AbstractMatching reservoir characteristics and completion technologyWith optimum drilling fluid formulation is important to the economics of any field development. In recent years, this need has become even more important with the growing application of open hole completions in high angle or horizontal wellbores that are now commonly drilled and completed with the same drill-in fluid. Openhole completions place several demands on the drilling fluids used in the payzone section. In order to maximize benefits, the properties of the fluid need to be optimised for the conditions prevailing in the reservoir. The drill-in fluid needs a rheology good enough to enable good hole cleaning, lubricity good enough to avoid frictional problems. Good inhibition with respect to imerstitial clays and interceded shales, and very importantly, to be minimally damaging to the permeability of the formation. Other factors to be considered include completion design and proposed payzone clean up procedure.The number of potential variables involved means it is very difficult to design a single fluid to cover all eventualities, but a range of altemative drill-in systems has been developed in which each fluid is designed to cover a limited set of circumstances. Most applications can be covered by at least one of the fluids. This paper will discuss four generic drill-in systems. each of which embodies a difierent approach to

Achieving the desired properties:~ water-based polymer systems which may be based onbrines of varying concentration and which may containwater or acid soluble bridging particleso all-oil systems, covering a wide density rangeI aqueous system based on a complex formed betweenbentonite and mixed metal silicate.0 solids-free water-based polymer systemFormulations, properties and some examples of fieldperformance are presented together with a discussion ofrelative advantages and disadvantages of each system type.The objectives of each fluid type are reviewed against actualfield experience. Factors influencing selection of fluid typeare discussed and guidelines are presented

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>introductionDuring the last decade the practice of completing wells withan openhole deviated or horizontal section has becomeestablished as a means of achieving improved productivity.Indeed the better reservoir draining and productivity possiblefrom horizontal wells has had a major impact on theeconomics of some dewzlopments. In some cases reduction inthe number of platforms required for olfshore fields has beenpossible and fields have been developed which would nothave been viable using other currently available techniques.in parallel with the increased use of operthole technologyhas been the development of specialized drill-in fluids. This

has followed from a recognition that the fluid used to drill theupper hole may not be appropriate for the payzone section;that for the reservoir a fluid should be devised which meetsthe specific characteristics of the reservoir and will beminimally damaging to the permeability. This lastrequirement is particularly important as in openholecompletions there is no perforation step to enablecommunication to be established through arty near well borepermeability damage.There are other advantages besides limitation ofpermeability damage to be obtained from changing to aspecific reservoir drill-in fluid. it means that specific aspectsof the payzone, such as stabilisation of clays and questions ofcompatibility with the connate water, can be addresseddirectly. It is not nwessaty to compromise with the difierentrequirements needed for the upper hole fluid.The main factors which are important for drill in fluidscan be listed as follows:

ERIC DAVIDSON. SPE, BAROIDLTD. SUSAN STEWART. BAROID CORP SPE/IADC 39254Temperature. The fluid needs to be able to withstand fullbottom hole temperature, particularly as at various stagesof the completion operation the fluid will be static forprolonged periods of time.Density. The density of the fluid needs to be formulatedto take into account pore pressure and requirementsregarding borehole stability.Clay stability. The deviated borehole may pass throughbeds of shale or there may be clay minerals in the interiorof the sandstone matrix. The fluid needs to befomtulated to be adequately inhibitive.Good control of fluid loss. This is necessary for tworeasons. First. to avoid excessive loss of filtrate into thefomtation. Secondly. many openhole wells are drilled inunconsolidated sediments and in this case a tightfiltercake (coupled with hydrostatic pressure) is requiredfor borehole stability.Hole cleaning. This is a very important aspect andspecial attention must be paid to the ability of the fluid totranspon cuttings along extended horizontal sections andround the particularly difficult 55° build section. Failureto achieve good hole cleaning can lead to seyeral drillingproblems and might interfere with the subsequentinstallation of any sand exclusion device or liner. Alsodevelopment of cuttings beds can markedly increase the

frictional forces acting on the string creating problems oftorque and drag.Cement. If cement contamination is likely the impacton fluid properties must be considered.Lubricity. This can be a major problem in wells withtortuous trajectories or large lateral extensions. Oflen themeasured depth achieved may be limited by frictionalconsiderations and this can be especially evident if thedrillstring is slid rather than rotated. Friction can beinfluenced by hole cleaning and pressure overbalance buta very important factor is the inherent lubricity of thedrilling fluid. ln the case of waterbased muds it is, ofcourse. common practice to add lubricants to the fluid butoil based fluids have a major advantage in this respect.Minimum damage. There are three main aspects of thisrequirement. First, it is important to ensure that the mudor mud filtrate is compatible with the connate water andhas no detrimental elfect on the wettability of the rocksurfaces. Secondly it is important that the fluid should beformulated in such a way to minimise the leakoff of mudfiltrate into the reservoir. To achieve this objective thebridging particles or fluid loss polymers need to beselected with a view to minimising the invasion of solids,polymers and filtrate into the fonnation. Thirdly it isessential that the filtercake formed should be easilyremoved; either by producing the well or by a suitablesolvent such as water or acid.Completion design and clean up procedure. These twofactors carmot readily be distinguished from each other.For example, if the completion comprises a sandexclusion screert it will be necessary to consider theimplications of attempting to flow the completion/drill influid through the screen. Solids in the fluid have thepotential of causing serious screen blockage and thispotential must be taken into account. Detailed treatmentof this question is outwith the scope of this paper, but itmust be borne in mind that the completion design, cleanup procedure and drill in fluid selection are interrelatedfactors.0 Environmental considerations. Disposal of cuttingswhich might be contaminated with mineral oil orconcentrated brine is a major consideration, and must betaken into account within the context of the proposedoperation.Several excellent reviews have been published dealing withdesign criteria versus reservoir conditions. Ezzat' inparticular highlighted risks of incompatibility between mudfiltrate and fonnation rock and/or connate the fluid. Dearing

