Study on the Mobilization Mechanisms of Microscopic Residual Oil in High-Water-Cut Sandstone Reservoirs

Abstract

As mature oilfields enter the high-water-cut development stage, significant amounts of residual oil remain trapped underground. To enhance the effectiveness of tertiary oil recovery, it is crucial to understand the distribution and mobilization patterns of this residual oil. In this study, polydimethylsiloxane (PDMS) was used to create a microscopic oil displacement model, which was observed and recorded using a stereomicroscope. The experimental images were extracted, analyzed, and quantitatively evaluated, categorizing the microscopic residual oil in the high-water-cut sandstone reservoirs of Dagang Oilfield into cluster-like, pore surface film-like, corner-like, and slit-like types. Polymer–surfactant composite flooding (abbreviated as SP flooding) effectively mobilized 47.16% of cluster-like residual oil and 43.74% of pore surface film-like residual oil, with some mobilization of corner-like and slit-like residual oil as well. Building on SP flooding, dual-mobility flooding further increased the mobilization of cluster-like residual oil by 12.37% and pore surface film-like residual oil by 3.52%. With the same slug size, dual-mobility flooding can reduce development costs by 16.43%. Overall, dual-mobility flooding offers better development prospects.

1. Introduction

During the development of mature oilfields in China, it is crucial to identify the distribution patterns of remaining oil, accurately calculate its volume, and determine the mobilization mechanisms to enhance recovery rates. After several decades of development, Dagang Oilfield has reached a high-water-cut stage, yet substantial amounts of residual oil remain trapped underground. Clarifying the distribution and mobilization patterns of this residual oil is essential for implementing secondary and tertiary recovery methods in mature oilfields. This also lays the foundation for selecting appropriate chemical agents, optimizing injection parameters, and conducting comprehensive adjustments in the later stages of oilfield development. Therefore, there is an urgent need to study the microscopic distribution and mobilization mechanisms of residual oil, providing data and theoretical support for the selection of chemical flooding techniques in the next phase [1,2,3,4,5].

Research on microscopic residual oil both domestically and internationally primarily focuses on macroscopic and microscopic domains, categorized into six main approaches: (1) reservoir engineering methods, (2) computer numerical simulation methods, (3) reservoir development geology methods, (4) sequence stratigraphy methods, (5) well logging data analysis, and (6) microscopic residual oil studies. The first five methods pertain to macroscopic research, with relatively fewer studies dedicated to microscopic residual oil. Common methods for studying microscopic residual oil include mathematical models and physical simulations. Mathematical models encompass the following: (1) one-dimensional models, (2) Bethe lattice models, (3) sphere packing models, and (4) network models. Physical experimental techniques include (1) nuclear magnetic resonance (NMR) technology, (2) cryogenic sectioning fluorescence technology, (3) laser confocal scanning microscopy, and (4) microscopic displacement models [6,7,8,9,10,11,12,13,14,15].

Among the existing experimental methods for microscopic residual oil, cryogenic sectioning fluorescence technology and laser confocal scanning microscopy focus on studying the distribution patterns of microscopic residual oil, which are static tests of experimental samples. In contrast, nuclear magnetic resonance (NMR) technology and microscopic displacement models emphasize the mobilization mechanisms of microscopic residual oil, analyzing the occurrence and mobilization patterns through dynamic testing. NMR technology is particularly used to analyze the differences in residual oil mobilization within pore throats of various sizes. This paper primarily investigates the mobilization mechanisms of different types of microscopic residual oil. By selecting microscopic displacement models for experiments, relevant experimental data can be obtained intuitively, elucidating the mobilization mechanisms of microscopic residual oil and providing a basis for enhancing the recovery rates of residual oil in subsequent stages [16,17,18,19,20].

