WorldWide Drilling Resource

19 OCTOBER 2021 WorldWide Drilling Resource ® Borehole Cleaning Part 1 of 2 by Ronald B. Peterson Drilling Products Specialist, Mountainland Supply Company I could have titled this article Drilling Fluid Rheology and Hydraulics ; the Thixotropic Properties of Drilling Fluids . I thought about it and decided very few would even finish reading the title. So I renamed it. In my article last month, we discussed marsh funnel viscosity, one of the most elementary of the fluid properties. Marsh funnel viscosity gives you the relative state of the viscosity, or thickness of the drilling fluid, and is an indication of any changes which are occurring during the drilling process. This month, we will discuss flow types and some of the other properties that impact the ability to clean the borehole. These properties are called thixotropic properties and are included in Rheology and Hydraulics. There are three basic types of fluid flow: • Laminar Flow - Occurs at low to moderate pumping rates when the layers of the fluid move past each other in an order- ly fashion parallel to the walls of the borehole. Friction between the fluid and the borehole walls is lowest for this type of flow. This results in lower fluid friction losses and pressures during the pumping process. • Turbulent Flow - Occurs at high pumping rates where the fluid moves in a chaotic fashion. Particles in turbulent flow are carried by random loops and current eddies. Friction between the fluid and the borehole wall is highest for this type of flow. This results in greater friction losses and pressures. • Transitional Flow - Occurs when the flow is shifting from laminar flow to turbulent flow and vice versa. The critical velocity of the fluid is the particular velocity at which the flow changes from laminar to turbulent or vice versa. There are two basic fluid types, Newtonian and non-Newtonian. Most drilling fluids are considered non-Newtonian. We are going to discuss how these properties and characteristics impact our drilling operations, but we will not go into how to obtain all the numbers. Annular velocity is the flow rate of the fluid moving between the drill pipe and the borehole. Annular velocity in feet per minute is calculated by using the following formula: (24.5 x your pump rate) / (borehole diameter squared - drill pipe diameter squared) Higher annular velocities result in turbulent flow. Turbulent flow can result in hole erosion and enlargement. In general, for a typical conventional mud rotar y drilling operation the ideal flow rate is between 90 feet per minute (ft/min) and 12 0 ft/min uphole velocity. Air drilling operations usually require much higher velocities, and stiff foa m drilling operations can actually be effective at lower velocities. Slip velocity is the rate of speed at which a cutting will fall through the flui d and settle out in the borehole or in the surface pit. Slip velocity is dependent i n part on the density of the cuttings, as well as the density of the drilling fluid . Different formations have different densities. In general, the density of water is 1 . The density of drill cuttings is usually approximately 2.5; however, some highl y mineralized zones, especially those containing metals, may approach a density o f 5. Slip velocities are also affected by the viscosity of the fluid and by the yiel d point. It is critical you have a high enough annular velocity to exceed the sli p velocity of the cuttings, to remove them from the borehole. The density of the fluid also affects the slip velocity. The higher the density o f the fluid, the greater the buoyancy or tendency of drill cuttings to float in the fluid . This can either slow or stop the particles’ tendency or ability to settle out. Be sure to look for Part 2 next month. Ron Ron Peterson may be contacted via e-mail to WTR 302 - 684 - 3197 FAX: 302 - 384 - 0643 142 Broadkill Rd. • Milton, DE 19968 www.atlantic - email: Manufacturers of Slotted & Perforated Pipe ranging from ½” to 24” diameter Atlantic Screen & Mfg., Inc. • Well Rehab. Products • Manholes • Bentonite • Filter Sock • Inline Chemical Mixers • Sampling Bailers • Clear PVC Pipe • Locking Caps Looking for Events? Click on this box in our online issue.