Characterization of Key Tight Oil Parameters and Mass Transfer of Counter-Current Imbibition in Fractured Tight Oil Reservoirs
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摘要: 在致密油藏水平井体积压裂开采过程中，压裂液通过缝网与基质接触并发生逆向渗吸作用，由于接触面积很大渗吸作用不可忽视；但目前关于表征致密储层的渗吸作用，从而研究渗吸对水平井体积压裂生产过程影响的研究尚未深入.为了解决以上问题，首先利用毛管束模型，通过考虑致密储层中边界层的特征，建立了解析的渗流参数计算表达式，用以计算致密储层的渗透率、毛管力、相渗曲线这3个关键渗流参数；同时，基于以上关键渗流参数和渗吸控制方程建立了适用于致密储层的渗吸速度计算模型；然后，将渗吸项作为源汇项加入到考虑缝网的双孔单渗模型中.最后，在真实水平井体积压裂开采过程中，耦合渗吸作用.研究表明，相比于不考虑边界层特征的致密油藏，边界层的存在将大幅度减弱储层的渗吸能力，同时也说明了在致密储层中，边界层的存在是不可忽视的，如果在渗吸计算中忽视致密储层的边界层特征会严重高估渗吸对致密储层产能的影响.Abstract: During the process of tight oil exploration, counter current imbibition effect is significantly different due to the presence of complex fracture network and flow characteristics in tight oil reservoirs. But at present, there is no proper model to simulate counter-current imbibition in fractured reservoir considering the characteristics of tight oil formation. In order to solve this problem, PEBI grids are used to match the complex fracture network, natural fractures and matrix are idealized as dual-porosity medium, rate of mass transfer of imbibition between matrix and fractures is treated as source or sink term in dual porosity model. A new semi analytical model for the calculation of mass transfer function of counter-current imbibition in the presence of complex fracture network is established by using radial integration boundary element method (RIBEM). In addition, to reflect the flow characteristics of tight oil, relative permeability and capillary pressure curve with the effect of boundary layer considered, and mixed wettability have also used in the mass transfer model. Besides, we show the capacity and practical application of the model with a field example from tight oil reservoir. From simulated results, it is concluded that counter-current imbibition plays an important role in improving the oil recovery and the existence of boundary layer reduces the contribution of imbibition to oil production dramatically.
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图 1 存在复杂缝网的逆向渗吸
Fig. 1. Counter-current imbibition in the presence of fracture network
图 2 边界层对致密储层流动的影响
a.存在边界层时流体在喉道中的流动；b.不存在边界层时流体在喉道中的流动
Fig. 2. The influence of boundary layer on the flow of tight throat
图 3 致密岩心OEO渗吸实验示意
Fig. 3. Schematic of tight core imbibition experiments with OEO boundary conditions
图 7 模型计算结果与实验数据对比
Fig. 7. Comparison between the calculated results and experimental data
图 9 不同情况下饱和度分布
a.初始时刻含水饱和度；b.50 d时不考虑边界层的含水饱和度；c.50 d时考虑边界层的含水饱和度
Fig. 9. Saturation distribution under the condition of without considering flow characteristics of tight oil reservoir
图 10 考虑致密储层流动特征情况下的采油速度及累计采油量对比
a.产油速度对比；b.累计采油量对比
Fig. 10. Comparison of oil recovery rate and cumulative oil production under tight reservoir flow characteristics
表 1 岩心基本参数
Table 1. Basic parameters of imbibition experiment under different boundary conditions
岩心编号 岩心直径(mm) 岩心长度(mm) 孔隙度(%) 束缚水饱和度(%) 模拟油密度(kg/m3) 地层水密度(kg/m3) S102 24.96 52.6 9.3 41.1 807 1 058 
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表 2 油藏基本参数
Table 2. Basic parameters of reservoirs
参数名称 数值 初始油藏压力(10-1 MPa) 250 井底压力(10-1 MPa) 5 致密油藏基质渗透率(考虑边界层)(mD) 1.28 致密油藏基质渗透率(不考虑边界层)(mD) 7.56 油藏天然裂缝渗透率(mD) 100 水力压裂缝的渗透率(mD) 2 000 基质的孔隙度 0.093 裂缝介质的空隙度 0.001 水力压裂缝的孔隙度 0.5 水力压裂缝的平均半长(m) 150
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