Experimental Investigation of Fracture Initiation and Propagation Characteristics in Deep Coal Seams

ABSTRACT Deep coal seams usually require hydraulic fracturing to enhance the coalbed methane productivity due to their low permeability and low porosity features. Identifying the initiation and propagation mechanics of deep coal seams could help to optimize the hydraulic fracturing design and operations and effectively stimulate the fracture network system in coalbed methane. This paper designed and conducted lab-scale hydraulic fracturing experiments to evaluate the effects of frac-fluid viscosity, water pump rate, and coal bed lithologies on fracture initiation and propagation. Totally 10 outcrop coal samples were collected and cut to 10cm × 10cm × 10cm for fracturing physical simulation. The in-situ stress (stress-difference) of the LinXing(LX) coalbed (2000m vertical depth, Shanxi China) was used as confining stress of the experimental samples. Stimulated fracture surface area calculated from computerized tomography (CT) scan and three-dimensional reconstruction was used as the primary indicator to analyze the significance of hydraulic fracturing operation parameters. Results showed that water pump rate, frac-fluid viscosity, and perforation angle were the three significant factors that could affect the complexity of the fracture network (fracture propagation). As the water pump rate increased from 3ml/min to 7ml/min, both fracturing pressure and loading time increased. The pump rate contributes to the complex fracture network. As the viscosity of the frac-fluid increased from 10mPa·s to 50mPa·s, the fracturing pressure increased, and the complexity of fractures reduced. Among mudstone, coal, and mud-coal samples, the fracturing pressure of mudstone is highest, than is mud-coal rock, while the fracturing pressure of the coal sample is lowest. Fracturing pressure increased with the perforation angle increase (90°>60°>30°>0°). The experimental results can provide fundamental data for the on-site deep coal seam fracturing design and optimization. INTRODUCTION Coalbed methane has been paid more and more attention in recent years (Cao et al., 2015; Li et al., 2017; Li et al., 2023). It is understood that the geological reserves of deep coalbed methane resources in China are about 22.45 trillion cubic meters, accounting for 61% of the total resources (Guo et al., 2015; Wang et al., 2022). As the unconventional oil and gas resource, the deep coal seam has poor physical properties, high stress, low permeability, and strong adsorption capacity, so the effect of conventional fracturing is not efficient (Qin and Shen, 2016; Yan et al., 2015; Chen et al., 2016; Li et al., 2018; Li, 2022).