mechanisms of formation damage and permeability

SPE 30320

Mechanisms of Formation Damage and Permeability Impairment Associated With theDrilling, Completion and Production of Low API Gravity Oil ReservoirsD.B. Bennion, F.B. Thomas, D.W. Bennion and R.F. BietzHycal Energy Research Laboratories Ltd

ABSTRACT 9, Formation damage issues peculiar to the drilling andcompletion of horizontal wells in heavy oil producing mnes.

The economic production of heavy oil reservoirs requires an in-depth understanding of the specific mechanisms of fonnationdamage which are unique to these types of formations. Thispaper will provide a brief description of some of the dominantmechanisms of permeability impainnent which can occur in lowAPI gravity oil-producing zones. These include:

This paper provides a general overview of many of the potentialtypes of fonnation damage which drilling, production andreservoir engineers as well as geologists and geophysicists shouldbe aware of when planning exploitation programs for heavy oilplays.

INTRODUCTION

Heavy oil reservoirs present a unique challenge to reservoir andexploitation engineers as they exhibit properties not commonlyfound in normal more conventional API gravity oil reservoirs.These include:

134.

Properties associated directly with the heavy oil such as highasphaltene (and possibly paraffm) content, extreme viscosity,solids content and a propensity to fonn stable water in oil orgas in oil emulsions.

6.

2 Many heavy oil producing fonnations are contained in poorlyconsolidated or unconsolidated sandstone fonnations in manylocations in the world. These particular types of formationspose their own particular challenge with respect to content ofreactive clays, potentially mobile fines, mineraltransformations and the physical problems associated withsand retention and control during nonnal productionoperations.

Drilling induced damage. solids entrainment. fines migration. rock/fluid incompatibilities. fluid/fluid incompatibilitiesReactive clay induced damageFormation of emulsions (water-oil, oil-water)Foamy oil phenomenaSand control and consolidation issuesCompletion induced damage, acidizing, solvent injection,etc.Thennally induced formation damage. mineral transformations. mineral and formation dissolution. wettability alterationsBiologically induced formation damage issues

References and illustrations at end of paper

2 MECHANISMS OF FORMATION DAMAGE & PERMEABILITY IMPAIRMENT SPE 30320ASSOCIATED WITH DRilLING, COMPLETION & PRODUCTION OF

lOW API GRAVITY Oil RESERVOIRS

This paper will attempt to quantify in a brief descriptive mannermany of the common mechanisms of formation damage whichshould be considered when drilling and completing heavy oilreservoirs, and provide indications of potential techniques whichmay be considered to attempt to reduce or eliminate some of theconcerns associated with these particular formation damagemechanisms.

Sand Control and Production Issues

A major problem associated with the production of heavy oilsand bitumens from many low API gravity crude oil reservoirs isthe fact that it is very difficult to produce the bitumen without

producing large quantities of entrained formation fines and sand.A variety of techniques to retain sand such as pre-packed screens,

openhole gravel packs, formation consolidation treatments, fracand pack treatments, etc. have been attempted, in many situationsover the years, with varying degrees of success. Generalconsensus among operators often is that, when sand productionis significantly reduced, oil productivity also tends to besignificantly reduced. A number of different authors haveinvestigated gravel packing as a technique for sand retention(3.6).Bennion et al(7) provides a detailed discussion of gravel pack

sizing criteria for the Rosary sands in the Battrum reservoir.This study illustrates significant reduction in gravel packpermeability and ultimate well productivity is achieved if poorgravel pack sizing is selected which allows the physical invasionof produced particulates and fines from the formation into thegravel pack. The authors also postulated, based on results inboth Canadian and Bakersfield heavy oil producing operations,that many of the problems associated with restricted gravel packproduction may be related to near wellbore disturbance anddamage phenomena associated with problems in the originaldrilling and completion operations. It was discovered that, ifinvasive formation damage and near wellbore disturbance couldbe minimized during the original drilling and completionoperations, near wellbore consolidation and sand controlproblems were much less problematic resulting in significantly(500%) greater productivity on the post-gravel packed wells.

