Solids Removal Region
The devices and associated tank arrangement in the removal region are: (1) Mud/Gas Separator,
- 2) Shale Shaker, (3) Sand Trap, (4) Degasser, (5) Hydrocyclones, (6) Mud Cleaner, and
- 7) Centrifuge(s).
- Mud/Gas Separator - Ahead of the entire sequence of solids removal equipment on most rigs is the Gas Buster. This is a large volume mud/gas separator vessel that allows gross amounts of gas to escape from the mud prior to going on to the shale shaker. It has a gas line laid far from the rig where gas can be flared; however, it does not remove all gas. The degasser downstream of the shale shaker removes the hard-to-remove gas and will be discussed later.
- Shale Shaker(s) - Shale shakers are vibrating screening devices and are fed by mud returning from the mud/gas separator or directly from the wellbore through the mud flowline. Total cuttings load goes onto the screens which can be a large mass per minute of solids during fast drilling. Mud should enter into the bottom of the cuttings box (possum belly) before going onto the screens. Mud flows upward and out of the box, providing a wide, slow flow over the screen surface. Cuttings should not be allowed to fill up the mud box, taking up the volume in the box which is intended to slow the mud flow before it passes onto the screen.
On some wells more than one type of shale shaker may be used. A low-speed, less efficient shaker may send its underflow onto the screen of a high-speed, fine screen shaker. The advantage of a cascading arrangement is that larger, coarser, more abrasive solids are removed or scalped by the rig shaker prior to sending fluid onto the finer-mesh shaker. Thus, solids loading is reduced and screening efficiency is improved. Also, in cases where more than one shaker is needed to handle the flow rate returning from the wellbore, shakers may be run in parallel to process the large volumes of fluid and cuttings. This is called a parallel shaker arrangement.
Shaker screens have capacity limits. Exceeding this limit means that mud will be lost over the screen(s). The capacity of shaker screens can be limited in three ways: (1) solids-load limit, (2) liquid-load limit, and (3) particle size plugging. The solids-load limit is reached in fast drilling of soft formations when the screen is handling so many solids that they cover the screen openings and thus, will not allow fluid to flow through. The liquid-load limit is simply excessive mud volume being pumped which is greater than the screen openings can handle. In either case, flow rate and/or screen size must be adjusted to allow the screens to handle the fluid. Particle size plugging is another factor which limits screen capacity. This is caused when screen openings are plugged by particles of the same size as the screen mesh openings. In this case, the screen size must be changed to prevent plugging (blinding) of the screen.
The shale shaker is positioned over the first tank above an unstirred compartment called the Sand Trap. Mud that passes through the shale shaker screens falls directly into the sand trap. Any mud that bypasses the shaker screen will also go into the sand trap. It is not recommended to bypass shaker screens.
- Sand Trap - The Sand Trap under the shaker has no agitation and allows particles an opportunity to settle and remain until the trap is periodically cleaned. Settling rate of particles depends on the rheological properties and density of the mud as well as the size and specific gravity of the particles in the mud. Sand traps are often built with sloping sides to allow particles to settle onto a solid surface and slide to the bottom. This shape also minimizes mud volume lost when cleaning out the
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- A sand trap is a good safety device to prevent extraneous large cuttings (from any source) from moving downstream and plugging the hydrocyclones and degasser, etc.
- Degasser - The degasser is an important part of auxiliary equipment in solids removal. It removes gas and air from the mud by one of several designs - usually by imposing a partial vacuum and flowing the mud as a thin film over a solid surface. If gas is allowed to exist in a mud, it can interfere with the centrifugal pumps which feed the solids removal devices. Also, if not removed at an early point in the solids removal sequence, gas could spread throughout the mud pit area creating a serious hazard.
- Hydrocyclones - hydrocyclones are the next piece of equipment in the processing sequence and must process more than 100% of the flow. These are cone-shaped devices which by their design cause fluid to spin within the cone. Mud which is under pressure enters tangentially at the large diameter of the cone. The mud spirals from the large end toward the small end of the cone. Solids are forced outward to the wall and then down the wall and exit at the adjustable opening at the small end of the cone. Mud goes back up the vortex finder in the center, exits at the large end and returns to the active mud system (see Figure 6).
Two sizes of hydrocyclones are commonly used for muds: desanders - 6-in. and larger diameter and desilters - smaller than 6-in. diameter. The desilter handles less volume per cone and requires more cones to process a given volume; but, it discards smaller-sized solids compared to the desander.
In general, hydrocyclones are not used on oil muds or on any type mud containing appreciable amounts of weighting materials because they will discard a high proportion of expensive materials.
Figure 6 Hydrocyclone
Figure 6 Hydrocyclone
- Mud Cleaner - The mud cleaner is a combination solids removal device which consists of a bank of hydrocyclones that discard onto a screening device (a small shale shaker). The mud cleaner is used for cleaning weighted muds and also for recovering the expensive liquid portion of any mud (such as KCl or oil mud). The desilter must be operated as usual with sufficient head to process the weighted mud through the cones. The discharge from the desilter cones spray into a tray and then flow out onto the fine-mesh vibrating screen, usually 150 to 200 mesh, where most of the bar-ite and liquid pass through. Some barite is discarded, especially when the mud cleaner is not being carefully monitored. Today, economics will usually suggest spending money on fine-mesh primary shale shakers rather than on mud cleaners in most situations where weighted muds are to be processed.
- Centrifuges - Centrifuges are of two basic types: low-speed or high-speed decanting, and fixed-speed, perforated-rotor machines. When the feed stream to a decanting machine is a weighted mud (with dilution water), its underflow is a damp mixture of barite and some drill solids. This mix is usually returned to the mud as recovered barite. The liquid overflow is usually discarded from the mud system - and contains colloidal solids and chemicals.
The perforated-rotor machine runs at fixed rpm and provides a liquid overflow stream and a liquid underflow stream. It can be adjusted to do a wider variety of separations than can the decanting machine; but, it does not discard damp solids. The perforated-rotor machine is limited to weighted water-based muds.
- For unweighted water-based muds - only the decanting type centrifuge is recommended. The damp underflow contains mostly drill solids and is discarded. The overflow contains the liquid phase and some silt materials and perhaps chemicals. In some cases, the centrifuge is fed from the underflow of hydrocyclones to increase the concentration of solids being processed to improve its efficiency.
- For weighted oil-based muds - where both liquid and solid phases are valuable, double centrif-ugation can be used. The primary low-speed centrifuge recovers barite. The liquid phase from the primary centrifuge is delivered to a second, high-speed centrifuge for silt removal. The liquid phase, with colloids, is returned to the active system. For unweighted oil-based muds, the decanting centrifuge can be used to remove unwanted drill solids and recover valuable liquid -oil and chemicals.
Today, as mud discards need to be less in volume and dryer for disposal purposes, one centrifuge may be used to feed its liquid overflow to another machine for further processing. This arrangement is generally used in closed-loop and zero-discharge operations.
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