Hole openers are used to increase size of well bore and there are two broad categories of hole openers: fixed diameter hole openers, and under-reamers.
Fixed diameter hole openers are usually made up of three “cutters” arranged around a mandrel, and mounted on “saddles” by strong retaining pins. Cutters may be milled tooth, PDC, or TC inserts, which will vary depending on the formation to be cut.
Fixed Diameter Hole Opener (Getech Equipments International, 2018)
Under-reamers, on the other hand, are hydraulically actuated hole openers that possess two or three arms. They are primarily used when a hole needs to be opened to a diameter larger than the casing which has already been set. Both of these forms typically feature a series of fluid passages, or “jets”, which are arranged to keep the cutters lubricated and help with the removal of cuttings. These need to be set up properly before use, to ensure a balanced mud flow both through and out of the hole opener.
Crew sets up the under reamer to enlarge the hole, (tamu.edu, 2003)
Shock subs, also known as vibration dampeners, are used to absorb vibrations and bit shock loads in drill collar strings. They usually feature long integral elastomeric elements, which serve to transmit torque and weight to the bit simultaneously. When drilling is being carried out at shallow depths, intermittent hard and soft streaks, along with broken formations, can transmit vibrations to the surface, where they are easily detected. With greater depths, though, these vibrations might not be detected because the drill string cushions them. However, they will still cause damage to the bit, as well as bottom hole assembly components and the drill string.
Shock Sub (Vibration Dampeners), Hunting (2018)
Some advantages of using a shock sub include:
- Offering faster drilling rates, since optimum bit weight and rotary speed may be used on the bit constantly.
- Increasing the bit length by reducing shock loads.
- Cutting damage to drill collars, drill pipe, and downhole tools by reducing bouncing.
- Reduced connection damage, because the elastomeric element absorbs both torsional and axial loads, so that connections are not at risk if the bit stalls.
- Reduced damage to surface equipment, including swivels, blocks, and wirelines.
Most modern drilling jars are hydraulic. They are also usually double acting, meaning they can deliver an extra-heavy impact should the bottom hole assembly become stuck. They are intended to work as an integral part of the drill string, and can withstand high pressures and temperatures over a long period of time, making them suitable for long-term use.
With almost the same length and diameter specifications as standard drill collars, and with a similar connection strength and slip setting area, they may be used as a component of a stand of drill collars without difficulty.
Usually, jars will be used alongside accelerators, which are run above the jar and work automatically. They serve to amplify the impact force of the jar, and can even double it in some cases. They commonly use the compression of silicon to give added stored energy and optimize jar impact and free-travel distance in both directions. They also have the added benefit of dampening the dynamic load in the drillpipe, since they transmit shock waves poorly, thus helping reduce damage to both string and surface equipment.
Drilling Jars Diagram (Slideshare, 2017)
Not only are reamers important for directional drilling, but they can also be useful in straight hole applications. Reaming assemblies can straighten out and smooth over crooked holes, restore undergauge holes to gauge, and get rid of any irregularities or keyseats. They also help to prevent excessive hole curvature in short intervals, which may be experienced when entering and exiting a section of hole which forms a sharp curve. Finally, reamers can reduce the rotational torque in a wellbore, and may therefore be used as a substitute for a conventional string or near-bit stabilizer.
Reamers are made by almost all major downhole tool manufacturers, and have the same core features: sealed or open (mud lubricated) bearings, cutter types – either “nobbly” or “smooth”, and either one (so called “3-point”) or two (“6-point”) sets of cutters in a tool.
Reamer (Courtesy of NOV, 2017)
Between the fracture pressure and the pore pressure of the formation, the hydrostatic pressure of drilling fluid will always be maintained according to conventional drilling practice. In order to control the transport cuttings to the surface as well as the formation fluids, the drilling fluid is held within the wellbore where it circulates. Furthermore, it also keeps the drill bit cool and lubricated as it acts as a stabilizing agent. For effective use, the fluid must be water- or oil-based and this leads to a maximum weight of 19 pounds for each gallon (minimum of 7.8 pounds). As an attempt at imparting fluid loss, density, and rheological properties, it also contains a mixture of liquid and solid products.
Figure 1 – Conventional Drilling
For many years, the conventional drilling has been the safest method when drilling a well but there are also some negatives to using the method. For example, fluid invasion is a common problem because the drilling fluid pressure is naturally above the pressure of the natural formation – this can cause permeability damage. Also, physical blockages and washouts are common as the solids and fluids lodge into the formation. Continue reading