Investigation on Mechanical Test and Fracture Propagation for Deep Coal
Authors: He Jiayuan , Li Xiaolong , Wang Lei , Li Danqiong , Wang Xinliang 
Published: August 2024

ABSTRACT:
The mechanical and fracture characteristics of deep coal and rock were clarified based on triaxial compression experiments. Considering the filtration of complex fractures, fluid flow in the reservoir matrix, and damage and fracture of the reservoir rock, a finite element model of hydraulic fracturing in the composite structure of deep coal rock roof limestone coal rock bottom mudstone was established Based on on-site fracturing testing of deep coal and rock, a complex fracture parameter inversion method was developed, and a reliable finite element model parameters was obtained. The research results indicate that (1) the average compressive strength of the selected coal rock is 41.08MPa, and the average elastic modulus is 5.61GPa. Compared to the top limestone and bottom mudstone, the mechanical strength of the coal rock is lower, and the degree of damage after loading is high, resulting in complex crack morphology. (2) By considering wellbore flow and perforation friction, adjusting finite element model parameters to calculate fitting bottom hole pressure until the bottom hole pressure matches the fitting bottom hole pressure, accurate finite element model parameters can be obtained through inversion. The research results of this article can guide the subsequent modelling of hydraulic crack propagation in deep coal and rock and further clarify the mechanism of hydraulic crack propagation in the combination of deep-roof coal seam floor coal and rock.

1. INTRODUCTION
China is rich in coalbed methane (CBM) resources, and the geological resources of CBM in the shallow depth of 2000m are about 29.82×1012 m3, and the recoverable resources are about 12.51×1012 m3 [1,2]. Domestic and foreign CBM exploration and development mainly focus on the shallow 1000 m. With the decrease of shallow CBM available reserves, the exploitation of deep CBM (burial depth greater than 2000 m) has attracted much attention [3,4]. Currently, horizontal wells and hydraulic fracturing reservoir modification are the key technologies for CBM extraction [5]. Given the complex geological conditions of deep core rock reservoirs, how to form a large-scale network of complex seams is a key issue for deep CBM production increase [6].
Ren Jianxi et al. used a triaxial compression test system combined with acoustic emission monitoring to study the macroscopic mechanical properties and damage characteristics of coal rock [7]. Liu Bin et al. used uniaxial compression and Brazilian splitting test, which analyzed the acoustic emission evolution law and microscopic damage mechanism of coal rock under compression and tension conditions [8]. Wang Lei carried out triaxial loading and unloading tests under different unloading rates on deep coal rock, and combined with CT 3D reconstruction technology, the damage characteristics of coal rock were clarified [9]. Chen Guangbo et al. used uniaxial compression test to study the rupture mechanism of core rock assemblage under different loading rates [10]. Zuo Jianping et al. clarified the energy evolution law in the destruction process of coal rock based on the uniaxial test of coal rock assemblage [11].