Discontinuity development patterns and the challenges for 3D discrete fracture network modeling on complicated exposed rock surfaces

Natural slopes usually display complicated exposed rock surfaces that are characterized by complex and substantial terrain undulation and ubiquitous undesirable phenomena such as vegetation cover and rockfalls.This study presents a systematic outcrop research of fracture pattern variations in a complicated rock slope,and the qualitative and quantitative study of the complex phenomena impact on threedimensional(3D)discrete fracture network(DFN)modeling.As the studies of the outcrop fracture pattern have been so far focused on local variations,thus,we put forward a statistical analysis of global variations.The entire outcrop is partitioned into several subzones,and the subzone-scale variability of fracture geometric properties is analyzed(including the orientation,the density,and the trace length).The results reveal significant variations in fracture characteristics(such as the concentrative degree,the average orientation,the density,and the trace length)among different subzones.Moreover,the density of fracture sets,which is approximately parallel to the slope surface,exhibits a notably higher value compared to other fracture sets across all subzones.To improve the accuracy of the DFN modeling,the effects of three common phenomena resulting from vegetation and rockfalls are qualitatively analyzed and the corresponding quantitative data processing solutions are proposed.Subsequently,the 3D fracture geometric parameters are determined for different areas of the high-steep rock slope in terms of the subzone dimensions.The results show significant variations in the same set of 3D fracture parameters across different regions with density differing by up to tenfold and mean trace length exhibiting differences of 3e4 times.The study results present precise geological structural information,improve modeling accuracy,and provide practical solutions for addressing complex outcrop issues.

The variational inequality formulation for unconfined seepage through three-dimensional dense fracture networks

The space block search technology is used to determine a connected three-dimensional fracture network in polygonal shapes,i.e.,seepage paths.After triangulation on these polygons,a finite element mesh for 3D fracture network seepage is obtained.Through introduction of the generalized Darcy's law,conservative equations for both fracture surface and fracture interactions are established.Combined with the boundary condition of Signorini's type,a partial differential equation(PDE) formulation is presented for the whole domain concerned.To solve this problem efficiently,an equivalent variational inequality(VI) formulation is given.With the penalized Heaviside function,a finite element procedure for unconfined seepage problem in 3D fracture network is developed.Through an example in a homogeneous rectangular dam,validity of the algorithm is verified.The analysis of an unconfined seepage problem in a complex fracture network shows that the proposed algorithm is very applicable to complex three-dimensional problems,and is effective in describing some interesting phenomenon usually encountered in practice,such as "preferential flow".

复杂缝网导流能力实验研究

缝网导流能力是影响致密油气藏储层改造效果的重要因素之一。设计新型导流室,通过室内导流能力实验研究闭合压力、铺砂浓度、砂堤高度、支撑剂及缝网结构等因素对缝网导流能力的影响,并利用正交试验分析各参数对缝网导流能力的影响程度。结果表明:随着闭合压力的增大,缝网导流能力先缓慢降低后大幅降低,最终下降趋势平缓;随着铺砂浓度和砂堤高度的增大,缝网导流能力增大;缝网导流能力随着支撑剂粒径的增大而增大,大粒径支撑剂承压能力差,随着闭合压力的增加,缝网导流能力下降较快;随着次生裂缝数量的增多,与出口端距离减小,缝网导流能力增大。各参数对缝网导流能力的影响程度分别为:闭合压力影响最显著,次生裂缝位置和支撑剂粒径影响较大,铺砂浓度和次生裂缝数量有一定影响,砂堤高度与支撑剂类型影响最弱。

页岩储层裂缝网络导流能力实验分析

为评价页岩储层压裂后形成的复杂裂缝的网络导流能力,改造常规裂缝导流仪,设计适合评价页岩裂缝网络导流能力的导流室,利用大理石和高硬度支撑剂组建不同结构的裂缝网络模型,进行裂缝网络导流能力实验,分析不同形态裂缝对裂缝网络导流能力的影响.实验结果表明:当裂缝宽度接近天然裂缝宽度时,平行于渗流方向的垂直裂缝对导流能力影响最大.现场施工时可使用较大铺砂浓度以增加裂缝宽度;压裂设计优选目标依次为平行于渗流方向的垂直裂缝、平行于渗流方向的水平裂缝和垂直于渗流方向的垂直裂缝.该研究结果为页岩储层压裂施工提供理论参考.

破碎压实岩体节理宏观力学参数研究

为解决随机分布节理面力学参数的科学估算,在实验室内选用不同粒径级配的破碎岩块进行压缩,获得不同压实率下的压实体试件。对压实体进行单轴压缩试验,记录其轴向应力应变曲线的同时分析压实体节理迹长概率分布函数和范围;其次,通过正交试验和数值模拟的方法构造单轴压缩数值模型,选用与试验轴向应力应变曲线最接近的模拟曲线的节理力学参数,作为该压实体的节理力学参数。最后结合实际现场,利用得到的压实体节理力学参数,研究不同压实程度下再生顶板巷道顶板稳定性。得出了在破碎岩块粒径越小,压实率越大的情况下,再生顶板越稳定的结论。

页岩气缝网优化的数值模拟

为定量评价页岩气储层压裂缝网参数对产量的影响程度,运用页岩渗流机理,通过Eclipse建立页岩气水平井压裂后形成的复杂缝网模型,进行局部网格加密,分析了改变单因素(段间距、簇数、簇间距、次生缝间距、主缝长、次生缝长、主缝导流能力、次生缝导流能力、天然裂缝渗透率)对累计产量的影响,再对所有参数进行多因素正交方案设计,找出影响累计产气量的主控因素先后顺序并得到产量最大化的方案。结果表明,随着段间距、簇数与簇间距的增加,累计产气量随之减少;随着主缝与次生缝的长度增加,累计产气量随之增加;随着天然裂缝渗透率的增加,累计产气量随之增加;影响累计产气量的第一因素是天然裂缝渗透率,排在2到5位的依次是段间距、主缝长、簇间距与簇数,其他4个因素对累计产气量影响不大;正交方案设计中累计产量最多的方案为段间距40m,3簇,簇间距40m,次生缝间距30m,主缝长530m,次生缝长40m,主缝导流能力7×10-3μm2·m,次生缝导流能力2×10-3μm2·m,天然裂缝渗透率0.0008×10-3μm2,累计产量为2.94×108m3。在实际生产中,尽可能提高主裂缝长度,压缩段间距、簇间距与簇数对页岩气产量有较大的提升。研究结果为页岩气压裂缝网部署提供了参考。

三维正交裂隙网络动水注浆耦合试验研究

注浆加固止水技术是水利工程中地下洞室建设的常用技术,其工程设计研究对提高堵水效果具有重要意义。文章在考虑温度场和应力场的条件下,进行三维裂隙网络的动水注浆耦合试验,并得出了封堵效果、浆液扩散过程以及动水流量方面的变化规律。