利用预应力固井方法预防水泥环微环隙研究

Study on Prevention of Micro-Annulus in Cement Sheath by Prestressed Cementing Method

深层页岩气水平井环空带压问题较为普遍,套管-水泥环界面处微环隙是导致环空带压的主要原因。针对该问题,运用力学实验手段和数值模拟方法,分析了预应力固井条件下微环隙的产生与发展,明确了不同预应力条件下水泥环耐受压裂段数。结果表明:套管内压越小,水泥环保证密封完整性时可承受的循环载荷次数越多;循环载荷作用下微环隙宽度为30.89μm是发生气窜的临界值。预应力固井显著降低了初次塑性变形量,增大了塑性变形增量;考虑预应力作用下套管产生的径向预应变,预应力固井技术显著降低了微环隙的宽度,增加了多级压裂过程中水泥环密封完整性的耐受压裂段数。预应力值越高,微环隙出现前的耐受压裂段数越多;压裂段数相同的情况下,预应力越大水泥环微环隙越小。现场应用结果表明,采用预应力固井技术及低弹性模量水泥浆,可以有效缓解深层页岩气水平井套管环空带压现象。研究结果可为页岩气水平井固井提供技术支持。

The annulus pressure is more common in deep shale gas horizontal wells,which is mainly caused by the micro-annulus at the interface between casing and cement sheath.To address this problem,the generation and propagation of micro-annulus under prestressed cementing were analyzed by mechanical experimental means and numerical simulation methods,and the number of fracturing-resistant sections of cement sheath under different prestress was determined.The results showed that the lower the pressure inside the casing,the more times that the cement sheath withstood the cyclic loads on the premise of keeping the seal integrity;the micro-annulus width of 30.89μm under cyclic load was the critical value for gas channeling.The prestressed cementing significantly reduced the initial plastic deformation and increased the plastic deformation increment;taking into account the radial prestrain generated by the casing under prestress,the prestressed cementing technology significantly reduced the micro-annulus width and increased the number of fracturing-resistant sections for cement sheath seal integrity in the multi-stage fracturing process.The higher the prestress value,the more fracturing-resistant sections before the micro-annulus appeared;with the same number of fracturing sections,the higher the prestress the smaller the micro-annulus of cement sheath.The field application results indicated that the annulus pressure of casing in deep shale gas horizontal wells was effectively reduced by prestressed cementing technology and cement slurry with low elasticity modulus.The results of the study provide technical support for the cementing of shale gas horizontal wells.