and Alia gave an excellent overview of most of the currentlyfavoured systems. whereas Hale ct al 3 and Hodge et al 4concentrated on identifying fluids for specific applications.These papers 3" dealt with actual cases and detailed the stepsfollowed to identify the fluid of choice and the preferredclean-up method for respective specific applications. In thispaper it is intended to review four types of drill-in fluid. Eachtype represents a difierent approach to achieving the desiredproperties and the purpose of this review is to discuss theproperties and limitations of the various systems in order todevise some guidelines for selecting an appropriate system forspecific reservoir conditions.Systems under considerationThe four basic systems to be dismissed are:a) Brine/polymer with bridging particles - viscosityprovided by polymerb) All-oil fluidc) Mixed metal silicate system - viscosity generated byinorganic interactions (i.e. water-based. shear- thinningwith bridging particles and fluid loss polymers)d) Water-based, solids- free non shear thinning polymerbased fluidThese systems will be discussed individually in detail insubsequent sections, but there are certain considerationswhich apply to them all.Design of rheology. As stated above hole cleaning is a veryimportant property, particularly if a highly deviated orhorizontal section is to be drilled. In order to optimiserheological properties considerable use is made of an in-househole cleaning mathematical model 5 .Fluid loss control. In order to obtain the best protectionfrom invasion of filtrate and from the possibility of

SPEIIADC 39264 OPEN HOLE COMPLETlONSI DRILLING FLUID SELECTION 3differential sticking it is preferred to optimise fluid lossproperties under dynamic conditions using Farm 90rheometer. The core used has pore throat dimensions assimilar as possible to those in the reservoir and thefomiulation is adjusted to minimise the Cake Depositionlndex (CD1) and the filtration rate. These parameters areknown to be good indicators of tendency to ditfereutialsticking. This process is utilised to identify the optimumparticle size distribution and fluid loss polymer concentration.For optimum overall perfonnance, of course, it may benecessary to seek a compromise between the respectivecompositions required for the best rheological and fluid loss

performances. In order to achieve maximum flexibility it isnecessary to be able to adjust all of the relevant parametersindividually and this is best done by ensuring all of theingredients are supplied as indiyddual components.In subsequent sections examples are given of typicalforrnulations and properties. but these are for illustrativepurposes only, for each type of fluid the properties can bevaried over a wide range.Water-based - polymer systemThe base formulation can be represented as :BrineViscosifying polymerFluid loss polymerBridging particlesVarious considerations influence the selection of theindividual components and it is useful to review the importantfactors.Fluid loss polymer. For application in this system thepolymer requires to have excellent fluid loss performance andto have no negative impact on the other importantcharacteristics of the formulated fluid. For example. it shouldbe easily removed at the clean-up stage and should haveminimal effect on important rheological parameters.Starch which has been subjected to a particular chemicaland physical modification has been found to perform verywell in this regard.Viscosifying polymer. Within the industry the polymerscommonly used are all of microbiological origin. Welangum and succinoglycan have found specialist applications;xanthan and, to a more limited extent, scleroglucan, are morecommon components of drilling systems. Succinoglycan islimited because of its restricted temperature range‘. Thetransition temperature for this polymer (ie the temperature atwhich the dispersed polymer molecules undergo a transitionfrom an ordered to a disordered configuration withconsequent decrease in viscosity) is dependent on thecomposition of the brine. The maximum is 80°C but inconcentrated calcilun based brines the transition temperatureis below 0“C. Scleroglucan has good compatibility withcalcium rich brines and performs over a wide temperaturerange, but the polymer has two disadvantageous features.First. there is the commonly observed need for heat to achievefull yield by the polymer, and secondly it is diflicult todegrade the polymer with strong acid at temperatures below40°C. A simple example of this feature is provided by sometests involving the dissolution of filtercakes. These weremade from fluids based on either xanthan or scleroglucan(equal quantities) and having the same composition with

