深部煤层气地质特征与开发对策

Geological characteristics and development countermeasures of deep coalbed methane

深部煤层气勘探与开发先导试验取得的重要突破展示了良好的勘探开发前景。但深部煤层气埋藏较深、非均质强,影响开发效果的地质、工程因素众多,合理的开发对策尚不明确。通过分析鄂尔多斯盆地8号煤层的成藏规律与开发地质特征表明:8号煤层有机质成熟度高,全盆地稳定分布,生烃潜力巨大;深部煤岩储层微孔、介孔、宏孔和微裂缝体积平均占比分别为78.0%、6.8%、2.1%和13.1%,为典型的多重孔-裂隙系统,储集条件复杂;深部煤层气通常位于临界深度以下,构造抬升幅度更小,埋藏深,储层相对致密,断裂不发育,水动力较弱,保存条件更好;深部煤层气含气量高且吸附气与游离气共存,煤体结构发育相对完整,更有利于压裂改造,气井排采早期产气量便快速提升,具有早期产量高、递减快的特征,按照解吸规律可划分为游离气采出、稳产和递减3个开发阶段。针对深部煤层气的开发难点,结合致密气与页岩气的开发经验,提出坚持三维地震先行、建立储层地质力学模型、工厂化大井丛建产模式、坚持先导试验4点针对性建议。以大宁—吉县区块为例,总结了先导试验取得的规律认识,以期为深部煤层气进一步开发提供借鉴。

Significant breakthroughs have been made in precursor experiments for the exploration and development of deep coalbed methane,showing good prospect for development.However,deep coalbed methane has relatively high buried depth with strong heterogeneity.Lots of geological and engineering factors may affect the development results,and reasonable development countermeasures are still not determined.This paper is a case study of No.8 coal seam in Ordos Basin,and analyzes its accumulation laws and development characteristics.The results show that No.8 coal seam has high maturity,stable distribution in the whole basin,and huge hydrocarbon generation potential;the average volume proportions of micropores,mesopores,macropores,and microfractures in deep coal reservoirs are 78.0%,6.8%,2.1%and 13.1%,respectively,as being a typical multiple pore-fracture system with superior hydrocarbon accumulation conditions;deep coalbed methane is located below the critical depth,characterized with a small-scale structural uplift,relatively tight reservoirs,undeveloped faults,weak hydrodynamic forces,and better preservation conditions.There is a high content of deep coalbed methane in the study area,which coexists with adsorbed gas and free gas.The coal structure is generally well developed,which is more conducive to reservoir stimulation by hydraulic fracturing.The production of gas wells quickly increases at early stag,characterized with high early production and rapid decline;according to desorption laws,the whole process can be divided into three development stages:free gas production,stable production,and decline.To address the challenges faced in the exploitation of deep coalbed methane,based on the experiences obtained during the development of tight gas and shale gas,four targeted suggestions are proposed:(1)initially applying 3D seismic technique;(2)establishing a reservoir geomechanical model;(3)building an industrialized large-scale well cluster construction mode;(4)keep doing precursor experiments.Finally,taking the Daning-Jixian block as an example,the understandings obtained from the precursor experiments have been summarized,will provide reference for further development of deep coalbed methane.