Increased solids reduce the life of the drill bits, pumps, and other surface equipment related to drilling fluid circulation.
By Ronald B. Peterson
The focus of this issue of Water Well Journal is pumps, and this column will explore the impact of borehole cleaning and drilling fluid properties on the formation, well performance—and pumps.
There are two ways to clean a borehole while drilling. The two ways are velocity and viscosity. High velocity is only accomplished with high energy, which by nature has a large impact on the borehole and borehole stability.
Velocity and Viscosity
Straight air drilling in the proper geological conditions is the fastest and most efficient way to drill a water well. Its advantages are there is no hydrostatic pressure on the borehole to hold down the bit chips and they come off the bottom quickly so that they can be removed by the airflow. This provides quick and efficient borehole cleaning due to velocity.
Air is a high energy environment with uphole velocities as high as 3500 feet per minute and commonly between 2000 and 3000 feet per minute. Velocities in that range can be erosive to the formation, so the formation for straight air drilling needs to be competent. Incompetent formations will probably be eroded and may result in hole instability and potential hole collapse.
When drilling with straight air, it is common for the air to build up in the formation; then when you turn off the air, the hole will unload or blow back until it reaches equilibrium. While the hole unloads, it will also blow the cuttings that have been pushed into the formation back into the borehole.
The amount of air needed to clean the borehole can be reduced by making the air thicker, increasing the viscosity. The viscosity of air can be increased by adding water with foaming agents to the airstream and even more by the use of polymers to make the resultant foam thicker and more durable (stiff foam), but that is a discussion for another time.
Stiff foam greatly reduces the amount of air required and the potential impact of the air environment on the borehole. If you are using an air hammer to drill, there are limitations as you must have the correct amount of air to trip the hammer. Using a stiff foam while conventional air drilling can reduce the effective uphole velocity in some cases to as low as 40 feet per minute.
Water-based fluids are another way to clean the borehole. Water by itself is usually not effective as it has a destabilizing effect on any water-sensitive zones. To compensate for the destabilizing effect of water, additives can be used which help protect and stabilize water-sensitive zones. These additives normally increase the viscosity, reducing the required velocity needed to clean the hole. The annular velocity required to clean a conventionally circulated fluid drilling system is from 90 to 120 feet per minute uphole velocity.
Water-based fluids have an associated hydrostatic pressure that helps control any formation pressures but can tend to push fluid along with any entrained cuttings into any sub-pressured formations. This may tend to plug the formations and impede aquifer potential.
Water by itself has a weight of 8.34 pounds per gallon, or a density of 1.0; anything higher than that is due to other material in the water. The other material can be dissolved or colloidal. You can have a viscous drilling fluid that has a weight between 8.34 and 8.6 pounds per gallon. If the weight is higher than 8.6, it is because we have added material to the fluid to increase the weight or because we have incorporated drilled solids into the fluid.
Increased mud weight can cause many problems, which include increased wall cake thickness in the borehole; increased wear on the circulating system components; increased hydrostatic pressure on the borehole; and greater potential for differential sticking of the drill pipe.
During the drilling process, the increased hydrostatic pressure on the borehole will result in a higher potential for drilled solids to be pushed into the aquifer, and higher potential for loss of circulation. It will also result in a reduced drilling rate because the higher hydrostatic pressure results in a greater chip hold down pressure, making it harder to remove the bit chips from the face of the borehole, which slows the advancement of the borehole. Higher drilling fluid weight also results in increased pumping cost and greater fuel consumption.
You need a firm thin wall cake that is quickly applied to the borehole. This wall cake will help prevent the cuttings in the drilling fluid from being pushed into the formation and control the amount of water that gets into the formation water, wetting and damaging water-sensitive formations.
During the completion phase, higher solids and the possibility of a thicker wall cake may cause problems in running casing. The higher mud weight and density will also increase the buoyancy of the casing, making it more difficult to float the casing into the borehole.
Drill solids are typically the single largest contaminant in a drilling fluid system. A borehole 9.875 in diameter drilled to a depth of 200 feet with an average specific gravity of the solids at 2.65 would produce 17,578 pounds, or nearly 9 tons of solids.
The weight or density of the drilling fluid is determined by using a mud balance which can be obtained from your drilling fluid supplier. The desirable mud weight is as low as possible and still in control of any anticipated pressures, artesian flows, and bore hole stability issues.
The mud balance has four scales on it to report the weight. The weight can be reported in pounds per gallon (pounds/gallon), specific gravity, pounds per cubic foot, or pounds per square inch (psi) per 1000 feet of depth. The accuracy if the mud scale can be verified by weighing water should be at 8.34 pounds per gallon.
The mud weight can be used to calculate the hydrostatic head of the drilling fluid, the total solids content of the drilling fluid, and determine the efficiency of any solids control equipment that you are using.
To calculate the hydrostatic head, use the following formula:
Hydrostatic Head (psi) = Drilling Fluid or Mud Weight (pounds/gallon) × Depth (feet) × .052
To calculate the solids content, use the following formula:
% (Percent) Solids* = (Drilling Fluid or Mud Weight [pounds/gallon] – 8.3) × 8
*Assumes 2.6 specific gravity of solids.
Why Do We Care?
During the drilling process, unless we take specific measures to control the drilling fluid weight, the drilled solids may be broken up or dissolved and become incorporated into the drilling fluid system. A high drilling fluid weight, as mentioned earlier, can push those solids out into the production zone of the well, which will result in the need for longer development time and the need for higher amounts of energy to effectively develop the well.
Solids pushed out into the aquifer can reduce the life of the pump and result in stubborn turbidity levels which can be recurring over time
The accumulation of drilled solids into the drilling fluid is seldom if ever a good thing. Increased solids make it difficult to control the density and the flow properties of the drilling fluid.
Increased solids reduce the life of drill bits, pumps, and other surface equipment related to drilling fluid circulation. Solids left in the formation can also reduce the life of the production pump.
The drilling fluid properties that are critical to the drilling process are viscosity, density, wall cake, and filtrate.
If you have any questions or a subject that you would like to see addressed, email me at email@example.com.
Ronald B. Peterson has been involved with the drilling industry for more than 40 years. He previously worked for Baroid Industrial Drilling Products and is now with Mountainland Supply Co., a supply company in Orem, Utah. He served as The Groundwater Foundation’s McEllhiney Lecturer in 2015 and was given NGWA’s most prestigious award, the 2013 Ross L. Oliver Award. He can be reached at firstname.lastname@example.org.