regard to modified starch and sized calcium carbonate.Filtercakes formed under identical conditions were immersedin excess 10% hydrochloric acid at about 30°C. Typically,alter two hours the xanthan based filtercake had dissolvedcompletely whereas more than 35% of the scleroglucan basedfiltercake still remained. The point of using acid solublecalcium carbonate as the sized particulate is to create theability to rise acid as a remedial treatment if necessary.Clearly any ingredient which reduces the activity of the acidis disadvantageous. At higher temperatures the reaction isfaster in both cases and the scleroglucan is less likely to causedifiiculty. However in terms of application to lowtemperature wells xanthan is the more applicable polymer.Xanthan has some limitations in brines containing veryhigh concentration of calcium. Transition temperaturedecreases with increasing concentration of calcium salt and attemperatures of around 100°C and above polymers tend todegrade7 Nevertheless, for general purposes the advantagesof the polymer greatly outweigh its disadvantages. Thematerial disperses easily, it gives the desired rheologicalperformance and is readily degraded by acid or oxidisingagents.Bridging particles. The objective is, as far as possible, toform a filtercake on the surface of the borehole wall.Therefore, for any particular application it is necessary tomatch the size distribution of the particles to the pore throatdimensions of the reservoir. Accordingly the sized particlesare available in a variety of size ranges for flexibility in theselection of grades to be used.in general there are two types of particulate commonlyused :Sized water soluble particles (sodium chloride)Sized acid soluble particles ( calcium carbonate- marble)lt is ofien preferred to clean up wells completed withcarbonate based systems without use of acid. In such casesacidisation is only performed if remedial treatment isnecessary.The same considerations with respect to particle sizedistribution apply for each type of solid but the two types ofsystem difi'er profoundly in terms of brine/particleinteractions. As will be made clearer below use of sizedsodium chloride has a major impact on;Composition of brine which can be usedMinimum density of brine which can be used.

4 ERIC DAVIDSON, SPE, BAROID LTD, SUSAN STEWART, BAROID CORP SPE/IADC 39284

Brine. Density is particularly important. It is preferred thatthe density of the whole fluid should be controlled by thebrine rather than solids loading. This is in order to obtaingood rate of penetration and formation of a thin filtercake.There are other important factors involved in brineselection including impact of brine composition on shaleirthibition and brine oompositionlconnate water compatibility.However, a primary point is the solubility of the particulateand it is appropriate to deal with the two cases separately.Sized calcium carbonate. In this case solubility of theparticles in the brine is not a concern. the particles can beregarded as being inert.Sized calcium carbonate is compatible with viater (sg L0)and with brines of other densities. for example:sodium chloride up to 1.2 sgsodium bromide tip to 1.5 sgpotassium chloride up to l.l6 sgpotassium formate up to l.57 sgcaesium chloride up to 1.9 sgcaesium fomiate up to 2.2 sgThat is. the range of possible brine density is from l to 2.2 sg(but of course the practicality of very high densities woulddepend upon whether the caesium salts ever becomecommercially viable) If desired calcium chloride brine canalso be used but this brine does have limitations. First atdensity higher than about 1.29 sg the brine is tooconcentrated to allow the polymers to hydrate properly.secondly the polymers have reduced thcnnal stability andthirdly the calcium ions introduce an increased risk ofincompatibility with connate water. However, particularly incool conditions calcium chloride brines are an acceptable basefor the fluid.If. for shale stabilisation purposes. a high concentration ofpotassium chloride were to be required. any concentrationfrom zero to saturated (372kg/m3) could be used. Also withinsolubility limits brine of any desired sodiumchloride/potassium chloride ratio could be used.Sized sodium chloride. Use of sized sodium chlorideplaces many constraints on the brine. Obviously it is a primerequirement that the brine must always be saturated withrespect to sodium chloride. otherwise some of the particulatesalt will dissolve and the particle size distribution may bedistorted. This means that the brine can never have a densitylower than 1.2 sg (i.e. the density of sanuated sodiumchloride brine). Thus the minimum density possible is alwayshigher than that achievable for sized carbonate systems.Further. the possibility of formulating a sized salt systemwith dense brines based on salts of monovalent cations is also

more restricted than is the case for sized carbonate. Solidsodium chloride is not compatible with potassium forrnate orcaesitun formate brine; in each case sodium formate wouldbe precipitated. Also the option of using dense caesiumchloride brines would not be possible as in this case aprecipitate of mixed chloride ( [Cs,.X (Nal~l;0),JCl ) of variablecomposition would be forrncdg.It is possible to use a brine containing calcium chloride,but it must be saturated with respect to sodium chloride.Furthermore the constraints which were described for thecalcium chloride/carbonate systems would apply in this casealso. The maximum acceptable brine density would beabout l.29sg.The maximum density achievable from a sized sodiumchloride system involves the use of sodium bromide (1.5 sg)as the base brine.As is well known there is also an incompatibility betweensolid sodium chloride and potassium chloride brine. ln thepresence of solid sodium chloride not more than 10%potassium chloride can be dissolved in saturated sodiumchloride brine at ambient temperature. Any attempt todissolve more potassium salt merely results in undissolvedpotassium chloride acciunulating in the solid phase. Asystem comprising sized sodium chloride in potassiumchloride brine would not be possibleThe elfect of cycling temperature, for example circulatingin a well. causes some of the solid salt to be dissolved andreprecipitated. It has been suggested’ that this could causedifficulties due to recrystallisation and crystal growth.However, this is not a problem for systems which containsolely sodium chloride. Crystallisation of sodium chloride ina single salt system can be readily controlled.Properties.Typical fannulatian. The formulations required toproduce the desired properties for any given applicationdepend upon requirements in respect of density, shaleinhibition, water compatibility etc. However, typicalformulations for carbonate and sized sodium chloride systemsare given in Table 1 together with typical rheologicalproperties. In each case minimum density formulations areexemplified.Shale stabilisation. Sized sodium chloride fluids possessa high degree of clay inhibition due to their highconcentration of dissolved salt. This can be further improvedby inclusion of up to 10% potassium chloride and 3% glycol.Sized carbonate systems can use similar brines or if necessaryhigher concentrations of potassium chloride. An example ofthe comparative effects of using saturated sodium chloride

and mixed brine with glycol is shown in Table 2. Theseresults are from Slake Durability Tests and illustrate thebeneficial results of potassium ions and glycol. The resultsshow t.hat inclusion of potassium ions and glycol greatlyreduced the degradation of the shale granules in contact withthe brine. ln the North Sea confimtation of these laboratoryresults have been available from the field. Thousands of feetof shales have been drilled with sized sodium chloride fluidshaving this type of brine phase composition without anymajor shale related ditficulties.If, for shale stablisation reasons it is necessary to use