2. PDMS Microscopic Oil Displacement Model Observation Method

2.1. Experimental Principles

The PDMS oil displacement model is a type of microscopic displacement model that uses polydimethylsiloxane (PDMS) as the model substrate. Combined with laser etching technology (Figure 1), this model is fabricated on a 1:1 scale based on the actual pore structure of natural rock cores. Compared to traditional glass etched models, the PDMS model more closely replicates the wettability of a real reservoir environment, provides a more detailed depiction of the pore structure, and achieves higher model accuracy.

Conventional microscopic displacement models use glass as the experimental substrate, combined with hydrofluoric acid etching, with patterns primarily drawn by hand, making it difficult to accurately replicate the shape and size of natural rock core pore throats. The traditional glass etched model has a precision of 50 μm, whereas the PDMS oil displacement model has a precision of 1 μm, closely matching the pore throat size of natural sandstone [21]. The PDMS model utilizes natural rock core data extraction, digital rock core modeling, and a 1:1 replication of the natural rock core pore throat structure (Figure 2). The accuracy of the pore throat replication in the PDMS model is far superior to that of the hand-etched glass models.

The experimental instruments include (Figure 3) a Zeiss SteREO Discovery V8 stereomicroscope (Zeiss Group, Oberkochen, Germany) with a magnification range of 4–150× and an observation accuracy of 1 μm; a Sony Alpha 7S III DSLR camera (Sony Corporation, Tokyo, Japan); ZEISS proprietary image analysis software DQ2.0 (Zeiss Group, Oberkochen, Germany); a Fluigent FLOW EZ micro-displacement pump (Fluigent Group, Paris, France); and a heating and insulation device manufactured by Jiangsu Huaan Scientific Research Instrument Co., Ltd. (Huaan Group, Nantong, China).

According to the experimental design requirements, the PDMS microscopic oil displacement experiment involves saturating the target reservoir’s crude oil and injecting formation water (Figure 4) or a chemical flooding system. The entire process is recorded using a digital camera and analyzed with specialized software to compare the occurrence, quantity differences, and mobilization patterns of microscopic residual oil before and after water flooding or chemical flooding.

2.2. Quantification and Classification of Microscopic Residual Oil

The occurrence state of microscopic residual oil in the reservoir, based on different mobilization mechanisms and its occurrence in porous media, can be classified into four categories: cluster-like, corner-like, pore surface film-like, and slit-like. According to the differences in mobilization mechanisms, it can be divided into two main types: mobilization by enhancing microscopic sweep efficiency and mobilization by improving microscopic oil washing efficiency.

Mobilization through microscopic sweep, which increases the mobility of the displacing phase, utilizes the crude oil in previously unswept areas, including corner-like residual oil and cluster-like residual oil.

Mobilization by improving microscopic oil washing efficiency, which reduces the oil–water interfacial tension or the mobility of the displaced phase, includes pore surface film-like residual oil and slit-like residual oil, thus utilizing the crude oil that was previously swept but not mobilized.

For the captured microscopic oil displacement images, self-developed microscopic residual oil quantification analysis software was used. This software performs a series of operations, including image capture, color differentiation, image recognition, region segmentation, pore identification, classification statistics, and quantitative analysis. Taking the identification of a single type of microscopic residual oil as an example, the extraction of the oil-containing area involves the following steps: (1) determining the pore throat boundaries, (2) extracting pore throat images, (3) calculating the image area, (4) determining the equivalent area circle, and (5) obtaining the equivalent radius [22,23].

To measure the mobilization degree of different types of residual oil by various chemical flooding methods compared to water flooding and their contribution to enhanced oil recovery, parameters SP and ASP were introduced [24]. These parameters respectively represent the contribution of polymer flooding and SP flooding to the mobilization of different types of residual oil and their respective contributions to enhanced oil recovery.

$$S_{P_i}={\displaystyle \frac{{A_{w_i}-A}_{P_i}}{A_w-A_P}}$$

(1)

$$S_{{SP}_i}={\displaystyle \frac{{A_{w_i}-A}_{{SP}_i}}{A_w-A_{SP}}}$$

(2)

In the formula, A_w represents the total oil-containing area in the microscopic images after water flooding, A_p represents the total oil-containing area in the microscopic images after polymer flooding, and A_sp represents the total oil-containing area in the microscopic images after SP flooding. (i = 1, 2, …, 5, represents the types of microscopic residual oil).

