Three Example of Solids Removal Efficiency, Mud Usage and Equilibrium Drill Solids Content

The concepts of solids removal efficiency, mud usage, and equilibrium drill solids can be related together with a simple equation. These concepts are used to post analyze wells, illustrating typical drilling fluid and solids control problems. Evaluation of the entire system of solids control equipment and control techniques and the mud system
can be accomplished by using several simple concepts and understanding their interrelationship. Solutions are suggested with quantified results needed for economical applications.

Good Mud System Performance (Example 1)

A well was drilled in early 1989 in the Green Canyon area of the Gulf of Mexico. Example 1 in Table 1 gives the pertinent information for a hole section of this well. The mud system used was a variation of a conventional KOH/Lime mud. These muds have demonstrated good performance in a number of areas 1n dnll1ng reactive shale formations.

The solids control equipment used consisted of conventional double deck shakers with 30 mesh screens over 40 mesh screens cascading onto premium linear motion shale shakers with 110 mesh layered screens. For the interval shown, the shakers were the only solids removal equipment used continuously. Intermittent usage (average of 4 hr /day) of a perforated rotor centrifugal separator for barite recovery was also made.

This combination of mud and solids control equipment resulted in an excellent overall drilling mud job. The drill
solids were controlled in a realistic, but low range at an average of 5.5 vol%. Mud usage of 1.60 bbl mud/bbl of gauge hole volume is at the low range of the value expected and the solids removal efficiency of over 85 vol% is very good for a water based mud. In making these calculations, beginning and ending values of depth, system volume, drill solids percentag, high gravity solids percentage were used. Hole s1ze and barite usage were also used. These values are listed in Table 1 for this and all the other examples given. Appendix A details the calculation of the. mud usage
factor, equilibrium drill solids, and solids removal efficiency for this first example.

Dipmeter logs showed an essentially gauge hole throughout the interval. Tight hole on a short trip at logging depth necessitated a slight increase in mud weight. After logging, casing was run and cemented with no problems.

Hole Washout (Example 2)

The second example illustrates the effects hole enlargement has on mud usage. When a wellbore enlarges, additional solids are generated and must be removed. Using material balance concepts, this means that the mud usage factor increases. With the increase in mud usage factor based on a gauge hole, a decrease in solids removal efficiency is calculated. The second example listed in Table 1 illustrates this.

This example is a section from a well drilled in the Green Canyon area of the Gulf of Mexico using a hydrate formation resistant NaCI polymer mud in 1988. In this well the calculation of mud usage yields almost 6
barrels of mud per barrel of gauge hole volume. This indicates that large volumes of mud were required to
drill the interval. This large volume could be due to inadequate solids control or to well bore enlargement (or
both). Without caliper logs for the well, the determination of the extent of wellbore enlargement is difficult. Monitoring amounts of barite required to change the system density is an approach that can be used to determine enlargement while drilling. Lag times and bottoms up measurements for trip gas do not give accurate information on hole size.

Given this starting point (assuming a gauge hole is drilled) for the mud usage factor and solids removal efficiency, these two values could be adjusted for different levels of washout. As the hole diameter from washout increases, the mud usage factor decreases for two reasons. The first reason is that the volume of hole made increases from washout. Also, the mud usage decreases because the amount of mud remaining in ·the system increases due to the extra volume in the washed out portion of the hole.

With a constant drill solids concentration and using equation 4, a new solids removal efficiency can be calculated for each mud usage factor determined for the different levels of washout. Figure 1 illustrates for this well interval, how the mud usage factor decreases from almost 6 with no washout to less than 2 with 50% washout. Over the same range of washout, the solids removal efficiency increases from about 60 vol% with no washout to almost 85 vol% at 50% washout.

Two arm caliper results indicated hole enlargement at 35% to 40% of bit diameter. With this much enlargement, a mud usage factor of approximately 2.5 and a solids removal efficiency of about 80 vol% is calculated. These values lead to the conclusion that improvements in solids removal equipment and solids removal efficiency will not result in major reductions in mud usage and mud costs in future wells of this type. The main solids control equipment on the well was three double deck shakers processing the mud and then processing with three linear motion premium shakers. This impressive solids control setup confirms the conclusion that improving solids removal equipment would have minimal benefits. The major cause of the excessive mud usage factor and poor solids control efficiency is the hole washout.

Planning Offset Well

In planning an offset well based on the two above examples, this analysis of the offsets leads to recommendations in two areas, solids control and mud type selection.

