system recognition was based on the 3D reconstruction technology. The maximal inscribed sphere algorithm was used to identify the certain throat part in varying diameter channels. Along with the connected channel, the shortest distance portion was categorized as the throat. While the connected spherical-shaped body is recognized as the pore. The principle of pore and throat recognition is shown in Figure9. The details of pore and throat classification methods and principles are reported in Denney and Yang et al. [35, 36]. The information of overall 3D distribution characteristics of irreducible water is extracted by identification of 3D coordinates of the irreducible water pixels existing in the pore and throat system as shown in Figure 10.Figure 9
Pore and throat recognition principle.Figure 10
3D structure: (a) pore system; (b) throat system. Note:length×width×height=750×750×750μm.
(a)
(b)According to the 3D reconstruction results based on CT scanning, the volumes of the pore system and throat system are determined to be 5.63 × 107 μm3 and 1.69 × 106 μm3, respectively. Moreover, the radius distribution, in either case, followed a normal distribution (Figure 10), where the peak-value regions of the pore and throat system are found in the 25–30 μm and 2–2.5 μm ranges, respectively (Figure 11). This fundamental information provides a better platform for less proportion of irreducible water saturation in the pore system than the throat system (Figure 12). The exact irreducible water saturation proportions are figured out to be (36%) in the pore system which is far less than in the throat system (64%) (Figure 13). This low percentage of irreducible water saturation in the pore system could be attributed to the difference in radius. It is a well-understood fact that permeability of a porous matrix is chiefly relying on the throat radius. However, irreducible water occupied the throat system either in the form of water film or being completely filled with it. Consequently, the effective throat radius would further be decreased in the flooding fluid. This obtained knowledge provided a rationalized answer to the high water-flooding pressure and low hydrocarbon recovery in a low-permeability reservoir. Characteristic parameters of irreducible water, including numbers and volume that exist in the pore and throat system with different sizes, are quantitatively studied in the upcoming part of the current section. In this connection, distribution of frequency number and irreducible water volume in different sizes of the pore and throat system is determined by following the same calculation method which is discussed in Section 3.3. As a consequence, the results are elucidated in Figures 14 and 15.Figure 11
(a) Pore; (b) throat radius distribution of the core sample.
(a)
(b)Figure 12
3D distribution of irreducible water: (a) pore system; (b) throat system.
(a)
(b)Figure 13
Histogram of irreducible water void space in the pore and throat system.Figure 14
(a) Volume; (b) frequency number distribution of irreducible water blocks with different sizes in the throat system.
(a)
(b)Figure 15
(a) Volume; (b) frequency number distribution of irreducible water blocks with different sizes in the pore system.
(a)
(b)Figure14 relates frequency number and volume number to irreducible water saturation in the throat system. It is shown that both characteristic parameters sharply decrease upon an increase in irreducible water saturation in the range of volume less than the average throat volume. Furthermore, this phenomenon can further be interpreted by throat radius determination. As it is widely accepted that the throat volume is proportional to the throat radius which subsequently provides a relatively larger flow area. Consequently, the exerted capillary force in a comparatively larger flow area would decrease, and it further resulted in the reduction in the throat’s capability to trap irreducible water during the process of oil displacement.Figure15 shows that the distribution frequency of the number of irreducible water in the pore system and throat system followed a similar trend, but the volume of irreducible water in the pore system and throat system varies slightly (Figures 14(a) and 15(a)). We can see that with the increase in the number of irreducible water blocks, the trend of the volume frequency distribution of irreducible water with different sizes first descended and then increased. This dramatic turning point (Figure 15(a)) in the trend happened when the volume of irreducible water was over 105 μm3. This obscure behaviour is explained as; since there are 13 irreducible water blocks in pore system which has volume over 105 μm3, these 13 irreducible water blocks were extracted. On this basis, the 13 pore spaces were retrieved that contained the corresponding 13 irreducible water blocks. In next step, SSwi′ was calculated. Consequently, results showed that SSwi′ fell in the 80–98% range. It can be inferred that this dramatic turning is due to poor sweeping of irreducible water during oil displacement.


