12 AUGUST 2021 WorldWide Drilling Resource® Horizontal Directional Drilling by Ronald B. Peterson Drilling Products Specialist, Mountainland Supply Company I would like to be able to never make a mistake, but unfortunately I am not. There is no way to sugarcoat it and make it any more palatable. In my WWDRJune 2021 article Artesian Flows, I reversed the numbers on pressure per foot of hydrostatic head. The correct relationship is 2.31 feet of head equals 1 psi of pressure. I apologize for the error. To complete the correction of the error, 3 feet of head would be 1.3 psi. At 50 feet, it becomes .026 psi per foot. You would need to raise the drilling fluid (mud) weight to 8.8 pounds per gallon to control that pressure of artesian flow. However, the increase of the fluid weight as a result of solids entrainment in the mud while drilling would more than likely accomplish your goal and make any further action unnecessary. Thanks Britt, Don, and David for letting me know about my mistake. I apologize to you all for any confusion this may have caused. Increase of drilling fluid weight as a result of drilling operations will control most minor artesian situations. Every situation is different and may have more than one potential solution. Always act quickly, evaluate the situation, and use the solution that is the most effective, the fastest, and the least expensive. Now that’s out of the way, let’s get on with this month’s discussion “Horizontal Directional Drilling.” Horizontal directional drilling (HDD) is still advancing a hole in the ground; however, the hole is horizontal instead of vertical. The fluid properties and the testing procedures are the same in horizontal drilling as in vertical drilling. Calcium or hardness and pH are still critical to know and adjust for. Filtrate or water loss and filter cake are still critical properties. The test procedures are basically the same, but the desired properties are different. Assuming we have good makeup water because we have tested and verified it, we need to use soda ash to adjust the calcium content and raise the pH as necessary. We then need a bentonite product designed with properties for horizontal boring. We need polymers to achieve desirable filtration control properties and stabilize active formation clays and shales. In very fragile formations, we may also need a lost circulation control material. Viscosity is still resistance to flow or a measurement of the thickness of the fluid, and can be a necessary evil. In vertical drilling, hole cleaning is achieved through velocity. If the velocity is insufficient, we simply raise the viscosity. When the solids-laden fluid reaches the surface, we want the solids to settle out in the pits. If the solids settle out in the pits, won’t they also settle out in a horizontal borehole? If you raise the viscosity to correct this situation it can be counterproductive. We know viscosity is resistance to flow, and we are trying to maintain flow. We are also very close to the surface, so the ground is very fragile. High-viscosity fluid plus high solids content results in reduced flowability. Reduced flowability means more pressure will be required to move the slurry. High viscosity causes high circulating pressures. High circulating pressures tend to break things and cause frac-outs and/or humps in streets, highways, and driveways. This is never good. We need to carry the cuttings by building different properties related to viscosity. These properties are yield point and gel strengths. Different bentonite products have different properties because they are designed for different drilling disciplines. Rather than depending on settling of solids in the pits to clean the drilling fluid, we now depend on solids removal equipment. Suspension is critical in HDD. Any recirculated solids need to be kept in suspension while in the borehole so they do not settle out and close off the bore. High solids content will cause additional wear on all of the circulating system components. If the borehole is deep enough and there is formation fluid present, there may be a need to be concerned with hydrostatic head. Very rarely, if ever, is a higher fluid density at the pump suction necessary in a horizontal borehole head. If higher hydrostatic head is needed, use properly designed weighting agents to achieve it. Remember, high-density fluids also require more pressure to flow. Gels are the tendency of the fluid to set up when idle. They need to be able to become fluid again when disturbed. This ability is referred to as fragile gels. The buildup of solids is not as much of a problem in a sacrificial fluid system, but it may be a major problem if recirculating or recycling the fluid. Sand content is every bit as critical in a horizontal borehole as in a vertical borehole for the same reasons. Remember, equivalent circulating density (ECD) is a combination of the hydrostatic head and the pressure required to move the fluid. Fracture gradient is the pressure at which the circulating pressure exceeds the strength of the formation. If the ECD exceeds the fracture gradient, you will have a frac-out or inadvertent flow to the surface, or into a stream, and/or an undesirable hump in an inconvenient location. Gels and viscosity, while related properties, are not the same. A thick fluid may not suspend cuttings, but a thin fluid can be designed to suspend cuttings. In horizontal drilling, flowability is the major concern. There is a popular statement that at the end of the bore if the fluid will flow, the product will go. We have only briefly scratched the surface on HDD fluids. If you have any questions on horizontal directional drilling fluids or if you have another topic you would like addressed, please contact Michele (below) and she will get your question to me. Ron Ron Peterson may be contacted via e-mail to michele@worldwidedrillingresource.com DIR
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