invert fluid flocculation –a novel technique for drilling ... · technique for drilling fluid...

INVERT FLUID FLOCCULATION – A NovelTechnique for Drilling Fluid Recycling

Karen McCosh, M-I SWACO

ABSTRACTSolids control equipment at the rig site is used to remove rock cuttings or low-gravity

solids (LGS) from drilling fluids to allow the fluid to return to the active system in a re-usablecondition. Typical equipment, such as shakers and centrifuges, remove LGS particles greaterthan 5-7 microns. Build up of finer solids in the fluid can create problems for efficient drillingsuch as drillpipe sticking, increased drillpipe torque and high viscosity issues. Ultimately thedrilling fluid must be diluted and re-formulated to reduce the concentration of LGS and returnthe fluid to acceptable physical properties. In severe cases, where dilution is uneconomical,the drilling fluid will be disposed as waste.

Techniques to remove fine solids from drilling fluids have significant benefits interms of fluids recycling, inventory control and reducing disposal volume as well asimproving drilling performance. With water-based drilling fluids, flocculation of the solidsand their subsequent removal using mechanical equipment is commonplace. However, forinvert fluids (water-in-oil emulsions) removal of the fine solids has previously beenproblematic.

This paper presents a novel technology for recycling invert fluids with recovery of there-usable components. Polymeric inverse emulsions and surfactants coupled with definedshear and mixing systems cause the fine solids within the invert drilling fluid to flocculate sothey can be removed using standard oilfield centrifuges. The result is the reclamation of thehydrocarbon component with no fine contaminant solids. This component can then be used asa base for new drilling fluids and therefore the treatment provides a unique method for invertfluid recycling.

INVERT FLUID FLOCCULATION – A Novel Technique for Drilling Fluid RecyclingKaren McCosh

INTRODUCTIONDrilling fluids are used in the oil and gas industry for the drilling of bore holes and

construction of oil and gas wells. The drilling fluid flows through the centre of the drillstring,out through nozzles in the cutting bit and then back up the annulus to surface (1) (Fig. 1).These fluids fulfil a wide variety of functions in drilling operations, including maintainingpressure in the formation rocks and helping to protect and support the borehole wall,preventing collapse. They are also designed to protect permeable zones from damage whiledrilling, increasing rates of hydrocarbon recovery. Drilling fluids also help to cool andlubricate the drill bit and drillstring and are essential for removing the excavated rock or “drillcuttings” from the borehole (1).

Drilling fluids are typically classified as either water- or oil-based/invert emulsions.Water-based fluids contain clays, weighting agents and other speciality chemicals in theaqueous phase, whereas oil-based fluids contain a hydrocarbon continuous phase with anemulsified internal aqueous “brine” phase in addition to clays, weighting agents and otheradditives (1) (Fig. 2). Typically, the types of solids present in the fluid are characterised bytheir reactivity, but also by size (Table 1) and play a major role in the physical properties ofthe fluid such as density, viscosity and filtration characteristics in addition to othermechanical and chemical properties (2). These properties are carefully determined andmonitored during drilling operations as they directly affect mud and well costs includingfactors such as drilling rate, hydraulics, dilution rate, torque and drag, surge and swabpressures, differential sticking, lost circulation, hole stability and bit balling (2). Some solidsare added to the drilling mud intentionally, such as weighting materials and clays to achievevarious desirable properties. Drill solids, however, consisting of rock and clays areincorporated into the mud continuously and become less tolerable with increasing muddensity (2). Above 2-5% volume percent, the drilled solids can generate excessive low-shear-rate and high-shear-rate viscosities, greatly reducing drilling rates, and increasing thelikelihood of the drillpipe sticking and increased torque and drag (2).

In order to remove the drilled solids but leave desired solids such as weightingmaterial in the drilling fluid, a series of solids control equipment typically is used at the rigsite. This includes shakers which are essentially vibrating screens that remove cuttings andother large solids from the drilling fluid and decanting centrifuges, which are mechanicaldevices that use centrifugal force to separate materials of different mass. The centrifugationstep is particularly important to enhance removal of finer solids from drilling fluids and isalso important in the flocculation technology presented in this paper. Solids separationperformance is often described using cut points – the size at which a stipulated percentage(usually 50%) of the feed solids is separated. Under ideal conditions many centrifuges arecapable of achieving a D50 cut point of 2 m, although actual cut points can be expected to besignificantly higher, typically 5-7 m (2).

