DEWATERING CUTS DRILLING MUD AND DISPOSAL COSTS

Gary West
Profco Inc.
Houston

Bob Pharis
Profco Inc.
New Orleans

Rig site dewatering of drilling fluids with recycling of processed water can help an operator to comply with environmental rules by reducing volumes of waste and reducing long term liabilities.

It can also reduce disposal costs and provide a cleaner drill site overall.

Rig site dewatering is the process of injecting coagulants or flocculating chemicals into the mud entering a large clarifying centrifuge. This coagulates the fine, drilled particles allowing them to be separated from the fluid which can then be handled separately. Fig. 1 shows a typical dewatering setup.

Most of the environmental concerns during the 1980s involved hazardous materials and toxic wastes. Drilling fluids, many of which are chemically benign, have escaped many of the difficult-to-comply-with rules and regulations. During the 1990s, however, operators may be required to submit a written plan for liquid waste reduction for even nonhazardous materials. Many states and local agencies may institute total bans on oil field wastes. Drilling rigs typically produce about 1 bbl of liquid waste for every 1 ft of hole drilled. Thus, a typical drilling operation can produce a large quantity of waste.

DEWATERING

Dewatering the mud can improve penetration rates, improve wall cake integrity and compressibility, reduce water and chemical use, and reduces overall well costs.

The mud dewatering process involves separating solids from the active mud system.

The dewatering engineer selects the relative dryness of the solids ejected and the solids content of the processed stream (effluent) returned to the active system.

The rig site dewatering system should have the flexibility to recycle either clean water or dirty water back into the mud system. Fig. 2 is a flow diagram of a typical dewatering setup. The setup includes the following:

653 DDS centrifuge--up to 200 gpm (69-in. long barrel x 20-in. diameter)
Neutralizing tank--400-bbl tank, centrifuge pump, and hopper
Injection unit--four pumps, manifold systems, and metering devices
Quality control tank--40-bbl tank with various transfer pumps.

The process is for a typical unweighted or low weight mud with zero liquid haul off. The following steps correspond to the numbers in Fig. 2:

Mud from the active system is pumped into either a separate tank or directly into an injection manifold. This mud is tested by the dewatering engineer immediately prior to processing. Through a series of standard tests, the operator determines the approximate amount and type of chemicals to give an optimum flocculation/ stabilization of the mud.
The injection manifold has a series of injection ports and internal static mixers. The calculated amounts of chemicals are metered into the ports with chemical injection pumps. These pumps have variable controls to continuously adjust the amount of chemical injected.
The treated mud then goes to a high-speed, high G-force clarifying centrifuge. The clarifying centrifuge separates the flocculated solids from the fluid. The amount of solids separated is determined by an economic analysis of the drilling operation to determine if clean water or dirty Water is to be returned to the active mud system.
The quality control tank allows the unit operator to monitor the effluent from the centrifuge returned to the active mud or storage system. The operator can then make further adjustments to the chemicals and equipment to ensure optimum dewatering. A bypass allows the operator to recirculate the centrifuge effluent if necessary.
The neutralizing tank may be used to store dewatered fluid and to make final treatments to the processed fluid, such as pH adjustments, before the fluid is recycled as makeup or dilution water. This tank is partitioned and can be used for prehydrating bentonite, for mixing chemicals, or for storing excess mud.

For weighted muds the dewatering unit is rigged up to draw from the effluent of a barite recovery centrifuge. A schematic of a weighted dewatering setup is shown in Fig. 3.

Running a scavenging centrifuge or a barite recovery centrifuge is a fairly simple operation. Usually, the equipment operator only needs to watch the settings on the centrifuge and use proper maintenance. However, a dewatering operation with a clarifying centrifuge can be very complex. Here are some of the operational variables that must be considered:

Polymer flocculent concentration
Polymer injection rate
Coagulant injection rate
Stages of chemical additions
Feed slurry rate to the centrifuge
Solids concentration of the feed
Distance between injection manifold and the centrifuge
Degree of feed agitation
Temperature and pH of the feed slurry
Centrifuge type and G-force
Pool depth in the centrifuge
Bowl speed/scroll speed differential.

