Application of the improved seismic AVAZ inversion method for fracture characterization of a tight sandstone gas reservoir in the Ordos Basin
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摘要: 致密砂岩气储层具有低孔、低渗的特征，裂缝的存在可以提高储层的渗透率，同时裂缝是油气重要的储存空间和运移通道，裂缝发育也有利于水力压裂过程中裂缝网络的形成，裂缝预测可为致密砂岩气储层的开发和部署提供重要依据. 地震振幅随方位角的变化可以提供储层中垂直裂缝的信息，本文针对HTI介质提出了一种改进的方位振幅差异反演方法，并结合岩石物理理论预测表征裂缝性质的裂缝弱度参数. 常规的反演方法一般同时反演弹性参数和裂缝参数，改进的方位振幅差异方法引入一个参考方位，构建消除各向同性背景的方位振幅差异道集，仅反演与各向异性项相关的裂缝弱度参数，充分利用方位各向异性响应，提高裂缝识别的敏感性与裂缝参数反演的准确性. 实际数据应用表明，方位振幅差异反演方法对裂缝参数预测的敏感性较常规方法有所提高，预测的裂缝弱度与测井渗透率曲线相吻合，并且与致密砂岩气储层产气性具有明显的相关性. 因此，利用方位振幅差异方法预测裂缝分布及其发育程度可为致密砂岩储层含气有利区的识别与开发提供可靠的指标.Abstract: A tight sandstone gas reservoir is characterized by low porosity and permeability. The existence of fractures can improve the permeability of the reservoir, and fractures are a vital storage space and migration channel of oil and gas. The development of fractures is also conducive to forming a fracture network during hydraulic fracturing. The prediction of fractures can provide an essential basis for developing tight sandstone gas reservoirs. The variation of seismic amplitude with azimuth can provide information on vertical fractures in the reservoir. This study proposed an improved inversion method of azimuth amplitude difference for the HTI (transversely isotropy with a horizontal axis) medium and combined with the rock physical theory, predicted fracture weakness parameter, characterizing the fracture properties. Conventional inversion methods invert elastic and fracture parameters simultaneously. The improved azimuth amplitude difference method introduces a reference azimuth, builds the track set of azimuth amplitude difference to eliminate the isotropic background, and inverts only the fracture weakness parameters related to the anisotropic terms. Using the only azimuth anisotropic response, the sensitivity of fracture identification and the accuracy of fracture parameter inversion are improved. The application of the field data shows that the sensitivity of the proposed inversion method to the prediction of fracture parameters is improved compared with the conventional inversion method, and the predicted fracture weakness is consistent with the permeability well logs and has a significant correlation with the gas production of tight sandstone gas reservoirs. Therefore, the prediction of fracture distribution and development degree based on the proposed method can provide a reliable indicator for the identification and development of gas-bearing favorable areas in tight sandstone reservoirs.
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图 1 He4段致密砂岩反射地震波双程旅行时等值线图. 红点表示气井，白点表示干井，红线为井间地震连线
Figure 1. Contour map indicating the two-way travel time (TWT) of seismic waves reflected from the He4 Member tight sandstones. Gas wells are indicated by red dots and the dry well by the white dot. Red lines indicate the cross-well seismic line
图 2 井B不同入射角叠前方位地震数据
Figure 2. Pre-stack seismic data of variable azimuths with different incidence angles adjacent to well B
图 3 He4段致密砂岩地震振幅平面图
Figure 3. Horizontal slice of seismic amplitude for the He4 Member tight sandstone
图 4 He4段致密砂岩VP/VS平面图
Figure 4. Horizontal slice of P- and S-wave velocity ratio (VP/VS) for the He4 Member tight sandstone
图 5 利用叠前弹性反演计算井A、B和C的VP/VS剖面. 图中曲线为三口井的含气饱和度测井数据
Figure 5. Cross-section of P- and S-wave velocity ratio (VP/VS) for wells A, B, and C, calculated by the pre-stack elastic inversion. The curves are the gas saturation logs of the three wells
图 6 HTI介质模型示意图
Figure 6. Schematic diagram of the HTI (transversely isotropy with a horizontal axis) medium
图 7 （a）含气饱和情况及（b）含水饱和情况裂缝弱度随裂缝密度的变化
Figure 7. Variation of fracture weakness with fracture density in gas (a) and water (b) saturation
图 8 （a）方位振幅Rpp和（b）方位振幅差异ΔRpp随入射角和方位角的变化
Figure 8. Azimuthal amplitude Rpp (a) and azimuthal amplitude difference ΔRpp (b) for varying azimuths and incidence angles
图 9 利用常规Rpp反演方法计算井A、B和C（a）法向裂缝弱度和（b）切向裂缝弱度平面图
Figure 9. Horizontal slice of normal (a) and tangential (b) fracture weakness across wells A, B, and C, calculated by the conventional Rpp inversion method
图 10 利用地震方位振幅差异ΔRpp反演方法计算井A、B和C（a）法向裂缝弱度和（b）切向裂缝弱度平面图
Figure 10. Horizontal slice of normal (a) and tangential (b) fracture weakness across wells A, B, and C, calculated by the seismic azimuthal amplitude differences ΔRpp inversion method
图 11 利用常规Rpp反演方法计算井A、B和C（a）法向裂缝弱度和（b）切向裂缝弱度剖面图. 图中曲线为三口井的渗透率测井数据
Figure 11. Cross-section of normal (a) and tangential (b) fracture weakness for wells A, B, and C, calculated by the conventional Rpp inversion method. The curves are the permeability logs of the three wells
图 12 利用地震方位振幅差异ΔRpp反演方法计算井A、B和C（a）法向裂缝弱度和（b）切向裂缝弱度剖面图. 图中曲线为三口井的渗透率测井数据
Figure 12. Cross-section of normal (a) and tangential (b) fracture weakness for wells A, B, and C, calculated by the seismic azimuthal amplitude differences ΔRpp inversion method. The curves are the permeability logs of the three wells
表 1 HTI介质模型参数
Table 1. Model parameters of HTI media
VP/
(km·s−1)VS/
(km·s−1)ρ/
(g·cm−3)V′P/
( km·s−1)V′S/
( km·s−1)ρ′/
(g·cm−3)φ/% 模型1 4.60 2.60 2.46 0.62 0 0.065 0.05 模型2 4.60 2.60 2.46 1.5 0 1.0 0.05 
下载: 导出CSV
表 2 双层介质模型参数
Table 2. Model parameters of the double-layer medium
VP /(km·s−1) VS /(km·s−1) ρ /(g·cm−3) ΔN ΔT ISO 3.90 2.30 2.46 0 0 HTI 4.60 2.60 2.46 0.3 0.1
下载: 导出CSV
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