泥页岩天然裂缝特征及其对页岩气成藏、开发的控制作用

Characteristics of Natural Fractures in Shale and Their Control Effect on Shale Gas Accumulation and Development

中国及北美地区页岩气勘探开发实践表明,泥页岩中发育种类繁多的天然裂缝,对于页岩气成藏和开发有着重要的控制作用,但各类裂缝所起的作用尚存在争议。首先根据产生裂缝的应力来源和触发机制的不同,将泥页岩内的天然裂缝分为构造裂缝和非构造裂缝两大类,进一步又划分出张裂缝、剪裂缝、构造压溶缝合线、层间页理缝、成岩缝和流体超压缝6类,并总结了每类裂缝的成因及发育特征。在此基础上,结合国内外页岩气勘探开发实例,认为泥页岩内部的微裂缝对页岩气的储集空间贡献微弱,大型天然开启裂缝可能破坏封存性能,不利于成藏;裂缝密度是页岩气开发的重要控制因素,一定范围内,裂缝密度越大,页岩气产量越高,超过一定范围,产量反而下降;被方解石充填的裂缝网络有利于水力压裂,而大型开启裂缝可能导致压裂液的漏失,诱发工程事故。

The success of shale gas exploration in North America as well as China has shown that there re abundant natural fractures in shales which must have an important effect on shale gas accumulation and development.However,it s still not clear how different fractures affect shale gas accumulation and development.A classification scheme of fractures was offersed based on the origin and mechanism of fractures and recognizes two major types of fractures,namely the tectonic and non-tectonic,which can be further divided into extensional fractures,shear fractures,tectonic stylolites,bedding-parallel fractures,diagenetic fractures and fluid-overpressure driven factures.The origin and characteristics of these types of fractures are summarized and their interaction with shale gas accumulation and development are explored using data in North America shale plays.The case studies suggest that microfractures might be of little importance in gas accumulation and larger,regional opening-mode fractures may be detrimental to shale gas accumulation.Fracture density has a positive correlation with gas production within a certain threshold and vice versa,if the seal hasn t been fractured and stays efficient.Large open-mode fractures may lead to fracturing fluid leakoff and geological hazards.