离散裂缝模型的多尺度混合有限元流动数值模拟研究

Nearly all hydrocarbon reservoirs are affected in some way by natural factures, yet the effects of fractures often result in strong heterogeneity and multi-scale characteristics. The present reservoir simulators are mainly based on the dual-porosity model or equivalent continuum model. However, these two models are too simplification at macro-scale to depict the multi-scale flow characteristic. The discrete fracture model is suggested due to its high calculation precision and more realistic results. However, it is rarely possible to be used for real oil-field problem because of its tremendous amount of computation by using present numerical methods. To this end, we propose an efficient multi-scale numerical scheme to discrete fracture model based on mixed multi-scale finite element method. In this numerical scheme, only coarse grids are generated, and the multi-scale basis functions are developed to capture the detailed micro-scale information. The macro-scale solutions are obtained by the global formulation, and then the micro-scale flow solutions can be obtained by using the multi-scale basis functions. Therefore, it can capture the small-scale effect on the large scales, but requires less amount of calculation. Furthermore, the computation time can be relieved by parallel computing. The corresponding lab experiments of two-phase flow are designed to verify the validity of the numerical scheme and parallel program. At the end, an efficient multi-scale numerical simulation theory and method will be developed for fractured reservoirs, which are suitable to the realistic discrete fracture model from the oil-field.

裂缝性油藏分布广、储量大，但具有强烈的非均质性和多尺度性。现有数值模拟主要基于双重介质和等效介质模型，但这两种模型过于宏观简化，均不能刻画其多尺度流动特征。离散裂缝模型虽具有计算精度高、拟真性好的优点，但对于此类多尺度流动问题，传统数值方法面临计算量大的瓶颈，仍难以应用于实际。对此，本项目将多尺度混合有限元引入到离散裂缝流动模拟中。该方法仅需在宏观尺度上进行粗网格剖分，通过在粗网格上求解局部微分方程来构造其多尺度基函数，把小尺度裂缝信息反映到多尺度基函数中，从而在获取大尺度解的同时也包含了小尺度信息，旨在降低计算量的同时捕捉小尺度流动特征，克服传统数值方法的缺点。结合并行算法，编制计算程序，进一步提高计算效率；并基于物理实验模拟来验证方法和程序的正确性。最终形成一套裂缝性油藏多尺度精细流动模拟理论与方法，为裂缝性油藏的高效开发提供技术支持。

裂缝性油藏分布广、储量大，具有很高的研究价值和意义。但裂缝性油藏具有强烈的非均质性和多尺度性，给数值模拟带来巨大挑战。现有数值模拟主要基于双重介质和等效介质模型，但这两种模型过于宏观简化，均不能刻画其多尺度流动特征。离散裂缝模型虽具有计算精度高、拟真性好的优点，但对于此类多尺度流动问题，传统数值方法面临计算量大的瓶颈，仍难以应用于实际。对此，项目将多尺度计算方法和离散裂缝模型有机结合了起来，首先对流动区域进行多尺度网格划分，在粗网格上求解局部精细流动问题，并通过构建粗网格多尺度基函数，反映介质中精细的裂缝信息，在保证计算精度的同时大幅减少了计算量，节省了计算内存，计算效率提高30%左右。通过本项目的研究，最终形成了一套缝性油藏多尺度精细流动模拟理论与方法，能够解决大规模裂缝性油藏精细数值模拟问题，并可以应用至地下水文学、地下核废料处理等其他领域。