页岩气储层四维地应力演化及加密井复杂裂缝扩展研究进展

Research progress on 4-dimensional stress evolution and complex fracture propagation of infill wells in shale gas reservoirs

油气藏流体运移及地层岩石形变贯穿油气开发始终,是油气开发的核心科学问题。页岩储层天然裂缝发育、地层流体流动机理多样、岩石力学参数呈现非均质性和各向异性等特征,致使页岩气储层气藏渗流—地质力学耦合问题异常复杂。页岩气井生产过程中井筒周围储层产生不同程度的压降,扰动压降区的原地应力,储层应力随开采时间不断演化,即四维动态地应力。准确预测页岩气储层四维动态地应力场是页岩气加密井压裂和重复压裂设计的前提。因此,本文系统总结了油气藏渗流—地质力学耦合及加密井裂缝扩展的数值模拟方法,深入讨论了页岩气藏多场耦合模拟进展和最新研究成果。目前油气藏渗流—地质力学耦合模型多种多样,按照耦合求解形式可划分为全耦合、顺序耦合、单向耦合及拟耦合,通过一种或多种软件结合实现复杂的耦合计算,但各类计算方法的计算时效性及适用性存在差异。由于页岩储层地质特征复杂,目前四维地应力演化模型在传统模型基础上进行了改进,其主要为基于全耦合方法的连续介质模型和离散裂缝模型,以及迭代耦合模型。页岩气开发过程中,三向地应力随孔隙压力的减小而降低,应力方向也会随之发生偏转。相对于连续介质,裂缝会影响储层地应力分布规律和变化趋势。这种地应力状态演化会使加密井裂缝扩展发生偏转及产生"Frac-hit"现象,并引起"微地震屏障"效应。页岩气藏开发过程中的储层渗流—地质力学耦合及裂缝扩展研究是多物理场、多维度、多尺度的耦合问题,本文建议深入研究地质工程一体化的解决方案,开展四维地应力演化条件下页岩气藏水平井重复压裂及加密井压裂过程中复杂裂缝扩展机理研究、页岩气储层立体化开发复杂裂缝空间干扰机理研究、重复压裂及加密井压裂时间优化研究,以及水平井压裂套管损伤机理研究等,为我国页岩气藏的持续高效开发提供理论支撑。

Fluid migration and rock deformation occur throughout oil and gas development, and they are the core scientific problems. The coupling of flow and geomechanics in shale gas reservoirs is extremely complicated due to natural fractures, complex flow mechanisms and the heterogeneity and anisotropy of rock mechanical parameters. Because of the pressure drop in the shale gas reservoir near the wellbores during production, in-situ stress is disturbed and changed over time, that is, 4 D stress evolution. Accurate prediction of stress evolution of a shale gas reservoir is the prerequisite of optimal design of parent well re-fracturing and infill well fracturing. In this paper, research progresses and results of simulation methods of flow and geomechanical coupling and fracture propagation are reviewed, especially in shale gas reservoirs. At present, there are various flow and geomechanical coupled models of oil and gas reservoirs. According to the types of coupling solutions, these can be classified as a fully coupled approach, iteratively coupled approach, partial coupled approach and quasi-coupled approach. Complex coupling calculation can be realized by combining one or more software algorithms, but there are some differences in the calculation timeliness and applicability of various calculation methods. Due to the complex geological characteristics of shale gas reservoirs, the current four-dimensional stress evolution models have been improved on the basis of traditional models, which are mainly continuous medium models and discrete fracture models based on the full coupled approach, as well as iterative coupling models. In the process of shale gas development, as pore pressure decreases, the magnitude of three principal stresses decreases as well, and the stress direction will be deflected. Compared to a continuous medium, fractures affect the stress distribution and change trends. This stress state evolution will cause deflection of hydraulic fracture propagation of infill wells and Frac-hits, and induce a "Microseismic Events Barrier" effect. The study of flow and geomechanical coupling in a shale gas reservoir and hydraulic fracture propagation during shale gas field development is a multi-physical, multi-dimensional and multi-scale coupling problem, which needs to explore the integrated geological and engineering solutions. Therefore, further research into the mechanism and simulation methods of complex fracture propagation during re-fracturing of horizontal wells and hydraulic fracturing of infill wells in shale gas reservoirs during stress evolution should be continued. And we suggest to focus on other research, such as the mechanism of spatial interference of complex fractures during the three-dimensional development of a shale gas reservoir, the optimization of fracturing timing in re-fracturing of parent wells and hydraulic fracturing of infill wells, and the mechanism of casing damage in horizontal wells during hydraulic fracturing. These are of great significance to the efficient development of shale gas reservoirs in China.