深层页岩气工程“甜点”地震预测技术探讨——以DS地区为例

A seismic prediction strategy for engineering“sweet spots”in deep shale gas exploration:A case study in DS area

中国深层页岩气具有良好勘探前景和巨大勘探潜力,随着埋深增加压裂难度增大,可压性预测及评价至关重要。以DS地区研究为例,明确了深层页岩气工程“甜点”敏感地球物理参数,提出了脆性、裂缝及水平应力差地震预测技术:①结合岩石力学实验,发现了泊松比对围压变化敏感,基于此构建了塑延性转换敏感因子及脆性指数地震预测新模型;②各向异性研究发现,页岩具有明显HTI介质旅行时方位各向异性特征,通过分方位地震旅行时解释及校正,利用椭圆拟合技术,形成了一种高效的裂缝地震预测新技术;③基于贝叶斯理论推导建立了组稀疏方位弹性阻抗同时反演方法,实现了各向异性参数及水平地应力差异系数的预测。预测结果表明:脆性指数、裂缝及地应力差是评价深层页岩气能否实现体积改造的工程“甜点”参数,脆性指数越高,储层裂缝越发育,页岩越容易压开,破裂压力越低;地应力差越小,越容易压裂形成复杂缝网。

China's deep shale gas resources are far greater than those in medium-depth and shallow layers.They possess good exploration prospects and substantial exploration potential.Deep shale gas exploration in DS and other areas has achieved major breakthroughs.Effective hydraulic fracturing is essential for commercial development.However,its implementation becomes more difficult as depth and geostresses increase.This makes the prediction and evaluation of compressibility very important.In this study,the geophysical parameters relevant to the identification of deep shale gas“sweet spots”are evaluated,and a targeted prediction method is proposed.It is found that①Poisson's ratio is a key parameter for the evaluation of the brittleness of deep shale;based on Poisson's ratio,the sensitivity factor of shale plastic ductility transformation is established,based on which a brittleness prediction method for deep shale is constructed.②Studying on anisotropy shows that deep shale has evident azimuthal anisotropy with respect to the travel time.On this basis,a new fracture prediction method is established.③Based on the Bayesian theory,a group sparse azimuth elastic impedance simultaneous inversion method is constructed.Furthermore,a seismic prediction method to evaluate anisotropic parameters as well as the horizontal geostress difference coefficient is performed.Prediction results showed that higher values of the brittleness index and of fracture strength correspond to lower fracture pressures in the shale.Smaller differences in geostress are conducive to the formation of a complex fracture network.It is clear that the brittleness index,fracture strength,and geostress difference are engineering“sweet spots”for evaluating whether deep shale gas can realize stimulated reservoir volume fracturing.