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.