SPE/IADC 39254 OPEN HOLE COMPLETIONS‘ DRILLING FLUID SELECTION Scalcium chloride brine then this is possible with either of thesized solids within the constraints described above.Density. Both carbonate and sized salt systems can beweighted up to some extent by addition of extra solid, but thepreferred source of the density is the base brine. Forcarbonate systems typical whole fluid minimum densitiesobtainable from usable, saturated brines (on the basis that thefluid contains 114 kg/m’ sized carbonate) are:Base Brine Fluid D€!lSl§[Water l.07 sgSodimn chloride( l.2sg) 1.26 sgPotassirun chloride(l.l6 sg) 1.22 sgSodium brormde(l.5sg) [.55 sgPotassium formate( l .57sg) L62 sgCaesium for-mate{2.2sg) 2.22 sgOf course. densities covering the entire range can be obtainedby diluting the saturated brines or by blending compatiblebrines, eg. sodium chloride and sodium bromide orpotassium formate and caesiinn formate.ln the case of sized salt systems the realistic possibledensity ranges are those which can be obtained with sodiiunchloride or sodium bromide brines, or blends thereof Theminimum densities obtainable (assuming 143 kg/mi of sizedsalt in the fluid) are :Base brine Fluid DensitvSodium chlor-ide( l .2sg) l.26sgSodium bromide(l.5sg) l.54sgA fluid density higher than 1.54 sg can be obtained byincreasing the content of sized salt in the system; the practicalmaximum is a fluid with a density of 1.67 sg and containingsrukg/in’ of sized saltTemperature. The controlling factor for both types offluid is the thermal stability of the polymers. The standard

polymers are polysaccharides and for optimmn perfonnancethese should be used in concentrated brine as this contributesto polymer stability’. Even so, polymer degradation becomesunacceptably rapid at about 100°C, but by the inclusion ofappropriate high pH ingredients in the fluids the temperaturestability can be increased significantly. By this means anupper temperature limit of 145-150°C can be achieved.Use of concentrated fomiate brines, particularly potassiumformate confers even greater thermal stability on thepolymers“). Using saturated potassium formats brine(l.57sg)the thermal stability of the polymers can be extended to aboutl75°C. However, since sized sodium chloride cannot be usedwith potassium fomtate brine this option is only available forsized carbonate systems.In order to obtain high temperature performance beyond175°C or with brines other than fomiates it is necessary to usespecial synthetic high temperature polymers,Compatibility between nrudflkrare and connare fluid. Itis preferred to base botlt types of system on brines made fromsalts containing monovalem cations rather than from calciumsalts. Not only are polymers more compatible with this typeof brine but risk of precipitation due to the formation ofinsoluble compounds is reduced . Most compounds of sodiumand potassium are soluble; this is not the case for alkalineearth metals.Precipitation of soluble salts due to concentration effects canoccur following the intermingling of highly concentratedbrines. For example, sodium chloride can be precipitated ifconcentrated sodium chloride and calcium chloride brines aremixed. Therefore, if there is a risk of mingling of saturatedor near saturated brines it may be preferred to opt for acarbonate based system because with such fluids the requireddensities can often be produced without the use of satiuatedsolutions. (For example, instead of saturated sodium chlorideat l.2sg a dilute solution of sodium bromide at l.2sg could beused),Ceniznr contamination. Being xanthan based the fluidscan be affected badly in the event of significant contaminationby cement. Both carbonate and sized salt systems need to betreated to remove the effect of high pH and calcirun ions.Clean up of filtercake. The optimum clean up procedurevaries according to the situation. In some circumstances it isadequate merely to produce the filtercalte off the formationbut in other circumstances a chemical attack by acid oroxidising agent might be necessary.However, in considering any clean up treatment it isessential to take into account aspects of procedure. Forexample, if a prepacked screen or other type of sand exclusion

device is to be used then appropriate steps must be taken tominimise risk of the screen becoming blocked by particles inthe drill in /completion fluid. Also in order to ensure that anytreatment is as eflective as possible it is important that thedisplacement steps should remove as much adherent fluid aspossible from the filtercake. lfacid or oxidising agent spendson adherent fluid it cannot attack the filtercake.In the case of carbonate based systems commonlysuccessful clean up methods are as follows:Gravel packingflro acidixing). The filtercake is lefl inplace while the gravel is placed: the well is subsequentlyproduced through the filtercake and gravel. _Producing the wzllfno wciifing). In this case the fluid isdisplaced to clear brine before the screens or liner areinstalled, following which the well is brought on.Acidisation aftlre well. This has been a traditionaltreatment for wells drilled with mrbonate based fluids.However, current opinion is that, if possible contact of acidwith the formation and completion steelwork is usually bestavoided. Also, particularly in the case of long horizontalsections in permeable sandstone. complete removal of thefiltercake by acid might be ditficult. If 5-10% l-lCl is appliedit will probably be necessary to use several flushes or tocirculate the acid. As pointed out by Hale’ if conditions arestatic there will probably be insuflicient acid in the well bore

