基于RVE尺度的水力压裂应力扰动模型

Modelling of stress disturbance due to hydraulic fracturing on RVE

应力扰动是水力压裂的伴生力学行为,是影响非常规天然气产出的重要因素。为了建立基于储层RVE的水力压裂应力扰动理论模型,首先通过数值模拟揭示RVE应力扰动的细观机理,将多孔介质骨架划分为颈部和根部,指出颈部应力为Biot有效应力,根部应力为单元总应力,等于初始地应力与本文所指的扰动应力之和;简要阐述有效应力演化产生体积张开度的基本原理;然后,在体积张开度模型的基础上,利用变形相容关系和孔弹性理论建立水力压裂应力扰动表达式。该模型的特点是反映断裂传播机制对应力扰动的影响。实例计算结果表明:①应力扰动与断裂传播机制有关,在裂缝张开方向和平行裂缝方向有分化。例如,对于黏性主导机制,裂缝张开方向的骨架应力峰值σ1 max约为静水应力2倍,断裂达到稳态后,降低为峰值的0.93倍;侧向应力峰值σ_(2,3max)约为静水应力2倍,达到稳态无应力降。对于韧性主导机制,峰值σ_(1max)约为静水应力1.7~2.7倍,达到稳态为峰值的0.59~0.73倍;侧向应力峰值σ_(2,3max)为静水应力2.19~2.91倍,达到稳态无应力降。②在断裂机理上,流体滤失有增强断裂黏滞性从而削弱其脆性的功能,使骨架拉伸强度峰值受到平抑和迟滞,因此导致体积张开度增大和应力扰动响应出现分化等一系列不同于无水条件的水力断裂力学特征。

Stress disturbance is a companying behavior with hydraulic fracturing volumetric opening,and has great effects on the unconventional gas production.In the present paper,in order to establish a theoretical model of stress disturbance on RVE,firstly,a numeric simulation on the meso-mechanism of stress disturbance is executed,by which,the skeleton of porous media are divided into neck and root according to the different stresses,that in the neck,the effective stress is generated,and in the root,the total stress is generated,which is sum of the initial in-situ stress and the stress disturbance.Next,the basic principle of effective stress change inducing hydraulic fracturing volumetric opening is clarified.and then,based on which,and using the compatibility of deformation,the stress disturbance model is established.The advantage of the model is that it can incorporate the effect of hydraulic fracturing propagation regimes on stress disturbance,which is the basic physics of hydraulic fracturing.Example shows that:①total stress is affected strongly by hydraulic fracturing regimes,and shows differentiation along fracture opening and parallel to fracture plane.For example,in viscosity dominated regimes,the peak stress σ_(1max) is about 2 times static stress,and falls to 0.93 times when fracture opening attains;while the peak stress parallel to facture plane σ_(2,3 max),is 2 times static stress and no stress falls in steady state.But in toughness dominated regimes,σ_(1max) is about 1.7-2.7 times static stress,and falls to 0.59-0.73 times when steady fracture opening attains;while σ_(2,3 max )is 2.19-2.91 times static stress and no stress falls.②the most important feature that hydraulic fracture different from the anhydrous conditions is that,fluid leak-off plays the part of enhancing the glutinousness of fracture,and thus ad reducing fracture brittleness,which,therefore,contributes to larger hydraulic fracture opening and total stress differentiation along fracture opening and parallel to fracture plane.