In many cases, multiple suction and discharge locations cannot be avoided.
For example, centrifuges that will process both unweighted and weighted systems must be located to permit routing both the cake and centrate streams to either the active system or to discharge. The following schematics show the fluid routing requirements for a solids removal system which must process either unweighted or weighted mud.
Note: The centrifuge to be used for barite recovery must be positioned so the solids may be routed either to discharge (unweighted) or returned to the active system (weighted mud). Use a high capacity machine for treating out coarse desilter underflows or recovering barite. The second unit should be a high-G machine capable of removing fine solids. If only one machine is used, it should be a high-G unit.
r SOLIDS CONTROL HANDBOOK
Tank Design and EquipmentArrangements
CONFIDENTIAL Fig. 6. Generic - complete system.
3 Summary
· The mud pits must contain enough usable mud to maintain mud properties and to fill the hole during a wet trip at maximum depth.
· The active circulating system is divided into two sections: Solids Removal and Addition-Suction. The purpose of each is self-explanatory.
Each section is further divided into enough compartments to carry out its designed function. Additional tankage includes the slug tank for mixing and pumping small pills, the trip tank for accurately metering pipe displacement during trips, and the premix tank discussed in Chapter 10, Addition/Mixing Systems.
· The best compartment shape is round with a conical bottom, followed by square with a V-bottom. Each must have enough surface area to allow entrained air to break out.
· Equalization height between compartments will depend upon the duty of the compartment. Refer to the discussion in this chapter for specific recommendations.
· The sand trap, located under the shale shakers, is the only settling compartment and should not be used in closed loop systems.
· Equipment arrangements for a variety of unweighted and weighted muds are illustrated in this chapter. Also included is a complete system arrangement when both unweighted and weighted muds must be processed during the course of the well.
Dewatering Systems
1 Introduction ... 1
2 Economic Overview ... 2
3 Monitoring Dewatering Costs and Efficiency... 7
4 Equipment Selection... 10 4.1 Dewatering Devices ... 10
5 Waste Management ... 11
6 Summary... 13 FIGURES
Fig. 1. Effect of solids on flocculent concentration. ... 5 Fig. 2. Evaluation of dewatering centrate. ... 6 Fig. 3. Material returned in centrate... 7 Fig. 4. Form for calculating dewatering efficiency... 8 Fig. 5. Dewatering costs, by interval... 9 Fig. 6. Dewatering system equipment. ... 10
1 Introduction
The use of on-line closed loop circulating systems to achieve drilling waste minimization is gaining popularity both in the domestic U.S. market and in other areas around the world. The recent introduction of dewatering devices to further close the loop of drilling fluid circulating systems and to dewater reserve pits is derived from technology used in the industrial and sanitary waste treatment industries.
The optimization of solids control equipment has been of primary concern to the drilling industry for many years. However, the emphasis in the past has been to utilize the solids control equipment to help optimize mud properties in order to control such variables as solids content, solids distribution, rheology, and fluid loss control. These properties affect important drilling parameters such as rate of penetration, stuck pipe, borehole stability, formation damage, and drilling costs. Because these objectives did not include entirely closing the circulating loop, significant volumes of liquid drilling wastes were generated. The recent advent of more stringent
environmental regulations and the better understanding of the economics of running a 100% efficient closed loop system has resulted in the introduction of dewatering technology to the drilling industry.
The term “closed loop” has been use quite freely in the drilling industry to describe various solids control layouts and drilling practices. In the context of this discussion, a closed loop system is one where all excess mud from either dilution or effluent from conventional solids control equipment is further processed using chemically-enhanced separation technology. This results in all solids being removed from the waste drilling mud while the liquid portion is recycled back to the active system. Ideally, all other liquid wastes generated on location are processed and also recycled. Using this technology often negates the need for a reserve pit.
There are numerous applications for a closed loop dewatering system.
Reasons may include restrictive environmental regulations, small locations where reserve pit space is limited, or locations where water is in short supply. Dewatering units can also be used in applications that do not require a fully closed loop system. The application where the primary desire is to recycle valuable chemicals or centrate has just recently been explored. This application may or may not require the fully closed loop system.
The options are limited for an operator faced with a zero discharge or reduced discharge scenario. A simple solution still widely used today is to haul off all cuttings and waste fluids to an offsite disposal facility. This can be expensive and there could be costs involving future liability if the disposal site is later declared a hazardous area. In certain areas the cuttings and waste fluids can be spread on nearby land. This can be a cheaper option but availability, meeting environmental specifications, and long-term liability can be a problem. Pumping waste fluid back down into the formation is sometimes used, but possible contamination of groundwater worries some regulators. Whatever method is used to dispose of drilling wastes, using good waste management techniques will usually result in substantial cost reductions. Savings of up to 50% have been realized on disposal and reclamation costs as well as reduced drilling days by operators using sound waste management practices.
