When primary cementing an oil well, it is necessary to displace one non-Newtonian fluid with another non-Newtonian fluid along a narrow eccentric annular geometry that can stretch for many kilometers, having a typical annular gap width of about 2 cm. If drilling mud remains stuck to the walls in the oil well, the fluid-bearing zones in the formation are not isolated and valuable production can be lost.

Schlumberger Dowell utilizes FIDAP at their Well Construction Research and Engineering Center at Clamart, France, to gain deeper insight into the fundamental mechanisms and parameters that control efficient mud removal during cementing.

The fluids pumped during cementing are more or less visco-plastic and the displacement flow problems depend on 8-9 dimensionless groups. Thus, interpreting results and generating sound physical understanding is a challenging task. CFD is used together with fluid flow experimentation and simplified models to address this challenge.

For example, consider the displacement of one Bingham fluid with another in a long 2 cm slot. The displacing fluid has a density of 1600 kg/m³ , yield stress of 9.8 Pa, and plastic viscosity of 2.8 cP. The displaced f luid has a density of 1600 kg/m³ , yield stress of 4.9 Pa, and plastic viscosity of 14.0 cP. The left image in Figure 1 shows the displacement profile computed with FIDAP after displacing with a mean velocity of 0.35 m/s for just over one second. On the right the displacement shown is when the fluid rheologies are reversed and the numerical experiment is repeated.

Figure 1. This cementing option shows a potentially damaging static layer along the wall in the image on the left. Although the displacement front is unstable on the right, the minimal static layer along the wall disappears as the simulation continues.

Which of these simulations shows a better result? In fact it is the second, which seems to show an asymmetric instability of the displacement front. Continuing the simulation, the blue fluid is displaced from the wall, whereas in the first simulation, a potentially damaging thin static layer is left attached to the wall.

These are characteristic of the computations Schlumberger makes with FIDAP. They also use FIDAP to study displacements through irregular shaped geometries, turbulent non-Newtonian wellbore flows, hydrodynamic stability and coupled thermo-fluid problems.

Fluid displacement and efficient mud removal are needed during primary cementing.

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