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不同層理頁巖常規三軸壓縮力學特性離散元模擬

Discrete element simulation of the mechanical properties of shale with different bedding inclinations under conventional triaxial compression

  • 摘要: 頁巖作為頁巖氣儲層,在沉積過程中形成部分弱面,在力學特性上表現出各向異性特征。所以,使用離散元軟件從微細觀層面探討深部頁巖力學各向異性特征具有重要實踐意義。基于頁巖室內常規三軸壓縮試驗結果,采用離元程序PFC2D對常規三軸壓縮下不同層理傾角頁巖進行了顆粒流模擬研究,分析了層理傾角及圍壓對頁巖力學特性的影響規律。結果表明:(1)頁巖峰值強度與黏聚力隨層理傾角的增加整體呈“U”形變化,但峰值強度在不同圍壓下的變化趨勢有所區別;而內摩擦角隨層理傾角的增大呈非線性變化。(2)層理傾角對頁巖周圍顆粒的位移方向及大小的影響隨著層理面與軸向應力的夾角的增大而減小。(3)同一層理傾角試樣最終破壞時的微裂紋總數隨著圍壓的升高有所增加;同一圍壓下,試樣最終破壞時的微裂紋數目,隨著層理傾角的增加呈現先減少后增多的趨勢。(4)同一層理傾角頁巖的脆性隨圍壓的增長整體呈下降趨勢;低圍壓情況下,頁巖脆性隨層理傾角的增加呈兩端大中間小的變化規律。

     

    Abstract: With the growth in energy demand, shale gas has attracted considerable attention as an unconventional clean and efficient energy source. In addition, the recoverable reserves of deep shale gas in China far exceed those with a depth less than 3500 m. Thus, deep shale gas is an important replacement field for shale gas production in China. Shale, as a shale gas reservoir, forms many weak surfaces in the deposition process and shows different degrees of anisotropy in the mechanical properties. Therefore, it is of great importance to use particle flow code (PFC) to explore anisotropy of shale from the perspective of micro-level for deep shale gas production in China. Based on the experimental results obtained from the shale specimens under conventional triaxial compression, PFC2D was used to simulate the triaxial mechanical properties of shale with different bedding inclinations. The effects of bedding inclination and confining pressure on the mechanical properties of shale specimens were analyzed. The following results are obtained. (1) With the increase of bedding inclination, the peak strength and cohesion of shale all display a "U"-type variation, but the trend of peak strength is different under different confining pressures and the internal friction angle varied nonlinearly with the bedding inclination increases. (2) The effects of bedding inclinations on the displacement direction and size of surrounding particles decrease with the increase of the angle between the bedding inclination and axial stress. (3) At constant bedding inclination, the number of microcracks at the final failure of the specimen increases with the increase of confining pressure. Under the same confining pressure, the number of microcracks in the final failure of the specimen first decreases and then increases with the increase in bedding inclination. (4) With increased confining pressure, the brittleness of shale with the same bedding angle decreases as a whole. Under low confining pressure, shale brittleness is larger at both ends and smaller in the middle with the increased bedding inclinations.

     

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