Mud is circulated through a wellbore to bring the cuttings to the surface. Here cuttings are separated out so that clean mud can be reinserted into the well.
The mud loop and solids control
A typical circulation system is presented in Figure 1 as an example from a fixed drilling platform, while Figure 2 presents the circulation system from another view; on a floating drilling unit. Here mud is mixed and prepared in the mud pits consisting of several large tanks, each typically 60 m³ large. One or two of the mud tanks are in active use for mud circulation, while the others are for transfer and storing. One reserve pit is for kill mud, where density is kept typically at 0.25 kg/l above the density in the active pits.
Both density and rheology are maintained in the active mud pits. Typical total volume of a mud pit is 200 m³, with a surface area of typically 50 m². A vertical height of two cm corresponds to a volume of 1000 liters! In the surface mud system in Figure 1, we see two pumps in parallel. On offshore rigs it is more common with three.
From the pumps, a high-pressure output line leads up to the drill floor, where, on the standpipe, a multy purpose junction is made, called the standpipe manifold. Here the driller can read the standpipe pressure, which as almost identical with the pump pressure, reduced only by pipe friction in the short distance between the pump and the standpipe manifold.
On its return to the surface, the mud is directed through a wide settling tank, where the largest particles are allowed to settle out: On other rigs this tank is called the sand trap, positioned in front of the shale shakers as an over flow tank.
Drilled out solids are separated from the drilling fluid and discarded in the mud-cleaning system. Figure 3 presents a typical mud-cleaning unit.
In a complete mud cleaning system, the following cleaning methods may be included:
- Mechanical separation
- Chemical treatment
Each method exhibits certain advantages and disadvantages. The most efficient form of solids control is a combination of several methods. Settling can remove particles down to colloidal size, but settling is time consuming and impractical. Mechanical separation can remove particles of sizes down to 5 mm as shown in Table 1.
|Table 1 Mechanical cleaning equipment|
|Shale shaker (Fine shaker)||Can remove solids > 74 μ (200 mesh)|
|Desander||Dependent on size of the cone, can remove solids > 30 μ|
|Desilter||Dependent on size of the cone, can remove solids> 15 μ|
|Decanter Centrifuge||Can remove colloidal solids > 5 μ|
Shale shakers are the most common cleaning method. With the quality and fineness of shaker filter screens available today, other cleaning methods have become almost obsolete. With a 60 mesh and a 300 mesh shaker screen in series, 99% of the cuttings are removed. A 60 mesh filter screen removes the largest cuttings and protects the extremely fine second filter screen from wear and tear. The Mesh-size system is presented in Table 2.
|Table 2 Important parameters for square Mesh shakers (Mesh # = # of openings pr. inch).|
|mesh||Wire Diameter||Opening Width||Smallest particle size removed|
Low gravity solids (LGS) enter the mud through dispersed or disintegrated cuttings and has a density of 1.8–2.8 kg/l. If its content (LGSC) is increasing above a certain limit, solids control must be intensified.
One issue with fine filter screens is the wear of filter screens; monitoring and maintenance is required.
The mud pump
Pumping drilling fluids through kilometre long pipe systems will result in large hydraulic friction, and correspondingly powerful mud pumps are required. One typical pump is shown in Figure 4.
When it comes to hydraulic friction and to Task 1, removal of crushed cuttings, it will be necessary to know the characteristics of a mud pump.
The mud pump characteristics are divided into two different operating ranges: Range 1 is defined through the pump’s smallest liner, and range 2 includes the rest of the liners. Table 3 presents a typical pump characteristic for large sized mud pumps.