Velocity String & Coiled Tubing

In simple terms, Velocity String is a type of small-diameter string normally applied in wellbore of gas wells to throttle and speed up fluid.

Many flowing oil and gas wells experience decreased production over time, and eventually can stop producing completely.

Factors causing this problem include declining reservoir pressure and gas velocities and liquid loading preventing reservoir fluids from entering thewellbore. Thus now many fields have installed a small diameter coiled tubing string (velocity or siphon string) to restore a well that is“liquid loaded”back to flow production. Deployed by coiled tubing (CT) device, connected by specialized wellhead tool, and with advancedly-designed length, 1.0”~2.0”CT will be hung into the original production tubing in the well to form new production string. Considering different production status, generally two production methods will be applied- gas production via CT or gas production via annulus between CT and production tubing.

The rationale behind installing a velocity string of smaller diameter than the production tubing is to reduce the cross- sectional flow area. The smaller cross-sectional flow area increases the gas velocity in the tubing. The higher gas velocity at the bottom of the tubing provides more transport energy to lift liquid up out of the well. Liquid no longer accumulates at the bottom of the well, and production is sustained.