Installing Water Wells in Sand Aquifers
In coastal areas and river bottoms, and most places not in hard rock country, water is often produced from sand. Sand is an excellent aquifer because it has the porosity and permeability to produce huge quantities of water. It also has great filtration properties.
Most sand aquifers started their geological lives as boulders or bedrock. Weathering and erosion breaks the massive rocks to particle size, and wind or water flow moves them to where we find them now. They are usually, eventually covered by layers of impervious clays and the like. The amount of time underground, and the depth and weight of the overburden, compacts the sand. An extreme example would be sandstone. This is just plain sand that has been compressed long enough to pretty much solidify. It can be hard enough to be used for building stone!
On the other end of the scale, we see new, young sands at shallow depths that are almost entirely unconsolidated. They drill so easily that they can often be penetrated without rotation. Just kick in the pump and jet down. These formations can be a headache because they may not be competent enough to hold the borehole. The usual cure is to use a good, competent mud that will build a good wall cake to hold the hole.
In between are the sand aquifers that we see, and which produce the most. Porosity and permeability vary greatly depending on area. It is important to understand those terms. Porosity defines the small spaces between grains of sand. Permeability is how well those small spaces are connected. For instance, if you filled a jar up with marbles, it would have both great porosity and permeability. But if you poured some cement in, the pore spaces would still be there, just not connected to each other.
Over time, the permeability of an aquifer may decrease due to cementation. In hard water areas, calcium, magnesium and other soluble elements may precipitate into the pore spaces, plugging them. This is called cementation. This can be good or bad in the construction of a well. A well-cemented sand is easy to drill, and the hole stays open long enough to install the well. The trick is to produce a sand that is not completely cemented and still has interconnecting pores. Commonly, these sands drill pretty fast, and the bit chatters, or “talks,” to the driller — a pleasant sound, indeed. That’s pay dirt!
In my area, most sands are well-enough cemented to make it possible to make an open-hole well. It sometimes takes a little more development time to remove all the loose sand near the wellbore, but it is cheaper because screens aren’t involved. The decision to screen or not is made on an individual and area-specific basis. This can be a trap. I know some poor-boy drillers who always make open-hole wells in sand because they are cheaper. If the sand is not consolidated or cemented enough, they just develop until the well clears. I’ve seen dump trucks worth of sand on the ground during development. Not really a first class practice.
When a sand aquifer is not consolidated or cemented well enough to “hold the hole,” it is time to consider setting a screen. Screen selection can be a little tricky. Too small, and you can lose efficiency and increase drawdown. Too large a screen, and you risk pumping sand forever. The federal government has a standard for sand, but I don’t go by it much. I prefer zero sand. Look at it this way: Even if the well makes a small, but allowable, amount of sand, if your customer’s wife sits on one grain of sand in the bathtub, you will hear about it. A better way is to collect samples of the sand in question, and use them to select the screen. Most screen companies will sieve and analyze your samples, and suggest a screen slot size. Or you can do like I do, and get a sieve set and do it yourself.
Different companies and manufacturers use a slightly different formula, but I have found that in most areas, I use a screen that retains about 80 percent of the natural formation. During development, the coarse grains form a filter pack around the screen and allow the fines to be developed out. This is exactly the same as a gravel pack, only natural.
For more Wayne Nash columns, visit www.nationaldriller.com/wayne.