Drilling Fluids: Velocity, Viscosity and Gel Strength
November 29, 2012
I recently was talking with a driller about a job he had just completed. He said that he and his crew had to over-drill the total depth by 30 foot in order to get the well completed to the depth they wanted. The formation had been mainly clay and shale with some gravel seams. The crew drilled the hole just using a PHPA polymer in its makeup water. Using a PHPA polymer works great for its clay and shale inhibition qualities and viscosity-building ability. However, a PHPA polymer has no gel strength. You can demonstrate this by taking a clear container, mixing water and the PHPA, then pouring a handful of gravel into it, and stirring it up. The gravel all falls to the bottom. With the gravel seams and no gel strength in their drilling fluid, they didn’t have enough pump capacity to create the velocity needed to carry the gravel cuttings out of the borehole.
In order to carry cuttings out of the borehole, the average fluid velocity must be greater than the settling velocity of the cuttings in your drilling fluid. The recommended ascending velocity should be between 80 feet and 120 feet per minute. The formula to check out your ascending velocity is below.
V= 25 x Q
D2 – d2
V = velocity of drilling fluid in ft./min.
Q = pump discharge in gallons per minute
D = diameter of borehole in inches
d = outside diameter of drill pipe in inches
One thing to keep in mind if you are using a centrifugal pump on your drilling rig: As the viscosity of the drilling fluid you are pumping increases, the pump performance is reduced. By increasing from 26-viscosity water to a 40-viscosity drilling fluid, your pump performance typically drops by 10 percent to 15 percent. Using a 60-viscosity drilling fluid will lower capacity by 25 percent to 30 percent. If you increase to an 80-viscosity drilling fluid, you can drop the flow rate of the pump by 40 percent to 50 percent.
Viscosity and gel strength can be a double-edged sword. It can help in suspending the cuttings to carry them out of the borehole. But it also can hold onto the cuttings when you need them to drop out in the mud pit or mud pan. Then you end up recirculating the cuttings, which slows your drilling rates and causes excessive wear on all components of the drilling fluid system.
One way to help remove the cuttings is to run hydrocyclones. Typically, you want to run an inlet pressure of four times the mud weight on the hydrocyclone to operate properly. A 4-inch hydrocyclone is rated at 50 gallons per minute, and a 5-inch unit is rated at 80 gallons per minute. When they are operating efficiently, they will have an umbrella-shaped spray pattern and make slurping sounds. However, when the discharge out of the hydrocyclone looks like a rope, the unit is overloaded with solids, as is shown in the picture above.
On this project, if they would have started their fluid mix with a high-quality sodium bentonite drilling fluid to give them gel strength, and then mixed in their PHPA polymer to inhibit the clay, they may have been able to complete the project without having to over-drill the borehole. When using any bentonite products, polymers and additives, please use them according to the manufacturer’s specifications to give you the best results for a successful drilling project. ND