Abstract: The flushing efficiency is a key index to evaluate the performance of flushing fluid, which directly affects the optimization, dosage, and then the cementing quality. However, the current evaluation methods for flushing efficiency are either at room temperature and pressure represented by the rotational viscometer method, and there is no process of water loss to form a mud cake, or there is little connection between the flushing speed of the device and the annulus displacement. To solve this problem, based on the principle of equal wall shear rate, the rotating speed model of the flushing efficiency evaluation device was established during mud cake formation and flushing, and the evaluation method of mud cake flushing efficiency under high temperature and high pressure was proposed. The method combines the flow of drilling fluid and flushing fluid in the well with the flow in the flushing efficiency evaluation device. The applicability analysis of high temperature and high pressure flushing efficiency evaluation method shows that the high temperature and high pressure flushing efficiency evaluation method has good repeatability, and the experimental results have high comparison, which can provide a basis for the optimization of flushing performance evaluation and application process parameters optimization. In addition, it is found that the flushing efficiency decreases with the increase of the formation time of mud cake water loss and the pressure difference, and increases quickly at first and then slowly with the increase of the flushing time and displacement.

Key words: wall shear rate, flushing efficiency, flushing fluid, cementing, high temperature and high pressure