Key Takeaways & Executive Findings
- •• Local contact in cleats reduces absolute permeability and increases starting pressure, impacting two-phase flow. • Microfluidic experiments reveal interface jumps in methane-water flow due to porosity alterations from local contact. • Jumping events cause additional energy dissipation, leading to potential overestimation of relative permeability in traditional models. • A dual-scale observation technique captures both micro-scale interface morphology and mesoscale flow characteristics in cleat systems.
Abstract
Cleat serves as the primary flow pathway for coalbed methane (CBM) and water. However, few studies consider the impact of local contact on two-phase flow within cleats. A visual generalized model of endogenous cleats was constructed based on microfluidics. A microscopic and mesoscopic observation technique was proposed to simultaneously capture gas–liquid interface morphology of pores and throat and the two-phase flow characteristics in entire cleat system. The local contact characteristics of cleats reduced absolute permeability, which resulted in a sharp increase in the starting pressure. The reduced gas flow capacity narrowed the co-infiltration area and decreased water saturation at the isotonic point in a hydrophilic environment. The increased local contact area of cleats weakened gas phase flow capacity and narrowed the co-infiltration area. Jumping events occurred in methane-water flow due to altered porosity caused by local contact in cleats. The distribution of residual phases changed the jumping direction on the micro-scale as well as the dominant channel on the mesoscale. Besides, jumping events caused additional energy dissipation, which was ignored in traditional two-phase flow models. This might contribute to the overestimation of relative permeability. The work provides new methods and insights for investigating unsaturated flow in complex porous media.
1. Introduction
CBM, a type of hydrocarbon gas known for its high energy content and minimal environmental impact, plays a crucial role in shifting our primary energy use from high-carbon fossil fuels to cleaner, low-carbon renewable sources [1]. It is becoming an important part of the global energy supply. Besides, theoretical and technical guidance is provided for enhancing the permeability of CBM reservoirs [2,3]. Nevertheless, CBM wells often see a quick drop in production after operation, as their actual daily output falls short of the initially high expectations [4]. Coal methane reservoirs, belonging to a typical porous media structure, primarily store CBM in these pores. Additionally, natural fractures in coal, known as cleats, act as the main pathways for fluid movement [5]. CBM enters water-bearing cleats from pores after drainage and depressurization during mining in the form of gas–liquid two-phase flow [6]. The economic efficiency of a CBM well is determined by the duration of the two-phase flow stage in the production process as well as the gas–liquid production and flow capacity [7].
Relative permeability is a crucial parameter indicating the flowability of methane and water in cleats. An accurate description of the two-phase flow behavior in cleats is the theoretical foundation for investigating the seepage mechanism of CBM [8]. Laboratory core seepage experiments are the most intuitive method to acquire relative permeability. The unsteady-state and steady-state methods are the primary approaches based on two flow assumptions [9]. Goodarzian and Sorbie [10] believed that when the traditional steady-state method is used to measure relative permeability, the results are affected by the capillary end effect. The traditional steady-state test method has been improved to increase the accuracy of the test. Regional values not affected by the capillary end effect are inversely calculated by analyzing the correlation between the reciprocal of the injected flow rate and water saturation. Thus, the accuracy of the test results is significantly improved. Hemmati et al. [11] optimized transient and steady-state laboratory test methods to determine the relative permeability of the core and obtain capillary force curves. A high-precision relative permeability model is constructed according to different relative permeability curves. The majority of laboratory
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ZHANG Jicheng, LV Dawei, ZHANG Jon Jincai, WANG Feng, YIN Dawei, YU Haiyang (2025). Dual-scale insights of two-phase flow in inter-cleats based on microfluidics: Interface jumps and energy dissipation. SinoTechIntel Verified Research. https://doi.org/10.1016/j.ijmst.2025.01.010
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Frequently Asked Questions
What is the main contribution of this study?
The study provides dual-scale insights into two-phase flow in coal cleats using microfluidics, revealing interface jumps and additional energy dissipation that are often ignored in traditional models, thus improving understanding of CBM production.
How does local contact affect two-phase flow in cleats?
Local contact reduces absolute permeability, increases starting pressure, narrows the co-infiltration area, and decreases water saturation at the isotonic point, thereby weakening gas flow capacity.
What are interface jumps in the context of this research?
Interface jumps are sudden changes in the gas-liquid interface morphology during methane-water flow, caused by altered porosity due to local contact in cleats. They affect flow direction and cause additional energy dissipation.
Why might traditional relative permeability be overestimated?
Traditional models ignore the additional energy dissipation caused by interface jumps, leading to an overestimation of relative permeability.
What experimental technique was developed in this study?
A microscopic and mesoscopic observation technique was developed to simultaneously capture gas-liquid interface morphology at pore and throat scales and the overall two-phase flow characteristics in the cleat system.
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