As drilling fluids are used to drill multiple holes, the particles unable to be removedby the solids control equipment build up and ultimately the concentration of fine solidsdetrimentally affects the drilling fluid properties and performance. Traditionally, dilution ofthe drilling fluid with the base fluid/oil and reformulation back to the required chemicalconcentrations and properties is required to recycle and re-use these fluids. This is often verycostly and also increases the volume of drilling fluid, leading to problems of storage, transportand disposal. In severe cases, the whole mud volume is disposed and fresh fluid mixed atsignificant cost.

The environmental impact of continued disposal of large volumes of drilling fluidmust be addressed and the disposal volume reduced. This paper presents a novel technologythat provides a unique method for invert fluid recycling and the reduction of waste disposed.


Polymeric inverse emulsions and surfactants coupled with defined shear and mixing systemscause the fine solids within the invert drilling fluid to flocculate so they can be removed usingstandard oilfield centrifuges. The result is the reclamation of the hydrocarbon component withno fine contaminant solids and this fluid can be used as base for new drilling fluids. The paperpresents a description of the technology principles, summarises the environmental benefitscompared to current oilfield practices and gives examples of the technology performance.

PRINCIPLES OF INVERT FLUIDFLOCCULATION

Flocculation of drilling fluids is commonplace where chemicals are used to coagulatethen flocculate solids in water-based systems which can then be removed using mechanicalequipment such as centrifuges. However, for invert fluids, removal of the fine solids haspreviously been problematic, due to the solids being held in an oil-continuous phase and thepresence of water-in-oil emulsions. This paper presents technology which, through theapplication of novel chemicals and equipment, the solid-liquid separation of invert fluids isenhanced such that high concentrations of colloidal drilled solids present in the drilling fluidare removed and a re-usable fluid is produced.

The flocculation process (Fig. 3) can be utilized both on active drilling projects toenhance solids control equipment efficiency as well as at storage facilities and wastetreatment plants for reconditioning drilling fluid returned from field operations. The first stageof the process is the chemical treatment of the drilling fluid (patent pending to CytecIndustries Inc.). A polymeric inverse emulsion is injected in-line. This product is tailored foreach drilling fluid system in that the carrier oil can be customized to suit the drilling fluidbeing treated to ensure there are no compatibility issues. The polymeric inverse emulsion isoften diluted further in compatible oil to varying degrees, depending on the treatment stream,and this serves to reduce both the viscosity of both the flocculation chemical and the drillingfluid to enhance the dispersal and mixing process. The flocculation process is driven byadhesion of the polymer particles to multiple particles of solids suspended in the invert fluid,creating larger aggregates and enhancing the solid-liquid separation using a decantingcentrifuge.The process can be qualitatively understood in terms of wettability differences andas such a demulsifying surfactant and water may be required to weaken the water-in-oilemulsion present in the drilling fluid and water-wet the solids. In these cases the waterdroplets will be demulsified and also removed from the drilling fluid during centrifugation.The exact chemical treatment regime is determined for each drilling fluid system andemulsion type through a detailed series of laboratory screening tests and a processoptimisation phase at the start up of any new operation. The operating concentration for theproducts is quite wide and the overdose concentration typically is at least twice that of theefficacious dose.

