In large-scale oilfield cementing operations, it is not uncommon to encounter phenomena like casing re-circulation after cementing. The causes of such phenomena are usually numerous. Below, I will share some common occurrences related to this phenomenon.

1. Incomplete Treatment of Mud

During the well circulation before cementing, the drilling mud constantly scrubs the seal valve core of the float collar and float shoe. If the mud is not properly treated and contains larger particles such as sand or impurities, prolonged scrubbing will damage the rubber sealing ring mechanism of the valve core, which in turn affects its sealing effectiveness. Before running the casing, it is important to filter the mud with a sand pump. If the particles are too large, the vibrating screen mesh should be replaced promptly.

2. Inadequate Mixing of Cement Slurry

During cementing operations, the cement and water are mixed to form cement slurry, which is then injected into the casing. If the cement slurry is not mixed thoroughly, it may contain large cement lumps or particles, which can also damage the valve core mechanism of the float collar and float shoe. Additionally, after cementing, when the float collar and float shoe valve core closes, cement lumps or particles may get trapped between the valve core and the sealing surface, preventing it from closing properly, creating gaps, and affecting the sealing.

3. Pressure Requirements during Pressure Testing

Generally, the cementing operation manual specifies detailed requirements for pre- and post-pressure testing. For example, when performing displacement during pressure testing, the displacement before pressure testing should be less than 0.8 cubic meters per minute to prevent damage to the float collar. It is recommended to reduce the displacement before pressure testing.

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