ABSTRACT
Key factors in the efficient removal of sand fill from deviated wells are the proper selection of a fluid and the pump rates. The operation should be designed to (1) reduce or eliminate the formation of beds of particles in the annulus between the casing and tubing, (2) maintain the particles in suspension and (3) transport the fill to the surface.
Currently, the only guidelines available for the preferred pump rate and fluid properties apply specifically to vertical wells. In essence, the recommendation is that the linear fluid velocity in the annulus should exceed twice the particle fall velocity. However, in a deviated well, the linear fluid velocity will not prevent the particles from falling to the lower side of the annulus and forming a bed. Unless the particles are perfectly supported by the fluid, formation of a bed can only be prevented by maintaining a sufficiently high level of turbulent activity.
A new design tool for coiled tubing (CT) cleanouts in deviated wells has been developed. Based on a mechanistic model of particle transport in deviated wells, it predicts the conditions under which a particle bed is formed, calculates the depth of the bed and determines whether the bed slides upward, remains stationary or slides back down the well. Moreover, it calculates the minimum pump rate required to achieve complete suspension of the fill for different fluid viscosities, sand pick-up rates and deviation angles, thereby allowing a simple assessment of the optimum design parameters.