In order to derive the most benefit from the products, it is necessary that they be wellmixed into the drilling fluid at an effective dose. The chemical dose can be controlled eitherby the ratio of flow rates between chemical injection pumps and the drilling fluid pump orusing flow meters installed in-line. Efficient mixing ensures that the flocculation polymer isin intimate contact with all the solids present in the drilling fluid thereby preventing over-treatment and ensuring full reaction. A chemical injection and mixing package has beendesigned and optimised for the viscosity of the chemical products and the drilling fluid toensure a homogeneous mixture at a constant ratio of polymer/mud solids. The mixingpackage employs a series of static and dynamic mixing elements. Static mixers are geometricelement(s) fixed within a pipe and use the energy of the flow stream to create mixing. Themixing unit utilises a variety of static mixer designs to ensure that surfactant and polymer are


adequately dispersed within the drilling fluid. The dynamic mixer operates by drawing thefluid into a chamber fitted with a bladed rotor. Centrifugal force drives the fluid to the outsideof the chamber where the fluid is subjected to a milling action between the ends of the rotorblades and the chamber wall. Afterwards, these fluids typically are expelled from the mixerthrough perforations.

Once the mud has been chemically treated and the solids flocculated, it is passedthrough a decanter centrifuge. These types of centrifuges typically operate on the principlethat, as drilling fluid enters and is passed through a rapidly rotating bowl, centrifugal forcesmove the heavier particles to the bowl wall, where they are scraped toward discharge ports bya concentric auger, which rotates at a slightly slower rate than the bowl (2) (Fig. 4). Thechoice of centrifuge is important in reducing the chemical consumption and increasing theseparation efficiency of the flocculated solids. The greater the centrifugal force and the longerthe residence time within the centrifuge the more efficient the process. The centrifugal forcewill be determined by the size of the centrifuge bowl and also the speed of rotation. Theresidence time can be controlled by the length of the centrifuge bowl and also the flow rate ofthe fluid entering the centrifuge.

During the centrifuge process, the flocculated solids present in the drilling fluids areremoved and the treated overflow from the centrifuge recovered. Figure 5 shows an exampleof typical solids expelled from a centrifuge and a picture of the solids once the flocculationchemical package has reacted with the solids and water present. As shown, the underflow hasbecome bulky and dry, with an elastic quality and this shows that the flocculation process isworking effectively.

THE FLOCCULATION EQUIPMENT PACKAGEThe process of chemical injection and mixing can be formalised as a standard

package offering, on a single-self contained skid (Fig. 6), which is easy to install on location.This package has been designed so that all the necessary equipment, piping and controls arein one location and easily accessible. The unit contains appropriately sized pumps for thedrilling fluid, flocculation polymers, base oil, surfactant and water so the fluid is pumped intothe skid, treated, mixed with static and dynamic mixers and pumped to the centrifuge. Inaddition, for ease of use the unit can offer fixed and removable chemical storage facilities anda storage tank for centrifuge effluent to allow sampling and process monitoring.

All drives for the pumps, the dynamic mixer and flow meters are controlled fromcentralised control-panels. The control panels are certified according to National ElectricalManufacturers Association (NEMA) standards and are available as NEMA 4 dust proofenclosures or NEMA 7 enclosures which are water, dust and hazardous atmosphere certified.

A compact unit, half the size of the full skid can also be provided for hazardous areaapplications such as at the rig site. This unit essentially operates in the same manner, howevercertain elements such as the storage facilities have been omitted to reduce the size and allowfor potential installation in situations where space is restricted. It is CE marked, which meansthat the product complies with the Canadian Electric (CE) Code. The CE mark shows that theproduct has been manufactured according to a certain design and procedure and acts like apassport that allows a product to be installed anywhere in the EU. The unit is also ATEXcertified. ATEX is an acronym of the French “Atmospheriques Explosives” and is a EuropeanDirective so that the equipment can be used in potentially explosive atmospheres. Thecompact unit is certified as suitable for use in Category 2, Zone 1 (Gas) areas, and therefore issuitable for use at the rig site, onshore and offshore.


In all, both the full scale skid and the compact unit provides standardised equipmentand the designed chemical injection and mixing package to ensure that the process of invertfluid flocculation is achieved successfully, consistently and efficiently.

