变温压条件下深部煤岩孔-裂隙结构的超声波传播特征研究

Ultrasonic wave propagation characteristics in pore-fracture structure of deep coal under coupled thermo-pressure conditions

  • 摘要: 煤岩弹性、物性参数及其变化特征是煤层气储层预测和“甜点区”圈定的基础,物性参数受温压影响较大。针对深部煤层气储层孔-裂隙结构在原位温压耦合下的动态响应机制不明确问题,以鄂尔多斯盆地临兴区块煤岩为研究对象,通过变温压超声波脉冲透射试验(0~30 MPa,11~99 ℃)系统分析波速与波形特征对孔-裂隙演化的响应特征,并采用改进双孔隙模型量化温压耦合约束机制,并构建波形相似系数与孔隙率的非线性映射关系。结果表明:波形对围压的敏感性显著高于波速,当围压从0增至30 MPa时,纵横波波形相似系数变化量均达60% 以上,较波速提升了2.96~5.99倍;随着围压升高煤岩孔隙闭合以刚性孔隙线性压缩与柔性孔隙指数闭合协同作用为主,温度升高(62 ℃、19 MPa)通过热膨胀效应使煤样总孔隙率较原条件(11 ℃、19 MPa)增加了0.55%;试验获波形相似系数-孔隙率映射模型(R2=0.786),结合孔隙反演约束(波形、双孔隙、温压),进而建立动态温压双孔隙率模型,以支撑煤层气“甜点区”多参数预测。研究结论可为煤层气储层物性精细表征与高效开发提供理论支撑。

     

    Abstract: The elasticity and physical properties of coal and rock, along with their variation characteristics, underpin the prediction of coalbed methane reservoirs and delineating "sweet spots". Physical properties are significantly influenced by temperature and pressure. This study aims to address the unclear dynamic response mechanism of pore-fracture structures in deep coalbed methane reservoirs under coupled in-situ temperature and pressure conditions, coal and rock samples were taken from the Linxing Block in the Ordos Basin. Variable temperature and pressure ultrasonic pulse transmission tests (0~30 MPa, 11~99 ℃) were conducted to analyze the response patterns of wave velocity and waveform characteristics to pore-fracture evolution. An improved dual-pore model was employed to quantify the temperature-pressure coupling constraint mechanism, establishing a nonlinear mapping relationship between waveform similarity coefficient and porosity. Results indicate: Waveforms exhibited significantly higher sensitivity to confining pressure than wave velocity. When confining pressure increased from 0 to 30 MPa, both P- and S-wave shape similarity coefficients changed by over 60%, representing a 2.96-fold~5.99-fold increase relative to wave velocity. As confining pressure increased, coal-rock pore closure primarily resulted from the synergy effects of rigid pore linear compression and flexible pore exponential closure. Temperature elevation (62℃, 19 MPa) increased the total porosity of coal samples by 0.55% compared to 11℃, 19 MPa conditions through thermal expansion effects. We obtained experimentally the waveform similarity coefficient-porosity mapping model (R2=0.786) and porosity inversion constraints (waveform, dual-pore, temperature-pressure), subsequently establishing a dynamic temperature-pressure, dual-pore fracture model for multi-parameter prediction of coalbed methane "sweet spots". These findings provide theoretical references for refined characterization and efficient development of coalbed methane reservoir properties.

     

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