Removal Section
Undesirable drilled solids and gas are removed in this section before new additions are made to the fluid system. Drilled solids create poor fluid properties and cause many of the costly problems associated with drilling wells. Excessive drilled solids can cause stuck drill pipe, bad primary cement jobs, or high surge and swab pressures, which can result in lost circulation and/or well-control problems. Each well and each type of drilling fluid has a different tolerance for drilled solids.
Each piece of solids-control equipment is designed to remove solids within a certain size range. Solids-control equipment should be arranged to remove sequentially smaller and smaller solids. A general range of sizes is presented in Table 5.1 and in Figure 5.2.
The tanks should have adequate agitation to minimize settling of solids and to provide a uniform solids/liquid distribution to the hydrocyclones and centrifuges. Concerning the importance of proper agitation in the operation of hydrocyclones, efficiency can be cut in half when the suction tank is not agitated, versus one that is agitated. Unagitated suction tanks usually result in overloading of the hydrocyclone or plugged apexes. When a hydrocyclone is overloaded, its removal efficiency is reduced. If the apex becomes plugged, no solids removal
Table 5.1
Size of Solids Removed by Various Solids-control Equipment
Table 5.1
Size of Solids Removed by Various Solids-control Equipment
|
Equipment |
Size |
Median Size of Removed Microns |
|
API 80 screen |
177 | |
|
API 120 screen |
105 | |
|
API 200 screen |
74 | |
|
Hydrocyclones (diameter) |
8-inch |
70 |
|
4-inch |
25 | |
|
3-inch |
20 | |
|
Centrifuge | ||
|
Weighted mud |
>5 | |
|
Unweighted mud |
<5 |
occurs and its efficiency then becomes zero. Agitation will also help in the removal of gas, if any is present, by moving the gaseous drilling fluid to the surface of the tank, providing an opportunity for the gas to break out.
Mud guns can be used to stir the tanks in the additions section provided careful attention is paid to the design and installation of the mud gun system. If mud guns are used in the removal section, each mud gun should have its own suction and stir only that particular pit. If manifolding is added to connect all the guns together, there is a high potential for incorrect use, which can result in defeating proper sequential separation of the drilled solids in an otherwise well-designed solids-removal setup. Manifolding should be avoided.
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