2.3. Experimental Procedure

The experiment mainly includes the following steps: (1) extract cast thin section data from core samples of the target reservoir according to the experimental requirements; (2) digitally model the relevant core thin section data; (3) use polydimethylsiloxane (PDMS) as the primary material for the model and perform laser etching; (4) process the model through casting, demolding, drilling, and bonding to complete the model fabrication; (5) conduct microscopic oil displacement experiments as per the design, injecting formation water and relevant chemical flooding systems while recording the entire process with videos and images; and (6) perform visual data analysis to calculate the mobilization of microscopic residual oil [25,26,27,28,29,30].

3. Quantitative Methods for Microscopic Residual Oil Mobilization Experiments

Traditional methods for identifying microscopic residual oil include image analysis, fluorescence capture, and reflection wavelength analysis. The latter two methods are affected by the interference of the rock in natural cores and cannot fully and clearly reflect the situation of microscopic residual oil. The PDMS oil displacement model, with light transmittance similar to glass, can accurately depict the distribution of crude oil in porous media. Using dynamic image quantification analysis software, it can precisely distinguish the types and quantities of microscopic residual oil.

3.1. Differences in Microscopic Residual Oil Mobilization Effectiveness at Various Development Stages

The distribution of microscopic residual oil in porous media after water flooding is influenced by the limited sweep efficiency of water flooding. The primary microscopic residual oil is cluster-like residual oil, resulting from areas not swept at the microscopic level. At the blind ends of the porous media, there is also corner-like residual oil, unswept at the microscopic level due to the influence of the main flow channels. In the swept areas, pore surface film-like microscopic residual oil is formed due to oil droplet adsorption, which is not carried away by the sweep. Additionally, slit-like microscopic residual oil in the swept areas is influenced by the capillary forces of the porous media, preventing its mobilization. Different microscopic residual oil forms are shown in Figure 5.

3.2. Differences in Microscopic Residual Oil Mobilization Effectiveness with Various Chemical Agents

Different chemical flooding methods exhibit distinct mobilization effects on various types of microscopic residual oil: (1) Polymer solutions primarily enhance microscopic sweep efficiency and secondarily carry oil through “pull and drag” washing effects. They mainly mobilize cluster-like and corner-like residual oil, with a secondary effect on mobilizing pore surface film-like and slit-like residual oil. (2) Surfactant solutions mainly improve oil washing efficiency. Due to their lower viscosity, the sweep effect is limited. They primarily mobilize pore surface film-like and slit-like microscopic residual oil, with a relatively limited effect on cluster-like and corner-like residual oil. (3) Polymer–surfactant SP flooding solutions combine the characteristics of polymer and surfactant solutions, effectively mobilizing all types of microscopic residual oil simultaneously [31,32,33].

4. Analysis of Experimental Results

After prolonged water flooding and chemical flooding development, Dagang Yangsanmu Oilfield exhibits low water flooding recovery rates and high water-cut levels. Although the water cut initially decreased with the use of chemical flooding, it quickly returned to high levels after a certain period. To explore the differences in the mobilization effects of various chemical flooding methods on crude oil, PDMS models were created using natural core samples from Yangsanmu. The mobilization of different types of microscopic residual oil by different chemical flooding methods was analyzed. To ensure experimental accuracy, three parallel experiments were conducted for each set, and the average value was calculated.

The experimental oil was sourced from Dagang Yangsanmu Oilfield, with an underground crude oil viscosity of 98 mPa·s and a reservoir temperature of 61 °C. The experimental water was filtered on-site injection water, with formation water classified as NaHCO₃ type, and it had a total salinity of 4507 mg/L. The polymer used was partially hydrolyzed polyacrylamide, product name: BHHP-113, at a concentration of 1800 mg/L. The surfactant used was petroleum sulfonate surfactant, product name: BHS-01B, at a concentration of 0.3%. The viscosity reducer was a nano viscosity reducer, product name: BHJN-04. All the chemical agents were provided by Dagang Oilfield Binhai Company in Tianjin. A PDMS model designed specifically for Yangsanmu Oilfield was used to conduct microscopic displacement experiments, including water flooding, SP flooding, and dual-flow flooding. These experiments aimed to determine the types and distribution of microscopic residual oil in Yangsanmu Oilfield after different displacement methods.