## 4.3. Microoccurrence of Irreducible Water in the Pore and Throat System

The 3D structure of irreducible water distribution in pore space is complicated due to the combined effect of porous media heterogeneity and the oil-water displacement process. Meanwhile, by 3D visualization, the irreducible water microoccurrence in pore and throat systems is categorized into each mode (Figures16 and 17), which are further explained below:
(a)
Irreducible water in the pore system:(i)
Conglobation shape: this kind of pores has comparatively small room and has a complicated structure. Moreover, they are produced due to dissolution at different stages and are referred to secondary pores. Referring to irreducible water presence, it occupied a small volume and its flow is governed by the bottle-necking structure at certain positions as shown in (Figure16(a)).(ii)
Cluster shape: the cluster shape exists in a primary pore with the large compartment. However, due to surface roughness and complex pore geometry, the irreducible water present in the corner is linked to the adhered water film at other parts of the pore space (Figure16(b)). This shape is more complicated than the water film but simpler than the conglobation shape.(iii)
Water film: this type of irreducible water appears as a water film which sticks to the relatively smooth surface of regularly shaped pores (Figure16(c)). However, it does not occupy all the rock surfaces.(b)
Irreducible water in the throat system:(i)
Curved piece-necking shape: this kind of throat structure featured with bottle-necking and curved segments besides microeffective seepage radius and large surface area. Consequently, it almost helps to fill the compartment entirely (Figure17(a)).(ii)
Slug-filling shape: irreducible water locates precisely at the neck in the slug-filling shape. As a result, it blocks the path for the displacing oil. At the same time, it breaks the continuous oil phase which leads to high water drive pressure. However, upon entering into the wide segment of the same throat, the capillary force suddenly disappeared which tends to cease the spontaneous imbibition phenomenon (Figure17(b)).(iii)
Water-film shape: we deal with that kind of irreducible shape, where it sticks to the surface throat in the form of a film, the so-called water film. Meanwhile, it reduced the effective radius for seepage. The channel radius and its complex geometry, the shape of irreducible water along with surface polarity of the pore-throat, are the controlling factors in the 3D distribution of irreducible water. Irreducible water sticks to the surface of the throat as the shape of the water film. Subsequently, the small seepage space is becoming much narrower (Figure16(c)).Figure 16
Different modes of irreducible water microoccurrence in the pore system: (a) conglobation pattern (ineffective oil reservoir space); (b) cluster pattern (partially effective oil reservoir space); (c) water-film pattern (effective oil reservoir space). Note: the yellow area represents a given pore or throat surface and the blue area represents irreducible water.
(a)
(b)
(c)Figure 17
Different modes of irreducible water microoccurrence in the throat system: (a) curved piece-necking pattern (complete blockage, ineffective percolation channel); (b) slug-filling (partially blockage, ineffective percolation channel); (c) water-film pattern (partial blockage, effective percolation channel). Note: the yellow area represents a given pore or throat surface and the blue area represents irreducible water.
(a)
(b)
(c)Figures16(b), 16(c), and 17(c) show that the wetting phase (irreducible water) did not spread all over the surface. It is considered as the “mixed” wetting characteristics of the rock surface. Since the sandstone core sample is water-wetting in nature.But due to the presence of certain clay, for instance, chlorite, oil may wet certain parts of the entire surface. XRD of the core sample has been conducted which showed that chlorite content contributed around 17.5% of the core sample as shown in Table 2. In a nutshell, the oil-water interface in natural sandstone porous media is complex due to the influence of capillary trapping and the existence of oil-wetting clay. This phenomenon is in agreement with Trantham and Clampitt and Feng et al. [37, 38].Table 2
XRD results of the core sample.
Clay minerals
Quartz
Plagioclase
Potash feldspar
Calcite
Illite
Chlorite
Wt. %
28.9
34.5
17.3
0.3
