Study on the Anisotropy Characteristics of Deep Shale in the Southern Sichuan Basin and Their Impacts on Fracturing Pressure
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摘要: 川南深层页岩地层地应力非均匀性强，钻进过程中漏失严重，破裂压力预测困难，在进行深层页岩微观组构观测和宏观力学试验的基础上，测试了深层页岩的各向异性特征，考虑各向异性特征建立了井周应力模型，结合深层页岩本体拉张破坏、裂缝剪切滑移破坏和裂缝张性破坏模式，建立了深层页岩地层破裂压力预测模型，分析了各向异性特征对地层破裂压力的影响规律。分析结果表明：页岩各向异性越强，地应力差异越明显，大斜度井裂缝越易滑移；较高的粘聚力可有效抑制裂缝弱面的错动能力；裂缝倾角大小主导着裂缝张性漏失。预测模型在川南彭水区块页岩气井地层破裂压力预测的结果表明，地应力差异较弹性差异对深层页岩破裂压力的影响更为显著，岩石粘聚力是诱导裂缝剪切滑移漏失的主因，相对裂缝倾角是诱导裂缝张性破坏的主因。裂缝发育的页岩地层以裂缝破坏为主，破裂压力受岩石粘聚力、裂缝倾角和地应力的影响显著，在预测破裂压力时应综合3种破裂模式判断破裂方式和预测破裂压力。Abstract: Deep shale formations in Southern Sichuan Basin show significant stratigraphic anisotropy and strong in-situ stress heterogeneity,which result in serious fluid loss during drilling and difficulty in fracturing pressure prediction.On the basis of microstructure observations and mechanical experiments on deep shale,the anisotropy characteristics of deep shale were systematically studied and a circumferential stress model was built for a wellbore.A deep shale formation fracturing pressure prediction model was established in combination with the tensile failure of deep shale rock body,shear slip failure and tensile failure of fracture to analyze the influence of anisotropy characteristics on the borehole fracturing pressure.The analysis results showed that stronger shale anisotropy and higher in-situ stress difference would lead to easier fracture slippage in highly deviated wells;higher cohesion would effectively restrain the dislocation of fracture weak planes,and fracture inclination would dominate the tensile leakage of fracture.The prediction model was adopted to predict the fracturing pressure of shale wells in the Pengshui Block of the Southern Sichuan Basin.Results showed that the impact of in-situ stress difference is more significant than that of elastic difference,and the cohesion of rock is the main factor in inducing shear slip failure,while fracture inclination is the main factor in leading to the tensile failure of the fracture.Fracture developed shale formations are dominated by fracture damage,and fracturing pressure is significantly affected by rock cohesion,fracture inclination and in-situ stresses.In practical fracturing pressure prediction,three types of fracture modes should be used integrally to determine the fracture mode and fracturing pressure.
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Keywords:
- shale /
- fractured formation /
- anisotropy /
- fracturing pressure /
- prediction model /
- Southern Sichuan Basin
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