Barrel yield for bentonite drilling fluids is an oilfield term for measuring the viscosifying capabilities of a bentonite clay and is defined as the number of 42 gallon barrels of 30 to 35 centipoise (or cP, a measure of viscosity) drilling fluid that 1 ton (2,000 pounds or 40 50-pound bags) of bentonite clay will produce. For example, 1 ton of 200 bbl yield bentonite will produce approximately 8,500 U.S. gallons of 30 to 35 viscosity slurry. So what does this mean to the everyday small to medium driller who utilizes between 500 and 2,000 gallons of slurry a day? Probably not much until a rep approaches them and points to the fact that they are using 200 bbl drilling fluid product versus a 220 bbl high yield bentonite drilling fluid product. Horizontal directional drilling (HDD) bentonite drilling fluid products, for example, which vary from 150 bbl to as high as 220 bbl, are specifically designed to meet the demands of horizontal directional drilling.
The biggest difference between high yield bentonite drilling fluid products and drilling fluid products specifically designed for HDD applications is in gel strength and fluid loss/filtration control. HDD specialty drilling fluid products contain additives to enhance gel strength (to suspend drill cuttings to carry them out of the hole) and filter cake qualities/filtration control (which enables a drilling fluid to stabilize or keep the hole open), whereas extra high yield bentonite products do not have enhanced gel strength and filter cake/fluid loss additives, and require higher viscosities to achieve the same properties.
A 220 bbl high yield bentonite drilling fluid product will produce 9,240 gallons of slurry (220 x 42) of 30-35 viscosity drilling fluid, therefore a 500-gallon tank will require approximately 108.65 pounds (just over 2 sacks) of product to achieve this result. A 200 bbl HDD specialty drilling fluid product will produce 8,400 gallons of slurry (200 x 42) of 30-35 viscosity drilling fluid and will require approximately 119.53 pounds (just over 2 sacks) of product to achieve this result for a 500-gallon tank. The end result is the high yield bentonite will yield a higher viscosity drilling fluid, yet lack the gel strength and filtration/fluid loss control of the HDD specialty drilling fluid product. Viscosity is the resistance to flow, therefore the higher the viscosity, the higher the annular pressures downhole. Frackouts occur when the pressure of the drilling fluid exceeds what the formation can withstand, therefore one must minimize annular pressures to prevent this from happening. Another example is 90 bbl yield API-13A oil field grade bentonite product which lacks extending polymers, and will yield 3,780 gallons of slurry (rarely used in HDD applications, if ever).
At first glance, a 220 bbl extra high yield drilling fluid product will save approx. 11 pounds per tank, therefore, over five tanks one should save one bag. But, if the soil conditions were fine to coarse sand and small gravel seams, additional bentonite (increasing viscosity) or polymer additives, such as fluid loss and gel-strength enhancers, would be required to achieve success. Utilizing a specialty HDD drilling fluids product in the 150 to 220 bbl yield range in the same application would provide superior gel strength and filtration/fluid-loss control at a lower viscosity, and require less additives to perform the required functions.
For contractors, the bottom line is always important, and many view polymers and additives as a luxury item because of the cost. For others, it’s about being successful day in and day out, and spending a little more up front to save costs in the short and long term. Many small to medium contractors look at how the river-crossing/maxi HDD drills are getting by with using just high yield bentonite, and wonder why they cannot do same. The simple answer is volume and horsepower. On large projects where contractors have long bores of over 1,000 meters with large diameters and they are going through anywhere from 30 to 40 skids a day, then the answer is simple. It’s about the bottom line. But in this type of drilling, high volumes are injected into the borehole to create a high velocity flow that helps carry out the cuttings and, in a lot of cases, certain additives are added. It also helps to have pullback capabilities from 500,000 to 1 million pounds and a lot of rotary torque. Also, almost 100 percent of these large and long HDD bores use a recycling system to recuperate as much bentonite as possible, and even then only 30 to 50 percent is recycled in the best of cases. Mud weight and sand content (indicating retained drill cuttings in suspension) dictate the available recyclable bentonite. High yield bentonite was designed for vertical applications such as well drilling, where high volume flow is required to achieve an annular ascending velocity of 85 to 120 feet per minute in order to carry the cuttings upward faster than gravity can settle the cuttings out. On the surface, many vertical well drilling contractors allow the cuttings to drop out of suspension into pits or tubs, via horizontal flow, so that the drilling fluid can re-utilized in an almost continuous loop.
For a small to medium HDD contractor, the best option is to utilize a specialty HDD bentonite drilling fluid that has some enhanced fluid loss control polymers and enhanced gel strength. In a lot of cases, contractors will need less bentonite and viscosity to achieve the desired yield and carrying capacity. A quick test, without getting too technical, is to scoop some of the yielded bentonite into a clear container (an empty water bottle with the neck cut off will do), then take some of the soil the contractor is going to drill into (dug up in the digging of the entry or exit pit), and dump it in and stir. If it settles to the bottom in the cup it will settle to the bottom downhole, and additional gel strength is needed. Either increase the amount of bentonite/raise the viscosity or, if the viscosity is already at the high end, use additives like xanthan gum that enhance gel strength while not increasing the viscosity. Of course, as with all HDD bores, the goal is to keep the hole open (filtration/fluid loss control) and continuously flowing at the entry or exit pits to ensure that contractors do not hydro-lock, stretch the product pipe or force inadvertent returns to ruin the day.
Another interesting fact, based on the three different barrel yield bentonite drilling fluid products discussed above, is that in order to achieve the 150 to 220 bbl yield, polymer extenders are added to the mix of both HDD and high yield bentonites. These are the polymers and other ingredients that are introduced during the bagging/packaging of the product to meet barrel yield volumes. Therefore, contractors are not using pure bentonite. Extending polymers are what make it possible to yield out to such high numbers versus non-treated bentonites, which normally yield at 90 bbls. One last thing to remember: Always adjust the pH levels of the mix water to between 8.5 and 9.5 with soda ash prior to introducing bentonite, in order to get the most yield, and the best fluid loss/filtration control and gel strength. If the mixing system is shut down for a while and there is a clear layer of water at the top of the mix tank, the bentonite is not providing the yield, gel strength and fluid-loss/filtration control that contractors are paying for when they purchase the bentonite products, and a simple addition of soda ash can make all the difference.
For an HDD contractor, with small to medium size drills, the differences between high yield bentonite and bentonite specifically formulated for HDD applications can clearly illustrate the advantages of using a product that is designed specifically for HDD drilling applications. Using a bentonite specific to HDD will yield a drilling fluid that performs all of the required functions, such as fluid loss/filtration control and gel strength, at a lower viscosity (lower annular pressure), while requiring fewer additives to achieve the desired results.