Why is drilling clay so different than drilling sand or rock? The reason is the reactive nature of the mineral. Imagine for a minute a small particle of clay of which there are many millions in a bore. If you enlarge that particle to the size of your local yellow pages, then you can see what individual clay minerals look like. They look like numerous individual sheets tightly bound together, but just like the yellow pages they are not so tightly bound that they can not be separated. If you took those yellow pages and were to leave them out in the rain, you would find that the book expanded significantly and no longer closes properly. This is because water is absorbed between the sheets of paper. This same phenomenon occurs in clay and is one of the peculiarities of drilling in clay.
The cuttings created by the reamer are exposed to both water in your drilling fluid and groundwater. If you can liken the swelling of a wet yellow pages to the swelling of millions and millions of clay particles in your bore, you can soon see how drilling in clay is significantly different than drilling in sand. Since bentonite is nothing more than specially refined reactive clay itself, drilling in clay ground with bentonite alone often can exacerbate the problem of thick sticky cuttings not flowing from the bore.
To minimize the swelling tendency of clay, a type of polymer called a PHPA or partially hydrolysed polyacrylamide is used. Polymers like POLY-BORE™ dry borehole stabilizing polymer, QUIK MUD® liquid polymer emulsion, EZ-MUD® liquid polymer emulsion, and EZ-MUD® GOLD dry beaded polymer possess a couple of interesting properties that bentonite doesn’t have that make them very effective at inhibiting the swelling of clay minerals. Firstly as the PHPA hydrates in the tank, the molecule becomes enveloped in a film of water, which creates electrostatic repulsion like two like ends of a magnet pushing each other apart. As the polymer continues to hydrate, it also uncoils and lengthens. This combination of repulsion and uncoiling creates the viscosity that commonly is observed in the tank. Secondly and more importantly, the hydrated polymer strand has a series of charged functional groups along its length that can interact with the clay particles in the bore. The edges of a clay cutting (i.e., the edges of the sheets in the yellow pages) are charged, so when the cutting is exposed to a drilling fluid containing a PHPA, the charges on the polymer attract to the clay’s surface and bind to the clay. Once the PHPA is bound to the clay particle, ground water or drilling fluid can no longer absorb into the clay. Simply stated, once the interaction between the cuttings and water is reduced, the more likely the yellow pages won’t swell much.
The PHPA polymers mentioned all are designed to prevent clay/shale swelling, so why is there more than one kind? The difference between them is the size of the molecule and the concentration of active ingredient, which makes them useful for different applications and drilling operations. For example, to build a polymer fluid of a desired viscosity (e.g., 40 sec/qt), it would take less of a larger molecule (e.g., POLY-BORE™ dry borehole stabilizing polymer) than it would with of a smaller molecule (EZ-MUD® liquid polymer emulsion). Moreover, a larger molecule like POLY-BORE™ dry borehole stabilizing polymer contains 100% active ingredient, while most liquid polymers are not (<40%).
Which PHPA should be used depends on three things: 1) the type of mixing system available, 2) whether bentonite is to be used as the base fluid, 3) and if the drilling fluid is to be recycled. If a venturi mixing system is available, then a POLY-BORE™ dry borehole stabilizing polymer type polymer is going to be the most cost effective method of inhibiting clay swelling. If the mixing system is limited, and/or bentonite is required as the base fluid and/or the fluid is being recycled, then a smaller molecule (e.g., EZ-MUD®
liquid polymer emulsion or EZ-MUD®
GOLD dry beaded polymer) should be used. The smaller molecule requires less energy to hydrate and helps avoid the excess viscosity that can develop when using bentonite as the base drilling fluid. However, it should be mentioned that there are times when drilling with polymer alone is insufficient for carrying some of the larger denser cuttings from the bore. Careful polymer selection will help ensure that you get the carrying capacity you need from bentonite combined with the clay inhibition from the right PHPA.
In addition to swelling, some clay can become very sticky when wet. When a clay is not water wet it can become sticky and bind onto the reamer (i.e., bit balling). To envisage what water wet means, think of mixing flour into water and the inevitable lumps that are created if not done correctly. These lumps are flour that is not water wet. The same principles often are observed when clay ground becomes sticky. We can make clay more water wet by using surfactants like PENETROL®
liquid penetration rate enhancer or detergents like CON DET®
liquid agent. The surfactant is able to neutralize the charges on the clay, allowing the clay cuttings in the hole to mix into the fluid slurry rather than clump and bind to the reamer. Historically, drilling detergents have been used as the primary method to increase wet ability of clay. However, with advances in chemistry, new blends of surfactants (e.g., PENETROL®
liquid penetration rate enhancer) now are more concentrated than even the very best of drilling detergents. This means that up to 70% less surfactant can be used compared to similar drilling detergents.
The other problem seen in clay ground is creating flowable slurry without having to add excess drilling fluids. Poorly mixed cuttings often have difficulty flowing out of the bore. This can lead to excess borehole pressures and ultimately frac-out situations. By adding more water or drilling fluid to the bore than necessary, the costs of cuttings disposal can eat into a significant portion of the overall job profitability. Repairing heaved driveways, curbs and roads resulting from frac outs also can make significant dents in job profitability. One effective way to minimize fluid addition and frac out situations is to use a small amount of a polymer dispersant such as AQUA-CLEAR™ PFD liquid dispersant to thin the slurry and make it more flowable. A dispersant thins thick clay slurry by increasing the repulsive forces between the clay particles, allowing them to slide past one another with less friction. Thinner slurry will result in a more open bore and less pressure build-up, thus reducing the frac-out potential. The net benefits can include: less cuttings to dispose of, reduced frac out potential and subsequent repairs, which can mean more profitability.
Next time you dig that entry or exit pit into a reactive or sticky clay, take a minute and think about how best to approach the bore. If you typically drill with water or bentonite alone, you may want to take a minute or two and consider the costs of crew and rig time on a protracted bore and potential surface repairs versus the cost of building a drilling fluid that has the right fluid properties that will improve drilling performance. Not all clay bores may require a PHPA, a wetting agent and a dispersant. However, I hope that now that you know how to identify whether a clay is swelling, getting sticky or getting thick, you will be better able to make informed decisions about how best to make the right selection from your drilling fluid tool box. For more information on drilling in clay and fluid selection, contact Robert Petrie of Baroid Industrial Drilling Products on +61 (0)414 557 917 or Robert.Petrie@halliburton.com