基于电阻率测井的高阶煤层割理孔渗评价方法及效果分析

Evaluation method and effect analysis of cleat porosity and permeability in high-rank coal bed based on resistivity logging

煤岩是一种双孔隙介质储层,主要包含基质孔隙度和割理(裂隙)孔隙度两部分,越高阶的煤岩割理越发育。割理是决定高煤阶煤岩孔渗的主要控制因素,也是决定煤层气井产能的重要因素。传统的裂缝孔隙度主要采用双侧向电阻率测井评价方法,该方法不适用于正交裂缝发育的高阶煤层。本文基于高阶煤层的气水赋存特征与测井响应特征分析,认为高阶煤层的割理系统中充满了地层水,可以看作饱含水裂缝型岩石系统,煤层的电阻率高低主要取决于割理孔隙度大小、地层水矿化度高低以及煤层割理孔隙系统连通性。因此提出了基于阿尔奇公式法的煤岩割理孔隙度评价方法,进而计算煤岩割理渗透率。在研究区计算的煤层割理渗透率与煤层气井平均日产量具有很好的相关性,表明基于阿尔奇公式法的割理孔隙度和渗透率评价方法是有效的,对准确认识高阶煤层的物性与产能具有重要的实用价值。

Coal rock is a kind of dual pore medium reservoir,which mainly includes matrix porosity and cleat(fracture)porosity.The higher the rank of coal rock,the richer developed the cleat.Therefore,cleat is not only the main controlling factor to determine the porosity and permeability of high-rank coal rock,but also an important factor to determine the productivity of coal-bed methane wells.The traditional fracture porosity is mainly evaluated by dual lateral resistivity logging,which is not suitable for high-rank coal beds with orthogonal fractures.Based on the analysis of gas and water occurrence state and logging response characteristics of high-rank coal beds,it is considered that the cleat system of high-rank coal beds is filled with formation water,which can be regarded as a water saturated fracture type rock system.The resistivity of coal beds mainly depends on the cleat porosity,the formation water salinity and the pore connectivity of coal bed cleat.Therefore,an evaluation method of coal cleat porosity based on Archie formula is proposed,and then the coal cleat permeability is calculated.The calculated coal cleat permeability in the study area has a good correlation with the average daily production of coal-bed methane wells,showing that the evaluation method of cleat porosity and permeability based on Archie formula is effective,and has important practical significance for accurately understanding the physical properties and productivity of high-rank coal beds.