Solids Control

Cascading of shale shakers in both examples resulted in solids removal efficiencies of 80 vel% or better. An offset well could expect to achieve good solids removal efficiencies by using at least two double deck shakers cascading onto at least two linear motion premium shakers. Since weighted muds will be used, a barite recovery centrifuge could probably be used economically and should be installed prior to spud.

Mud Type Selection

If a well were to be drilled in shallow water, the mud system recommendation would be the KOH/lime system. In the deeper water, it is possible for hydrate formation to occur if a gas kick occurs. Concern over gas hydrates has led most operators to select hydrate resistant mud formulations for drilling in a deep water environment. Most hydrate resistant mud formulations contain 12 wt% to 20 wt% NaCI. If this mud were to be used, several recommendations would be made.

The first recommendation would be to increase polymer additions above the level seen in the example well. This
would improve the encapsulation of the cuttings and wellbore wall resulting in an increase in mud inhibition.
Also, additions of lignosulfonate dispersants should be limited and their use avoided entirely, if possible. Finally,
addition of a Gilsonite based shale inhibition additive has been shown to improve wellbore stability in these types of formationg, and would give additional improvements in controlling differential sticking tendencies.

Inadequate Solids Control

The third example (example 3 in Table 1) is an analysis of a well drilled in Indonesia. This well was drilled in the
first half of 1989. Analysis of the interval between 4300 ft and 7800 ft indicated an average drill solids content of
above 7 vel%, a mud. usage factor of about 3, and a solids removal efficiency of about 70 vel%. The rig solids control equipment consisted of tandem double deck shakers operated with 40 mesh shaker screens. Caliper logs indicated near gauge hole through hard mudstones, sandstones and carbonates. The slow drilling rates experienced in the bottom portion of the well concerned drilling personnel. Examination of mud records indicated that drill solids were high (above 7 vol%) in the bottom portion of the well. In addition to the high drill solids contents, high plastic viscosities were also seen. Improvement in penetration rates could be expected with a reduction in drill solids and plastic viscosity. A goal of reduction in drill solids content to less than 4 vel% would be expected to provide the most
increase in penetration rate.

The operating cost reductions from an increase in rate of penetration (ROP), of course, would be offset by increases in mud and 1 or solids control costs. If the cost of reducing the drill solids content of the mud exceeded the savings from increasing ROP, then well costs would be higher. It is difficult to determine the expected increase in ROP, but estimation of the cost of reducing the drill solids is practical using material balance concepts. In this situation, the cost of upgrading the solids control equipment using rental premium shakers can be compared to the cost of dilution to reduce the drill solids. Rather than examine costs directly, however, the mud usage factor can be used to calculate changing costs and consumption when the cost/barrel of mud and hole volume to be drilled is known. For this topic, the change in mud usage factor will be used as an indicator of changing mud consumption and cost.

Figure 2 shows the increase in mud usage factor as the drill solids are decreased to 3 vel% assuming that the
solids removal efficiency remained at 70 vel% and using equation. This percent increase would also be reflected in the mud costs. At the desireable 4 vol% drill solids, mud consumption would be expected to increase about 100%. This situation would not be economical. If the solids removal efficiency could be improved by 10% to 80 vol% removal, then the mud consumption would be about 30% higher at the 4 vol% drill solids level. The figure also indicates that solids could be reduced to 5 vol% with no change in mud consumption.

The results of this analysis indicates that dilution to reduce drill solids to 4 vol% would not be economical. Rental of premium shakers that would allow 80 vol% solids removal efficiency would be expected to economically achieve the desired 4 vol% drill solids.

These concepts have proven useful for solids removal efficiency, mud usage and equilibrium drill solids content:

1. Comparison of mud systems in terms of material usage.
2. Planning of offset wells where hole sizes and geometry changes occur.
3. Analysis of problems with high mud usage in order to determine nature of excessive usage.
4. Estimate the impact of upgrading of solids control equipment.
5. Analyzing costs associated with hole enlargement.

When the solids removal efficiency and the mud usage factor are known, an equilibrium drill solids concentration can be determined from these two values. This drill solids concentration represents the drill solids content that will result from continued drilling with the existing efficiency and mud usage. If the drill solids content of a mud remains nearly constant for the drilling of an interval, then the solids control and mud system are stable and mud usage and drill solids content can be used to determine the removal efficiency for the entire drilling system. If the drill solids content increases, the rate of change can be used to determine removal efficiency.