非均质弹塑性煤体水压致裂裂纹形态研究

Study on fracture morphology of hydraulic fracturing in heterogeneous elastoplastic coal

常规储层煤层气开采,主要使用水力压裂的方法对煤体进行增渗改造,准确模拟弹塑性煤体在水力压裂后的缝网形态,可为施工工艺的改进和储层开采价值的评估提供参考。有限离散元方法(FDEM)是一种模拟弹塑性材料内部裂纹扩展形态的有效方法,为获得模拟所用本构模型中基于B-K准则的混合断裂能,对煤进行了变预制裂缝倾角的半圆盘三点弯曲(SCB)试验;提出一种使用环氧树脂包裹煤样的立方体试件进行水力压裂研究的试验方法,还原原位状态下水力裂缝的扩展过程,验证数值模拟方法的准确性。考虑到天然状态下煤储层的非均质性,引入统计学中的Weibull概率分布,将密度函数中的形状参数m定义为非均质度系数,按照Weibull分布对数值模型中的黏聚型单元的强度和断裂能进行随机赋值,模拟不同非均质程度煤储层在水力压裂作业中的裂缝扩展形态,并充分考虑了裂隙流从Darcy流向Poiseuille流的逐渐转化。结果表明:①有限离散元法适用于模拟弹塑性煤体中水力裂缝的扩展,以二次名义应力准则为裂纹的起裂判据,以B-K混合断裂能准则为裂纹的延伸判据,可准确模拟复杂应力状态下水力裂缝的扩展形态。②单簇射孔情况下,非均质度系数m=2~5的煤层中,水力压裂模拟得到的注液压力-时程曲线呈现出明显的压力峰,符合实际工程中的注液压力曲线。③非均质度系数m=1.5~5.0的煤层中,水力裂缝的扩展会出现分叉和偏转现象,非均质度系数过低(m<1.5)的煤层只会形成单条的长直裂缝,过高(m>5)的煤层会形成对称的双翼曲线裂缝,其中m=2的煤层在水力压裂后形成的裂缝形态最为复杂。

Conventional reservoir CBM mining,which mainly uses hydraulic fracturing to modify the coal body for seepage enhancement,accurately simulates the seam network morphology of the elastoplastic coal body after hydraulic fracturing,which can provide a reference for construction process improvement and reservoir recovery value assessment.The finite discrete element method(FDEM)is an effective method to simulate the internal crack extension morphology of elastoplastic materials.To obtain the mixed fracture energy based on the B-K criterion in the intrinsic model used for the simulation,a semi-circular bending(SCB)test with variable prefabricated fracture inclination was performed on the coal;a test method for hydraulic fracture studies using a cubic specimen of epoxy-coated coal samples is proposed to restore the in-situ The test method is proposed to restore the expansion process of hydraulic fracture in the in-situ condition to verify the accuracy of the numerical simulation method.Considering the non-homogeneity of coal reservoirs in the natural state,the Weibull probability distribution in statistics is introduced,the shape parameter m in the density function is defined as the non-homogeneity coefficient,and the strength and fracture energy of the cohesive type units in the numerical model are randomly assigned according to the Weibull distribution to simulate the fracture expansion patterns of coal reservoirs with different non-homogeneity degrees in hydraulic fracturing operations,and fully consider the gradual transformation of fracture flow from Darcy flow to Poiseuille flow.The results show that:①the finite discrete element method is suitable for modelling the extension of hydraulic fractures in elasto-plastic coals,using the quadratic nominal stress criterion as the crack initiation criterion and the B-K mixed fracture energy criterion as the crack extension criterion,which can accurately simulate the extension morphology of hydraulic fractures under complex stress states.②In the case of a single cluster of injection holes,the hydraulic fracturing simulated fluid injection in coal seams with non-homogeneity coefficients m of 2-5.③In coal seams with non-homogeneity factor m of 1.5-5.0,the hydraulic fracture extension will show bifurcation and deflection,and coal seams with non-homogeneity factor too low(m<1.5)will only form a single long straight fracture,while coal seams with too high(m>5)will form a symmetrical biplane curved fractures,with coal seams with m=2 forming the most complex fracture pattern after hydraulic fracturing.