Coupling law of nuclear magnetic relaxation and mechanical responses of multiple types of coal and rock masses at same scale

The spatial distribution of multi-scale pore-fracture structures in deep coal-rock reservoirs have a controlling effect on their mechanical strength and permeability behavior. Efficient and accurate detection of this structure is of great significance for resource reserve assessment and revealing the damage and failure mechanism under stress disturbance. Based on the process of core drilling in underground coal mines, if the pore structure and stress state along the core can be clearly identified, it will be helpful to reconstruct the flow field and stress field distribution within the reservoir and achieve the engineering geological goal of “one hole, multiple uses”. Currently, there is a lack of research on the pore-mechanical coupling mechanism of coal-rock at the same scale. Therefore, low-field nuclear magnetic resonance technology integrated with uniaxial/triaxial loading and failure experiments are carried out to systematically study the correlation between the nuclear magnetic relaxation characteristics and mechanical response parameters of various types of coal and rock, and establish a quantitative coupling relationship between pore structure parameters and mechanical parameters. The experimental results show that different types of coal and rock exhibit significant differences in relaxation responses. The T₂ spectrum obtained by the “original state/saturation-centrifugal” combined measurement reveals that there are obvious differences in the internal pore-fracture size distribution. With the increase of confining pressure, the peak strength σ_p and elastic modulus E of coal-rock increase. The T₂ parameters (geometric mean T_2gm, arithmetic mean T_2am, geometric mean T_2gmf and arithmetic mean T_2amf of effective connected pores) of various types of coal-rock are negatively correlated with their corresponding σ_p and E. This indicates that the matrix around large-sized pores and fractures with higher T₂ values is more prone to stress concentration, inducing the initiation, expansion and coalescence of new fractures. At the same time, weak cementation structure and smaller effective bearing area will weaken the cohesion and increase the average stress level. T_2gm and T_2gmf dominate the crack instability propagation path and fracture toughness during the loading and failure process, while the negative correlation between T_2am and T_2amf and mechanical parameters reflects the weakening effect of the original pore defects on stiffness and effective bearing capacity. The correlation between the T₂ spectrum sorting coefficient (T_2gm/T_2am, T_2gmf/T_2amf) and σ_p, E indicates that the wettability of coal and rock and the anisotropy of pore-fracture spatial distribution directly affect the tortuosity and structural strength of the T₂ spectrum. σ_p and E are positively correlated with the T₂ cutoff value (T_2cutoff) and negatively correlated with the nuclear magnetic porosity φ, indicating that T_2cutoff and φ can jointly reflect the connectivity and effective stress level of pores and fractures. Under the same confining pressure conditions, the failure modes and particle sliding directions of different coal and rock samples are different, and their stress sensitivity and relaxation response characteristics during loading and failure jointly affect the correlation between parameters. The ranking of the correlation between relaxation and mechanical parameters of different samples further indicates that the sensitivity of coal and rock to mechanical characteristics in relaxation response is jointly regulated by multiple factors such as mineral composition and pore size distribution.