Deep Coal Coring Made Cheaper by Mapping the Fight Between Heat and Pressure

As China’s coal mines march relentlessly downward, engineers face a stubborn problem: the deeper they drill, the harder it becomes to bring a piece of coal to the surface in anything close to its original state. By the end of 2025, the average mining depth of the country’s production mines had approached 700 meters, with more than 60 mines operating beyond 1000 meters, and recoverable reserves below 1000 meters now account for roughly 53 percent of what remains. Whether these deep resources can be assessed, mined and managed safely depends on knowing the coal’s true in-situ physico-mechanical properties, and the only direct route to that knowledge is fidelity testing on cores that have not been altered by the journey upward. A new study published in Results in Engineering offers a surprising shortcut, showing that in many deep settings it is not necessary to preserve every physical field at once, because two of the most important, temperature and pressure, spend much of their time fighting each other.

The research team, led by Kunchen He, Haichun Hao and Bengao Yang, together with colleagues from institutions including Sichuan University’s State Key Laboratory of Intelligent Construction and Healthy Operation and Maintenance of Deep Underground Engineering, started from an awkward truth about existing coring technology. Pressure-holding coring tools date back to the 1960s and were refined through the Deep Sea Drilling Project, the Ocean Drilling Program and later the European HYACE and HYACINTH systems, which added the Pressure Core Analysis and Transfer System. More recently, a so-called five-preservation concept has been proposed to maintain pressure, temperature, mass, moisture and light simultaneously. But comprehensive fidelity comes at a price: costs balloon and success rates fall. Because prior work had already shown that in weakly water-bearing formations the dominant deep factors are temperature and pressure, and that these two factors interact competitively rather than additively, the researchers set out to determine exactly when each one rules the behavior of deep coal.

The experimental material came from the No. 8 cross-heading in the southern part of a colliery in the Qinshui coalfield, sampled at a vertical depth of 568.2 meters within the No. 3 coal seam of the Lower Permian Shanxi Formation. The seam sits in a weak aquifer horizon, so the natural moisture content of the fresh samples was only 2.67 to 3.18 percent, allowing the team to justify excluding pore water pressure and focusing on the coupling of confining pressure and temperature. The coal is a bright, hard black anthracite with an original gas pressure of 2.5 to 2.75 megapascals and gas contents between 20.62 and 27.49 cubic meters per ton. Proximate and elemental analyses confirmed the material’s high maturity: fixed-carbon contents near 81 percent, very low to low total sulfur of 0.41 to 0.45 percent, and average maximum vitrinite reflectance between 3.15 and 3.17 percent.

Because deep anthracite is notoriously brittle and riddled with endogenetic fissures, only 27 standard cylindrical specimens of 50 millimeters in diameter and 100 millimeters in height could be prepared from the block samples, which were sealed in bubble wrap, taped, and packed in foam-lined wooden boxes on site. Physical properties varied considerably, with densities between 1.38 and 1.5 grams per cubic centimeter and P-wave velocities spanning 1.19 to 2.43 kilometers per second, a fluctuation of up to 51.2 percent. To prevent this heterogeneity from contaminating the results, the team applied a local outlier factor algorithm using density and wave velocity as characteristic variables. The filter retained 19 statistically similar specimens, cutting the mean square deviation of density by 58.4 percent and that of wave velocity by 60.6 percent, a crucial step for a study that hinges on detecting subtle temperature-driven changes.

The core of the work was a two-factor, four-level orthogonal scheme spanning 16 experimental groups, with temperatures of 20, 50, 80 and 95 degrees Celsius crossed with confining pressures of 1, 22, 44 and 55 megapascals, chosen using formation gradients of 30 degrees Celsius per kilometer and 22 megapascals per kilometer to represent depths down to 2500 meters. Testing used the RTRX-140-65 GCTS rock mechanics system, capable of 1000 kilonewtons of axial load, 70 megapascals of confining pressure and heating to 140 degrees Celsius, with a Micro-II acoustic emission system tracking crack growth in real time through six sensors. A loading path of temperature first, confining pressure second, was adopted to protect the apparatus, with a gentle heating rate of 0.5 degrees Celsius per minute, followed by a two-hour temperature-pressure retention experiment and then real-time strain-controlled triaxial loading at 0.05 percent per minute.

