基于改进的岩石物理模型表征页岩天然裂缝特征

Characterizing the characteristics of natural fractures in shale based on the modified petrophysical model

为了给四川盆地下志留统龙马溪组页岩气勘探开发提供技术支撑,以长宁、涪陵2个页岩气田的龙马溪组为研究对象,利用氦气法、X射线衍射、真密度测试等方法,借助改进的岩石物理模型,定量表征了龙马溪组页岩中天然裂缝的发育特征,并探讨了天然裂缝的发育类型、成因机制以及对页岩气开发的影响。研究结果表明:①运用改进的岩石物理模型可以精确刻画长宁页岩气田页岩孔隙系统,拟合度达0.74;②页岩中天然裂缝的发育具有区域控制因素的差异性,其中长宁页岩气田裂缝局部限制发育,钙质充填,裂缝孔隙平均值为0.15%,由基底逆冲断层、滑脱层和内部褶皱作用控制,而涪陵页岩气田裂缝则普遍较发育,硅质未充填或半充填,裂缝孔隙平均值为1.30%,主要受控于逆断层和滑脱作用;③地层条件下,天然裂缝的开启性存在着差异,长宁页岩气田基本处于封闭状态、渗流能力弱,而涪陵页岩气田则处于相对开启状态、渗流能力强;④天然裂缝影响页岩气的赋存方式,内部以游离气为主体,页岩气井的初始产气量相对较高。结论认为:①引进回归系数计算现今实际的总有机质含量,使得改进的岩石物理模型能够更加精确地刻画基质孔隙和裂缝孔隙;②四川盆地页岩产层段中天然裂缝发育对页岩气富集与开采相对有利,但原始地层条件下会削弱天然裂缝的渗流能力。

In order to provide technical support for the shale gas exploration and development in the Lower Silurian Longmaxi Formation of the Sichuan Basin,this paper takes the Longmaxi Formation in the Changning and Fuling shale gas fields as the research object to quantitatively characterize the development characteristics of natural fractures in Longmaxi Formation shale by means of helium pycnometry,X-ray diffraction(XRD),true density testing and other methods,with the aid of the modified petrophysical model.Then,the development types and genetic mechanisms of natural fractures and their influences on shale gas development are discussed.And the following research results were obtained.First,the modified petrophysical model can accurately describe the pore system in the Changning shale with a fitting rate of 0.74.Second,the development of natural fractures in the shale is different in different regions.The natural fractures in the Changning Shale Gas Field,controlled by basement thrust faults,decollement layers and internal folds,are locally developed and filled with calcareous,and the average fracture porosity is 0.15%.In the Fuling Shale Gas Field,however,natural fractures,mainly controlled by reverse faults and slippage effect,are commonly more developed and unfilled or semi-filled with siliceous,and the average fracture porosity is 1.30%.Third,under the formation conditions,the opening of natural fractures is different.The natural fractures in the Changning Shale Gas Field are basically closed with a weak flowing ability while those in the Fuling Shale Gas Field are relatively open with a strong flowing ability.Fourth,the occurrence mode of shale gas is influenced by natural fractures and it is internally dominated by free gas.The initial gas production of shale gas well is higher.In conclusion,the regression coefficient is introduced to calculate the actual total organic matter content,which promotes the modified petrophysical model to describe matrix pores and fracture pores more accurately.What's more,the development of natural fractures in the shale pay zone of the Sichuan Basin is relatively beneficial to shale gas enrichment and exploitation,but its effect will be weakened under the formation conditions.