樊庄北部3号煤层现今应力场分布数值模拟

The In-situ Stress Field Distribution Numerical Simulation of No.3 Coal Seam in the North of Fanzhuang CBM Well Blocks

为了制定针对樊庄区块北部煤层气井区3号煤层的有效缝网改造措施,利用工区二维地震资料对石炭系一一二叠系的构造样式及断层特征进行解释,在此基础上,利用压裂法、成像测井及有限元数值模拟方法对山西组3号煤层现今应力场特征进行研究。结果表明,3号煤层主应力满足σ_H>σ_v>σ_h,各主应力随深度的增加而增加,σ_v、σ_H及σ_h的应力梯度分别为0.025 MPa/m、0.018 MPa/m及0.013 MPa/m。3号煤层的σ_H及σ_h均略高于其顶底板砂泥岩地层1.0~2.5 MPa,这种应力特征易造成压裂穿层,缝高难于控制;3号煤层σ_H-σ_h为2.0~6.0MPa,且随深度增加而增加,这是造成工区埋深相对较浅的煤层压裂效果较好、产气量相对较高的重要原因。研究区现今地应力方向在SN及NNE之间,与喜马拉雅中晚期构造挤压运动相关。有限元模拟显示,煤储层应力场平面分布特征受控于埋深、岩性、褶皱及断层等因素。3号煤层σ_H为15.6~21.0MPa;σ_h为12.5~16.0MPa,预测结果与实测值相符。

To develop effective measures for improving the fracture net of the No.3coal seam in the northern Fanzhuang wellblocks,we analyzed Permo-Carboniferous strata tectonic patterns and fault characteristics of the CBM well blocks using2-D seismic data.Accordingly,the stress field characteristics of the No.3coal seam in the Shanxi Formation were analyzed using multiple approaches,including the fracturing method,image logging,and finite element simulation.The results show that values of the three principal stresses follow the order_H>_v>_h;they all increase with depth._v shows the largest stress gradient of about0.025MPa/m,followed by_H,at about0.018MPa/m,and_h at about0.013MPa/m.The values of_H and_h of the No.3coal seam are slightly higher than those in the sand/shale formation of the roof and floor by about1.0-2.5MPa,leading to the formation of a fracturing wear layer and making fracture height control difficult.The values of_H-_h are mainly distributed in the range of2.0-6.0MPa.This is also the main cause for the high fracturing effect and gas production of the relatively shallow No.3coal seam.The imaging logging induced fracture analysis shows that the in-situ stress direction in the study area is located between SN and NNE,which is associated with the Himalayan middle-late tectonic compression movement.The complex local structure characteristics and the existence of faults can result in a change in this direction.Results from finite element simulation of the study area show that the stress plane distribution characteristics are dominated by multiple factors such as buried depth,lithology,folds,and faults.The values of_H and_h of the No.3coal seam from simulation are distributed between15.6-21.0MPa and12.5-16.0MPa,respectively,and are consistent with the measured results.