Foam drilling can be a success when used correctly.
By Jeff Blinn
I received a call recently from a friend and colleague asking for help troubleshooting a foam drilling issue.
Specifically, the person was having trouble cleaning the hole. So, for this column, I will leave the liquid mud businessfor now and address one of my favorite drilling methods: air/foam drilling.
Foam drilling and its subcategories, stiff foam and gel foam, are modifications to straight air drilling used in areas where straight dry air drilling causes borehole instability, excessive borehole erosion, and borehole collapse.
Foam can be successful in areas with severe loss of returns or where a large influx of formation water makes lifting the water out of the hole with straight air impractical. Additions of bentonite, PHPA, and PAC polymers improve foam performance and help overcome formation problems.
The overriding principle leading to the success of these methods is to use the minimum air volume required to generate the foam column and to circulate the foam from the bit to the surface. This is the “Less Air Approach” to air drilling.
Using Compressed Air
To help understand the benefits of a foam drilling system, we should first review traditional straight air drilling which uses compressed air as the drilling fluid.
Yes, air or any gas, by definition, is a fluid. In dry air drilling, compressed air is the carrier medium; drill cuttings are blown out of the hole by air volume and air velocity. To remove a drilled cutting from the drill bit to the surface, an uphole air velocity of 3000 feet per minute to more than 7500 feet per minute is required.
This high-energy solids-laden air stream can cause borehole wall erosion and formation instability. The air stream has poor lifting characteristics to remove formation water from the borehole because it can only handle a small influx of water.
Dry air drilling is ideally suited for hard competent rock with little formation water. This is what all air drillers are taught from the beginning—lots of air is good, more air is better, and always get a bigger compressor!
The following equation can be used to calculate the minimum air volume, in cubic feet per minute, necessary for any hole size and tooling configuration:
CFM= Air Velocity × 〈D²h–D²p〉
CFM = cubic feet per minute from the compressor
Dh = hole diameter in inches
Dp = drill pipe diameter in inches
183.4 = conversion constant.
As an example, using a bit diameter of 8 inches and 4½-inch drill pipe, air velocity of 3000 feet/minute for dry air drilling, solving the equation:
CFM= 3000 × 〈8²–4.5²〉
CFM = 716
Gives a minimum air volume of 716 cfm required to effectively lift cuttings.
Now for a little larger hole diameter, let’s use 12¼ inches, and the same 4½-inch drill pipe. We calculate the required cfm:
CFM= Air Velocity × 〈8²–4.5²〉
CFM= 3000 × 〈12.25²–4.5²〉
CFM = 2123
Increasing the hole size from 8 inches to 12¼ inches increases the air volume required from 716 cfm to 2123 cfm! Quite a substantial increase.
From the equation and our examples, you can see increasing the hole size or decreasing the drill pipe size will increase the air volume necessary to achieve 3000 feet per minute uphole air velocity. Let’s put it another way: Increasing the annular space requires more air, and sometimes a lot more air.
Less Air Approach
If air drilling is a high-velocity, high-energy environment that can be detrimental to hole stabilization, what can we do to tame this energy while still using air to drill with? We can use the Less Air Approach to air drilling: air/foam and modified air/foam drilling techniques.
Foam and modified foam drilling methods do not rely on air alone to lift the drill cuttings. Foam drilling involves injecting a foaming agent into the air stream to create drilling foam with the consistency of shaving cream.
Small, strong bubbles are necessary to float the drill cuttings. The drilled cuttings are floating on the bubble structure and removed from the hole by the upward movement of the foam column. The foam column helps support the borehole and allows for air drilling in less stable geologic environments than straight air drilling.
Drilling with foam also allows drilling to progress in the presence of groundwater influx to the wellbore.
The foaming agent should be diluted in water before injecting into the air stream. Concentrations of foaming agent range from 1/16% by volume of the injection fluid to 2% by volume. Concentrations of 1/2% to 1% foaming agent concentration are most typical.
With foam drilling, an uphole velocity of only 200 feet to 400 feet per minute is required to remove cuttings from the hole. The reduction in uphole velocity from that required by dry air is achieved by reducing the air volume output from the compressor. This is done either by controls on the driller’s console or by plumbing a bypass valve into the airline.
Using the CFM equation and keeping the same hole configuration and tooling as the first example, changing from dry air to a foam system and reducing the air velocity to 200 feet/minute, a minimum air volume of 48 CFM would be necessary.
CFM= 200 × 〈8²–4.5²〉
CFM = 48
The second example with a hole diameter of 12¼ inches and using a foam system reduces the air volume requirement from 2123 cfm to 142 cfm. That is a huge reduction in air volume and a much smaller compressor is needed to achieve success.
CFM= 200 ×〈12.25²–4.5²〉
CFM = 142
The reduced air requirement from 716 cfm to 48 cfm, or from 2123 cfm to 142 cfm, lowers the energy of the system against the formation and lowers the potential for hole erosion. To work properly, the contractor must use less air than dry air drilling. Typically, less air is a learned skill as it goes against the conventional wisdom of an air driller.