The flocculation equipment package can be operated in several ways (Fig. 7). It canbe used as a stand-alone unit feeding the centrifuge and the treated fluid either being returnedto the active mud stock or into storage for later use. An additional decanter centrifugedesigned to remove high gravity solids (intentionally added weight material used to increasefluid density) can be installed up-stream of the flocculation unit. This would allow thevaluable solids to be removed from the drilling fluid and only the residual colloidal solids tobe flocculated. This reduces the chemical consumption and loading on the flocculationcentrifuge and reduces waste as the recovered weighting material can be recycled and addedback into the fluid at a later stage. Another variation of the process flow is where dilution ofwith clean oil is required for viscosity reduction. If the treated fluid has high oil content, thiscan be cycled back into the process and injected into the mud instead of the clean oil diluent.This will therefore reduce the cost and volume requirements of new product.

ENVIRONMENTAL BENEFITS OFFLOCCULATION TECHNOLOGY

The waste hierarchy classifies waste management strategies according to theirdesirability. The waste hierarchy has taken many forms but the basic concept has remainedthe cornerstone of most waste management strategies. The aim of the waste hierarchy is toextract the maximum practical benefits from products and to generate the minimum amountof waste. The traditional approach is termed the 3R’s – Reduce, Reuse, and Recycle. Theenvironmental benefits of the flocculation technology can be described under all threestrategies.

In the drilling fluid life cycle, equipment that can efficiently remove drill solids fromthe fluid, whilst returning as much of the fluid to a re-usable condition means drilling fluidsdo not need to be disposed of and can be used several times. However, the limitations of thistype of equipment eventually results in a build up of fine drilled solids in the fluid, which canreach high enough concentrations as to detrimentally affect the physical properties of thefluid. The flocculation technology can be used at the rig site as a fluid management techniqueto increase the efficiency of the total solids removal equipment package available and preventthe formation of un-usable stock. This will extend the life-span of the fluid and will decreasethe need for dilution to control solids build up. The added benefit is reduction in fluid volumeto be transported at the end of drilling operations to storage facilities with the associateddecrease in chances of spills etc.

Drilling fluid returned from the rig site is stored in local or centralised facilities. If notreatment options are available, the drilling fluid has to be disposed of as waste. If sufficientstorage space and chemicals are available, this mud may be diluted and treated back to theoriginal properties. This is not only a costly exercise but generates excess volume that has tobe stored for eventual use or disposal. The flocculation technology offers a system to treat theun-usable inventory, without the need for excessive dilution and increased volume, such thatfine solids (and water) are removed and the fluid returned to acceptable properties. Thetechnology can be used to recondition the fluid so it can be re-used in the next drillingoperation or used to recover valuable components of the fluid for re-use in building newstock.


This novel technology reduces the drilling fluid waste stream from 100% (if thewhole fluid is disposed) to approximately 30% of the fluid volume. The waste stream fromthis technology is the flocculated solids that are removed from the centrifugation step andmust be disposed of in an environmentally acceptable manner. The flocculation polymerswhich bind firmly to the solids in the system do not dissolve in the liquid oil phase so theycan be removed with the solids from the centrifuge. Analysis of the recovered fluid has shownless than 0.1% polymer (detection limit of the equipment) is retained in the centrifugeeffluent. The surfactant has a high HLB value (Hydrophobic-Lipophilic Balance) andtherefore will partition mainly in the water phase of the mud, which also is removed with thesolids in the centrifuge underflow. Laboratory testing of recovered fluids after thisflocculation process has shown that fully formulated drilling fluids can be mixed withdesirable properties, thereby showing that the concentrations of polymer and surfactantretained in the fluid are negligible and not detrimental to the re-use of the fluid.

FLOCCULATION TREATMENT PERFORMANCE

As the flocculation technology is a chemically enhanced solids-removal process,capable of eliminating the majority of fine solids from invert drilling fluids, and/or decreasingthe quantity of water present, the result after treatment is a fluid with decreased density,increased relative oil content and no drill solids. This treated drilling fluid will be a higherperformance drilling fluid that can increase the drilling rate and reduce non-productive timethat results from drilling problems such as the drill pipe becoming stuck.

