基于核磁共振和低温氮吸附的煤层酸化增透效果定量表征

针对某些煤储层中的孔隙和裂隙被方解石、白云石等矿物或杂质堵塞致使煤层渗透率降低的问题,提出一种煤层酸化增透方法。利用核磁共振(NMR)和低温氮吸附实验表征了煤样酸化前后的比表面积、孔隙体积、孔隙度、渗透率、孔径分布及核磁成像等特性参数,分析了酸化作用下煤的细观结构变化规律。测试结果表明,煤样经酸化处理后,饱和水状态的NMR信号量增加,残余水状态的NMR信号量降低,T_2谱的中大孔峰(T_220 ms)的变化较大,NMR所表征的总孔隙度、有效孔隙度、渗透率均增加,残余孔隙度、T_2截止值降低;低温氮吸附所表征的比表面积和总孔隙体积均增加,且主要集中增加在10 nm左右的孔隙,但高煤阶煤增加不明显。NMR图像直观地反映了煤样酸化前后内部孔隙结构的变化特征,为揭示酸化增透的微观演变过程提供了新的手段。煤样初始孔隙越发育或孔隙裂隙中可溶性矿物质越多,所表征的特征参数变化越大,酸化增透效果越好。

Quantitative characterization of the effect of acidification in coals by NMR and low-temperature nitrogen adsorption

A lot of pores and fractures in some coal reservoirs are blocked by minerals or impurities,such as calcite and dolomite.The permeability of those coal seams is low.This paper puts forward a new method to increase permeability by acidification.In order to study the law of meso-structure of coal under the action of acidification,those parameters,such as specific surface area,pore volume,porosity,permeability,pore size distribution and magnetic resonance imaging,are measured by low temperature nitrogen adsorption and NMR experiments.Test results show that the coal samples’saturated water state NMR signal increases,irreducible water state NMR signal cut down,and the changes about mid-large pores peak (T2>20 ms) is much bigger than small pores peak in T2 spectrum.The parameters that quantified by NMR,such as total porosity,effective porosity,and permeability were increased,and the residual porosity and T2 cut-off values were decreased.The specific surface area and total pore volume were increased,which quantified by the low temperature nitrogen adsorption,and the increase was mainly concentrated at the pore size of 10 nm,but the increase of high rank coal was not obvious.The nuclear magnetic resonance image directly reflects the change characteristics of the internal pore structure before and after the coal sample acidification,which provides a new way to reveal the microscopic evolution law of acidification.The comparison shows that the initial porosity of coal sample is more developed or more carbonate,silicate and other soluble minerals in the pore,the greater the change of the characteristic parameters,the better the effect of acidification.