深层页岩气有效开发中的地质问题——以四川盆地及其周缘五峰组-龙马溪组为例

深层页岩气是中国页岩气勘探开发的主要接替领域。四川盆地及其周缘五峰组-龙马溪组深层页岩气具有高温、高压、高地应力的地质条件,深层覆压的增加使储层的孔隙度降低15%~20%、渗透率降低90%~95%,页岩中裂隙对渗透性的贡献显著下降。不同地区五峰组-龙马溪组深层页岩含气量有一定差异,但总体上与已投入开发的中-浅层页岩差别不大,含气量主要受原始沉积条件和后期保存条件联合控制。深层页岩所经历的构造抬升剥蚀较少,保存条件相对较好。深层页岩地层温度较高,地应力较大且应力差值高,页岩塑性增强,导致裂缝起裂及延伸困难,且裂缝闭合压力高,支撑剂破碎和嵌入影响压裂支撑缝宽,裂缝导流能力递减快,长期导流能力难以维持。WF2-LM4笔石带硅质页岩生物成因石英含量高、抗压实作用强,有利于有效储层的形成和保存,由于脆性大,天然裂缝更发育,页岩气"甜窗"的底限可能超过5 km,是深层页岩气最优开发层段。深层页岩气的富集程度主要与先天沉积成岩条件的优越程度、埋藏及生-排烃历史以及后期构造抬升与变形的差异性有关。优选出具有高脆性、高孔渗性、高压力系数、高含气量等特性的深层页岩分布区,建立与深层页岩相适应的体积压裂改造技术,不断优化形成合理的生产制度,才能最大限度地采出页岩气,成功实现深层页岩气规模性有效开发。

Geological problems in the effective development of deep shale gas: a case study of Upper Ordovician Wufeng-Lower Silurian Longmaxi formations in Sichuan Basin and its periphery

Deep shale gas is the main alternative field for shale gas exploration and development in China. The deep shale gas in Wufeng Formation and Longmaxi Formation in Sichuan Basin and its periphery has a geological environment of high temperature, high pressure and high crustal stress. With the increase of deep overburden pressure, the reservoir porosity is reduced by 15%-20%, the permeability by 90%-95%, and the contribution of fractures in shale to permeability is decreased significantly. The deep gas content varies in different regions in Wufeng Formation and Longmaxi Formation, but generally varies little in the middle-shallow shale that has been exploited. The gas content is mainly controlled by the original sedimentary conditions and later preservation conditions. Deep shale underwent less structural uplift and erosion, and had good oil and gas preservation conditions. Deep shale formations are characterized by high temperature, large crustal stress, and high stress difference, which can easily lead to the enhanced shale plasticity, difficulties in fracture initiation and propagation, and high closure pressure in shale. For this reason, the proppant was cracked, thus influencing the propping degree of fracturing fractures; fracture conductivity decreases rapidly, and it is difficult to maintain the long-term conductivity. The biogenic siliceous shale in the WF2-LM4 graptolite zone is characterized by high quartz content and strong resistance to compaction, which are conducive to the formation and preservation of effective reservoirs. However, due to the large brittleness and rapid development of natural fractures, the lower limit of "sweet window" of the shale may exceed 5 km, which is the optimal interval for deep shale gas development. The enrichment degree of deep shale gas is mainly related to the superior congenital sedimentary and diagenetic conditions, burial and hydrocarbon generation and expulsion history, amplitude of later uplift, and differences in structural deformation. Shale gas can be fully exploited to successfully achieve the large-scale effective development of deep shale gas by optimizing the distribution areas of deep shale with high brittleness, high porosity and permeability, high pressure coefficient and high gas conten, establishing a suitable method of volume fracturing treatment in deep shale, and a reasonable production system through continuous optimization.