The chemical and equipment package performance has been proven in severallocations worldwide on various types of invert fluids. Throughout the world, invert fluidsvary in the type of oil used – from diesel to low-aromatic-content mineral oils to syntheticoils. The emulsifier package used to produce the invert emulsion (water-in-oil) present inthese fluids also varies. Through testing on fluids that comprise a range of oils and emulsifierpackages, it has been proven that the flocculation technology is widely applicable. For eachnew system, there is an optimisation phase to ensure maximum performance and the processdesign can be varied depending on the treatment criteria, performance and environmentaldrivers. The system can be operated to remove the fine solids to produce an intact drillingfluid, or in cases where volume reduction is important or only the oil component is needed,the chemical dosage can be increased with the chemicals removing solids and water toproduce a clean centrifuge effluent that has a low density and a very high (98-99%) oilcontent, which can be reused and recycled. Tests have shown that this effluent also containsvaluable components of the original drilling fluid, such that less product is required to convertthe effluent to fully formulated drilling fluid compared to using clean base oil.

Performance Optimisation

When the flocculation unit is first installed, a series of standard treatment regimes aretested, where increasing concentrations of polymer and surfactant are added to the drillingfluid and the properties of the centrifuge effluent determined, such as the density, the ratio ofoil to water and the concentration of drilled solids, using standard oilfield techniques (3). Theoptimum flow rate needs to be determined and it is generally the case that the slower the flow,the greater the chemical reaction time and residence time within the centrifuge, aiding bothflocculation and separation. For example, the data presented in Table 2 show that the densityof the centrifuge effluent, when treating a diesel-based drilling fluid decreased from 1.05 to0.91g/cm3 with decreased polymer concentration required, when the flow rate was decreased2.6 times.


Depending on the drilling fluid type and properties, the flocculation process can befurther aided by increasing the degree of dilution with oil, which helps reduce the viscosity ofthe fluid and aids dispersion and mixing of the chemical additives. Dilution also decreases theconcentration of solids enabling the polymer to flocculate the solids more effectively at lowerconcentrations. An additional benefit is seen where base oil is added in quantities equivalentto the volume of waste generated by the process. This enables the original fluid volume to bemaintained. As illustrated in Table 2, at reduced flow rate, increasing the dilution of a diesel-based drilling fluid reduced the concentration of polymer required to achieve an effluent witha low density. Therefore, the cost savings by reducing the chemical consumption is greaterthan the additional cost of the oil required. The additional oil has not been seen to increase theoil content of the centrifuge discharge and therefore there is no increased impact to theenvironment compared to traditional centrifuge operation.

In almost all circumstances the performance of the process can be further improvedby the addition of the surfactant, and for the same fluid as discussed above, lowconcentrations only were necessary to further reduce the centrifuge effluent density from 0.95to 0.89g/m3. In a similar manner the various surfactant products available can be tested inorder to select the optimum product.

Performance Examples

An example of how this technology can be applied was illustrated in the Middle East(Abu Dhabi), where diesel-based drilling fluid was stored with excessive amounts of low-gravity solids or severely contaminated with water, as a result of cementing operations.Dilution with diesel to reduce solids contamination would increase volume considerably andin the past had put significant pressure on the plant capacity, so much so that it restricted thereceipt of fluids from active drilling rigs in the area. In this scenario, large volumes ofcontaminated fluid were being disposed of, either on land or by a process of injection down adisposal well at the rig site. Through a series of optimisation steps using the flocculationtechnology, drilling fluid which was heavily contaminated with drilled solids wassuccessfully treated so the fine solids were removed and the density of the fluid was reducedfrom 1.2 to <0.9 g/cm3. The oil content of the recovered fluid in some cases was as high as98%. This recovered fluid was used in subsequent operations as a dilution fluid instead ofneat diesel and effectively eliminated the high dilution required to recondition the drillingfluids and significantly reduce disposal volume and associated cost.

