Drilling Fluids Play Critical Role in North Carolina Power Installation

A new high yield drilling fluid solution from Baroid Industrial Drilling Products, was recently applied in the installation of power lines in North Carolina. This drilling fluid is a specially formulated, bentonite-based drilling fluid system designed to readily disperse, provide suspension, and maintain low fluid loss for horizontal drilling applications. It is particularly suitable for small diameter, short length bores.

In Morehead City, N.C., power line installations were completed using QUIK-BORE high-yielding, bentonite-based drilling fluid. Four 4-in. pilot holes, 200 to 400 ft in length, were drilled and then reamed to 6 in. (with a fluted reamer) to install 4-in. power lines. Although the drilling and installation method for each bore was consistent, the subsurface conditions were not. The first bore was comprised of beach sand, the second a silt, sand, clay mixture and the third and fourth were primarily clay with interspersed silt and sand. As a result, the drilling fluid was adapted to soil conditions. The bentonite-base fluid remained the same with polymers added as needed.

Since a drilling fluid system is comprised of products with specific characteristics and functions, variety in soil conditions requires variety in drilling fluid products and properties.

Bentonite is typically used as a base for drilling fluids in industrial applications and provides suspension, carrying capacity, viscosity, and bore stability (filtration control). QUIK-BORE Wyoming bentonite provides all of the aforementioned properties at reduced concentrations.

In North Carolina, high-yielding bentonite-base fluid initially was mixed at 25 lb/100 gal with a standard HDD drilling fluid mixing system. At the applied shear, superior mixing and dispersion were evident as no un-yielded material was observed after five minutes. The same level of efficiency in mixing was evident for the duration of the trial.

The first bore was drilled through a beach sand environment, and high-yield fluid was used at 25 lb/100 gal with no additives. Once drilling commenced, it was observed that they were boring through groundwater. The wet and porous environment of beach sand and groundwater can pose difficultly in an HDD bore, especially without the use filtration control additives, but using high-yielding bentonite-base fluid the drillers were able to drill this hole in about two hours. There were no frac-outs or loss of circulation, indicating the base fluid provided adequate hole cleaning and stability. With this fluid mixture, time and money were saved with the use of high-yielding bentonite fluid because the bore was completed in a timely manner and less product was required.

After drilling the first hole through a sandy environment, the fluid was adjusted with the high-yield fluid reduced to 22.5 lb/100 gal and inclusion PAC filtration control additive. The fluid was altered to provide filtration control at lower viscosity. However, once drilling commenced, it was discovered that the new area consisted of a mix of silt, sand and clay. Despite the difference in geologic conditions, the high-yield drilling slurry with PAC filtration control additive maintained appropriate hole stability, fluid loss and suspension. This was proven by observing consistent cuttings laden fluid flow out of the hole (indication of hole cleaning), no frac-outs, subsidence and no pressure spikes from the pump.

High clay content was evident in the third and fourth bores, so a PHPA (partially hydrated polyacrylamide) clay/shale stabilizing polymer was added to the formulation. PHPA polymer increases the viscosity of drilling slurries and prevents the erosion of drilled cuttings and the bore, hence preventing the cuttings from swelling and causing the rods to get stuck or pressure to spike; both of these occurrences could result in loss of circulation, bore enlargement, and subsidence. As with the second bore, high-yield fluid was used at 22.5 lb/100 gals to account for viscosity generated by the PHPA polymer.

For all the formulations, the high-yield, fluid-based slurry maintained gel strengths and yield points. These properties are indicative of cuttings suspension and carrying capacity, which are crucial for horizontal directional drilling.

Fluid properties were measured at the exit pit of each bore, prior to reaming, and mud weights ranged from 10.9 to 11.3 lb/gal. The hole volumes were calculated to have a capacity of 130.5 gal. By using the mud weights at the entrance and exit pits, it was calculated that the fluid at the pits contained 20 to 25 percent solids. Based on drilling rate, pump rate, cuttings and hole characteristics, this level of solids loading indicated sufficient hole cleaning.

Conclusions

This installation provided the first application of QUIK-BORE high-yield drilling fluid and produced evidence that the product is capable of building superior suspension properties and adequate filtration control when used alone and when combined with drilling fluid additives.

Even at decreased concentrations of bentonite, the fluids supported an efficient installation. By varying the fluid additives used based on the formation encountered, the drilling fluid accomplished hole stability and cleaning in four different bores over the duration of this field trial. It provided fluid loss control and stability in a formation of strictly beach sand, without the presence of additives. However, when drilling through sticky clay, silt and sand, PAC and PHPA polymeric additives could be added and the fluid maintained good suspension, hole cleaning, hole stability, and filtration control.

When using this high yielding bentonite drilling fluid, the use of additives can be minimized on a given site when the right conditions exist, thus the cost of drilling short-shot boring jobs is lowered and money can be saved. A high-yield bentonite should only be used when high solids are not necessary. This bentonite provides an alternative solution and tool for drilling fluids that can reduce the amount of product required.
As demonstrated in North Carolina, it can be effectively applied by working closely with the drilling contractor, considering geologic conditions and specific drilling requirements, using adequate equipment on site, and maintaining the fluid properties required.

Laura Kuri is an associate chemist at Halliburton, parent company of Baroid Industrial Drilling Products. Mark Whittle is a Southeast and Texas account representative for Baroid Industrial Drilling Products.