Experimental study of gas-water two-phase flow patterns in fracture: Implication for enhancing coalbed methane production

Abstract

It has been long considered that the gas and water in coal seam fractures are produced in the form of stratified flow; however, recent researches and production practices have proved that two-phase flow in fractures shows various flow patterns. In this paper, visualized experiments were conducted to investigate the diversity of two-phase flow pattern in fracture and their controlling factors. The results show that the flow pattern can be bubbly, stratified, slug, wavy, annular-mist and mist flow under different combinations of gas and liquid flow rates. Among them, the slug flow is characterized by strong disturbance, which may easily cause serious reservoir damage during the drainage process of coalbed methane (CBM) wells. It has been observed that the variations of flow rate, fracture surface roughness, aperture, tortuosity as well as liquid viscosity and surface tension not only determine the proportions of each flow pattern but affect the intensity of slug flow greatly. Based on the observations, it is found that slug flow could be mitigated during the processes of reservoir stimulation and drainage. During the reservoir stimulation process, low-damage fracturing fluid, multi-graded proppant and surrounding rock fracture network stimulation are conducive to the prevention of slug flow. Meanwhile, during the drainage process, a two-phase flow pattern identification template can be used to predict flow pattern distribution, and a continuous but slow drainage strategy could be adopted to prevent slug flow and promote CBM production. This study can guide the establishment of CBM development strategy. • Multiple two-phase flow patterns have been observed in fractures through visualized experiments. • Flow pattern distribution and slug flow strength are impacted by flow rate, fracture and fracturing fluid properties. • Optimizations of stimulation technology and drainage schedule are crucial for inhibiting slug flow during CBM development.