Stiff Foam Drilling
Stiff foam drilling is a modified foam drilling method. Adding polymers to the foam mixture increases the strength of the bubble structure. Stronger bubbles can support and remove larger cuttings from the hole and add additional stabilizing characteristics to the foam column.
Hole cleaning can be achieved with an uphole velocity of 100 feet to 200 feet per minute. The reduction of air volume requirement again lowers the energy in the borehole and increases borehole stability.
The polymer additives also add versatility to the system by imparting the same system effects as if these products were used in a liquid drilling fluid system. The polymer additions allow for drilling water-sensitive or unstable formations such as overburden or soft sections.
Additions of partially hydraulized polyacrylamide (PHPA) type polymers are used for shale and clay inhibition, and poly anionic cellulose (PAC) type polymers help with filtration control and sand stability. Foam and polymer enhanced stiff foams are compatible with down-the-hole hammers. And always remember to follow the hammer manufacturer’s recommendations for minimum air requirements.
A typical stiff foam system would contain the following products for 100 gallons of injection mixture:
- Soda ash: 1/2 pound/100 gallons
- PHPA liquid polymer: 1 quart/100 gallons
- Foaming agent: 1 pint to 2 quarts/100 gallons
To be successful, air volume from the compressor must be reduced. Less air for stiff foams.
Gel foam is a stiff foam using bentonite as the stiffening agent. Small additions of bentonite to the injection mixture create a small and strong bubble. Low uphole velocities are needed to remove cuttings and clean the hole. Some research has shown that an uphole velocity as low as 40 feet per minute will remove cuttings, with 100 feet per minute being typical. This is a low energy system.
Reduced air volume from the compressor is required for this system to be effective. Solving the CFM equation using the same hole size and drill pipe as in our previous examples and decreasing the uphole air velocity to 100 feet per minute gives a required minimum air volume from the compressor of 24 CFM.
CFM= 100 × 〈8²–4.5²〉
CFM = 24
Gel foams are especially effective for drilling unstable or fractured formations. The effectiveness of this system will be enhanced by additions of polymers. This system is not recommended for down-the-hole hammers and requires the use of tricone or fixed cutter bits.
A typical gel foam system would contain the following products for 100 gallons of injection mix:
- Soda ash: 1/2 pound/100 gallons
- Bentonite: 10-12 pounds/100 gallons
- PAC polymer: 1/2 pound/100 gallons
- Foaming agent: 1 pint to 2 quarts/100 gallons
Definitely less air for gel foams.
Many air rigs use an old barrel or small trough to hold their injection mix. This works for just adding a foaming agent as foamers only need to be stirred into the water to disperse. (I’ll address a better way in a moment.) Stiff foam and gel foam systems begin to resemble some liquid drilling fluid mixes and therefore have some of the same mixing requirements.
The use of a two-tank system is recommended for mixing and using foam and modified foam systems. This allows you to know and maintain consistent product concentrations at all times.
It also allows for easy alteration of product concentrations as subsurface conditions change. You need to know where you are before you can change to where you need to be.
For foam drilling, both tanks are used as injection tanks. This allows drilling to continue uninterrupted while the foam mix is being built. The injection pump suction is plumbed to switch between tanks, always pulling from a ready mix while foaming solution is being mixed in the second tank.
For modified foam systems, the first tank is used as a premix tank and the second tank is the suction tank. The pre-mix tank is used to mix bentonite and polymer additives. A small centrifugal pump and venturi mixing hopper are needed to mix these products.
Bentonite and polymers need some time to shear and hydrate to be fully effective. The base fluid is transferred from the pre-mix tank to the suction tank where the foaming agent is added. Foamers only need to be stirred into the mix.
The rig injection pump then picks up and injects this final mixture into the air stream. The drilling foam is created as the air and foam mixture exit the drill bit. Adding foamer to the pre-mix tank would cause premature foaming of the mixture and unable to be injected into the air stream.
Answering a Friend’s Question
I mentioned at the beginning of this column a problem with a foam system cleaning cuttings out of the hole. Here is the answer to that and this is the No. 1 reason for the failure of foam systems: Too much air was being sent downhole.
Foam systems are three-phase systems. They have solids (drill cuttings), liquid (foam injection mixture), and gas (air for drilling). The gas and liquid phases need to be stable to create foam to suspend and lift cuttings. Too much air blows through the liquid mix and either destroys the bubble structure or prevents it from being established in the first place!
So, as I’ve said several times already: air/foam systems are a less-air approach to air drilling. This is a necessity and not just a good suggestion.
Air drilling is in reality a progression of drilling techniques, spanning the range from high-energy, get-after-it dry air drilling to foam and modified foams including low energy. I can’t believe this works gel foam with its low-free energy to affect the formations being drilled.
Our examples show air volumes can be reduced to drill effectively using smaller compressors and can help a small rig act like a big rig. These techniques are tools to extend the ability of the contractor and increase the chances of success.
Jeff Blinn has had a 40-plus year career as a professional drilling fluids engineer. Beginning with mud school in 1978, he has worked in many drilling disciplines including minerals exploration, water well, oil and gas, geothermal, geotechnical, and horizontal directional drilling. He has held positions as field sales engineer, engineering supervisor, account representative, technical services representative, and training manager. He can be reached at email@example.com.