中国煤储层渗透率主控因素和煤层气开发对策

摘要

Objectives:The permeability of coalbed reservoirs in China is generally 1 ～ 2 orders of magnitudes inferior to that of US,leading challenges in coalbed methane (CBM) development.In this paper,the controlling factors of coalbed reservoir permeability are discussed and CBM development strategy is proposed.Methods:Based on in-situ permeability data and related geological data from 33 areas/mine sites covering almost all sedimentary basins under CBM exploration and development,this paper carries out a comprehensive analysis of the main controlling factors of coal permeability.Results:The study shows that the permeability is controlled mainly by coal body structure,megascopic fracture and microscopic cleat/fracture density and cleat/fractue filling due to tectonic evolution and related fluid activities as well as by the in-situ stress.Complex coal body structure,highly filling status and high in-situ-stress,are the main factors leading the permeability to be low in sedimentary basins in China.Conclusions:Moderate tectonic deformation producing megascopic fracture enhances coal permeability and strong tectonic deformation is detrital to coal permeability.Mineral filling in cleat/fracture system and higy in-situ stress is detrital to permeability preservation.Based on logging interpretation,the mylonite coals should be screened out for fracturing operations in highly stressed situations.Due to highly filling status of cleat/fracture system in Carboniferous—Permian coals in northern China,the development and application of acidization-fracturing technology is a new and helpful direction.The in-situ stress amplitude and stress types are to be appropriately evaluated for fracturing design.Finally,due to high in-situ stress conditions,the drainage of CBM well should be controlled with a schedule of step-wise depressuring.%我国煤储层渗透率比美国煤储层渗透率低1～2个数量级,低渗透率是制约我国煤层气勘探开发的主要因素之一,本文系统分析了我国煤储层渗透率的主控因素.研究表明,煤储层的渗透率主要受煤体结构、宏观裂隙以及割理/裂隙系统充填状况和现今地应力等因素的控制,适度构造变形产生的碎裂煤中因宏观裂隙发育导致其渗透率高于原生结构煤的渗透率,但强烈构造变形形成的碎粒煤和糜棱煤则使渗透率降低;割理/裂隙系统矿物充填和高应力不利于渗透率保存.在不同地区,控制煤储层渗透率的关键因素不同,针对性对策是煤层气开发的关键:针对复杂煤体结构,在压裂井层优选时要在煤体结构测井解释的基础上考虑避开糜棱煤;针对我国华北地区石炭—二叠系煤层割理/裂隙普遍以方解石充填为主的煤储层低渗透成因,建议探索和开发酸化压裂一体化储层增透技术;针对高应力和地应力类型在垂向上的转换,在压前搞清应力强度和类型的基础上,控制水力压裂隙高度以避免沟通煤层围岩含水层;针对煤储层的应力敏感性和高应力状态,建议采用逐级降压制度,以提高单井的累计产气量.