4.1. Distribution of Microscopic Residual Oil in the High-Water-Cut Stage after Water Flooding

In Dagang Yangsanmu Oilfield, the underground crude oil has a viscosity of 98 mPa·s, and the reservoir temperature is 61 °C. The water used in the experiments is filtered injection water from the field. The extraction methods include water flooding, SP flooding, and dual-mobility composite flooding. By using PDMS models designed specifically for Yangsanmu Oilfield, microscopic displacement experiments were conducted to determine the types and distribution of microscopic residual oil after water flooding in Yangsanmu Oilfield.

After water flooding, the residual oil content is 62.38%, with the following distribution: cluster-like residual oil accounts for 34.12%, pore surface film-like residual oil 17.03%, corner-like residual oil 6.05%, and slit-like residual oil 5.18% (

Table 1. Microscopic water drive oil experiment data of Yangsanmu Oilfield.

Experiment No.	Residual Oil after Water Flooding, %	Cluster-Like, %	Pore Surface Film-Like, %	Corner-Like, %	Slit-Like, %
Yang11-7	58.98	31.13	16.33	5.18	4.23
Yang19-9	65.77	37.11	21.73	8.92	8.13
Average	62.38	34.12	17.03	6.05	5.18

).

By analyzing the data from the microscopic oil displacement model experiments of Yangsanmu Oilfield (Figure 6), the following can be discerned: (1) Due to the insufficient sweep efficiency of water flooding, the dominant form of microscopic residual oil after water flooding is cluster-like residual oil, with a significant amount of crude oil in the porous media remaining unswept, accounting for 54.70% of the total residual oil. (2) Additionally, on the surface of the porous media, a considerable amount of oil is not carried away by the water flooding sweep zone and remains adsorbed on the porous media surface, accounting for 27.30% of the total residual oil. (3) The blind end effect of pore throats leaves residual oil at the blind ends of the pore throat media unswept and unmovable by water flooding, accounting for 9.70% of the total residual oil [34]. (4) In the swept areas, some narrow regions of the flow channels are affected by capillary forces, preventing the mobilization of residual oil, accounting for 8.30% of the total residual oil.

4.2. Mobilization of Microscopic Residual Oil after Different Chemical Flooding Methods

To enhance the field development effectiveness, after water flooding, SP flooding is carried out. In the SP flooding, the polymer used is BHHP-113 with a concentration of 1800 mg/L, and the surfactant is BHS-01B with a concentration of 0.3%. The SP slug size is 0.8 PV.

After SP flooding, the residual oil content decreased from 62.38% to 35.11%. Specifically, the proportion of cluster-like residual oil decreased from 34.12% to 18.03%, pore surface film-like residual oil decreased from 17.03% to 9.58%, corner-like residual oil decreased from 6.05% to 3.13%, and slit-like residual oil decreased from 5.18% to 4.37% (

Table 2. Microscopic SP flooding oil experiment data of Yangsanmu Oilfield.

Experiment No.	Residual Oil after Water Flooding, %	Cluster-Like, %	Pore Surface Film-Like, %	Corner-Like, %	Slit-Like, %
Yang 11-7	31.77	16.09	8.79	2.68	3.77
Yang 19-9	38.45	19.97	10.37	3.58	4.97
Average	35.11	18.03	9.58	3.13	4.37

).

