注触变性水泥停泵安全启动压力预测

触变性水泥浆是现场防窜、堵漏的有效手段之一，但是在注水泥过程中若临时停泵，再启泵时启动压力会随停泵时间增加而增大，启动压力过大可能压漏薄弱地层，甚至无法启泵。针对上述情况，在分析多种触变性模型的基础上，提出了描述触变性水泥浆特征的模型；并将该模型与压降方程结合，得出了注触变性水泥停泵安全启动预测模型。用该模型分析触变性水泥浆在不同井身结构下的启动压力，实例计算结果表明:停泵时间越长，环空间隙越小，所需要启动压力越大，停泵30或60 min后启动压力分别达到13.94和21.12 MPa，选用φ139.7 mm的套管停泵30 min的启动压力比用φ177.8 mm的套管低了4.66 MPa，停泵60 min的启动压力降低了7.51 MPa，触变剂加量改变影响附加压耗，触变剂加量由5%降为2.5%后，30 min的启动压力降低了42.73%。因此触变性水泥浆应在环空间隙较大的井使用，可降低因故停泵重启泵时的风险。 

Prediction of Safe Starting Pressure after Pump Halt for Thixotropic Cement Slurry Injection

The use of thixotropic cement slurry is an effective means of anti-channeling and controlling lost circulation in feld operations. In pumping thixotropic cement slurry, if pump halts, the pressure of re-starting the pump will increase with time. Too big an initiation pressure may result in lost circulation to weak formations, or lead to the pumping that is unable to be re-started. To solve this problem, a model describing the characteristics of thixotropic cement slurry has been presented based on the analysis of several thixotropy models. A combination of this model and pressure drop equations gave birth to the model for predicting the safe re-starting pump pressure in injecting thixotropic cement slurry. The re-starting pressures for thixotropic cement slurry injected into wells with different well profles were calculated, indicating that the longer the halt of pump and the smaller the annular clearance, the higher the re-starting pressure required. After pump halted for 30 min and 60 min, the re-starting pressures were 13.94 MPa, and 21.12 MPa, respectively. The re-starting pressure when a φ139.7 mm casing was used was 4.66 MPa lower than when a φ177.8 mm casing was used, if the pump halted for 30 min. If the pump halted for 60 min, this re-starting pressure decrease was 7.51 MPa. The concentration of thixotropic agent also affected the additional pressure loss. When the concentration of the thixotropic agent was reduced form 5% to 2.5%, the 30 min re-starting pressure was reduced by 42.73%. From these data it is understood that thixotropic cement slurry should be used in wells with big annular clearance to minimize the risk experienced in restarting the pump.