Solids control may be defined as the control of the quantity and quality of suspended solids in the drilling fluid so as to reduce the total well cost.
The following equation may be used to estimate the volume of solids entering the mud system whilst drilling:
Vc = (1-φ)d^2(ROP)/1029
- Vc = volume of cuttings (bbl/hr)
- φ = average formation porosity
- d = hole diameter (in)
- ROP = rate of penetration (ft/hr)
Thus for a typical North Sea well (d = 26″, ROP = 62 ft/hr, φ = 0.25)
Vc = (1-0.25)*676*62/1029 = 30 bbl/hr
Therefore 30 bbls of solids have to be removed by the solids control equipment every hour. Solids control is the most expensive part of the mud system since it is operating continuously to remove unwanted solids. It is generally cheaper to use mechanical devices to reduce the solids content rather than treat the mud with chemicals once the solids have become incorporated in the drilling fluid.
The solids which do not hydrate or react with other compounds within the mud are described as “inert”. These may include sand, silt, limestone and barite. All of these solids (except barite) are considered to be undesirable since:
- They increase frictional resistance without improving lifting capacity.
- They cause damage to the mud pumps, leading to higher maintenance costs.
- The filter cake formed by these solids tends to be thick and permeable. This leads to drilling problems (stuck pipe,increased drag) and possible formation damage.
It is these solids which must be removed to allow efficient drilling to continue. However some particles in the mud (e.g. Barite, Bentonite) should be retained since they are required to maintain the properties of the mud. If these desirable solids are removed they must be replaced by more additions at surface which will increase the mud cost.
For most practical purposes the mud solids can be divided into two groups according to their density:
- Low gravity solids s.g. = 2.5 – 3.0
- High gravity solids s.g. = 4.2 (barite).
Drilling fluids will contain different proportions of each type of solid (e.g. to maintain hydrostatic mud pressure high gravity solids must be added, and so this type of mud should contain fewer low gravity solids). Solids control in muds containing barite (weighted muds) requires special procedures to ensure that barite is not discarded along with the undesirable solids. Muds containing low gravity solids only (unweighted muds) have a density of 8.5 – 12 ppg.
There are three basic methods used to control the solids content of a drilling fluid:
A shale shaker uses a vibrating screen to separate the solids according to size. Material too large to pass through a given mesh size will be discarded while the finer material will undergo further treatment.
For natural settling of solid particles under laminar conditions Stokes Law applies:
Vs = 2gdc*2(ρm-ρs)/92.6μ
- Vs = slip or settling velocity (ft/sec)
- g = acceleration due to gravity (ft/sec^2)Vs = slip or settling velocity (ft/sec)
- dc = largest cutting diameter (ft)
- ρm = mud density (lb/ft^2)
- ρs = cutting density (lb/ft^2)
- μ = mud viscosity (cps)
Basically the solids will settle out more readily when:
- The solid particles are large and heavy.
- The mud is light and has a low viscosity.
- The gravitational force can be increased by mechanical means.
When the viscosity of the mud is increased (to improve lifting capacity) solids settling becomes more difficult. For practical purposes the natural settling rate is far too slow, so mechanical devices are introduced to remove the solids. Hydrocyclones and centrifuges increase the gravitational force on the solid particles, and so the process is sometimes called “forced settling”.
After passing through all screening and settling stages there will still be a very fine solids content which remains in the mud. This can either be discarded or diluted. Due to the limited capacity of the active system some mud is usually discarded (together with desirable solids and other chemicals) before the remainder can be diluted and conditioned for re-circulating.