Continuing the data analysis of the microscopic oil displacement model for Yangsanmu Oilfield (Figure 7), the following can be discerned: (1) The polymer solution in the SP flooding enhanced the microscopic sweep efficiency, while the surfactant solution improved the oil washing capability. Due to the flow control ability of the polymer, the microscopic sweep range was expanded, reducing the proportion of cluster-like residual oil from 34.12% to 18.03%. Additionally, the viscoelasticity of the polymer increased the mobilization of corner-like residual oil, decreasing its proportion from 6.05% to 3.13%. (2) The surfactant improved the oil washing efficiency of the microscopic residual oil in the swept areas, reducing the proportion of pore surface film-like residual oil from 17.03% to 9.58%. The slit-like residual oil also experienced some mobilization, with its proportion decreasing from 5.18% to 4.37%.

4.3. Mobilization of Microscopic Residual Oil after “Dual-Flow Flooding” Chemical Flooding

Based on the core concept of “dual-phase (direction) mobility” control, the initial stage focuses on reducing the mobility of the displacing phase and enhancing oil washing efficiency through the SP flooding system, thereby promoting the enrichment of residual oil. At the optimal time, a periodic transition to a viscosity reduction system is employed to increase the mobility of the displaced phase, further expanding the sweep volume. This approach synergistically improves oil washing efficiency with surfactants, maximizing the effectiveness of dual-mobility control in composite flooding to expand the sweep volume and enhance oil recovery efficiency [35].

For the field application in heavy oil reservoirs, after water flooding, SP flooding combined with a viscosity reducer forms the dual-mobility system. The polymer used is BHHP-113 with a concentration of 1800 mg/L, and the surfactant is BHS-01B with a concentration of 0.3%, with the SP slug size being 0.5 PV. The viscosity reducer used is BHJN-04, with the viscosity reducer slug size being 0.3 PV.

After the dual-mobility composite flooding, the residual oil content decreased from 62.38% to 29.81%. Specifically, the proportion of cluster-like residual oil decreased from 34.12% to 13.81%, pore surface film-like residual oil decreased from 17.03% to 8.98%, corner-like residual oil decreased from 6.05% to 2.87%, and slit-like residual oil decreased from 5.18% to 4.15% (

Table 3. Microscopic dual-flow flooding oil experiment data of Yangsanmu Oilfield.

Experiment No.	Residual Oil after Water Flooding, %	Cluster-Like, %	Pore Surface Film-Like, %	Corner-Like, %	Slit-Like, %
Yang 11-7	27.88	12.77	7.73	2.21	3.78
Yang 19-9	31.74	14.85	10.23	3.53	4.52
Average	29.81	13.81	8.98	2.87	4.15

).

$$M_{wo}={\displaystyle \frac{{\mathsf{\lambda }}_{\mathrm{w}}}{{\mathsf{\lambda }}_{\mathrm{o}}}}={\displaystyle \frac{K_w}{{\mu }_w}}/{\displaystyle \frac{Ko}{{\mu }_o}}={\displaystyle \frac{K_w{\mu }_o}{Ko{\mu }_w}}$$

(3)

The data analysis of the microscopic oil displacement model for Yangsanmu Oilfield using dual-mobility composite flooding shows the following (Figure 8): The polymer solution in the dual-mobility composite flooding enhanced the microscopic sweep efficiency, while the surfactant solution improved the oil washing capability [36]. Additionally, the viscosity of the crude oil was reduced, which not only lowered costs (as the unit price of the viscosity reducer was less than that of the equivalent SP flooding slug) but also further improved the oil washing and displacement efficiency [37].

To further validate the results of the microscopic dual-flow flooding experiments, indoor oil displacement experiments were conducted using artificial rectangular cores (a three-layer heterogeneous model, dimensions 30 cm × 4.5 cm × 4.5 cm, with permeabilities of 300 × 10⁻³ μm², 600 × 10⁻³ μm², and 2000 × 10⁻³ μm² for each layer, respectively). In the dual-flow flooding experiment, 0.3 pore volumes (PVs) of the viscosity reducer BHJN-04 were injected first, followed by 0.5 PV of the SP flooding system. The changes in water content and injection pressure in the displacement experiment are shown in Figure 9.

