Computer modeling of free fall phenomenon to improve primary cementing design

Abstract

Free fall is defined as the unrestrained motion of a body, in this case a wellbore fluid, in a gravitational field. It takes place in cementing since the frictional pressure cannot compensate for the hydrostatic pressure difference between casing and annulus. The occurrence of the free fall causes the fluid level to fall below the wellhead and creates void space above the top of the wellbore fluid. Therefore, the wellbore fluid flow rate is no longer in control. A pre-job simulation is needed to check whether the rate schedule and fluid properties are appropriate for successful cementing. An algorithm for modeling this phenomenon during primary cementing has been written. The model assumes piston-like displacement so that there is no transition region at the interface between two fluids. It also assumes incompressible fluids and no fluid loss into the formation. The fluids' properties are considered constant. The wellbore allows multi hole diameter. The model allows for examining the effects of several parameters (annulus restriction, location of annulus restriction, cement properties, pump rate, and annulus clearance). A user interactive simulator of the primary cementing that includes the free fall phenomenon has been developed. It is developed by using Microsoft Visual Basic for Windows, and it can be run on IBM compatible personal computers. It is easy to use and it can also be run simultaneously with other Windows based programs. The simulator provides information during a simulated cementing job in graphical and numerical ways and it is dynamically driven by the user. It provides an alternative way to design the rate schedule that allows the user to save time in obtaining the appropriate rate schedule.