Experimental insights into the nucleation and propagation of hydraulic fractures in anthracite coalbed methane reservoirs

Abstract

Developing low-permeability Coalbed Methane (CBM) reservoirs can significantly benefit from a comprehensive understanding of hydraulic fracture nucleation and propagation mechanisms, particularly in anthracite CBM reservoirs. This study employs true-triaxial hydraulic fracturing experiments to investigate these mechanisms, with variables including injection flow rate, horizontal stress difference (sH-sh), and bedding orientation. Additionally, we conduct corresponding numerical cases to validate the experimental conclusions. The research also considers re-fracturing instances. For the first time, we utilize a combination of Kaiser tests and the stress transfer function in ANSYS Workbench finite element analysis to accurately restore the confining pressure of the coal sample. The findings suggest that a high initial injection flow rate during hydraulic fracturing can promote fluid leakage and aid in maintaining substantial fracture pressure. Enhanced fracturing efficiency can be achieved through higher injection rates, and it can ensure optimal fracturing efficiency, minimizing roof and floor fracturing in coal reservoirs to prevent fracturing fluid leakage. The presence of a high horizontal stress difference facilitates hydraulic fracture propagation along the direction of the maximum horizontal compressive stress, requiring a greater hydraulic pressure to produce more fracture systems in coal reservoirs. Additionally, a minor deviation in the wellbore injection direction from the bedding orientation assists in creating a complex hydraulic fractured network, although this also requires higher hydraulic pressure to initiate new fractures. In the case of multiple hydraulic fracturing, the second initiation pressure tends to be significantly higher than the first, indicating that a sequential increase in hydraulic pressure aids the formation of additional fractures. Moreover, a simplified numerical simulation has been conducted to corroborate the experimental findings. These insights are crucial in optimizing hydraulic fracturing processes to enhance the permeability of anthracite CBM reservoirs.