致密砂岩油藏复杂壁面裂缝内支撑剂耦合运移铺置行为研究与模拟

The suitably placement of proppant and effective propped fracture are critical to sustain the high conductivity and enhance oil and gas production after the volume hydraulic fracturing in tight sandstone oil reservoir. In the practical, rough and tortuous fracture with complex wall, the migration law and step type development of proppant are essential different from the results based on the ideal fracture with smooth wall. In engineering practice, it is lack of necessary theoretical support in how to make the proppant settling at the optimum location in the fracture with complex wall during the hydraulic fracturing. Hence, in this project, a large visual experimental rough fracture model will be set up to focus on resolving three problems with the methods of physical simulation, theoretical analysis, numerical calculation and computer simulation. First, some research will be done to describe the relationship between the macro morphology and micro roughness of fracture and the mechanical property, also to describe the relationship between macro morphology and micro roughness of fracture and the mineral composition of rock, with the method of fractal geometry on the base of experimental results and analysis results of the field monitoring data. Second, an large scale and visual fracture model with complex wall will be set up to scientific simulate and show the evolution mechanism of movement behavior of proppant under the synergistic effect of multiple parameters. Third, the prediction model of engineering scale proppant placement shape in the fracture with complex wall under various physical forces is established, which will be used to guide the fracturing design. The research achievements have important theoretical and practical significance on optimization of volume fracturing design for tight sandstone oil reservoir and scientific construction, which also have important guiding significance for other complex oil and gas reservoir.

支撑剂在压裂裂缝内的合理布放和有效支撑是关系到致密砂岩油藏体积压裂后裂缝能否保持高导流能力、提高油气产量的重要因素。而支撑剂在实际粗糙、迂曲的复杂壁面裂缝中的运移规律和阶跃式生长现象与现有基于理想光滑壁面模型得出的结论存在着本质区别，其工程实践缺乏必要的理论支撑。故本项目应用理论分析、物理模拟、数学建模和计算机模拟的方法主要研究：1）结合现场监测资料和实验测试结果，用分形几何方法刻画复杂壁面压裂裂缝宏观形态和微观粗糙度与岩石力学性质、矿物组成等的关系；2）建立大尺度可视化复杂壁面裂缝模型，科学地模拟、展示多参数协同作用下复杂壁面裂缝中支撑剂耦合运移行为演化机制；3）建立满足现场工程尺度条件多种物理力作用下复杂壁面裂缝中支撑剂铺展形态预测模型，并用于指导压裂施工设计。研究成果对优化致密砂岩油藏体积压裂设计、科学施工具有重要理论和现实意义，对其它复杂油气藏开展体积压裂作业也具有重大指导意义。

致密砂岩油藏水力压裂后裂缝整体呈现出宏观垂直、带有一定倾角、有一定迂曲度、相互交错，且裂缝壁面具有一定粗糙度的复杂裂缝。压裂液携支撑剂在粗糙、迂曲的复杂壁面裂缝中流动、运移、铺置是压裂液-复杂裂缝壁面-支撑剂三者耦合作用的结果。针对这一多相流领域复杂边界条件下的流固耦合热点问题，项目以鄂尔多斯盆地长8致密砂岩油藏为对象，用分形几何方法刻画了裂缝壁面的粗糙度，建立了大尺度可视化复杂壁面裂缝模型，通过室内实验系统研究了裂缝壁面粗糙度、迂曲度、携砂液性质、支撑剂性质、裂缝开启几何形状、注入制度、支撑剂组合和注入顺序等对支撑剂运移规律的影响。研究结果表明，支撑剂在粗糙、垂直裂缝中的运移机制与平滑、垂直裂缝完全不同；在粗糙垂直裂缝中，支撑剂不会像光滑壁面裂缝中形成规则的砂丘，而是形成具有分形特征的不规则形状砂团；在粗糙壁面垂直裂缝中支撑剂会发生桥接现象；裂缝壁面的粗糙度不仅影响支撑剂的铺置面积，还影响支撑剂在裂缝中的铺置密度；通过优化粒径和加砂程序支撑剂铺置面积能达到80%。通过研究为致密砂岩油藏压裂过程中支撑剂铺置形态预测及导流能力计算提供理论依据。