The results revealed a stark antagonism. At a confining pressure of just 1 megapascal, temperature ran riot: peak strength fell by 14.73 to 69.65 percent and elastic modulus by 5.5 to 32.6 percent as temperature rose, with the decline accelerating, so that the strength loss jumped from 15.95 percent between 50 and 80 degrees Celsius to 57.65 percent between 80 and 95 degrees Celsius. Stress-strain curves at 80 degrees Celsius and above developed periodic fluctuations, the first acoustic emission event arrived progressively earlier, from 5.41 to 1.4 minutes, and the fraction of shear cracks climbed 2.8 times to 51.62 percent, signaling a shift from brittle toward ductile failure. But once the confining pressure reached 22 megapascals, the thermal fingerprint vanished almost entirely. Strength and modulus values stayed within the normal fluctuation ranges across all temperatures, acoustic emission characteristics became nearly indistinguishable, and shear crack proportions settled between 26.38 and 32.82 percent regardless of temperature.

The mechanism behind this tug-of-war turned out to be largely a battle between internal pore pressure and external confinement. During low-pressure, high-temperature retention tests, the researchers observed mass loss of 0.55 percent, a collapse of moisture content by 91.79 percent, a 22.44 percent drop in volatile components, and thermal shrink film inflated by escaping colorless gas. Drawing on the classic firedamp drainage observation that anthracite desorbs roughly 0.8 percent of its methane per degree Celsius, and applying the ideal gas law, the team estimated that pore pressure could rise more than six-fold as temperature climbs from 20 to 50 degrees Celsius. Above roughly 60 degrees Celsius, desorbing methane and water vapor generate pressures strong enough to carve gas-erosion crack networks along natural weak directions, explaining the earlier crack initiation, larger fissures and erratic strain paths. At 22 megapascals or more, high confining pressure suppresses molecular activity, inhibits methane desorption and moisture evaporation, raises effective stress, and effectively strangles the gas-erosion process before it begins.

To quantify exactly where control flips from one field to the other, the team devised high-temperature confrontation tests in which samples were heated first and then subjected to gradually increasing confining pressure while deformation was tracked. By analyzing strain rate responses, they defined the controlling confining pressure, the pressure that fully cancels the thermal damage of a given temperature, and the controlling temperature, the temperature that overwhelms a given pressure. The numbers were striking: at 95 degrees Celsius, confining pressures up to 2 megapascals were powerless, with the absolute volumetric strain rate rising 4.5-fold, but at 15 megapascals the sample stabilized with deformation below instrument accuracy. The fitted relationships, a quadratic curve for controlling confining pressure and a linear one for controlling temperature, each achieved a correlation coefficient of 0.99.

Plotting these two curves together produced the study’s headline deliverable: a two-dimensional optimization atlas that divides the deep sampling environment into three preservation zones. Above the quadratic boundary, pressure governs and temperature damage is negligible, so a coring tool needs to maintain only the pressure field. Below the linear boundary, temperature dominates and only the temperature field must be preserved. Between the curves, both fields matter and dual preservation is required. For engineers designing the next generation of fidelity coring tools, the atlas promises a direct path to lower costs, higher success rates and scientifically valid cores, because it replaces blanket five-field preservation with a tailored strategy grounded in the genuine in-situ conditions of each target depth. As China’s mines push past 900 meters into first-level high-temperature zones, that kind of targeted efficiency may determine whether the deep coal frontier can be opened both safely and economically.

Subject of Research: An experimental and theoretical study of the antagonistic temperature-pressure mechanisms controlling the physical and mechanical properties of deep coal and the resulting selection strategy for fidelity coring.

Article Title: Selection strategy for preserving physical fields in deep coal fidelity coring

Article References: He, K., Hao, H., Yang, B., Xie, J., Xu, L., Duan, H., & Gao, M. (2026). Selection strategy for preserving physical fields in deep coal fidelity coring. Results in Engineering, 32, Article 113000. https://doi.org/10.1016/j.rineng.2026.113000

Image Credits: AI Generated

DOI: Not provided

Keywords: deep coal, fidelity coring, temperature-pressure coupling, thermal damage, confining pressure, acoustic emission, methane desorption, pore pressure, triaxial testing, anthracite, pressure preservation, coring technology