~AJOR FORMATION DAMAGE MECHANISMS INHEAVY OIL RESERVOIRS -- ---

Major formation damage mechanisms in heavy oil reservoirs canbe subdivided into four major classifications, these being:

Mechanically induced fonnatjon damageChemjcally induced fonnation damageBiologically induced fonnation damageThennally induced fonnation damage

2.

3.

4.

Each of these will now be discussed in greater detail

Mechanically Induced Formation Damage Mechanisms

Physical Mi2ration of In-Situ Fines

Many heavy oil reservoirs are clastic formations which containa high concentration of potentially mobile, in-situ fmes andparticulates. These could include clays such as kaolinite, detritalrock fragments, pyrobitumen or other potentially mobilematerials. Heavy oils occurring in carbonate reservoirs are alsonot immune to fines migration phenomena as documentedinstances of the migration of dolomite or carbonate fines,pyrobitumen, etc. have also been observed in this type ofreservoir application.

Solids Entrainment Phenomena-

Figure 2 provides a schematic illustration of the mechanism ofsolids entrainment into homogeneous matrix type systems whichare commonly encountered in most heavy oil applications.Fines migration is controlled by a number of factors including

wettability of the porous media (fines generally tend to migratefairly exclusively in the phase that wets the rock). Thisphenomena is illustrated in Figure 1. Pore size distribution andsize of fines, as well as velocity of the fluids flowing in theinterstitial space, also strongly control the severity of problemsassociated with fmes mobilization. Fines migration can becontrolled in a number of fashions in heavy oil reservoirs,ranging from reducing apparent velocity by utilization ofopenhole completions, selective fracturing jobs or, in somesituations, potentially mobile clays can be stabilized through theuse of chemical clay stabilization treatments which have beendescribed by a number of authors(l).

Solids invasion is a common occurrence which happens duringoverbalanced drilling and completion operations due to the factthat the hydrostatic pressure in the circulating fluid system isusually greater than the formation pressure. Due to the relativelyhigh penneability of most low API producing gravity oilhorizons, this results in relatively large pore throats and asignificant propensity for the physical invasion of both artificialsolids (ie. weighting agents, fluid loss control agents or artificialbridging agents) or naturally occurring drill solids (silicate,carbonate, dolomite or other fonnation fines) into the formation.

SPE 30320 D.B. Bennion, F.B. Thomas, D.W. Bennion, A.F. Bietz 3

General screening criteria suggests that the long-term buildup ofan external stable filter cake causing significant reductions inpenneability during water injection, can be avoided by ensuringthat filtered injection water is filtered to a size distribution ofparticulates of less than approximately 200/0 of the median porethroat size diameter of the target formation.

In general, many studies conducted indicate that the fonnationacts very much like a large filter element. filtering out themajority of the entrained solids in an area relatively close to thewellbore. In many situations, if a cased and perforatedcompletion is being contemplated, a relatively shallow amount ofinvasive fonnation damage (less than 2 cm in depth) is usuallynot significant. as perforation charges will usually penetrate wellbeyond this radius. However, if extremely high permeabilityformations are encountered, or if drilling and completion inhighly overbalanced conditions in a pressure depleted formationoccurs, it is documented that invasive solids damage can extenda significantly greater distance into the fonnation. Also, a largenumber of completions in heavy oil reservoirs, particularly withrespect to horizontal wells, tend to be either openhole, slottedliner or gravel pack type completions. In this type of situation,the remaining zone of high skin induced by the invasion ofartificially or naturally occurring drill solids remains aproblematic barrier to production and must be physicallyproduced through, and therefore becomes of much greatersignificance with respect to impaired productivity. Pastexperience with laboratory studies and field studies indicates thatthis particular mechanism of formation damage in many openholehorizontal wells which have been completed in low APIproducing reservoirs plays a significant role in reducingproductivity .