By comparing the results of the above core experiments (

Table 4. Results of the artificial core dual mobility composite flooding experiment.

Experimental Content	Viscosity, mPa·s	Water Flooding Recovery Factor, %	Chemical Flooding Recovery Factor, %	Ultimate Recovery Factor, %	Cost per Cubic Meter of Liquid, CNY
Water flooding	--	32.77	--	36.45	--
SP flooding	57	33.07	57.31	62.26	122
Dual-flow flooding	56	33.71	57.91	69.01	100

), it can be seen that dual-flow flooding can improve the recovery factor by approximately 6.75% compared to SP flooding. Based on the chemical agent procurement prices from PetroChina Dagang Oilfield, and calculating the cost per cubic meter of liquid, the costs for SP flooding and viscosity reducers are CNY 122 and CNY 63, respectively. The cost per cubic meter of liquid for dual-flow flooding is CNY 100. Compared to the CNY 122 per cubic meter cost for polymer–surfactant binary flooding, the agent cost is reduced by 16.43%.

4.4. Differences in Microscopic Residual Oil Mobilization Among Various Systems

From the analysis of the above Figure 10, it can be concluded that SP flooding effectively mobilizes the primary types of residual oil: cluster-like and pore surface film-like. Specifically, 47.16% of the cluster-like residual oil and 43.74% of the pore surface film-like residual oil were effectively mobilized. Additionally, there was some mobilization of corner-like and slit-like residual oil.

Based on the SP flooding, the dual-mobility flooding further increased the mobilization of cluster-like residual oil by 12.37% and pore surface film-like residual oil by 3.52%. Although the improvement in mobilization efficiency slowed compared to the SP flooding, the unit cost of the viscosity reducer was lower than that of the SP flooding. With the same slug size, the development cost was reduced by 16.43%. Considering these factors, dual-mobility flooding provides a better overall development effect.

5. Conclusions

(1)

The PDMS oil displacement model uses polydimethylsiloxane (PDMS) as the model substrate, combined with laser etching technology, to create a microscopic displacement model on a 1:1 scale based on the actual pore structure of natural rock cores. The PDMS oil displacement model has a precision of 1 μm, making it closer to the pore throat size of natural sandstone. Through natural core data extraction, digital core modeling, and the 1:1 replication of the natural core pore throat structure, the model’s pore throat fidelity is significantly higher than that of traditional hand-etched glass models.

(2)

Microscopic residual oils can be categorized into four main types based on their occurrence state in porous media and different mobilization mechanisms: cluster-like, corner-like, pore surface film-like, and slit-like. The corner-like and cluster-like residual oils are mobilized by enhancing microscopic sweep efficiency, while the pore surface film-like and slit-like residual oils are mobilized by improving microscopic oil washing efficiency.

(3)

Different chemical flooding methods have distinct mobilization effects on various types of microscopic residual oil. Polymer solutions primarily enhance microscopic sweep efficiency with a secondary “pull and drag” oil washing effect, mainly mobilizing cluster-like and corner-like residual oils. Surfactant solutions focus on improving oil washing efficiency, but their low viscosity results in limited sweep efficiency, primarily mobilizing pore surface film-like and slit-like microscopic residual oil. Polymer–surfactant flooding solutions combine the characteristics of both polymer and surfactant solutions, effectively mobilizing all types of microscopic residual oil, although the solution cost is relatively high.

(4)

SP flooding effectively mobilizes both cluster-like and pore surface film-like residual oil, with 47.16% of cluster-like residual oil and 43.74% of pore surface film-like residual oil being effectively mobilized. Corner-like and slit-like residual oils are also mobilized to some extent. On the basis of SP flooding, dual-mobility flooding further increases the mobilization of cluster-like residual oil by 12.37% and pore surface film-like residual oil by 3.52%. Although the rate of improvement slows compared to SP flooding, the development cost is reduced by 16.43% with the same slug size. Overall, dual-mobility flooding provides better development effectiveness.