The use of chemically enhanced dewatering devices is proving to be a reliable method of reducing wastes generated at the rig site. Several dewatering devices have been investigated as possible candidates for oilfield application, including a belt press, horizontal belt/vacuum filter, a vertical screwpress, and decanting bowl centrifuges. Further detailed studies of using chemically enhanced dewatering to increase the solids control efficiency in drilling applications have been documented.
2 Economic Overview
Dewatering flocculation units are practical devices for the control of solids and liquids. They are not, however, cost effective in all situations. Since they are often used as an alternative to disposing of liquid mud, operating the unit in this mode would have to be less expensive than the disposal costs. If an
inexpensive mud is to be discarded as waste on location (with no associated treatment costs), it is unlikely the dewatering unit would be beneficial.
However, if the liquid phase is expensive, or the mud has to be disposed of at a commercial waste disposal site, then the use of the dewatering equipment should be investigated further to prove its feasibility.
Some of the costs that should be considered when determining whether or not the dewatering unit will be cost effective are as follows:
· Disposal Costs: The proper use of the dewatering unit can negate the necessity to dispose of liquid mud until the well is completed. Solids will have to be disposed of in a manner according to local or national government regulations. If the estimated disposal costs without a dewatering unit are higher than the costs associated with the dewatering unit, then the dewatering unit is definitely cost effective.
· Centrate Cost: If the centrate (filtrate) of the liquid water base mud is expensive to formulate (i.e., saturated brine, glycol, etc.), then recovering the liquid could be extremely beneficial and cost effective.
· Solids Control Equipment: The efficiency of the overall solids removal equipment will increase considerably with the use of a dewatering system. The dewatering unit will remove almost all of the insoluble solids and very little of the dissolved solids. Other than makeup volume, usually no additional dilution (that would otherwise be needed without the use of the dewatering system), will be required unless lost circulation occurs.
· Location Costs: The use of the dewatering unit will allow smaller reserve pits to be built, thereby decreasing overall location costs. Since no liquid will be discarded, reserve pits can be constructed to accommodate only solid material. Often reserve pits can be eliminated completely if solids can be immediately spread on the land or taken off site for disposal.
To determine the cost effectiveness of using a dewatering closed loop system, follow this logical order when calculating the economics:
First, look at the costs that would be incurred if a dewatering unit was not used:
1. Choose the solids control equipment that will be needed and determine the costs that will be incurred. Estimate the overall efficiency of this equipment as this will be needed to determine how much of the drilled solids will be removed.
2. Calculate the total solids per interval that will be generated (including washouts) as a result of the hole drilled. Determine the amount of solids that will be removed with the solids control equipment and the cost of disposing of these solids. Disposal rates at commercial facilities usually do not vary significantly between the mud and cuttings. Transportation rates, however, will differ considerably if road transportation is used.
Keep in mind that the solids generated will not be dry, but rather will contain a significant amount of liquid.
The amount of liquid will usually depend on the size and type of solids generated and can be determined through analysis. For estimation purposes, a reasonable solids-to-liquid ratio is 1:1 or 50% liquid by volume.
3. Calculate the dilution volumes that will be required to maintain the desired drilled solids content. The efficiency of the solids control equipment selected will play a crucial part in determining this number.
Since this volume will have to be disposed of before dilution can be added, use this volume to determine the liquid disposal costs. Disposal rates will usually range from $5.00 to $10.00 per barrel (plus transportation) depending on the type of mud being discarded.
Next, look at the costs of the dewatering, closed loop system:
4. Dewatering system costs include the equipment, personnel, and the chemicals used in the flocculation process. Equipment and personnel costs are relatively fixed, but chemical usage will vary and will be the most difficult to quantify. The chemical costs will depend on the product cost and the concentrations required to achieve the correct flocculated state. Flocculent concentration increases significantly as the solids content of the feed fluid increases, particularly when the measured solids is above 5% by volume. Fig. 1 graphically illustrates this point as the amount of flocculent needed increased from 325 ppm at 4.85%
solids to almost 600 ppm at 5.1% solids to 750 ppm at 5.5% solids.
This demonstrates the need for good solids removal ability upstream of the dewatering unit.
