Impact of Stimulated Fractures on Tree-Type Borehole Methane Drainage from Low-Permeability Coal Reservoirs

Author:

Zhang LiangORCID,Qi Qingjie,Chen Xuexi,Zuo Shaojie,Deng Kai,Bi Ruiqing,Chai Jiamei

Abstract

Tree-type hydraulic fracturing (TTHF) is a promising method applicable to the effective development of methane in low-permeability coal seams. However, a large-scale application of this technique is limited due to the unclear impact of stimulated fractures by TTHF on the effect of post-fracturing methane drainage. To address this issue, a multi-scale methane flow model of coupled thermo-hydro-mechanical (THM) processes in stimulated coal seams by TTHF was developed and verified against laboratory-based measurements. Using this proposed model, a systematic evaluation of the influence extent of hydraulic fractures connecting sub-boreholes in a tree-type borehole on the drainage effect under different fracture apertures, initial permeabilities of the cleat system, and remnant methane pressures was performed. Detailed simulated results showed that the presence of highly permeable fractures induced by TTHF greatly enhanced, as expected, the drainage efficiency of coal seam methane between the ends of adjacent sub-boreholes, and led to a significant increase in the homogeneity coefficient β. Furthermore, increasing the stimulated fracture aperture and initial cleat permeability or reducing the remnant methane pressure also resulted in a larger value of β, but in turn shortened the lead time of the tree-type borehole. The β’s growth rate for different investigated cases compared to identical simulations without stimulated fractures presented an overall trend of increasing at first and then slowly decreasing with sustained drainage time. Meanwhile, large-aperture hydraulic fractures and lower remnant methane pressure are more beneficial to the drainage effect of tree-type boreholes in the initial stages of drainage. These results portrayed herein can be employed to better understand how fractures generated by TTHF play a role in post-fracturing drainage programs and provide theoretical assistance in engineering applications.

Funder

China Postdoctoral Science Foundation

Key R&D Plan Program of Hebei Province of China

Fundamental Research Funds for the Central Universities

Publisher

MDPI AG

Subject

Geology,Geotechnical Engineering and Engineering Geology

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