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Coal Geology & Exploration

Abstract

Objective Existing dynamic-to-static elastic parameter conversion models for coals fail to fully account for the actual in situ pressure and temperature conditions of deep reservoirs. This leads to significant deviations in directly evaluating the mechanical properties of reservoirs using log-derived dynamic parameters. To address this challenge, this study investigated dynamic-to-static elastic parameter conversion under the in situ stress and geotemperature conditions of strata. Accordingly, high-precision parameter conversion models were established. Methods This study examined coal samples from the No. 8 coal seam at a burial depth of approximately 1200 m in the Daning-Jixian block along the eastern margin of the Ordos Basin. Using a high-temperature and high-pressure triaxial test system, this study simultaneously determined the dynamic and static elastic parameters of the coal samples under varying confining pressure and temperature conditions. Among these, the dynamic elastic parameters were determined in real time using the ultrasonic transmission method, whereas the static parameters were derived from the elastic regions of stress-strain curves. Using experiments, the impacts of temperature (30-70 ℃) and confining pressure (0-37.5 MPa) on both dynamic and static elastic moduli and Poisson's ratios of the coal samples were examined. Results and Conclusions The results indicate that both P- and S-wave velocities increased with confining pressure but decreased with increasing temperature. Notably, the S-wave velocity was more sensitive to temperature variations. Both dynamic and static elastic moduli exhibited distinct responses to temperature-pressure coupling. These elastic moduli increased with confining pressure but decreased with rising temperature, while the static elastic modulus was more sensitive to temperature and pressure variations. The static Poisson's ratio increased significantly with temperature, whereas the impact of confining pressure on Poisson's ratio was temperature-dependent. Notably, the dynamic and static elastic parameters differed the most significantly under low temperature and confining pressure conditions, while the differences gradually decreased with increasing temperature and confining pressure. Based on these observations, nonlinear, logarithm-based conversion models were established. These models enable high-precision conversion from log-derived dynamic parameters to in situ static parameters, overcoming the limitation of conventional methods, which tend to overestimate formation stiffness. In combination with energy evolution analysis, it can be inferred that the brittleness index of coals vaies nonlinearly with increasing depth. Specifically, the brittleness index increases slightly in shallow to moderately shallow parts (400-1200 m) but initially increases and then decreases in moderately deep parts (1200-2 000 m). Practically, the results of this study can provide a quantitative basis for static pressure optimization, drainage flow rate control, and in situ stress evaluation for the fracturing of deep coal reservoirs. Therefore, these results serve as a guide for efficient coalbed methane (CBM) exploitation under similar temperature and pressure conditions.

Keywords

coalbed methane (CBM), dynamic-to-static elastic parameter conversion, temperature-pressure coupling effect, elastic modulus, Poisson's ratio, Daning-Jixian block

DOI

10.12363/issn.1001-1986.25.12.0922

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