模拟压裂作用对水泥环密封性破坏及改善研究

利用全尺寸水泥环密封性评价装置,研究了多段压裂作用下水泥环的密封完整性。分析了密封性破坏的机理;并提出了相应改善措施和评价标准。试验结果显示基浆水泥环在压裂交变应力作用下,加卸载次数较少时,水泥环密封完整性即遭到破坏;密封性失效发生在卸压阶段水泥环与套管之间的界面处。表明在套管内压高应力作用下,弹性模量较高的基浆水泥环中产生较大的应力,除了发生弹性变形还产生了塑性变形;卸载时套管弹性回缩,水泥环中塑性变形不可完全恢复而存在残余应变,导致二者在界面处的变形不协调一致而引起拉应力。随着压裂应力交变次数的增加,水泥环塑性变形不断累积,卸载后的残余变形和拉应力也随之增大。当拉应力超过界面处的胶结强度时出现微环隙,水泥环密封性破坏。采用掺入胶乳、弹性粒子等形成弹塑性水泥石,弹性模量降低并保持较高的抗压强度,压裂作用时水泥环中产生的应力相对其抗压强度较低,产生的塑性变形较小,因此提高了水泥环保持密封完整性时压裂应力交变的次数,改善了水泥环的密封性。

Investigation on sealing failure and improving of cement sheath under simulated staged#$NBSfracturing

The sealing integrity of cement sheath under simulated staged fracturing is investigated by using sealing equipment, the mechanism of sealing failure is analyzed, improvement and evaluation criterion are proposed. Experimental resultsSindicate that under high alternating stress, the sealing of pure cement sheath was destroyed under fewer loading times and it occurred at the interface between cement sheath and casing when unloading. It shows that great stress was produced in pure cement sheath of high elastic modulus under fracturing,plastic deformation was produced in addition to elastic one. The casing retracted elasticlly and plastic deformation of cement sheath could not recover completely when unloading, so the deformationsof the two werenot coordinated and thus caused tensile stress at the interface. With increase of the fracturing number of times, the plastic deformation was accumulated. When the tensile stress exceeded the bond strength of the interface, micro annulus appeared and it caused seal failure of the cement sheath. By incorporation of latex and elastic particle and so on, elastic plastic cement stonewas produced, its elastic modulus decreased and compressive strength kept high, the alternating stress times to preserve sealing were improved.