深层海相页岩脆性特征分析与表征

Brittleness Analysis and Characterization of Deep Marine Shales

针对中国南方深层海相页岩气开发尚未突破的现实,以四川盆地五峰组—龙马溪组页岩为研究对象,聚焦深层海相页岩的脆性表征,建立了基于全应力—应变曲线的脆性定量评价指标,以峰值应变和峰后坡降陡度分别表示页岩发生脆性破坏的难易程度和充分程度。在此基础上,通过对硅质页岩、钙质硅质页岩、钙质页岩和黏土质硅质页岩开展高围压三轴压缩岩石力学实验,明确了围压对页岩脆性的影响,建立了适用于研究区深层海相页岩的脆性评价方法,开展了深浅层页岩脆性对比。结果表明,低围压时,围压主要表现为对页岩抗压强度的强化,脆性随围压增加而增强,高围压时,围压则主要表现为对裂缝产生与扩展的抑制,脆性随围压增加而减弱;以弹性参数为岩性指示因子,以断裂韧性为围压指示因子,建立了力学脆性指数,可作为矿物法和弹性参数法的补充,有效应用于3500m以深页岩储集层的脆性评价;硅质页岩和钙质硅质页岩在4500~5000m仍具有优质储集层的潜力,但是深层海相页岩脆性的弱化使得优质储集层厚度减小。

Aiming at the challenges of deep-marine shale gas development in the southern China,and taking the Wufeng-Longmaxi shale in the Sichuan basin as the research object,the paper focuses on the brittleness analysis and characterization of deep marine shale,establishes a quantitative assessment index of shale brittleness based on the complete stress-strain curve,and uses the peak strain and steepness of post-peak slope as indicators to express the degrees of brittle failure complexity and adequacy,respectively.Then according to the results of high-confining-pressure triaxial compression tests for main shale lithofacies including siliceous shale,calcareous-siliceous shale,calcareous shale and clay-siliceous shale,the paper analyzes the influence of confining pressure on shale brittleness,establishes a brittleness evaluation method for deep marine shales and compares the brittleness of deep and shallow shale reservoirs.The results show that under low confining pressure,the pressure mainly enhances the compressive strength of the shale and the shale brittleness increases with the enhancement of the confining pressure;while under high confining pressure,the pressure mainly restrains the initiation and propagation of fractures and the shale brittleness decreases with the weakening of the confining pressure;using elastic parameters as the indicators of lithology and fracture toughness as an indicator of confining pressure,a mechanical brittleness index is established,which can be effectively used in the brittleness evaluation of the deep shale reservoirs below 3 500 m as the supplementary of the mineral method and elastic parameter method;the siliceous shale and calcareous-siliceous shale still have the potential to be favorable reservoirs at the depth of 4 500-5 000 m.However, the deep marine shales have thinner“sweet spot”layers and lower development potential due to the brittleness weakening.