In July, 2017, we hosted our first webinar in a series covering drilling fundamentals, the various technologies and their applications. There were plenty of great questions posed in this first webinar in our drilling series, so if you missed it, we thought we’d take the opportunity to answer a few of the most salient ones.
There isn’t a single best method. The various methods each have strengths and weaknesses as they run into clay. For example, air rotary struggles as the clay balls up and becomes hard to lift up. Also, when clays get wet they can swell, so you really have to be careful if you’re drilling with fluid or mud rotary. The depths to which you are drilling also changes equation. Take for example the, the corcoran clay in Central CA, that at 700-800 ft can cause problems. While you cannot hollow stem auger to that depth, if you are only putting in a 75 foot well, auger is really good at getting through the clay.. Air methods definitely have challenges as well. It is difficult for sonic because clay attenuates a lot of energy, much like hitting rock with a hammer as opposed to a pile of playdough.
Hollow stem auger or direct push.
Yes, but it depends on what is meant by “long.” Hundreds of feet is going to be hard on the tooling: you will go through a lot of core barrels, wear out tools and put stress and strain on the rig. It is capable of doing it, but it also depends on the competence of the rock. Sonic works great in siltstone, clay stone or weathered rock. Pure granite is going to be different and so definitely depends on how long you are talking about. At 20 – 30 feet, the headache of switching over doesn’t offset the difficulty of penetration, but it is worth considering other methods if you’re dealing with hundreds of feet.
You can’t measure something without changing it, and there is no drilling method that doesn’t disturb a sample. However, from the geotechnical perspective, Shelby tubes pushed with auger or rotary wash are pretty undisturbed. The sonic method of penetration liquefies the particles in the immediate vicinity of the bit interface so it’s disturbing the soil as it is going down. If you stop and push a Shelby tube once you get away from that bit interface, you should have an undisturbed sample at some point, but it is hard to know exactly how far the extent of the disturbance goes. The amount of disturbance is generally on the scale of inches or fractions of an inch. If you need to get through a shallow material that’s impenetrable to auger or rotary wash methods, Sonic at least gets you to where you need to be in order to push a sample. Afterall, it’s better to have some data than no data.
When you override the casing, like a 4 inch diameter to a 6 inch outside casing, that couple inches of material between the two has to go somewhere. The short answer is that is get displaced into borehole wall. If you are circulating water when running the outside casing, commonly known as washing over the casing, it can cause cuttings to return back out of the hole. Drillers typically have a pan at the surface to catch the returns coming back up. So, not all cuttings are getting shoved into the formation. As for how much water to use - that varies a lot on geology and driller experience.
Generally you do not use mud rotary for core sampling. You can take a split spoon sample by tripping out the tri cone bit and drill pipe, running in a split spoon barre and then driving a discrete soil sample. If you need to collect a core, there is a coring system called 94 mm wireline using wireline coring for unconsolidated material. You run the wireline in the mud hole with the drilling fluid to advance the core system. If you want a continuous core with mud rotary, wireline coring would be the best recommendation.
During the webinar we mentioned that the flush threaded casing is able to isolate different aquifers from one another. The best way is by telescoping a smaller diameter casing into the larger casing. For example, say aquifer A is shallow and aquifer B is deep, and you want to make sure those aren’t talking to each other during the drilling process. You would drill down to the barrier that separates the two aquifers, set an 8 in diameter casing, then continue to aquifer B with 4in x 6in override, thus telescoping the smaller into the larger.
Mud rotary is always good because you are able to use the hydrostatic pressure of the mud. You might, however, have to run a different kind of fluid program (usually denser mud maybe with polymers). Sonic is also a good option due to the flush threaded casing. That allows you to hold the head pressure on the formation as you drill. With hollow stem auger, the augers are bolted together and have open joints so they can’t hold the head pressure as well. With sonic, once you get past whatever the zone is that has the heave, you are able to cast it off, and then physically isolate it with steel so that it’s not continually giving you problems.
There are four parameters to track during mud rotary drilling: density, viscosity, filter cake, and sand content. The filter cake gets measured in two different ways: (1) thickness that is created on the borehole wall, and (2) relative impermeability of the filter cake. This is determined by a Filtrate Loss Test where you take a sample of mud and put it under pressure on filter paper. The amount of water able to get through in terms of volume and again and the thickness of filter cake, tells you what you need to know. Keep an eye out for a Drilling Fluid Fundamentals webinar, because that’s an hour long discussion in and of itself.