基于寻优算法及微分等效介质理论模型煤系地层岩石孔隙纵横比反演

Inversion of rock aspect ratio in coal measure strata based on optimization of algorithm and DEM theory model

沁水盆地太原组、山西组及下石盒子组煤系地层气测异常现象极为普遍,具有巨大的天然气勘探潜力。但其中烃类聚集程度差异较大,裂缝发育程度是影响烃类聚集差异的主导因素。岩石孔隙纵横比反演对孔缝形态基础研究、地震反演、物理模拟、天然气疏导保存及井区优选等方面具有重要参考价值。以沁南樊庄区块固县井区山西组一口煤层气井的全波列测井资料为基础,提出基于寻优算法+微分等效介质理论组合模型的岩石孔隙纵横比α反演方法。首先基于寻优算法提取原生结构煤岩及其顶板砂岩、砂质泥岩基质矿物及干岩样骨架模量,然后利用微分等效介质理论模型对煤系地层岩石孔隙纵横比α进行反演。研究结果表明,所研究原生结构煤以近球形孔隙空间系统为主;顶板致密黏土质砂岩以微裂缝空间为主;砂质泥岩中存在孔隙和裂缝多重空间系统。利用该方法可以对地层裂缝发育段进行有效识别,吻合率为92%。对于砂质泥岩或泥包砂类型岩石而言,微裂缝主要存在于砂质中,该类岩石α与孔隙度负相关,与泥质含量正相关。无论砂岩、泥岩还是煤岩,裂缝系统的沟通或疏导能力都要强于孔隙系统,当存在有利气源、疏导体系、裂缝系统及保存条件时,理论上均可富集成藏。α研究在裂缝参数提取、岩石存气能力、孔隙沟通能力、渗透能力及天然气疏导保存成藏等方面均具有一定参考价值,对指导煤层气或煤成气科学勘探开发选区具有现实意义。

Gas logging abnormality is very common in Taiyuan Formation,Shanxi Formation and Lower Shihezi Formation in Qinshui Basin,which has a high potential for gas exploration.But there is a huge difference in the extent of hydrocarbon accumulation,which is dominated by fracture development degree.Inversion of rock pore aspect ratio has reference value for basic research of hole seam,seismic inversion,physical simulation,gas channel conservation and wellblock optimization.Making a coalbed methane Well in south of Shanxi Formation in Fanzhuang block,Guxian well block of Qinnan as an example,based on optimization algorithm,we extract the matrix mineral and rock skeleton modulus of coal and its roof sandstone and sandy mudstone,then inverse the pore aspect ratio using DEM theory model.It shows,the nearly spherical pore space system is dominant in the studied coal rock;the roof dense clayey sandstone is given priority to micro cracks;there are multiple space systems of pore and fracture in sandy mudstone.The use of this method can effectively identify the formation with fracture development,the coincidence rate was92%.For sandy shale,sand or mud package type rock,micro cracks mainly exist in the sand,αis negatively related to the porosity of rock,and positively related to shale content in this kind of rock.Regardless of sandstone,mudstone or coal rock,communication or drainage capacity of fracture system is superior to the pore system.When there are good gas source,drainage system,fracture system and preservation conditions,coal-derived gas reservoir will be formed.Study ofαhas certain reference value in fracture parameters extraction,rock ability of saving gas,pore communication ability,permeability ability,and gas channel conservation accumulation etc,and it has positive significance to guide CBM or coal-derived gas exploration and development.