Characterization of the length of structural fractures in low permeability reservoirs and its application: a case study of Longwangmiao Formation in Moxi-Gaoshiti areas, Sichuan Basin
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摘要: 为明确低渗透储层构造裂缝长度定量表征方法,以四川盆地磨溪—高石梯地区寒武系龙王庙组为例,综合采用岩心裂缝统计及岩石力学实验方法,从构造应力场的角度推导低渗透储层构造裂缝长度的关系式。该方法将裂缝长度与裂缝体密度、应变能密度及岩体应力状态联系起来,建立了裂缝长度与裂缝体密度之间的定量计算关系。结果表明:裂缝长度与裂缝数量呈负指数幂关系;裂缝体密度与应变能密度呈正比线性关系;裂缝长度与裂缝体密度呈负指数幂关系。将推导的裂缝长度公式应用于磨溪—高石梯地区龙王庙组,数值模拟结果显示:裂缝体密度值普遍介于1~5 m2/m3,最高为9 m2/m3,高值区主要分布于断层及周边地区;裂缝长度主要介于1~20 m,断层及周边区域裂缝密而短,长度普遍小于3 m。Abstract: In order to classify the quantitative characterization method of structural fracture length in low-permeability reservoirs, the Cambrian Longwangmiao Formation of Moxi-Gaoshiti areas in the Sichuan Basin is taken an example in this study. The relationship between structural fracture lengths of low-permeability reservoirs was derived from the perspective of structural stress field by the means of core fracture statistics and rock mechanical experiment. Quantitative relationships between fracture length and fracture volume density, strain energy density and rock mass stress state were established. Results showed a negative exponential power relationship between fracture length and number, a proportional linear relationship between fracture volume density and strain energy density, and a negative exponential power relationship between fracture length and volume density. The derived fracture length formula was applied to the Longwangmiao Formation in Moxi-Gaoshiti areas. Numerical simulation results showed that fracture density generally ranges from 1-5 m2/m3, and the highest value is 9 m2/m3, mainly distributed in fault and its surrounding areas. Fracture length is mainly between 1-20 m. The fractures in fault and surrounding areas are dense and short, usually less than 3 m.
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表 1 四川盆地磨溪—高石梯地区不同应力承载比例下裂缝长度及数量
Table 1. Fracture length and quantity under different stress levels in Moxi-Gaoshiti areas, Sichuan Basin
承载比例(σ/σc) 裂缝长度/mm < 2 2~3 3~4 4~6 6~8 8~10 >10 长度平均值 0 72 16 2 0 0 0 0 1.32 0.50 133 40 7 1 0 0 0 1.45 0.65 142 42 7 4 0 0 0 1.49 0.85 188 43 8 8 2 1 0 1.52 1.00 232 52 18 16 5 4 4 1.90 1.18 286 73 23 18 6 5 5 1.88 裂缝条数均值 175.5 43.3 10.8 7.8 2.2 1.7 1.5 表 2 四川盆地磨溪—高石梯地区龙王庙组样品实验参数
Table 2. Experimental parameters of samples from Longwangmiao Formation in Moxi-Gaoshiti areas, Sichuan Basin
试样编号 实验应力σ/ MPa 实验应变ε/ 10-4 承载比例(σ/σc) 裂缝体密度Dvf/(m2·m-3) 应变能密度ω/(J·m-3) 样品状态 ② 165.8 53.0 1.18 44.0 439 370.0 完全破碎 ③ 70.3 17.7 0.50 3.2 62 215.5 发育微裂缝 ④ 91.3 24.3 0.65 6.6 110 929.5 发育微裂缝 ⑤ 119.4 31.7 0.85 13.4 189 249.0 产生宏观裂缝 ⑥ 140.5 42.5 1.00 40.6 298 562.5 宏观裂缝扩展 表 3 四川盆地磨溪—高石梯区块地质模型的岩石力学参数
Table 3. Rock mechanical parameters of geological model in Moxi-Gaoshiti areas, Sichuan Basin
岩体 弹性模量/GPa 泊松比 密度/(kg·m-3) 断层(古) 85 0.30 2 780 断层(今) 50 0.19 2 200 储集体 79 0.29 2 744 夹层 65 0.31 2 600 围岩 80 0.28 2 750 -
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