6 ERIC DAVIDSON, SPE. BAROID LTD, SUSAN STEWART, BAROID CORP SPEIIADC 39284to consume all of the carbonate. Also there is an importantpoint which needs to be bome in mind concerning rate ofreaction. At bottom hole temperature conditions acid andcarbonate will react very rapidly. Therefore rapidconsumption of carbonate is likely at the point at which acidis introduced to the well bore. This can lead to localisedclean up, excessive local losses and inability to circulate acidto the rest of the open hole section. If acid is to be usedconsideration should be given to a weak organic acid. foamedl-ICI or some other form of retarded acid.In the case of sized sodium chloride syst.ems similarconsiderations apply. Contrary to superficial expectationsonly rarely can the flltercake be cleaned up solely withnndersaturated brine. This is due to two main causes: firstthe polymer in the filtercake protects the salt from the water,and secondly the filtercake usually contains significantquantities of water insoluble drill solids. In Nonh Seaoperations it tends to be standard practice to attack sized saltfiltercake with the same sort of aggressive reagents as are

used for carbonate systems i.e. 5-15% HCI or oxidisingagent.One useful feature of the sized salt system is that it ispossible to attack the polymer with an aggressive reagent withminimal effect on the sized salt. The principle of thisapproach is to use saturated NaCl brine as the carrier solutionfor the oxidising agent or acid; thus the fluid can attack thepolymer while leaving the salt unafiected. Then havingremoved the polymer the residual salt can be dissolved by awash with undersaturated brine. This approach has givensatisfactory results. but in pemieable fomtations there is therisk of localised leak-ofi" of water into the formation resultingin loss of returns and failure to wash the full length of thesection.Salt based systems do olfer a particular advantage forwater producing or water injection wells. In this case it canbe expected that the injected/produced fluid will tend todissolve any residual particulate.Selection of salt based versus carbonate based systems.Disposal of cuttings requires due attention and may beinfluential in choice of system. Oflshore it is usuallyacceptable to discharge saline material, with some exceptionssuch as restrictions on potassium discharge in the Gulf ofMexico or discharge of saline fluids in brackish conditions.When such restrictions apply. if density requirements permit,a carbonate system could be used with very low salinity brine.This is not an option for sized salt systems. Cuttings disposalfrom sized salt systems always presents a more pollutingpotential than is the case for carbonate systems. This can bevery significant in onshore locations.In terms of density there are many applications for whichthe salt system is not an option. However, there is a densityrange for which both types of lluid could be applied. In thiscase sodium chloride systems offer advantage if use of acidbased remedial treatments will not be acceptable or if thewells to be drilled are for the production or injection of water.Unless such conditions apply the final choice is best guidedby retum penneability studies in the laboratory preferablyusing core from the reservoir.A point to consider when judging the relative merits ofsized salt or sized carbonate systems is the relativeeflectiveness of water and acid in dissolving the respectiveparticles. In this connection it is worth remembering that as ageneral rule the chemical driving forcc between acid andcarbonate is much larger than the chemical forces driving thedissolution of salt in water. This means that over a givenperiod of time there will tend to be more complete dissolutionof carbonate by acid than of salt by water. This is a

significant point particularly if it is desired to rcmovcfiltercake from the lower side of a horizontal hole.Examples.Gravel packing — carbonate systenr A very successfulexample of a carbonate system used in a grave] packingapplication is provided by recent operations by Statoil in theNorwegian sector of the N. Sea. In these cases since therequirement was for a fluid density in the range 1.6 - l.67sgthe fonnulation was based on sodium bromide brine (l .5sg).Pilot holes were drilled with synthetic ester oil based mud andthe water based system was used for the underreaming stage.The screens were installed while t.he fluid was in the hole andthe displacement and gravel packing steps were perfonnedwith the filtercake still in place. No acid was used.production was through the filtercake and gravel pack. Theobjectives of this operation included good hole cleaningavoidance of fluid loss during the drilling and gravel packingoperations, minimal fomiation damage and the ability tobring the well on withmrt acid treatment. All of theseobjectives were met very satisfactorily. Production from thesewells exceeded expectations. A major factor in their successwas that all aspects of the operation were optimised inadvance by careful testing and planning.Producing the well - carbonate system - no acidDevelopment of a gas reservoir in the southem sector of theNorth Sea (by BP) involves the borehole being circulated toclean brine ( density l.l3sg) before the screens are installed.Then the well is brought on without acidisation or otherchemical treatment of the filtercake. The objectives of thisapproach included good hole cleaning and retention of goodfluid loss control afler the hole had been displaced to cleanbrine. All objectives were met. including excellentperforrnance. Produaion was higher than expected.Sized salt sysrenu Several operators including Chevron,BP, and Kerr McGee have relied very heavily on sized salt fordevelopments in the North Sea. The reservoirs involved hada density requirement which fell comfortably in the sized saltrange (eg. 1.25 - 1.35 sg) and which were demanding interms of the need for shale inhibition. The productive sands

