Last month, we started a discussion on drilling fluids viscosity with a look at some terms drillers should know and the basics of viscosity testing. This month, we get into detail to help drillers understand how viscosity affects mud pump performance.
First off, know that all mud pumps are calibrated with water at sea level which, to remind readers, has a 26s viscosity. That means that, at 40s viscosity, you lose 10-15% capacity; at 60s viscosity, up to 30%; and at 80s viscosity, up to 50%. Operators need to take this into account when calculating flow requirements. The gauge on the control panel of the drill does not automatically calculate viscosity, nor is there a magic dial on the pump to take viscosity into account. It is up to us to account for this and apply it correctly.
Here’s an example for a pilot bore of 6 inches in clay. In a perfect world, diameter squared divided by 24.5 equals gallons per foot. Plugging “6” in, we get:
(6 x 6) / 24.5 = 1.469 gallons
Multiply that by three (volume ratio) to get 4.407 gallons per foot. Think about that. You dial in 4.5 gallons on your control panel, but if your viscosity is 40s, in reality you’re only sending 3.82 gallons per foot downhole. The solution is to take the 4.5 gallons and add 15%. Set the dial at about 5.175 gallons to overcome the loss to viscosity.
The above example is for a pilot hole. As tools get bigger, the gaps in capacity increase exponentially. For a 16-inch bore, the math calls for 31.34 gallons per foot; however, if you dial in 32 gallons at 40s, you only pump a little over 26 gallons. Adjust up, and dial in 37 gallons. The difference in volume is directly related to failure or success of our bore.
Determining the proper viscosities for ground conditions is another critical step of the process. Coarse, unconsolidated soils require gel strength to suspend the cuttings and a strong filter cake to prevent fluid loss. Higher viscosities, in the 50s to 80s range, will give you higher gel strengths and more bentonite platelets to tighten that filter cake. If we attempt a bore in sand/gravel with a 35s viscosity fluid, we are destined for failure. This is due to a lack of carrying capacity and a weak, ineffective filter cake.
The reverse applies applied to clays where we require less viscosity and gel strength when drilling. The natural clays break down into fines and work in concert with the bentonite. This means the fines will increase our viscosities and, although a filter cake is required, there is less chance of fluid losses in clay. Using a 35s to 40s fluid is recommended. If we use a 60s to 80s fluid, densities skyrocket quickly, and it will be difficult to flow out the cuttings. We would be just re-constituting the ground conditions behind the drill head and increasing the risk of inadvertent returns on the bore path.
To wrap up our foray into viscosity, last month we attempted to more clearly describe some of the terms you hear in the industry. We also familiarized ourselves with the Marsh funnel and cup to determine viscosity. (People with testing questions can, of course, ask their drilling fluids rep for a demonstration.) This month, we talked about using the proper viscosity for our ground conditions, and wrapped with tips to adjust your flow rate based on the viscosity to ensure the proper amount of fluids downhole and — ultimately — success.
Of course, for the purpose of these columns, we only talked about bentonite. There are a lot of polymeric additives out there to deal with specific ground conditions; some will increase viscosities slightly if using low-viscosity (LV) types and others a little more. In any event, these additives are available and will increase your chances of success (just like understanding the effects of viscosity). Contact your representative or local distributor for further advice and support. Stay safe and we will all get through these difficult times together.
For more Drilling Fluids columns, visit www.thedriller.com/drillingfluids