CENTRIFUGES

An integral part of a complete solids control system is the centrifuge, often the final piece of solids control equipment. Centrifuges have been used extensively to remove ultrafine-size solids. A well-designed system of solids control, dewatering, or recycling should have a proper centrifuge.

The decanting centrifuge was introduced into the drilling industry in the 1960s as a means to control mud costs, primarily in weighted systems. Three basic types of decanting centrifuges are widely used. Table 1 shows a basic comparison of the types of centrifuges used in drilling operations.

Barite recovery centrifuges were the first centrifuges used in the 1960s. These are used primarily to reclaim barite from weighted muds and are typically short machines with 14-16 in. diameters. Barite recovery centrifuges are low volume machines that process about 20-50 gpm of mud. Because they are used to reclaim barite, a very heavy material with a specific gravity of approximately 4.2, the bowls are turned at a slow speed. Thus, these units produce a lower G-force, typically about 650, than the other types of centrifuges.

Classifying centrifuges are larger centrifuges that are 14-18 in. in diameter and 42-50 in. long. They were first used in the late 1970s to discard low gravity drill solids from unweighted mud (below 10.0 ppg). They can process 4080 gpm of mud.

Most closed-loop systems use one barite recovery centrifuge and one classifying centrifuge. A classifying centrifuge has a gear ratio that allows faster rotation of the bowl to create G-forces around 900. A higher G-force is needed to remove the lower specific gravity drill solids (about 2.6 sp gr).

Clarifying centrifuges are the newest generation of large centrifuges with diameters around 18-24 in. and lengths of 60-70 in. They were introduced into the drilling industry in the late 1980s in conjunction with dewatering equipment. Large clarifying centrifuges are capable of handling 100-200 gpm of effluent from a barite recovery centrifuge or whole mud weighing less than 10.0 ppg. When the clarifying centrifuge is used in a dewatering operation, flocculating chemicals are injected into the feed line of the centrifuge and clean, reusable water is produced. These centrifuges run at high speeds creating G-forces from 1,400 to 1,800.

A centrifuge consists of 6 major components:

Bowl
Drive motor
Conveyor
Feed pump
Gear box
Skid.
Centrifuges typically have four adjustments:
Feed rate--This affects the torque load and solids retention time.
Pool depth--The liquid depth inside the bowl affects the cut-point and dryness of the discharged solids.
G-force--The higher the G-force, the finer the cut
Differential speed--The difference between bowl and conveyor speed of rotation influences the solids load tolerance and solids dryness.

Centrifuges are very technical pieces of machinery that must be manufactured to exact tolerances. As such, they are expensive. Typical costs for new centrifuges are given in Table 1.

The larger, high-volume centrifuges offer a significant savings on environmentally sensitive wells. These centrifuges are solids control equipment that also help reduce liquid mud disposal costs and drilling fluid costs.

SURFACE AREA

Most mud engineers are aware that surface area--not solids content--determines the amount of water and chemical dispersant required in a mud.

For instance, 1 lb of 5 mu glass beads has a surface area of 234.5 sq ft. The same beads crushed to 0.003 mu have a surface area of 2,930,000 sq ft (about 60 football fields). It takes much more water to cover the surface area of the 0.003 mu beads than the 5 mu beads. Water consumption is directly related to surface area.

In a typical closed-loop system, one usually only sees the equipment actually involved in removing solids. These systems also may contain as many as six centrifugal pumps breaking up solids particles into finer sizes. This equipment, no doubt, increases the surface area of the solids in a mud.

Table 2 is an example showing the possible effects on a 1-bbl volume of mud after completely passing through a closed-loop system. Although the solids content is reduced, the surface area is increased 66%. As the surface area increases, water and chemicals must be added regardless of the total solids content of the mud.