SPE/IADC 39284 OPEN HOLE COMPLETlONS DRILLING FLUID SELECTION 7occurred largely as lenses within beds of shale. so drilling inthe shale was unavoidable. A main objective of theseoperations was to maintain borehole stability within the shaleand this was achieved by boosting the inherent inhibitive

properties of saturated soditun chloride brine by additionalpotassium chloride and gl3.."ls (see Table 2). The otherprincipal objectives included good production and avoidanceof damage. In these developments the tiltercake has beendegraded either by 5 - 15% hydrochloric acid or an oxidisingagent Although there has been some variability in the resultsit has been considered that the objectives have been satisfiedwell enough to justify the continued use of the sized saltsystem.BP has used sized salt for drilling the final sections ofwells for water injection and water production. In this casethe objective was to exploit the expected benefit of using asized particulate which would be soluble in theproduced/injected fluid. The clean up stage involved anattack on the filtercakes with 15% hydrochloric acid followedby flow of water. Several hours were required for the well toclean up afler which excellent performance was attained.All-oil systemMany of the common drilling problems erioounterod eg.shale stability, lubricity etc. are addressed very satisfactorilyby use of conventional oil based invert emulsion mud.However, regular invert emulsion includes powerfulsurfactants to ensure that the mineral components of thesystem (e.g. cuttings. well bore and weighting/bridgingmineral particles) remain wetted by the oil phase rather thanby the dispersed aqueous component. lf the powerfulsurfactants invade the fonnation they can cause the reservoirrock which is normally water wet to become oil wet resultingin a large decrease in the permeability of the reservoir to oil ". This elfect can seriously impair productivity. A probableexample of this phenomenon was described by Dearing andAli: They summarised a development which involved theuse of a conventional oil-based invert fluid. Five wells weredrilled with this lluid and in all cases the productivity indexwas poor. Remedial treatment with acid did not achievedesired results. For subsequent wells oil-based mud wasabandoned; sized salt was used and the perfonnance wasgreatly improvedUse of an all-oil fluid offers a means of using an oil basefluid but avoiding wettability problems. Since allcil fluidscontain no aqueous phase they do not need to containpowerful oil wetting surfactants. To deal with ingress ofwater a passive emulsification system can be used. ln thepresence of water such an emulsifier is automaticallyactivated, but only by enough to emulsify the quantity ofwater involved. Surplus emulsifier remains in a non activecondition and since it is a weak surfactant it leaves the rock ina water wet condition.

The wettability of this fluid system can be demonstratedby tests involving oil/water contact angles and the Arnottwettability test procedure‘. For example, in the case ofoontact angle measurements the tests were done on quartz asthis is the predominant mineral in most reservoirs. Cleanquartz was flooded with 2% sodium chloride solution and adroplet of filtrate from the fluid in quesnon was placed on thequartz surface. Then the angle at the droplet/quartz interfacewas determined. The angle observed is a measure ofwettability. for example:0 if surface is totally water wet the oily filtrate does not wetthe surface and contact angle is 0°.0 if surface has neutral wettability contact angle is 90°1 if surface is completely oil wet contact angle is >l35°In short. contact angles of less than 90° indicate preferentialwater wettability.The other test procedure (Amott Wettability Test) involvesusing penneable core material (e.g. Berea). The methodmeasures the ability of water to spontaneously displace oilfrom art oil saturated core and the ability of oil tospontaneously displace water from a water flooded core. Theresults are represented as water index and an oil index.,Water indices of l and 0 indicate respectively complete waterwetting and complete oil wetting. Oil indices of l and 0indicate respectively complete oil wetting and complete waterwetting. Intermediate values indicate varying degrees ofwater and oil wettability.Some examples of behaviour of different all-oil systemscontaining passive and active emulsifiers are given in Table 3and the data demonstrate that presence of passive emulsifierin the mud filtrate gave results for contact angle and waterindex tests which indicated that the mineral had retained itswater wettability. In the cases in which active emulsifier wasused the rock had become much more oil wet.An alloil mud ofiers several other advantages:v being water free it can be used in strata containing shaleswithout the risks which attend the use of conventionalwater based system.o frictional problems are minimised due to the inherentlubricity confened by the oil phase0 low density: all-oil systems have the ability to produce afluid density of less than l. The minimum achievabledensity is set by the density of the base oil; that is, about0.8 sg. Water based systems all have densities greaterthan 1.0 sg.0 temperature stability of the system is very good. In largemeasure the maximum is set by the base oil used but thetemperature maximum is in excess of 250°

- cement contamination is not a problem for this fluid as itis water free.This type of fluid can be formulated with any grade of oilranging from fairly crude petroleum fractions such as dieselor kerosene to environment friendly biodegradable syntheticesters.

