交变温度、压力及其耦合作用下“套管-水泥环-地层”完整性研究

The shale gas becomes important substitute resource for natural gas in China. However, during hydraulic fracturing the failure of cement sheath integrity which is caused by fluctuation, continuous change, alternating loading and unloading of temperature and pressure will seriously threaten wellbore integrity. Hence, this project is performed based on full-scale experiment and finite element simulation. Firstly, the new testing and evaluation method of casing-cement sheath-formation system integrity under alternating temperature, alternating pressure and its coupling effect is presented, and a finite element analysis model of casing-cement sheath-formation is set up to simulate the cement sheath integrity during multistage fracturing. Secondly, taking the study on cement sheath integrity as the main subject of this project, the internal structural (such as formation of cracking and micro annulus), anti-channeling performance and interface mechanical behavior of cement sheath are measured and characterized respectively under different loading-unloading ways and cycle times. Thirdly, taking the main measured data such as the interface mechanical behavior of the cement sheath as a bridge, the correlation between full-scale experiment and finite element simulation is established, and then the finite element model is modified and improved. Finally, from the perspective of mechanics and material composition, the effect of key elements on failure of cement sheath integrity is comprehensively analyzed, and the failure mechanism of cement sheath integrity is clarified under alternating temperature and pressure as well as its coupling, by which a set of recommended methods of maintaining the long-term cement sheath integrity will be formulated. This project firstly combining full-scale experiment and finite element simulation studies the failure mechanism of casing-cement sheath-formation system integrity under alternating temperature, alternating pressure and its coupling effect, which contributes to lower the risk of wellbore integrity failure during fracturing.

深部页岩气已成为国家重要接替能源，但压裂过程中因井筒内的温度、压力波动及持续变化、循环加卸载引发的水泥环完整性问题将威胁井筒完整性。本项目以实物测试和有限元模拟为基础，提出交变温度、压力及其耦合作用下全尺寸实物“套管-水泥环-地层”组合体完整性测试评价方法，建立套管-水泥环-地层组合体完整性分析的有限元模型；以水泥环完整性研究为主线，测试表征不同加卸载方式及循环次数下水泥环内部结构特征、抗窜能力及界面力学行为；以水泥环界面力学行为等主要实测数据为桥梁，建立实物测试与有限元模拟两者之间的联系，修正完善模型，从力学和材料组分角度综合分析影响水泥环完整性失效的关键要素，弄清交变温度、压力及其耦合作用下水泥环完整性失效机理，提出保持水泥环长期完整性的推荐作法。项目首次结合实验和模拟研究交变温度、压力及其耦合作用下套管-水泥环-地层组合体完整性失效机理，有助于降低压裂过程中井筒完整性失效的风险。

深部页岩气已成为国家重要接替能源，但压裂过程中因井筒内的温度、压力波动及持续变化、循环加卸载引发的水泥环完整性问题将威胁井筒完整性，而避免水泥屏障失效的关键在于弄清压裂诱导的交变载荷下水泥环完整性失效机理。为此，本项目以实物测试和理论分析为基础，构建了一种交变温度、压力及其耦合作用下全尺寸实物“套管-水泥环-地层”组合体完整性测试评价方法。建立了交变温度、压力及温-压耦合作用下套管-水泥环-地层组合体完整性分析的理论及数值模型：（1）得到了交变载荷下套管-水泥环-围岩组合体及界面应力-应变及分布规律；（2）明确了水泥环弹塑性服役行为及界面接触力演化特征；（3）揭示了交变载荷下水泥环界面密封完整性失效机理：水泥环一旦产生塑性应力/应变，卸载后将产生不可恢复的残余应变/应力，导致“界面拉-压应力反转”，造成界面脱离，形成微环隙。采用自主研发的实验装置，测试表征了不同加卸载方式及循环次数下水泥环内部结构特征、抗窜能力及界面力学行为：（1）获取了普通和高强度两种水泥环在3种交变热载荷（30℃↗90℃↘30℃↗、30℃↗120℃↘30℃↗、30℃↗150℃↘30℃↗）和五种交变压力（0↗30↘0MPa，0↗50↘0MPa，0↗60↘0MPa，0↗70↘0MPa和0↗80↘0MPa）作用下密封完整性失效的循环次数及密封完整性失效前后套管-水泥环界面力学性能；（2）得到了交变温度、压力峰值及循环次数对水泥环完整性的影响规律，揭示了水泥环完整性在交变载荷作用下的失效机理及过程，明确了水泥环密封完整性与力学完整性之间的相关性；（3）发现了高强度水泥环耐交变温度及压力的密封/抗窜能力及界面力学性能均明显优于普通水泥环。以水泥环界面力学行为等主要实测数据为桥梁，建立实物测试与理论分析两者之间的联系，修正完善了理论模型，从力学和材料组分角度综合分析了影响水泥环完整性失效的关键要素，提出了保持水泥环长期完整性的控制措施及推荐作法。研究成果有助于压裂井水泥浆体系优选及施工参数优化，降低井筒完整性失效的风险。