泥页岩井壁稳定影响因素分析

ANALYSIS OF EFFECT FACTOR IN SHALE WELLBORE STABILITY

考虑泥页岩钻井液体系中电化势渗透产生的流体流动和离子运移以及与固体变形的联合作用,提出泥页岩井壁稳定耦合新模型。通过有限元计算泥页岩井壁周围孔隙压力场和应力场,分析泥页岩及钻井液性能参数对井壁周围地层坍塌破坏系数和坍塌压力的影响。研究结果表明,泥页岩渗透率增大,溶质扩散系数增大,泥页岩单位表面电荷数减小有利于泥页岩井壁稳定。通过使用高浓度、高反射系数的钻井液能够提高泥页岩井壁的稳定性,对于泥页岩地层使用过高的钻井液密度反而会导致井壁不稳定。泥页岩钻井液体系膨胀系数减小,泥页岩水化程度能够减轻。在钻井液浓度大于泥页岩孔隙流体浓度的情况下化学反渗透可能使泥页岩失水,与大尺寸井眼相比,小尺寸井眼坍塌破坏指数随时间变化较显著。

A new shale wellbore stability model considering fluid flow and ion transmission induced by shale-drilling fluid system electrochemical potential osmosis, and solid deformation resulted by solute diffusion, ion transmission and hydrate expansion is proposed. Pore pressure and stress field around the wellbore wall is computed by finite element method, and the effect of shale and drilling fluid parameters on collapse index and caving pressure is analyzed. The research results demonstrate that large shale permeability, large solute diffusion coefficient and small shale per unit surface charge are favour of shale stability. Drilling fluid with high concentration and reflection coefficient is beneficial for shale stability. High mud weight can sometimes result in instability for shale formation. While the swelling coefficient of shale-drilling fluid system decreases, the shale hydration can be alleviated. Especially for the case that the drilling fluid concentration is larger than the shale pore fluid, the chemical reverse osmosis may cause shale dehydration. The collapse index of small shale wellbore altered with time is more obvious than large wellbore.