考虑复杂应力分布的数值缝宽计算模型及其应用

Numerical fracture width model considering complex stress distribution and its application

缝宽的预测精度,对于裂缝延伸模拟和压后产能评价具有显著影响。但目前的缝宽计算方法都对裂缝壁面的应力分布情况进行了简化,不能真正有效地利用现场实测数据,对动态缝宽的预测也不一定准确。基于经典模型的缺陷,建立了可以考虑任意复杂应力分布的数值缝宽计算模型,该模型首先将原始的复杂分布应力离散成多个矩形均匀分布应力,再通过叠加原理得到缝宽表达式。经解析解对比验证,数值缝宽模型的计算精度随着离散单元个数的增加而提高,具有很高的实用价值。地应力扰动分析显示,随着地应力的波动幅度增加、频率降低,经典简化模型的计算结果与实际缝宽的偏差逐渐增大;当地应力波动的频率很高时,缝宽会出现明显的震荡,极大地增加缝内流动摩阻。将该模型应用于现场实例中,模拟输出的缝宽会在高应力夹层处出现明显的颈缩现象,在进行该类储层的施工时,应该通过加大前置液用量或者增加排量等方式来提高净压力,以保证支撑剂的铺置效率。裂缝三维延伸的模拟结果显示,当地应力存在严重波动时,需要考虑裂缝壁面粗糙度对缝高的抑制作用。

Fracture width simulation accuracy has a significant influence on fracture propagation simulation and post fracturing produc tivity prediction. However, stress distribution on fracture faces is simplified in the current calculation methods of fracture width, so it is unable to effectively use the field measured data, which may lead to errors in dynamic fracture width prediction. Based on the above defect of classic models, a numerical fracture width calculation model considering arbitrary complex stress distribution was established, in which original complex stress is dispersed into a number of rectangular uniform stresses and then the final fracture width expression is obtained through superimposition principle. After the contrast validation of analytical solution, the simulation ac- curacy of numerical fracture width model is improved with discrete unit number rising, indicating a very high practical value. The geo stress disturbance analysis shows that with the increase in amplitude or decrease in frequency of geo stress fluctuating, the differences between the calculation results of classic simplified model and actual fracture width will be enlarged gradually; in case of a very high fluctuating frequency of geo stress, fracture width is volatile significantly, thus greatly increasing the intra fracture flow resistance. When this model is applied to field case, an obvious necking phenomenon will occur to the fracture width obtained through simulated output in high stress inter layers. During the construction in such reservoirs, net pressure should be enhanced through increasing prepad fluid consumption or displacement, so as to ensure the laying efficiency of proppant. The simulation results of 3D fracture propagation reveal that in case of severe geo-stress fluctuations, it is required to consider the inhibition of fracture face roughness on fracture height.