Selection strategy for preserving physical fields in deep coal fidelity coring

Selecting preserving parameters is key to deep fidelity coring and cost reduction. For achieving cost-effective fidelity coring under the dominant controlling factors, first, this study employed local outlier factor algorithm to screen homogeneous samples. Then, the physico-mechanical response laws of coal and antagonistic mechanism were investigated through orthogonal test of temperature and pressure. Finally, a mathematical model for preserving parameter optimization was established and the corresponding two-dimensional map was constructed. The results demonstrate that the homogeneity of selected samples was improved by more than 60%; Elevated temperatures were found to accelerate the thermal damage efficiency of coal mechanical properties increase the proportion of shear fractures and large cracks, and alter the developmental characteristics of the stress-strain curve. Conversely, high confining pressure enhanced strength parameters of coal and inhibited crack propagation, indicating a strong antagonism between temperature and confining pressure. At the microscopic level, high temperature promoted the release of volatile component and moisture from coal, increased pore pressure, and created "gas-erosion" fracture networks, resulting in thermal damage to coal samples. Confining pressure, however, can suppress methane desorption and increase effective stress to counteract thermal damage. Controlling confining pressure (CCP) and controlling temperature (CCT) were defined. The CCP exhibited an inverted U-shaped quadratic relationship with temperature, whereas the CCT varied linearly with confining pressure. On this basis, the two-dimensional optimization atlas for preserving parameters was divided into three zones: pressure-preserved zone, temperature-pressure preserved zone, and temperature-preserved zone. These findings provide valuable insights for the efficient development of deep coal resources.