基于低场核磁共振技术的液态CO_(2)循环致裂煤体孔隙特征演化规律

Pore evolution of coals affected by cyclical liquid CO_(2)fracturing based on the low-field nuclear magnetic resonances

煤层低孔渗特征成为制约瓦斯高效率抽采的主要瓶颈,现多采用强化致裂增透技术来改造煤储层孔隙结构,旨在提高煤体渗透率。液态CO_(2)循环致裂能够通过循环热应力、相变致裂及疲劳损伤的耦合效应协同致裂煤体,可联合低场核磁共振技术实现煤体孔裂隙结构的定量表征,而基于核磁共振技术对液态CO_(2)循环作用煤体孔隙的几何特征演化规律研究较少。基于自主研发的液态CO_(2)循环致裂试验平台,对多个褐煤试样进行循环冲击作用,采用低场核磁弛豫技术对循环作用煤体的孔裂隙结构进行统计监测,并结合几何分形理论探讨了渗流孔分形维数D s、有效贯通孔分形维数D e、T 2截止值、孔隙率、渗透率的耦合关系。统计分析液态CO_(2)循环作用后煤体端面不同裂隙形态的差异性与基质非均质性、原始孔裂隙结构的相互关系;“饱水-离心”联测法获得的T 2弛豫谱曲线表征液态CO_(2)循环作用煤体的总孔隙率φt、有效孔隙率φe、核磁渗透率k SDR-d及其增长率Δk SDR-d均有所增加,利用基于T 2谱构建的几何分形计算模型,论证了渗流孔和有效贯通孔具有较好的分形特征,而吸附孔不具备分形特征。D s与T 2截止值存在“快速增大—缓慢增大”的指数拟合关系,分别与φt,φe,k SDR-d和Δk SDR-d存在负相关关系,而D e分别与φe和Δk SDR-d呈正相关,揭示了液态CO_(2)循环作用能够促进煤基质内多尺度孔裂隙结构的扩容及延伸,裂隙间的贯通率和渗透特性大幅提升。

The low porosity and permeability of coals have been the main bottleneck restricting the efficient gas extraction.Nowadays,the pore structure of coal reservoirs is mostly reformed by the fracturing and anti-reflection technology to improve permeability.The liquid CO_(2)cyclical fracturing can synergistically destroy the coals by the coupling effect of thermal cycling,phase change fracturing and fatigue damage,and the low-field nuclear magnetic resonance technology can be used to achieve a quantitative characterization of pore-crack structure of coals.However,few studies focus on the evolution law of pore geometric characteristics of liquid CO_(2)affected coals based on the nuclear magnetic resonances.Based on the liquid CO_(2)cyclical test platform dependently developed,the lignite specimens are affected by multiple liquid CO_(2)impacts,and the pore change are recorded by nuclear magnetic relaxation technology.The fractal geometry theory is adopted to explore the relationships among the seepage pore fractal dimension D s,the effective pore fractal dimension D e,the T 2 cutoff value,porosity,and permeability.The relationships among the crack morphology difference,the matrix heterogeneity and original pore structure of affected coals by liquid CO_(2)are statistically analyzed.The T 2 spectra obtained by the“saturation-centrifuge”method indicates that total porosityφt,effective porosityφe,nuclear magnetic permeability k SDR-d and its increase rateΔk SDR-d all increase.Geometry fractal calculation model is built based on the T 2 spectrum,demonstrating that the seepage pores and the effective connected pores have better fractal characteristics and the adsorbed pores do not have fractal characteristics.D s and T 2 cutoff values have an exponential fitting relationship with“rapid increase-slow increase”,and have negative relationships withφt,φe,k SDR-d andΔk SDR-d,respectively.De positively correlates withφe andΔk SDR-d,respectively.These reveal the fact that the liquid CO_(2)cyclical impact could enhance multi-scale pore expansion and crack extension,and the crack penetration and permeability of coals substantially increase.This method could provide some basic supports for the quantitative evaluation of pore structure under the effect of liquid CO_(2)cyclical fracturing in the field application.