考虑天然水平裂缝的页岩气三维运移模型

A Three-Dimensional Numerical Model for Shale Gas Transport in Porous Media Considering Effect of Natural Horizontal Fracture

页岩气藏中分布着大量的天然裂缝,天然裂缝既是气体的储集场所也是气体的渗流通道。为了分析天然水平裂缝对页岩气产能的影响,将页岩气藏的基质和裂缝简化为双重介质模型,建立了三维地质渗流模型,并采用有限元程序COMSOL Multiphyscis 5.1进行了求解,同时利用现场测井数据验证了该模型。在此基础上研究了天然水平裂缝的宽度、空间展布情况、充填以及密度等参数对产能的影响。三维地质渗流模型与二维地质模型相比由于考虑了水平向的裂隙更接近于实际情况,当天然水平裂缝的导流能力比人工裂缝导流能力强时,考虑天然水平裂缝时的页岩气产量是不考虑时的两倍。裂缝的宽度实际变化不大,裂缝的导流能力主要取决于其填充情况对渗透率的影响。在本模型的几何尺寸下,人工裂缝比天然裂缝的渗透率高1个数量级,且1 000d时,和不考虑天然裂缝相比,累计产量多出了近3倍。页岩气的累计产量还随天然裂缝垂向分布密度的增加而增加,1 000d时,裂缝由0增加到1条时,累计产量可增加近4倍,裂缝由2条增为3条时,累计产量增加量为5×108 m3。在实际开发过程中,人工裂缝的设计需考虑到储层中天然水平裂缝的相关参数对产能的影响。

Shale gas reservoirs are distributed with a large number of natural fractures.They are both gas storage sites and gas seepage channels.In order to analyze the influence of natural horizontal fractures on shale gas production,the matrix and fracture of shale gas reservoirs are simplified into a dual porous media model.The three-dimensional geological seepage model is then developed and solved by the finite element based program COMSOL Multiphyscis 5.1.The logging data are used to verify the proposed model.On the basis of this,the effect of the parameters including width,spatial distribution,the filling materials and the density of the horizontal fracture on the shale gas productivity is studied.The results show that the predicted results of the proposed three-dimensional geological model are closer to the actual situation than those obtained by the two-dimensional geological model taking the natural horizontal fractures into account.When the conductivity of the natural horizontal fracture is larger than that of the artificial fracture,the production capacity is two times greater than that without considering the effect of the natural horizontal fracture.The diversion capacity of the fracture depends mainly on the effect of the filling on the permeability.Under the geometric size of the model,the artificial fracture is more than an order of magnitude higher than that of natural fractures,and 1 000 days,compared with the natural fractures,the cumulative output is more than 3times the production capacity.The cumulative production of shale gas also increased with the increase of the spatial distribution area and vertical distribution of natural fractures.At 1 000 days,by0increasing to 1,the cumulative production can be increased by nearly 4times.When the number of increases from 2to 3,the cumulative production can be increased by 5×108 m3.In the actual mining process,the design of artificial fractures should take into account the relevant parameters of the natural horizontal fractures in the reservoir.