Borregard’s latest article about rheological additives for oil & gas drilling describe how microfibrillated cellulose (MFC) is evaluated at ultra-low shear (0.0001 s¯¹) to demonstrate the superior suspension capability of MFC for proppant, cuttings, and solids as compared to typical rheological modifiers, like hydrocolloids, for field conditions with high water cuts.
However, what if you don’t have access to an instrument that can measure the viscosity of your fluid at ultra-low shear? Is there a simple test that can be attempted to distinguish the performance of MFC verse the incumbent in your drilling fluid?
Stokes’ Law of Sedimentation
Stokes’ law of sedimentation describes the velocity of a suspending particle is directly proportional to the particles size and inversely proportional to the viscosity of the medium (η) as seen in Figure 1. Therefore, if we can alter the viscosity of medium by changing the rheological additive’s ability to suspend particles from xanthan, a hydrocolloid that doesn’t suspend solids well, to BioDrill RX, a product that has superior suspending ability at low shear, then should be able to affect the rate of sedimentation (ѵ).
This is because BioDrill RX has a robust network that suspends solids well and should have a higher viscosity at near static conditions or at rest; therefore, increasing the viscosity of medium which would decrease the rate of sedimentation (ѵ) as compared to xanthan.
Figure 1. Stokes' Law of Sedimentation.
Suspension drop test: an indirect testing method
Taking Stoke’s Law of Sedimentation into consideration, Borregaard developed the suspension drop test that can be easily utilized to determine if BioDrill RX was incorporated into your formulation. This test utilizes two graduated cylinders, BioDrill RX, incumbent (in this case xanthan), proppant, and a stopwatch as seen in Figure 2. One of the two the graduated cylinders was filled to 100 mL mark with 0.28wt.% of BioDrill RX and the other graduated cylinder was filled to the 100 mL mark with 0.28wt.% of xanthan. A 40/70 mesh proppant with a specific gravity of 2.9g/cm3 was dropped from the top of the graduated cylinder. The stopwatch recorded the time for how long the proppant took to fall to the bottom of the graduated cylinder.
In the case of BioDrill RX and xanthan, the time that was recorded for the proppant to reach the bottom of the cylinder was sixteen minutes and forty-one seconds and thirteen seconds, respectively. Therefore, by changing the viscosity of the medium from xanthan to BioDrill RX a chemist can observe that BioDrill RX has superior suspending capability, impeded the rate of sedimentation of the proppant, and observe that BioDrill RX is incorporated into the formula.
Figure 2. Suspension drop test of 0.28wt.% of BioDrill RX Vs. xanthan
In summary
In some instances, an indirect testing method maybe required to determine the incorporation, or the performance benefit of product as compared to the incumbent in a formula. Performing the proppant drop test would not only show that BioDrill RX is incorporated into your formula, but also distinguish the effect of sedimentation of proppant, cuttings, fines, and solids of a formulation with BioDrill RX as compared to a formulation with xanthan.
In this case, BioDrill RX took sixteen minutes and forty-one seconds, while xanthan took thirteen seconds for the proppant to fall to the bottom of the graduate cylinder. Therefore, the addition of BioDrill RX maybe what your next formulation needs to impede or inhibit the sedimentation of solids, cuttings, fines, and proppant to prevent your next possible failure.