非常规油气复杂裂缝网络动态扩展模拟及其对井筒完整性分析的定量影响

The wellbore integrity assessment is rather challenging due to the complex fracture network system in the reservoir caused by the volume fracturing technique used in unconventional oil and gas production. This project proposes an approach using the "L system theory" and "fractal recursive algorithm” in the plant morphological structure evolution in the formation of complex fracture morphology, and put forward a simulation method for complex fracture network caused by volume fracturing formation and its dynamic growth. This study introduces the complex fracture network and fracturing operation into the "casing and cement ring - formation" analysis system, and establishes model for the stress field in the surrounding of the borehole under the influences of the complex fracture network. Using this model, fracturing damage response analysis can be carried out for the casing-cement ring with complex perforating defects and damaged under the coupled action of temperature and stress. With the test method, mechanics performance change of perforating holes on casing material can be quantitative studied. Based on analysis of different cement loop damage from hole casing constraint conditions, the whole hole casing physical mechanics calculation model can be built, casing cement ring stress-strain calculation expression can then be derived. The dynamic extension in the form of complex fracture network simulation method proposed in this project is promising to solve the unconventional oil and gas Wells reservoir exploitation of ground stress, and can be used as a reference in a variety of complex shape and structure analysis.

针对非常规油气开采体积压裂增产技术产生的储层复杂裂缝网络系统，在井筒完整性评估上带来的一系列急需解决的技术难题，项目将研究植物形态进化构造的“L系统理论”和“分形递归算法”用于复杂裂缝形态生成，从而提出一种体积压裂引起的复杂裂缝网络形成及动态扩展模拟方法。研究将压裂作业和复杂裂缝网络引入“套管-水泥环-地层”分析系统，建立复杂裂缝网络影响下的井筒周围地应力场求解模型，对含复杂射孔缺陷和破损形态的套管-水泥环进行温度、压力耦合作用下的损伤响应分析。借助试验手段研究聚能射孔对套管材料力学性能的定量改变程度，并依据水泥环破损形态分析不同井段套管约束情况，建立全井段套管物理力学计算模型，推导出套管应力应变计算表达式。项目研究提出的复杂裂缝网络形态的动态扩展模拟方法，不仅限于对非常规油气井储层开采地应力的求解，还可以在各种涉及复杂形态结构分析的领域中为数学分析模型的建立提供研究思路和技术参考。

项目研究针对体积压裂增产技术产生的储层裂缝网络系统，在井筒完整性评估上带来的一系列技术问题展开研究。首先，通过对压裂储层进行岩样取芯，获取目标层段的自然节理分形维数，借鉴蒙特卡罗方法和粘性带模型，建立同源应力干涉下的储层压裂应力场，利用MATLAB-PYTHON-FEM平台的交互开发环境，对分段压裂影响下的裂缝动态扩展进行求解，分析天然裂缝复杂程度、压裂液排量、地应力不均匀系数、分段压裂间距等参数对裂缝网络扩展的影响规律。其次，将压裂作业引起的井筒周围地应力场的非均匀性引入“套管-水泥环-地层”分析系统，计算分析地应力不均匀系数、水泥环卸载能力大小、多裂缝间应力干扰下的压裂层段套管力学性能。再次，重点对高内压引起的射孔段套管开裂行为、含不同射孔孔眼形状（圆形、椭圆形、五边形、六边形）和孔边裂纹的射孔段套管力学性能影响，进行了有限元数值模拟和解析公式推导研究。最后，将推导得出的公式与非API套管接头相关力学行为的研究结果相结合，通过Visual Basic编写了井下套管易损部位的安全性能分析软件。

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