B ERIC DAVIDSON, SPE, BAROID LTD, SUSAN STEWART, BAROID CORP SPE/IADC 39284Limitations of system High densities represent a difficulty.The density of the system cart be increased only by weightingup the system with solids. Acid soluble carbonate can be usedto produce up to 1.5 sg but for higher densities non acid-soluble materials such as barite or haematite would berequired. Obviously in the density range for which brinebased fluids can be considered tlte all-oil system willinevitably contain a higher loading of suspended solids.Depcndiuguponthetypeofoiluseddisposalofthecuttings in marine locations might represent a limitation.However difliculties in this respect could be significantlyreduced by using biodegradable synthetic ester as the base oil.Clean up. The preferred technique is to produce the well butif the sind bridging particles are calcium carbonate it ispossible to carry out remediation with acid treatments. lnsuch cases it may be appropriate to treat the filtercake withmutual solvent/surfactant prior to. or during, the acidisationstage to ensure better wetting of the carbonate and moreeffective attack by the acid.Cuttings disposal Restrictions on dumping oily cuttings inmarine environments is becoming more difficult throughoutthe world. Synthetic biodegradable ester would probably beacceptable, but if other less biodegradable types of oil wereused then the implications of disposal restrictions have to beconsidered. For land operations appropriate containment ofthe urttings would be needed.Example. Typical formulations and properties for all oi]fluids of densities 0.85 and 1.2 sg are giwan in Table 4.An all oil system has been used in applications in Sharjahwhich exemplify extremely well some of the strengths of thistype of fluid". The circumstances called for a fluid whichhad :low fluid density (0.83 - 0.96 sg)good hole cleaning rheologygood lubricityminimal damage potentialThe fluid was optimised in preliminary laboratory work andthe milling operations went extremely well. The objectives of

the operation included drilling a horizontal section of 2000feet, but due to the excellent hole cleaning and lubricity thislength was actually exceeded by 1600 feet i.e. to a total of3600 feet. Furthermore the minimally damaging nature ofthe fluid was oonfinned by production data which indicatedzero skin. Thus the original objectives were exceeded in thefield.Mixed metal silicate systemsThis system exploits the ability of a special mixed metalsilicate (calcium aluminium silicate) (MMS) to form acomplex with predispersed bentonite to yield a fluid withunusual rheologiml properties. The fluid is shear tbimting32Oproviding a flat rheological profile with very high carryingcapacity.In terms of rheological behaviour the MMS fluid is similar tothat of mixed metal hydroxide (MMH). As is well known theMMH system also relies on the interaction of bentonite withdispersed MMH particles but although the rheologicalbehaviour of the two systems is similar it is believed that thenature of the interaction between bentonite and the respectivemixed metal compounds is ditterent for each case. Evidenceon this point comes from a study of clay slun-ies in theCapillary Suction Test (CST). The effect of increasing MMI-land MMS on CST results was studied and the results areshown in Figure l. As can be seen the eiiect ofMMH at lowconcentration was to cause an immediate decrease in the CSTvalue whereas over the same concentration range MMScaused an increase in CST. Diflerent results imply differentbonding mechanisms.Both MMS and MIMH interact with bentonite to yieldfluids with very flat rheological profiles. A typicalfonnulation and rheology of a base MMS fluid in Table 5which demonstrates the low plastic viscosity but very highyield point and high low shear rheology. This rheologicalprofile gives the fluid excellent suspension and cuttingstransport properties.The MMS fluid is tolerant to starch derived fluid losspolymers and to high loading of particles such as sizedcarbonate and/or baritc. ln addition the high viscosityreduces the leak of rate of fluid through pore throats andfissures. The type of fluid has been used to drill sectionswith difficult fluid loss potential and there have been cases inwhich fluid loss problems have been cured by increasing thelow shear viscosity.The composition of MMS systems cart be changedaccording to the proposed end rise. For example, for simple

milling inside casing a basic MMSlbentonite fluid would beadequate. Whereas for drilling in a permeable formationpolymers and sized particles would be included to meet fluidloss and density requirements.Limitations of MMS systems Any ingedient which willinterfere with the MMSlbentonite bond or the state of thebentonite mineral will impair the properties of the fluid. Forexample powerful anionic thinners such as lignosulphonate orpolyacrylamides cause irreversible loss of rheology. Similarlycontact with brincs of high ionic strength or containingcations which affect bcntonite (e.g. calcitun, potassium etc.)has a deleterious eflect. The fluid cannot tolerate a highsalinity brine. Seawater can be used but significant densitycannot be contributed by the aqueous phase. This means thatweighting up is only possible by increasing concentration ofsuspended solids in the fluid. If calcium carbonate is used asthe weighting agent the practical density limitation isprobably about 1.5 sg. For higher densities complete orpartial substitution of carbonate by other minerals such as

SPE/IADC 39284 OPEN HOLE COMPLETIONSI DRlLL|NG FLUID SELECTION 9iron carbonate or acid insoluble materials e.g. barite orhaematite is necessary.MMS systems are not compatible with all types oflithology. The fluid performs well in most types of drillsolids but beds of bentonitic shale lead to thickening of themud, and evaporite beds would tend to destroy rheology.Also care is necessary when displacing out previous fluids. Itwould be nwessary to ensure that any calcium brines orstrongly anionic products had been removed from the wellbore and could not contaminate the MMS mud.The fluid is fairly tolerant to cement contamination butonly if enough sodium carbonate is used to ensureprecipitation of the soluble calcium.Clean np. As is the case for any drilling fluid the clean upmethod is greatly influenced by the type of completionequipment used. Prepacked screens require care to ensurethat any fluid remaining in the annulus can be producedthrough the screens without causing blockage. To aideflicient displacement it might be necessary to cause therheologyoftheMMSfluidtobereducedandthiscanbedoneby using an anionic thinner. This will help to reduce theviscosity of the fluid adhering to the filtercake and makeremoval easier.Acid can be used to simultaneously dissolve the sizedcarbonate and degrade the modified starch but in this case

