Geotechnical, Mineral Exploration Drilling Fluids and Grouts
Geotechnical and mineral exploration contractors have been using drilling fluids and grouting products provided by bentonite manufacturers for years. This article will attempt to cover most of the products that have been used over the years and some new ones that are more specific to certain formation conditions. These products are also used in various other drilling markets such as water well, horizontal directional drilling and geothermal heat loop installations.
In geotechnical and mineral exploration, there are several types of drilling that use minimal amounts or no drilling fluids and these include auger drilling, both solid stem and hollow stem, along with coring. The two main types of drilling that employ the use of drilling fluids are mud rotary drilling and air drilling. It is these two areas that will be the focus of this discussion.
Those drillers using mud rotary most often prefer a high-yield, low-solids drilling mud, usually 200 barrel yield plus or minus. This type of drilling mud is very popular across a number of drilling methods. There are a couple of other drilling mud options available, as well such as higher-yielding products that range up to 230-240 barrel yield, and low-yielding products in the 90 barrel range. This latter product is used extensively in oil well drilling, but can be used with other drilling applications where a higher-solids drilling mud is needed to stabilize a borehole. Barrel yield refers to the number of 42-gallon oil well barrels yielded from one ton of product at 15 centipoises of viscosity.
Advantages and functions of bentonite drilling mud include:
- Cooling and lubricating the bit and drill string
- Stabilizing the borehole
- Cleaning the borehole, providing gel strength to suspend and remove cuttings
- Controlling fluid loss to the formation
- Reducing torque in clays and shales
- Helping to prevent water intrusion from the formation
Air drilling is another drilling method that can require drilling fluid for completion of the borehole. Air and water are commonly used to begin the drilling process and then, as the bit gets deeper, drilling foam is injected into the air stream. Drilling foam is a surfactant. Laundry and dish soaps are also surfactants. The difference is in the concentration of the surfactant and the bubble size and strength. Drilling foams are much more concentrated with a smaller, tougher bubble structure. Laundry soaps and dish soaps have larger bubbles with less strength to perform in the borehole.
Advantages and functions of air foam drilling include:
- Faster penetration rates
- Lower pressures in the borehole
- Almost instant evaluation of the formation
- Proper cleaning of the borehole
- Easier on the drilling equipment and formation
The use of a good quality foam with air drilling results in:
- Lubrication and cooling of the bit and drill string
- Increased penetration rates
- Reduction of the amount of air needed
- Reduction of hydrostatic pressure
- Reduction of the amount of water needed for drilling
- Reduction of the amount of dust from drilling
Both mud drilling and air drilling with foam are widely used and provide protection in the borehole. In either type of drilling, bentonite mud and quality drilling foam help get the job done efficiently and in a timely and more profitable manner.
However, there are formations that may require bentonite mud and drilling foam to be enhanced in order to get the job completed in a reasonable and timely manner. This can be accomplished by adding a variety of polymers and additives available today. In the early days of drilling, there weren’t many additives available. At one time, it was only materials like lime and starch that were added to drilling fluids. Today, there are a number of products available that deal with specific problems in drilling. These will be discussed below.
Controlling Clays and Shales
Today there are a number of additives available that will coat clays and minimize swelling and sticky conditions for a time during drilling to make the drilling easier and faster. These are called PHPAs (partially hydrolyzed polyacrylates/polyacrylamides). There are a wide assortment of these: long chain and short chain, wet and dry, with varying amounts of active ingredient. These can be added to both bentonite and air/foam systems for enhancement.
There are also other products that can be added that actually break the clays down and liquefy them so they can be pumped out of the borehole. These are the best products for the toughest clay conditions, especially with the high swelling, sticky ones. These products also come in liquid and dry formulations.
There are also surfactants that can be added to the products above to help with sticky clays and bit balling problems. These are liquid products and should be added after the polymers or clay-breaker products are hydrated. One note of caution: Bentonite drilling mud must always be hydrated before adding polymer or other additives!
Controlling Unconsolidated Formations
Formations encountered that have sand, gravel, cobble or rock may require a different type of polymer to be added if the drilling mud or foam is not able to stabilize the borehole. These polymers are called PACs and they are modified natural cellulosic polymers. They are very popular because they have the ability to develop a thin, tough filter cake thus stabilizing the borehole wall and dramatically lowering fluid loss. These PACs come in liquid and dry form.
In addition to bentonite drilling mud, drilling foam, polymer and other additives for drilling, bentonite producers offer several grouting products for sealing geotechnical and mineral exploration boreholes. There are both dry sealing materials that can be poured (or tremied) into the borehole and there are grout slurries that can be pumped down the borehole through a tremie pipe. The dry sealants need to be hydrated as they are placed in order to be an effective grout seal. The slurries will set up after placement over varying amounts of time, dependent on the product used, and become like peanut butter or a stiff pudding.
Below is a list of most of these products:
- Bentonite chips
- Bentonite tablets
- Coated bentonite tablets
- Granular bentonite (in dry boreholes)
- Granular bentonite and polymer
- Bentonite and cement
- Bentonite and sand
- High solids bentonite slurries (20 percent and 30 percent solids)
- Enhanced, viscous drilling fluid (sometimes allowed in mineral exploration boreholes, state regulations need to be checked for this to be used)
Please check with your local distributor or bentonite manufacturer with any questions about all of the products mentioned above. Feel free to contact me through National Driller.
For more Drilling Fluids columns, visit www.nationaldriller.com/drillingfluids.