The volume and type of solids in a drilling mud system can adversely affect mud properties, reduce penetration rates, cause damage to drilling equipment, and increase total drilling costs. Efficiency controlling the solids content of the mud system is an important phase of an efficient and cost-effective drilling program. The three basic methods of removing solids are dilution and/or displacement of whole mud, settling and mechanical solids-control equipment.
The dilution method increases the total volume of the circulating mud system by the addition of dilution water and/or base mud, reducing the relative volume occupied by drilling solids. The amount of dilution necessary to reduce the volume of drilled solids to the desired level may be calculated with the following formulate:
Vwm = Vm(Fct-Fcop) /(Fcop-Fca).
Where: Vwm = barrels of dilution water or base mud.
Vm = volume of mud in circulating system.
Fct = fraction, % total LG solids in system.
Fcop = fraction, % total optimum LG solids desired.
Fca = fraction, % LG solids (bentonite and/or chemical) added.
The displacement method removes a portion of the active mud system containing drilled solids and replaces it with an equal quantity of fresh water or base mud, maintaining a constant circulating volume. The amount of displacement required to lower drilled solids to a specified level can be computed as follows:
Vwm = Vm(Fct-Fcop) /(Fcop-Fca).
Where: Vwm = barrels of dilution water or barrels of mud to be jetted to maintain constant circulating volume.
In effect, both methods create a surplus of mud to lower the relative solids content of the system. This surplus mud, which must be discarded or stored, has certain economic costs. If dilution water or water hauling is expensive, the value of surplus mud high, or the disposal of the excess a problem, these methods impact unfavorably on overall drilling costs.
The settling methods, by itself, is a slow and inefficient process which takes place in a sand trap or settling tank. Settling rates of solids in drilling mud may be increased by flocculating and viscosity reducing chemicals. The settling rate can also be enhanced mechanically by various centrifugal devices which increase gravity, accelerating the separation process.
Solids-control equipment is the most effective method of solids control. There are several types of equipment available, each designed to operate most efficiently given the characteristics of the particular fluid system and of the drilled solids from the formation. Properly designed and applied systems can approach optimum efficiency (where the removed of solids from the mud system equals the influx of drilled solids). All solids-control equipment is designed to remove particles based on their size or specific gravity. Solids control equipment being used today are shale shakers, desanders, desilters, hydrocyclone centrifuges, and bowl centrifuges. The size, capacity, and operating range of each is shown in Fig 1.
Drilled solids brought to the surface from the borehole are removed from the mud stream by a shale shaker’s vibrating screen. The circulating volume of mud determines screen opening size which range from a 10*10 mesh to a 200*200 mesh. Most shakers on rigs utilize the 10 to 20 mesh size to remove large drilling cuttings (normally 30 to 40 percent of all drilled solids). In recent years, some rigs have adopted modern-type shakers using finer mesh screens. These finer screens remove a higher percentage of drilled solids from the mud stream but are limited by circulating volume handling capacities.
After the shale shaker, hydrocyclones such as desanders, desilters and mud cleaners (shaker, desander, desilter combined) are the most widely used pieces of solids control equipment. They are designed to remove low specific gravity drilled solids with particle sizes in the 20 micron and above range. The desander is capable of removing sand-sized particles of 45 micron and above; the desilter removes sand-size particles down to approximately 20 micron. To determine the performance of hydrocyclone in the field, a monograph may be used (Fig 2.). This equipment is used to process a weighted mud system. Barite is discarded alone with low specific gravity drilled solids. This makes the use of desanders and desilters on weighted mud systems impractical from a cost basis.
Hydrocyclone centrifuges and bowl centrifuges provide an economical means for removing colloidal-sized particles under 10 micron without discarding usable barite in a weighted mud system. Some barite will still be lost when hydrocyclone and bowl centrifuges are used on weighted mud system. However, this barite is ultrafine and causes increased rheological properties and gets strengths.
The high specific gravity barite is recovered by the hydrocyclone centrifuge through the underflow stream in a weighted mud system. The overflow stream containing low specific gravity solids in the 5 to 10 micron range is discarded. The process is reversed for unweighted muds since drilled solids are concentrated and discarded in the underflow while retaining the overflow. Bowl centrifuges separate out particles in the 2 to 5 micron range still remaining in the mud stream after processing by other pieces of solids-control equipment.
For weighted muds containing barite determine the percent volume of solids in the discard sample by using the mud record. Care should be taken in running the record to get a correct reading.
Efficiency is considered optimum when the percent of solids in the mud stays in the desired range without the addition of excess water for dilution purposes (solids removal should be based on a system approach). For maximum efficiency equipment should be installed, as recommended (Fig 3). Each piece of equipment must operate efficiently or the subsequent pieces will likely be overloaded and/or lose efficiency.
If improved solids control efficiency is needed, the following can be changed, adjusted or added to the solids control equipment in use:
- Shale Shaker – Determine if a smaller size screen mesh can be installed without losing mud over the screen. If not, consider installing an additional shale shaker based on cost of controlling solids.
- Desander and desilter (mud cleaner) – Check the type of underflow discharge. If it is a rope discharge, then increase the apex size to obtain a spray discharge. Check the feed pressure requirement [ or the existing unit and correct if necessary. Make sure the desander is always run ahead of the desilter; otherwise it will overload and reduce the efficiency of the desilter. Check the feed inlet parts for possible blockage in the cyclone action.
- Hydrocyclone centrifuge – When processing unweighted muds apply necessary back pressure on over flow discharge or increase the size of apex to improve the efficiency. In processing weighted muds, adjust the vacuum by opening the valve on overflow discharge.
- Bowl centrifuge – Decrease the feed solids rate by dilution with water when processing weighted muds to discard LG solids with overflow. On unweighted mud, increase the efficiency by removing most LG solids through the underflow.
- Additional solids control equipment – If satisfactory results are not achieved, considerable thought should be given to installing additional solids control equipment.
- The most effective and economical method to remove the solids is by the use of solids-control equipment.
- Proper selection of type and size of equipment is determined by hold size, circulating volume, rate of penetration, density and type of mud.
- All solids control equipment must be installed in accordance with manufacturer’s recommendations to achieve maximum efficiency.
- Solids separation systems are only as effective as the care given them by the field personnel who should be properly trained to understand and operate the solids control equipment.
- The method to evaluate the efficiency of solids separation equipment described in this topic may be used by field personnel to maximize the benefits derived from solids control equipment.