localised Ink off of acid is possible. Thus, as is the case forother systems containing carbonate, use of a retarded acidcould be advantageous.Disposal 0|’ cuttings. In an otfshore situation discharge ofcuttings into the sea should not be a problem. High pH is themain characteristic but high concentration of salts is not aconsideration. On land containment of the cuttings and runoff would be required, as the high alkalinity would bedetrimental to plants and animals. However, the fluid doesnot represent a long term hazard and on exposure to air thehigh alkalinity would decrease as hydroxides are converted tocarbonates.Examples An example of an uweighted MMS system isgiven in Table 5. The Table contains details of theformulation and rheology. 'The principal objectives in using a fluid of this type are totake advantage of the rheology to obtain optimum holecleaning and fluid loss control. Horizontal displacements of4000 - 7500 feet were achieved in Austin chalk and in looseporous sand in otfshore Qatar developments. In all cases theoperations were accompanied by frmdom from wellboreproblems and excellent hole cleaning fliroughout. Infonnations in Iran where control of losses was normally amajor problem good control was maintained by adjustingrheology.On the negative side attempts to drill in bentonitic shalesin Israel had to be abandoned because the fluid incorporatedexcess clay and became too thick.Solids tree fluidThis approach is possible only for low permmbility reservoirsas the fluid comprises only brine and visoosifying polymers.The fluid contains no bridging solids and no attempt is madeto build a filtercake. The fluid is designed for use infonnations with permeability of no more than a fewmillirlarcies. The viscosifying agents are nonionic polymerswhich do not yield shear thinning fluids, therefore subsequentdiqalacement of any fluid which has invaded the formation isrelatively easy.The lack of suspension propenies is helpful in cleaningthe circulating fluid. Any solids remaining in suspension aflertreatment by the shakers rapidly settle in the pits.Being solids free the density of the fluid is controlledsolely by the base brine used and in practical terms the rangeis l - 1.75 sg. The temperature range over which this type ofsystem can be used is fairly limited. The polymers nomtallyused will disperse in concentrated sodium chloride orpotassium chloride brine, but depending upon theconcentration, will be reprecipitated as temperature is

increased and the tendency to reprecipitation increases withincreasing brine concentration. An identical phenomenonwas reported rwently for similar types of polymer used iniluid loss fluids”. Funhermore the preferred polymers arenot compatible with formate brines except at very lowconcentration of disolved formate.For saturated sodium chloride or potassium chloridebrines the effective maximum temperature is around 90° C. Inthe case of sodium bromide brine or dense calcium basedbrines precipitation is not an issue but polymer degradation isexpected for temperatures above l00° C.Despite its limitations this type of fluid can havesignificant advantages in low permeability conditions such aslimestone and tight consolidated sands. The lack ofsuspended solids is beneficial with respect to rheology andcirculating pressure losses and the fact that no significantflltercake is fonned makes the clean up stage very simple.Also since the polymers normally used are nonionic cementoontarnination is not a significant problem unless very densebrines are used. Nonnally, irrespective of the type ofcompletion used the well is simply cleaned up by producingit.Limitations of the system This fluid is suitable only for afairly restricted set of conditions. Low permeability andfreedom from fractures is necessary and the practicaltemperature limit is relatively low at 100°C. Also it is verytmlikely that this system could be used in an unconsolidatedsection.Cuttings disposal. Oflshore, similar considerations will

10 ERIC DAVIDSON, SPE, BAROIDLTD, SUSAN STEWART, BAROID CORP SPE/IADC 39284apply as for those which exist for the brinelpolymer/carbonatesystem. That is, in most circumstances there is not aproblem, unless the fluid is based on very dense brine forwhich discharge restraints apply. For onshore locationcontainment of the cuttings would be necessary as run off of ,highly saline solution would not be acceptable.Examples. An example of a low density fluid is given inTable 6. It is clear that this rheology is different from that ofthe other fluids in that the low shear viscosity is very low.This type of fluid can only be considered for low permeabilityrock but of course such formations are the most susceptible todamage. Accordingly this fluid has only been used alter coreflow studies have demonstrated the likelihood of high returnpermeabilities. In opting for this approach the objective is to

keep the fluid as free from solids as possible and to keep thecirculating pressure as low as practicable.In some applications of this fluid in the USA therequirement for a gas reservoir was for a density of 1.45 sg.In a typical case a horizontal section of I650 feet was drilledwith good hole cleaning, excellent directional control andhighly satisfactory production.Fluid SelectionIt is suggested that the process of selecting a suitable fluidshould proceed through the following steps:- Lay out the proposed design of the operation, including:details of completion equipment,reservoir characteristics and downhole conditions.logistical implications, eg disposal of cuttings.0 Identify the parameters which are important and consultthe summary in Table 7. This Table summarises theapplicability of the 4 systems over a range of conditions.o From Table 7 select the types of fluid which would beacceptable for the conditions in question. The otherscan be eliminated from consideration.I For each of the possible candidates formulate fluids forthe reservoir conditions eg. density, bridging particles etc0 For each of the candidate systems carry out laboratorytests to measure compatibility of the fluid ' ( ie wholefluid and filtrate) with:formation rockcomiate waterproduced oil.- Carry out return penneability tests, as far as possiblefollowing the same clean up procedure as is intended foruse in the field.To meet unusual well conditions it may be necessary tomodify one or more of the formulations summarised in Table7, or indeed to investigate radically difierent designs. Ineither case the requirement for laboratory and retumpermeability testing remains