考虑泥页岩黏弹性的钻井液密度计算方法

泥页岩地层受到外来流体侵入后,岩石具有明显的流变效应,易造成井壁坍塌。为了研究泥页岩井壁受入井流体影响产生的黏弹性力学行为对井壁稳定性的影响,建立了黏弹蠕变本构模型来反映井壁流变失稳破坏的力学机理与演化规律。根据井壁围岩受力推导出平衡方程、几何方程,结合物理方程和边界条件,得到考虑泥页岩黏弹性特性的钻井液密度方程。结合现场钻井数据给出不同时间、不同收缩速率下泥页岩的钻井液密度图版。结果表明:泥页岩地层钻进时,井眼的收缩速率越小,则所需的钻井液密度越大;井越深,钻井时间越长,所需钻井液密度越大。

Calculation method of drilling fluid density based on viscoelastic characteristics of shale

Due to the intrusion of foreign fluid, mud shale presents obvious rheological effect, so borehole collapse tends to happen easily. In order to investigate how the borehole stability in mud shale is affected by viscoelastic mechanical behavior which is caused by fluids in the well, a viscoelastic creep constitutive model was developed to show the mechanical mechanisms and evolution laws of borehole rheological instability and deterioration. Then, balance equation and geometric equation were derived according the force on the surrounding rocks of borehole. Combined with the physical equation and boundary conditions, the drilling fluid density equation based on viscoelastic characteristics of shale was established. And finally, density charts of drilling fluids in mud shale with different shrinkage rates over the time were plotted based on actual drilling data. It is indicated that the lower the borehole shrinkage rate is, the higher the drilling fluid density shall be. As the well depth and drilling time increase, the density of drilling fluid shall be increased.