HF-Technologie

The principle of sonic drilling

Sonic Drilling, Rotasonic, Rotosonic, Sonicore, Vibratory or Resonantsonic Drilling, are some of the many names given to a dual cased drilling system that employs the use of high frequency mechanical vibration to take continuous core samples of overburden and most bedrock formations, and to advance casing into the ground for well construction and other purposes. Any of the names above can be used because they all describe a high frequency vibratory drilling system that is basically the same. The only differences are the rig designs, the operators, and some of the downhole tools and methods of operation that various sonic drillers or companies use. For ease, and to be consistent, we will refer to this system or method as sonic drilling throughout this article.

The word sonic appears in most of these names because this drilling technique vibrates the entire drill string at a frequency rate between 50 and 150 hertz or cycles per second. This frequency falls within the lower range of sound vibration that can be detected by the human ear, thus the term sonic has been commonly used to describe this drilling system. The Rota- or Roto- part of the drilling technique refers to the rotational power that can be applied in hard formations to slowly rotate the drill string to evenly distribute the energy and the wear at the drill bit face.

A sonic drill rig looks and operates very much like any conventional top-drive rotary or auger rig. The main difference is that a sonic drill rig has a specially designed hydraulically powered drill head or oscillator which generates adjustable high frequency vibrational forces. The sonic head is attached directly to the core barrel, drill pipe or outer casing, sending the high frequency vibrations down through the drill steel to the face of the drill bit creating the displacement, fracturing or shearing action depending upon the foundation being drilled. The oscillator uses two eccentric, counter rotating balance weights or rollers that are timed to direct 100 percent of the vibration at 0 degrees and 180 degrees. There is an air spring system in the drill head that insulates or separates the vibration from the drill rig itself.

The Process

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step 1: “Crowd in” – moves all the bit face material into the core barrel.

step 2: “Crowd out” – moves all the bit face material into the borehole wall.

step 3: “Neutral” – lets the bit face material seek the path of least resistance

step 4: repeat core advancement

The processes which result in borehole advancement are fracturing, shearing and displacement. Drilling through cobbles, boulders and rock is caused by, fracturing of the material by the inertial moment of the drill bit. Shearing takes place in dense silts, clay and shale’s, provided the amplitude of the drill bit is high enough to overcome the elasticity of the formation material. Displacement occurs when unconsolidated formation material is moved away by the vibrating drill bit.

This system has many advantages, but the one that seems to be the most unique is the ability to obtain large diameter (2 inch to 8 inch) continuous core samples of almost any overburden formation without the use of air, fluid, additives, and with or without rotation. It also can drill and sample through boulders, wood, concrete and other construction debris which with conventional rigs usually causes refusal and necessitates moving and redrilling.

The outer casing prevents cross contamination and formation mixing. When drilling through multiple aquifers, the outer casing can be vibrated down past an aquifer while a bentonite grout mixture is being pumped under pressure into the small annular space on the outside of the casing. This process completely seals off aquifers in the upper borehole allowing for uncontaminated soil and water sampling at greater depths from aquifers that are isolated from other aquifers in the borehole.