Solids control is the most important function in mud treating. Control of mud properties centers around the treatment necessary to counteract the continual influx of drilled solids into the active mud system. Almost all of the costs of treating a mud can be directly attributed to solids control.
Undesirable solids increase drilling costs in a number of ways in addition to increased mud costs. If not properly controlled, they lead directly to such problems as lost circulation. differential pressure sticking, and reduced penetration rate.
There are basically three mechanisms for reducing the solids content of a drilling fluid. These are dilution, screening, and settling or centrifuging. Dilution is accomplished by adding liquid to reduce the concentration of solids in a given volume. Solids can also be reduced by passing the mud over a screen and discarding those particles that will not pass through the screen openings. Solids can be separated from a fluid by settling or by imposed centrifugal force. This is accomplished in settling pits, hydrocyclones, and centrifuges.
All these methods are used and have a place in mud treating. However, the method that will work best and most economically is a function of mud type, density, viscosity, and cost. The degree of solids control necessary is a function of the severity of the drilling problems, and the type of formation being drilled. This will also influence the method of solids control that should be employed.
Our basic aim in solids control is to reduce the formation solids content of a mud to a practical limit that will allow us to maintain the filtration and flow properties in a range to best handle all of the drilling problems. This means that we must pay special attention to control of the particles less than 2 microns in size, since they are dominant in determining the mud properties. Unfortunately, the small particles are the most difficult to remove. To reduce the concentration of these particles, it is necessary to either dilute and build mud volume or to centrifuge the mud. When centrifuging, these small particles are discarded along with a portion of the liquid phase which contains a large portion of the chemicals in the mud. Both dilution and centrifuging are expensive. Centrifuging is normally considered to be less costly in muds weights above 12 lb./gal.
The problems and cost of removing the small particles makes it imperative that we remove as much of the large particles as possible before they are reduced to the fine size range. This is accomplished primarily by screening weighted muds and by using desanders and desilters on unweighted muds. The cost of controlling solids goes up as the mud density increases.
The cost of reducing the solids content in a 10 lb./gal mud is approximately half that of a 12 lb./gal mud and one-fourth that of 17 lb./gal mud. This means that we should begin weighting up a mud which contains a minimum amount of drilled solids. This will insure a minimum cost and better mud properties at all times.
The addition of water to a mud system directly results in the cost of converting that water into mud of the proper density and chemical content. The volume of new mud which is built increases the total volume of the mud system, thereby reducing the concentration of drilled solids in the mud.
The first rule for minimum dilution cost is to maintain the mud system at minimum volume. A volume of mud equal to the volume of new mud to be built should be removed from the mud system prior to beginning dilution. This results in a maximum percent dilution from the addition of a given volume of new mud. The mud discarded prior to dilution is also of less value than the mud discarded after dilution, because it contains more of the undesirable solids.
This leads to rule number two. The mud should be diluted to the desired solids content in one step rather than a series of smaller steps. The mud discarded at each step contains some of the new mud from the previous dilution. Consequently, the total cost to reduce the solids content to a predetermined level will be considerably more if done in small stages than by a single dilution or displacement of part of the mud system.
These conclusions are illustrated in Figure 1. Consider the situation where it is desired to reduce the drilled solids content by 50 percent. The least expensive way to accomplish this is to discard half the mud system and replace with new mud. If the system is diluted by a series of discard-and-replace steps, the required dilution volume and cost will increase as the size of replacement steps decreases.
If the mud system is allowed to increase in volume, the costs will be even higher. If the system is allowed to increase in volume by 25 percent and then discarded back to original volume, the cost will be 76 percent higher than partial displacement cost. The most expensive dilution method is to allow the volume to continually increase. This will result in twice the cost of a partial displacement to reduce the drilled solids content by 50 percent.
Obviously, the expense of dilution can be minimized by proper application of these principles. The dilution program should be planned rather than left to chance. The solids content can be allowed to build during intervals where careful control is not necessary and then reduced by massive dilution before proceeding into more critical situations.
The cost of controlling solids by dilution is a function of mud weight and the amount of drilled solids incorporated into each barrel of mud. Figure 2 illustrates the dilution cost per pound of drilled solids as a function of mud weight and volume percent drilled solids carried in the mud. For the purpose of this figure, the bentonite and chemical costs were assumed to be $2.50/bbl at all mud weights and a barite cost of $.0365/lb.
As the mud weight increases and the amount of drilled solids allowed to incorporate in the mud decreases, the cost of controlling the drilled solids concentration increases. In other words, as mud weight increases, the cost of a barrel of new mud increases; and as the allowable concentration of drilled solids in the mud is decreased, the barrels of new mud required to maintain a given percent drilled solids increases. This gives us two more methods for reducing dilution costs. First, dilute the mud and reduce the drilled solids content at low mud weights rather than waiting until the mud weight has been increased and it becomes necessary to dilute in order to achieve the desired properties. This is simply a matter of planning ahead. If the mud contains more drilled solids at the low mud weight than can be tolerated at the highest expected mud weight, the solids should be reduced at this time to a level that will allow increasing the mud weight without subsequent dilution.
Figure 3 may be used as a guide to the solids content at each mud weight that will allow increasing the mud weight to the highest expected value without further dilution. For example, if the current mud weight is 10 lb./gal and the highest expected mud weight is 16 lb./gal, the 10 lb./gal mud should contain no more than 11 volume percent solids. To allow for future incorporation of drilled solids, it would be well if the mud contained considerably less than 11 percent solids. At each mud weight along the way, adjustments in the solids content can be made as necessary.
When the solids content increases with increased density more than predicted by this chart. it indicates that solids removal is not keeping pace with the solids influx. This signals a need for more solids removal capability by such means as finer screens or more centrifuging.
The other factor controlling dilution cost is the percent drilled solids that we allow the mud to contain. Obviously. the mud maintenance cost is reduced if higher drilled solids concentrations are allowed. However, the problems that high concentrations of drilled solids cause often far outweigh any savings in mud costs. Our first concern should be to control the solids to such a point that we can achieve and maintain the required mud properties.
The solids content outlined in a mud program should be realistic. If it is lower than needed. the mud maintenance costs will be forced to be higher than necessary. If it is not consistent with other designated mud properties it will lead to confusion.