Fine solids (less than 5 mu) can cause a number of problems:

Produce high surface areas, requiring dilution and chemical additions
Increase mud costs
Cause high, progressive gel strengths
Cause thick and sticky filter cakes
Slow drill rates
Increase total disposal and well costs.

A centrifuge will return most particles smaller than 5 mu to the active mud system. Thus, it is necessary to chemically hook together these small particles such that they are large enough to be removed by the centrifuge.

Typically, the engineer uses either coagulants or flocculating chemicals. For optimum, economic solids removal, experimentation is needed to determine the appropriate amount and type of chemicals. In addition, it has been determined through experimentation that larger centrifuges (bowl length 48 in. or longer) in combination with chemical injection can remove all the solids from a mud to produce clear water.

This combination of chemical injection and larger centrifuges is dewatering. The dewatering unit works to remove solids that a conventional solids control system or closed-loop system can not.

FLOCCULATION

There are three main steps involved in the development of a flocculated mud (Fig. 4). The first diagram in Fig. 4 shows finely divided particles suspended in water. Each particle has an electrical charge that repels the like-charged particles surrounding it. Based on the type and amount of solids and the characteristics of the water phase, there is a certain radius around each particle. In this condition, the particles are extremely small and are difficult to remove with solids control equipment alone. The charge radius must be reduced to allow the particles to flocculate.

The second diagram in Fig. 4 shows the particles after the addition of a coagulating polymer. The repellant charges are neutralized and the individual particles come into contact with each other. The long-chained polymers used result in "tails" extending from the particles. These help in particle bridging as individual pin flocs are formed. The beginning of flocculation has started.

The third diagram in Fig. 4 shows the formation of microflocs as more particles become coagulated. An alternative to the use of a polymer in the coagulation phase is the use of an inorganic material that contains alum or a ferric material to cause charge neutralization.

The last diagram in Fig. 4 shows the formation of large flocs that can settle. The larger the floc, the easier it can be removed. Care must be taken not to agitate these flocs too much because they are subject to being separated back into smaller flocs.

If the dewatering unit is run to return clean water, the selection of the most economical chemical coagulants is determined by how the water will be used after dewatering.

If the water will be recycled into the active mud system, the floc polymer must be treated out or be compatible with the mud system. If the water will be injected into a disposal well, the polymer carry-over must be destroyed to prevent well bore plugging. After choosing an appropriate method for handling the water, the operator can determine the most cost-effective and acceptable clarity of the water.

RECYCLING

The process whereby the water is recycled into the active mud system requires the following:

Coordination of drilling mud products and dewatering products with regard to clay chemistry and polymer chemistry.
Communication and cooperation between the mud engineer and dewatering engineer.
An understanding by both the mud engineer and the dewatering engineer of downhole conditions, such as, temperature, salinity, alkalinities, and formations.
Total solids removal from a drilling fluid requires a slight overtreatment with flocculating polymers. If the processed water is to be recycled into the active system, it must be collected in a holding tank and neutralized. Recycled water must be treated so that it has no apparent effect on the drilling fluid properties. The neutralizing agents for processed water include the following:
Oxidizing agents
Encapsulating agents
pH modifiers.

The mud engineer should run a pilot test of the processed water prior to adding it back to the active system.

The technology required to produce water that can be recycled into the active system requires an understanding of the drilling fluid and the dewatering chemicals. The coordination of products used for recycling processed water into a mud must be studied and fixed prior to the spudding of the well. During the planning stages, the following should be determined:

Solids control equipment design and coordination
Mud type and mud weights
Method of recycling for the processed water (into the active mud system, down the backside of the casing, or by other means of disposal)
Compatibility of dewatering chemicals with active mud system
Experience level of mud engineer and dewatering engineer
Liquid storage requirements (for displaced mud while running and cementing casing)
Location (land, offshore, or inland waters)
Method of handling dry solids.

Proper planning, experience, and good rig site communications are essential for economical and safe recycling of processed water into the active mud system.