1.5
17.5Irreducible water in the pore and throat system can be studied by 3D visualization technique. In this context, irreducible water distribution can be classified according to specific essential parameters, such as shape factors and irreducible water saturation. The standard classification is illustrated in Table3.Table 3
Classification of microcharacteristics of irreducible water in the pore and throat system.
Unit
Irreducible water patterns
Shape factor ranges
Range of saturation of irreducible water in a single pore or throat
Pore system
Conglobation
G < 0.1
90 <SSwi′ < 100
Cluster
0.1 <G < 0.2
30 <SSwi′ < 90
Water film
0.2 <G < 0.4
10 <SSwi′ < 30
Throat system
Curved piece-necking
G < 0.2
90 <SSwi′ < 100
Slug-filling
0.4 <G < 0.7
30 <SSwi′ < 90
Water film
0.2 <G < 0.4
10 <SSwi′ < 30Figure18(a) shows the irreducible water frequency proportions in three kinds of irreducible water microoccurrence. In detail, the respective proportions in conglobation shape, cluster shape, and water film shape are 14.2%, 68.4%, and 17.4%, respectively. Figure 18(b) depicts the volume frequency of irreducible water microoccurrence in the throat system which is the curved piece-necking shape (16.9%), slug-filling shape (8.6%), and water-film shape (74.5%). Therefore, the main irreducible water microoccurrence in pores and throats is in cluster and water-film shapes.Figure 18
Histogram of irreducible water modes distribution: (a) pore system; (b) throat system.
(a)
(b)At the same time, from the results of the irreducible water microoccurrence, the water-film shape irreducible water is found to be unflowable in the pore-throat network. It is mainly attributed to the attractive force between the water film and the rock surface. As far as the other two kinds of irreducible water patterns, for instance, conglobation shape and cluster shape, are concerned, irreducible water should be confined by geological structure. It should be noted that these latter types of irreducible water could be fluidized using advanced technology, such as hydraulic fracturing.Compared with irreducible water contained in the pore system, the microoccurrence of irreducible water in the throat system has severely influenced the oil-water two-phase flow behavior. In the process of spontaneous imbibition, due to the capillary force, water is imbibed along narrow channels and crude oil is driven out from the pores through a comparatively larger size channel. As the saturation of irreducible water in the throat system is much higher than that in the pore system, the amount of imbibed water in the throat system will significantly be reduced. Additionally, some of the channels for crude oil are plugged due to the presence of irreducible slug water (curved piece-necking and slug-filling) in the throat system. Therefore, an inverse correlation between the saturation of irreducible water and spontaneous imbibition recovery could be concluded from this study. Last but not least, the role of lower rock permeability always linked with higher irreducible water saturation could also be explained in the above micromechanism.


## 5. Conclusions

(1)
The micro geometric structures of the pore-throat systems have a significant influence on the microoccurrence of irreducible water in the low-permeability sandstone core sample. Certain core slices showed significantly different irreducible water saturation (29.57%–50.69%) even though these slices had the same areal porosity.
(2)
Due to the influence of capillary trapping and the existence of oil-wetting clay, the irreducible water saturation in the throat system (64%) is much larger than that in the pore system (36%). Furthermore, the wetting phase (irreducible water) did not spread all over the surface of the pore-throat network. Subsequently, oil-water two-phase interface becomes much more complicated.
(3)
In the micro scale, the main irreducible water occurrence in the pore system is much different from that in the throat system. In the pore system, the irreducible water principally existed in the corner of the pores which are linked through the water film. While in the throat system, the irreducible water occurrence is dominated by the water film. However, 25.5% of the throats are blocked by the irreducible water (curved piece-necking and slug-filling) which plug the crude oil drainage channels.



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*Source: 6062475-2018-08-06.xml*