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.
One of the major uses of coiled tubing (CT) is the removal of sand or similar fill from a wellbore. The operation involves the circulation of a fluid through the CT to the sandface where the sand is picked up by the jetting action of the nozzles. It is then transported to the surface through the annulus between the CT and production tubing or casing. An important consideration in designing cleanout operations is the proper selection of the fluid and pump rate. They should be chosen so that bottomhole pressure is within the stated bounds to control the well. The pressure within the CT should not exceed the safe operating limits, and the sand or fill should be efficiently conveyed to the surface. This paper is concerned only with this last aspect of the operation; the computer models described here generate the minimum pump rate and the optimum fluid viscosity for efficient hole cleaning. The pressures in the wellbore should then be checked to define a maximum possible pump rate.
There is an obvious similarity between sand cleanouts and drilled cuttings removal. Many of the differences between the two operations are of scale (pipe and tubing dimensions and particle size) and operating conditions (drillpipe is rotated, CT is not), but perhaps the most important difference is in the rheology of the carrier fluid.