Fig. 1. Effect of solids on flocculent concentration.
Note: Flocculent consumption can increase dramatically as solids concentrations increase.
Different mud systems will also require different flocculating polymer concentrations. Dispersed muds need more flocculent to achieve desired results than do nondispersed. Optimum concentrations of the flocculent are needed to provide the best “floc” for the lowest price.
Since any excess flocculent used will be returned to the mud system, keeping this concentration to a minimum is important. Elevated chemical costs can make the overall dewatering system cost prohibitive.
5. Solids disposal costs will be slightly higher when using a closed loop dewatering system as more solids are removed from the mud. It is assumed that the dewatering unit will be able to remove all the solids necessary to maintain the drilled solids content at desired levels. This assumption is based on the fact that enough solids removal equipment is utilized to help the dewatering unit achieve this goal. If these solids are to be spread on location, add the costs of the spreading. If the solids are to be disposed of at a commercial facility, add the costs of disposal, plus transportation. Assume all liquids not associated with the solids can be recycled back to the mud system or dewatering unit.
6. Recovering a costly centrate can be a definite economic saving. If the mud in use is a basic inexpensive fresh water system and if fresh water is readily available, the liquid phase cost will be minimal. However, if the centrate contains salts, glycols, or expensive polymers, recycling this
liquid must be included in the economics and may be a significant factor in deciding whether or not to use a closed loop system with a dewatering unit. Fig. 2 clearly shows that the amount of material returned in some centrates can be significant. As shown, a considerable amount of polymer, fluid loss control agents, and soluble salts return to the active system in the centrate. As expected, barite, bentonite and low gravity solids are almost totally removed and discarded as waste. Fig. 3 shows an example of the cost of the chemicals salvaged by the dewatering unit versus the cost of the mud in use. As can be seen, a substantial portion of the mud makeup cost can be returned.
Fig. 2. Evaluation of dewatering centrate.
Note: The amount of valuable material returned in the centrate can be significant.
Fig. 3. Material returned in centrate.
Note: The value of the centrate must be considered when estimating dewatering economics.
7. Subtract the portion of the location costs that would not otherwise be incurred if the closed loop system were not applied. This will normally include the preparation of the reserve pit system, larger location, location clean-up and backfill of pits.
After all calculations are completed, compare the costs of having a dewatering system versus not having one, and decide if a dewatering closed system is economically warranted. These figures may be crude at first, but with more precise data and increased experience, the values will become more accurate.
If the cost per barrel of dewatering is less than the cost per barrel of disposal, it is obviously economical to proceed in this direction. The spreadsheet program DEWATER has been provided to assist in making these calculations. Refer to Appendix A, Solids Control Programs.
3 Monitoring Dewatering Costs and Efficiency
If it is decided that a dewatering system is warranted, monitoring the cost efficiency on a daily basis is imperative. To approach this, equate all costs associated with the dewatering unit to a “dollar ($) per barrel of mud processed” figure. By equating all costs to $/barrel, comparisons against disposal costs can easily be made. Fig. 4 is a sample form that can be used to keep track of these expenses as well as the mud volumes processed. The
contributing factors in determining overall cost efficiency are: a) dewatering equipment, personnel and chemical costs, and b) volume of liquid processed. The centrate returned may contribute to the cost savings as well and should be determined by multiplying the centrate value times the volume returned.
Fig. 4. Form for calculating dewatering efficiency.
As hole size and process volumes decrease, the cost of dewatering ($/bbl) increases. At some stage it may become evident that the dewatering cost will be greater than disposal costs. Fig. 5 shows the interval cost per barrel of a dewatering operation that lasts through five intervals. Note that the cost usually increases with each subsequent interval. Hole sizes are smaller and therefore circulating volumes are less. At the point where the cost per barrel approaches the cost of disposal, a decision will have to be made to either remove the dewatering equipment, or treat the mud on a “batch” basis. In this example, that point is reached at the end of interval #3. Continual processing of mud in interval #4 is more costly than disposing of the liquid volume. Two options are available: (1) Cease dewatering operations, or (2) place the unit on standby until a sufficient volume is accumulated to warrant the operating cost to dewater. As stated before, the $/bbl efficiency of dewatering can be decreased either by lowering the costs ($), or increasing the processed volume (bbl). The economics of maintaining the unit on standby will depend on the standby rate and anticipated frequency of use.
Fig. 5. Dewatering costs, by interval.
Note: Intervals 4 and 5 are uneconomic to dewater in this example since the liquid disposal cost is less.