Relative Penneabilitv Effects

Water trapping and retention or aqueous phase trapping (APT) isnot usually considered to be a significant permanent problem inmany heavy oil reservoirs due to the fact that:

The majority of heavy oil reservoirs exist at some initialwater saturation comparable to what would be considered theultimate irreducible water saturation.

2 The majority of heavy oil reservoirs exhibit highpermeability and porosity characteristics and correspondinglylow apparent capillary pressures which also tend to reducethe propensity of damage associated with permanent aqueousphase trapping.

Bennion(9.IO) further discuss this phenomena. Figure 4 providesa schematic illustration of relative permeability curves showingthe mechanism of an aqueous phase trap in an oil or gassituation. In general, due to the high permeability pre-existingin heavy oil reservoirs, they may susceptible to a non-pennanentphenomena which is called an aqueous phase load. In thissituation, a substantial amount of water-based fluid is lost to diefonnation directly adjacent to the wellbore. This creates anadverse relative permeability effect in the highly water saturatedzone which results in a significant reduction in the apparentrelative penneability to oil. This phenomena generally (if it isnot accompanied with other damage mechanisms such as thefonnation of emulsions, fines migration or clay reactions) can begradually cleaned up over a period of time if a sufficient pressuregradient can be mobilized within the formation over the invadedzone to produce die fluid back to the well bore. This may resultin impaired productivity for a period of time and hence, althoughpermanent reductions in penneability in heavy oil reservoirs dueto die establishment of aqueous phase traps, may occur. short ormedium term reductions in production rates can be readilyapparent when wells are unnecessarily killed or subjected to highamounts of invasion with water-based fluids. The lower thepenneability of the fonnation, the more significant the propensityof aqueous phase trapping and permanent retention of waterproblems becomes. Figure 5 provides a schematic exampleshowing die relative severity of potential aqueous phase traps asa function of initial water saturation and permeability in porousmedia. If a low penneability heavy oil play at a low watersaturation is under consideration. care should be taken to avoid

This problem can be obviated in many situations by appropriatedesign of a fluid system with the appropriate size distribution ofgranular bridging agents to create an effective, sealing,impermeable filter cake very rapidly upon the face of thefonnation, thereby inhibiting continual losses of small solids andpotentially damaging mud filtrate into the fonnation. Thisphenomena is further illustrated in Figure 2.

A variety of bridging agents are available for this purpose,including calcium carbonate, oil soluble resins, cellulosicmaterials, sized salt, etc. Care should be taken in the selectionof a bridging agent with respect to the completion practicesproposed for the well to ensure that. if pennanent entrainment ofthe bridging agent does occur, an acceptable stimulationtechnique is available to remove the bridging agent withoutdamaging the formation.

Another potential mechanism of permeability impairmentassociated with solids entrainment can occur during waterinjection or water disposal operations in heavy oil applications.Strict filtration criteria must be maintained in order to avoidlong-term plugging of the sand face by suspended solids. Thisphenomena is illustrated in Figure 3. Bennion(l) providesadditional criteria with respect to water filtration and waterquality concerns associated with water injection and disposaloperations.

4 MECHANISMS OF FORMATION DAMAGE & PERMEABILITY IMPAIRMENT SPE 30320ASSOCIATED WITH DRilLING. COMPLETION & PRODUCTION OF


the introduction of potentially damaging aqueous phase fluidsinto the formation.

porous media, has been documented to cause significantreductions in the apparent productivity and overall permeabilityto oil. Therefore, the radical expansion effect caused by theentrapment of the residual gas saturation is partially counteractedby reduced productivity and permeability impairment associatedwith the entrainment of the foamy oil. This also results inincreased pressure drawdowns and gradients near the weUbore,and may aggravate problems associated with sand control. Therehas also been some evidence that the stable interfaces betweenthe gas and oil phases in the foamy oil emulsions can act astransportation sites for small fines which may exacerbateproblems with fines migration, sand transport and formationconsolidation.

