Study and application of the key technologies for wellhead hazard treatment

The tradition well killing method for treating wellhead hazards usually brings serious damage to reservoirs and the mechanical plugging technology is affected more by objective factors. The frozen temporary plugging technology is beyond the restriction of the size of pipe string and the structure of counter balance valve, and it can be used for the back pressure test of frozen temporary plugging bridge so as to realize the replacement of leaked wellhead device under pressure. And it has been gradually become the mainstream technology for wellhead hazard treatment in China. In the operation process of frozen plugging technology, the key technical parameters (e.g. the formula of temporary plugging agent and the freezing time) are all determined by the engineers based on their experience, so they cannot be valued accurately, leading to safety hazards. In this paper, the key technologies to ensure the smooth implementation of frozen plugging operation were studied based on laboratory experiments and field operation, including formula of temporary plugging agent, injection rate of temporary plugging agent, freezing temperature and freezing time. It is indicated that the optimal formula of temporary plugging agent is water, clay and viscosifier with the volume ratio of 2∶1∶1, the calculated injection rate of temporary plugging agent shall be optimized with the correction coefficient of 1.2-1.4, −20 ℃ is the critical temperature for the formation of frozen temporary plugging bridge, and the moment when the wellhead pressure begins to rise after dropping to the minimum value is the critical freezing time. The research results present good field application effects and are of great significance to optimize the construction design and reduce the operation cost and labor intensity.