深部煤层气储层各向异性岩石物理建模及应用

Anisotropic rock physics modeling and its application for deep coalbed methane reservoirs

  • 摘要: 岩石物理建模作为一种定量刻画油气储层性质的重要手段,鲜少应用于煤层气储层,已建立的岩石物理模型大多针对中浅部煤层气储层,主要考虑煤层气的赋存状态和水平裂缝的影响,忽略了煤层气储层固有各向异性的影响。基于深部煤层气储层特征与微观物性特征,聚焦深部煤层气储层复杂的孔隙结构难以表征和固有各向异性成因不明的问题,提出了一套适用于深部煤层气储层的双重孔隙各向异性岩石物理模型,并通过超声实验数据和实际测井数据验证了模型的有效性和可靠性。微观观测结果表明:研究区深部煤样煤体结构较为致密,以圆形气孔为主,但仍含有少量随机分布的微观裂缝。有机质构成深部煤样岩石骨架,呈条带状且定向分布,部分黏土矿物在成岩过程中呈水平定向排列,使得深部煤层气储层表现出较强的固有各向异性特征。将构建的深部煤层气储层岩石物理模型应用于鄂尔多斯盆地东南缘某工区某井,实现了裂缝结构参数和层状因子的反演与速度各向异性参数的定量预测,预测的纵、横波速度与实际测井数据吻合良好,误差小于3%。目标储层反演的裂缝孔隙度为0.88%~3.15%,裂缝纵横比为0.01~0.1,层状因子为1~120,预测的速度各向异性数值较大,说明目标储层有较强的地震各向异性。在目标深度范围内,速度各向异性参数与有机质和黏土含量大体呈正相关关系,与层状因子呈负相关关系。研究结果可为揭示深部煤层气储层的地震岩石物理机理和后续的工程“甜点”关键参数的高精度预测提供理论支撑。

     

    Abstract: Rock physics modelling, as an important method to quantitatively characterize the properties of oil and gas reservoirs, is seldom applied to coalbed methane reservoirs, and most of the established rock physics models are targeted at shallow and medium coalbed methane reservoirs, which mainly take into account the influence of the storage state of coalbed methane and the horizontal fracture, and neglect the influence of the intrinsic anisotropy of coalbed methane reservoirs. Based on the characteristics of deep coalbed methane reservoirs and microphysical features, focusing on the problems of difficult to characterize the complex pore structure of deep coalbed methane reservoirs and the unknown cause of the intrinsic anisotropy, a set of dual-pore anisotropic rock physics models for deep coalbed methane reservoirs is proposed. The validity and reliability of the model are verified through ultrasonic experimental data and actual well logging data. The microscopic observation results show that the coal body of the deep coal samples in the study area is relatively dense in structure, dominated by circular pores, but still contains a small number of microscopic cracks with random fracture distribution. The organic matter constitutes the rock skeleton of the deep coal samples, which is banded and oriented, and some clay minerals are horizontally oriented in the process of diagenesis, which makes the deep coalbed methane reservoirs show strong intrinsic anisotropy properties. The constructed rock physics model of deep coalbed methane reservoir was applied to a well in a work area in the southeast edge of Ordos Basin, and the inversion of fracture structure parameters and lamination factors as well as the quantitative prediction of velocity anisotropy parameters were realized, and the predicted P- and S-wave velocities matched well with the actual logging data with an error of less than 3%. The fracture porosity of the target reservoir inversion ranges from 0.88% to 3.15%, the fracture aspect ratio is 0.01–0.1, the lamination factor is 1–120, and the predicted velocity anisotropy values are relatively great, which indicates that the target reservoir has strong seismic anisotropy. In the target depth range, the velocity anisotropy parameter is roughly positively correlated with the organic matter and clay content, and negatively correlated with the lamination factor. The results can provide theoretical support for revealing the seismic rock physics mechanism of deep coalbed methane reservoirs and subsequent high-precision prediction of key parameters in the engineering ‘sweet spot’.

     

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