Slugs of skim oil or enb'ajned oil during water injection andwater disposal can also have adverse relative penneability effects,particularly if the targeted injection zone is an aquifer or entirelywater saturated disposal zone. Further infonnation on thisphenomena is contained in Bennion(l). This phenomena isschematically illustrated as Figure 6 and is especially problematicwith heavy oils, due to both the difficulty in separating producedwater and heavy oil emulsions prior to re-injection and the tarrynature of heavy crude oils and their ability to rapidly wet andfonn high residual oil saturations in porous media.

In order to eliminate concerns with skim oil injection, suspendedoil in water content should be kept less than 100 ppm with thesuspended oil being injected as a finely divided oil in wateremulsion, rather than a nucleated solution where free oil dropletscan coalesce and, instead of moving with the bulk injected fluidstream, can be entrapped in the porous media as a residual oilsaturation.

Emulsions

Emulsions are a problem associated with many heavy oiloperations where both oil and water are simultaneously beingproduced. Bennion(IS) discusses the fonnation of emulsions in-situ at elevated temperatures in porous media.With respect to water in oil emulsions, two different types ofemulsions are possible - the water-in-oil emulsion, which tendsto be the most problematic as it exhibits very high apparentviscosity in comparison to clean oil, and the less problematic oil-in-water emulsion.

Foamy Oil Phenomena

Considerable interest has been expressed in recent years in thegeneration of stable foamy oil emulsions, both at downhole andsurface conditions, as both a mechanism for the increased and, insome cases, reduced production of oil from heavy oil reservoirs.A number of authors have discussed foamy oil behaviotll-14).

Water in oil emulsions are generated by a number of documentedphenomena including turbulence, the presence of sand, silt ordispersed fines, paraffins, iron sulphide, asphaltenes and resins,and a variety of organic acids and cyclic and aromatichydrocarbon compounds.Foamy oil emulsions are created due to the high inherent

viscosity and interfacial tension characteristics exhibited betweengas and crude oil, as pressures are reduced below the bubblepointpressure. General research at Hycal indicates that a certaincritical threshold pressure must be exceeded to generate theformation of a true foamy oil emulsion. This pressure, in pastexperience, has been a function of the oil gravity, temperatureconditions and oil composition which exists in the reservoir, butgenerally falls in the pressure range of approximately 700 - 1400kPa. Upon the formation of a stable gas in oil emulsion, a largeportion of gas which would otherwise be liberated as free gasremains entrapped in the oil in an emulsion form. This causesa reverse formation volume factor phenomena and a rapidexpansion in swelling of the crude oil which is postulated to bethe primary mechanism associated with the high production rateswhich are often associated with foamy oil reservoirs. In-situviscosity measurements indicate that foamed oil emulsions canexhibit significant increases in apparent viscosity due to theentrainment of the gas in solution. This increase in viscosity,coupled with the presence of the gas saturation both entrappedwithin the oil. and as a critical free gas saturation within the

The major problem associated with the formation of water in oilemulsions is the extremely high viscosity exhibited by thesefluids. Over four orders of magnitude increases in viscositycaused by the generation of stable water and oil emulsions hasbeen documented in the literature. Some examples in recentlaboratory evaluations illustrated produced emulsion fromBattrum area ftrefloods with reservoir temperature viscosities inthe range of approximately 3S 000 mPa.s in comparison to cleanoil viscosities from the same field of 190 mPa.s.

The fonnation of emulsions downhole results in the generationof a high viscosity block which can impair fluid flow towards thewellbore. Research generally indicates that, although emulsionsdo tend to fonn in-situ in porous media, the majority of theextremely tight and highly viscous emulsions that are oftenencountered on surface appear to be generated in downholepumping and surface transfer equipment.

SPE 30320 D.B. Bennion, F.B. ThorT8S, D.W. Bennion, A.F. Bietz 5

A variety of other potential agents may result in the formation ofstable in-situ emulsions. One of these agents which will bediscussed in greater detail later is hydrochloric acid which, in aspent fonn, particularly in the presence of high concentrations ofacid, can spontaneously emulsify with many heavy oils togenerate extremely viscous emulsions which may result in atemporary or permanent blocking effect in the near wellboreregion.

crude oils). It is only when the asphaltenes are destabilized andflocculate from solution as solid bodies that significant reductionsin both in-situ penneability and plugging in surface productionand treating equipment may become problematic. Asphaltenesare typically destabilized by reductions in temperature andpressure or by contact with precipitative agents such asunsequestered hydrochloric acid or a variety of organic materialssuch non-compatible oils or diesels or gaseous treating agentssuch as LPG or carbon dioxide gas.