In an operation northwest of Alberta, Canada, at the foothills of the RockyMountains, mineral oil-based drilling fluid was being used. The driver for treatment of thisfluid was to increase the drilling rate and reduce well costs associated with the drilling fluidby decreasing the density of the fluid from 1.20g/cm3 to 0.95g/cm3. Traditionally, this wouldhave been accomplished by disposal of the full mud volume and then blending a completelynew system. Considerable time and money would be required in addition to the high chemicalconsumption and large disposal volume. Attempts to treat the fluid using conventionaltechniques, including dilution with base oil and centrifugation, only achieved a density of1.02g/cm3. By applying the novel fluid flocculation system as described in this paper,successful treatment of the existing stock to meet the customers’ specifications was achieved.On site, the mud density was reduced to 0.95g/cm3 and eliminated the need to dispose of 7063ft3 of mud per well. Higher levels of chemical treatment achieved even lower mud densities.With an average of 25 wells per year, this flocculation technology would eliminate the needto dispose of 5,000m3 (~176,500ft3) of fluid annually while at the same time saving rig time.


CONCLUSIONS

This paper has presented a new technology to increase the efficiency of traditionaloilfield solids control equipment to remove colloidal fines and water from invert drillingfluids such that the fluid life span can be increased and valuable components recovered andreused. The technology has been shown to employ chemical optimization and effectiveequipment design to flocculate colloidal solids which previously could not be removed andwhich have a deleterious effect on drilling fluid properties and performance. The performancecan be enhanced by using a centrifuge up-stream of the process to remove high gravity solids(fluid weighting agent), which can be added back into the fluid system after treatment,reducing the volume of waste and the cost of treatment. The addition of surfactant causes theemulsion of the drilling fluid to weaken allowing water to also be removed from the system,resulting in a fluid with decreased density, increased oil content as well as reduced colloidalsolids that can be used as a base for new drilling systems.

The benefits of such a system are three fold. The process produces higherperformance fluids with increased life span that permits higher drilling rates and thereforelower well costs. Secondly, the process can be used part of an inventory management systemto reduce the stock of un-usable fluid and reduce storage requirements and costs. It alsoreduces the volume increase, chemical consumption and transportation costs compared totraditional methods of dilution used to reduce solids content. Last, but by no means least, thesystem has associated environmental benefits as the volume of spent or excess fluid that mustbe disposed will be reduced as fluids are re-used and recycled.


TABLES AND FIGURES

Table 1. Size classification of solids2

Category Size (m) Types of ParticlesColloidal <2 Bentonites, clays, ultra-fine drilled solidsSilt 2-74 Barite, silt, fine drilled solidsSand 74-2,000 Sand, drilled solidsGravel >2,000 Drilled solids, gravel, cobble

Table 2. Flocculation Technology Performance on a Diesel based Invert Drilling Fluid* Flow rates and product concentrations are provided as multiples of the lowest value for the data set.

Feed Rate(gpm)

Relative PolymerConcentration*

Relative SurfactantConcentration *

Relative Base OilConcentration*

Effluent Density(g/cm3)

Effect of Flow Rate2.6 4.6 0 1.05 1.051.0 3.1 0 1 0.91

Effect of Base Oil Dilution1.0 3.1 0 1 0.911.0 2.3 0 1.5 0.911.0 1.3 0 2.9 0.90

Effect of Surfactant Addition1.0 1.0 0 2.7 0.951.0 1.0 0.5 2.7 0.89

Figure 1. Drilling fluid flows through the centre of the drillstring, out through the bit nozzlesand then carries the cuttings up the annulus for removal


Figure 2. Generic oil base drilling fluid components (*Others include; organophilic clay,lime & fluid loss control agents)

Figure 3. Invert Fluid Process Flow

Figure 4. Decanter Centrifuge Mode of Operation (6)


Figure 5. Flocculation effect as seen in the centrifuge solids discharge

Figure 6. Flocculation Unit (Full Size)

Figure 7. Flocculation Unit Set-Up Options


REFERENCES CITED

1. Darley, H.C.H. and Gray, G.R., Composition and Properties of Drilling andCompletion Fluids, 5 th ed, Gulf Publishing Company, Houston (1988).

2. ASME Shale Shaker Committee, Drilling fluids Processing Handbook, Elsevier(2005).

3. American Petroleum Institute, Recommended Practice 13B-2: RecommendedPractice for Field Testing of Oil-Based Drilling Fluids, Fourth Edition, API (2005).

invert fluid flocculation –a novel technique for drilling ... · technique for drilling fluid...

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