Study of sand particle transport characteristics and different critical velocities in sand-producing wells via indoor experiments.

Due to the widespread application of new technologies such as increased production and injection, the output of many old wells has improved, but this has also led to varying degrees of sand production. Increased sand production has affected oil well production, increased equipment wear, wellbore plugging, and safety risks. To increase the sand particle transport performance within wellbores, indoor experiments have been conducted to study the sand transport characteristics, transformation relationships, and critical flow velocities for fluid carrying sand. The experimental results indicate that sand particle transport within wellbores can be classified into four modes: stationary, rolling, skipping, and suspended. The flow pattern characteristics of the sand particles were further subdivided into eight categories, and the transformation relationships between these flow patterns were established. Experiments on the critical flow velocities for different grain sizes of sand in inclined wellbores have shown that the fluid velocity required for lifting sand particles is greater than that for rolling and sliding. When the wellbore inclination angle is greater than or equal to 70°, the fluid velocity required for lifting sand particles is approximately 1.9 times greater than that for rolling, and the rolling velocity is approximately 1.6 times greater than that for sliding. In vertical wellbores, the critical velocity for the sand particle suspension is approximately 0.81 times the terminal settling velocity of the sand particles in water. This research provides important scientific evidence for improving and optimizing sand removal techniques in oil wells, as well as a systematic approach and experimental foundation for further studies on sand transport in complex wellbores.