Clay Drilling Techniques: Best Practice


Drilling in clay is a vastly different experience to drilling in dirt or rock. If you have drilled in clay before you may have observed the cuttings beginning to swell and the borehole constricting as a result. This is because when clay is exposed to water or bentonite drilling fluid, its properties change dramatically. If you are drilling in dirt or rock and are facing problems such as your auger balling up, the ground heaving, difficulty creating slurried flow or frac-outs, chances are you have hit clay.

The Composition of Clay

Clay particles are composed of what appear to be hundreds of tiny ‘sheets’ bound closely together. These ‘sheets’ are separable unless they become wet. If exposed to water, moisture is absorbed by the space in between each sheet and the particle swells until the sheets become inseparable. The effect is comparable to dousing a copy of the Yellow Pages in water: the pages will stick together and cannot be separated and the book swells up and won’t close properly. In a similar vein, wet clay particles swell up when exposed to water and become difficult to drill in.

The problem can also be visualised by comparing clay drilling to making a cake. If you use a mixmaster to stir a cake mix without enough liquid in it, the cake mix begins to swell as it is mixed. It starts to stick to the mixmaster and doesn’t combine properly. The same thing happens during clay drilling when water or bentonite-based drilling fluid is used as lubricant: The clay solution swells up to the extent that it is too thick and sticky to flow from the borehole.

How Drilling Fluids Improve Drilling Efficacy

On a molecular level, the edges of a clay particles (the edges of the sheets) are charged. This phenomenon can be taken advantage by using a bespoke drilling fluid during excavation. Charged particles contained in custom made drilling fluid bind to the edges of the clay and prevent water getting into the medium. Without water causing the clay particles to swell up, cuttings cannot stick together and will flow from the borehole smoothly.

When drilling in clay, it is best practice to use partially-hydrolysed-polyacrylamide-based drilling fluid (PHPA) to achieve this effect. PHPA particles are enveloped in a thin film of water in the drilling fluid tank. This catalyst causes the PHPA particles to repel each other, uncoil and lengthen. As the particles undergo these changes, viscosity in the fluid tank increases. The combination of water and PHPA also produces a charge on the PHPA particles. When charged, the PHPA particles bind to the edges of clay particles and prevent water entering into the medium. This ensures a smooth flow of cuttings from the borehole.

There are several types of PHPA solutions available on the market, and the variety which should be used depends on the context of its application. The primary difference between each solution is the size of the particles contained within. If a venturi mixing system is available, then a PHPA made from larger particles should be used. If the mixing system is limited, and/or bentonite is required as the base liquid and/or the drilling fluid must be recycled, then a PHPA made from smaller particles should be used.

Not all clay requires a bespoke drilling fluid solution, but should your drilling medium start to swell, clump together and become sticky, the use of a PHPA drilling fluid solution is advised. 

In conclusion, if you are drilling in clay and are using only a polymer/bentonite solution as drilling fluid, it is certainly worth considering the costs of longer job completion time, the cost of crew for this time and the need for surface repairs after boring is complete. Using a drilling fluid containing PHPA can reduce drilling time, costs and surface repairs, and is therefore the most effective clay drilling technique.

Contact Tebco

If you are looking for an auger to use for clay drilling, just call Tebco today to place an order. We have a vast selection of augers available for sale. Check out our online store, call us on (02) 4677 2649 or reach out via our contact page to ask any questions or place an order for any auger in stock.