Clay ReactionMost heavy crude oils do not naturally destabilize asphalteneswith nonnal reductions in temperature or pressure, but may befairly susceptible to the fonnation of asphaltic sludges whencontacted with some of the aforementioned precipitative agents.Therefore, before any type of acidiZBtion or chemical stimulationb'eatment is attempted, it is usually advisable to conductextensive compatibility tests to ensure that the fonnation of eitherasphaltic based sludges or emulsions does not occur in-situ uponcontact with the potential stimulation or treating fluid.

Since of the majority of heavy oil producing formations are oftencontained in clastic formations, a variety of different types ofauthigenic and detrital clays may be present such as kaolinite,smectite, mixed layer clays, chlorite and illite. Each of theseparticular clays has their own potential sensitivity with respect todifferent types offormation damage mechanisms. Kaolinite clay,as already mentioned, has a propensity to be susceptible tovelocity induced migration associated with high flow rates andpressure shocks. Mixed layer or smectitic clays (Figure 7) aresusceptible to swelling caused by contact with either low salinityor fresh water. The expansion of the smectitic or mixed layerlattice can cause a constriction in the pore system and result insubstantial reductions in permeability. This phenomena has beenwell documented by a number of authors(16).

Paraffins are more problematic in some situations with heavy oilsand are generally conuolled by reductions in temperature. Inmany situations, paraffin problems tend to be more of aproduction issue rather than a downhole issue as generally, atreservoir temperature conditions, temperature remains sufficientlyhigh enough to inhibit the formation of crystalline waxes.Detailed cloud and pour point measurements can be undertakenon produced heavy crudes to ascertain the precipitationcharacteristics of the crudes as well as solid hydrocarbon analysisto quantify the fraction of white vs black wax can be undertakento ascertain whether wax treatments are better accomplishedusing thermal or chemical suppression treatment methods.Thomas(17,11) provides additional insight into the methods of theprediction and mitigation of problems associated with solidsprecipitation in porous media.

Another potentially significantly damaging mechanism ofpermeability impaimlent in unconsolidated sandstone reservoirscontaining heavy oils is that of clay deflocculation. Claydeflocculation is less well understood than clay swelling andrepresents a disruption of electrostatic bonds which are holdingthe clays together in a bound or flocculated state. Abrupt contactwith a pH shock or significant alteration in brine chemistry (suchas the introduction of fresh or low salinity water) can cause adisruption of these electrostatic forces and result in dispersion ofthe clays and subsequent migration and significant reductions inpermeability due to blocking and bridging. This phenomena isdiscussed in detail in the literature(8) and is schematicallyillustrated in Figure 8.

Biologically Induced Formation Damage Mechanisms

Bacteria can be introduced into the fonnation at any time duringdrilling, completion, stimulation or workover operations whenaqueous phase fluids are utilized and improper bacteriologicalcontrol is maintained. Bacterially induced fonnation damage isa particularly insidious type of fonnation damage in that theapparent deliterious effects of the introduction of the bacterialagents are usually not readily apparent. but are of a delayed andusually significant onset. Bacteria which can be problematic inheavy oil reservoirs fall into two types, classified as aerobic andanaerobic. Aerobic bacteria require a constant source ofdissolved oxygen to survive and are usually only problematic inlong-term water injection operations. Anerobic bacteria requireno dissolved oxygen and tend to be more widespread andproblematic in a number of different scenarios.

In many cases, producing heavy oil formations are inadvertentlydamaged by the use of low salinity or fresh water simply becausepreliminary petrographic analysis indicates an absence of classicalfresh water swelling clays such as smectite. A variety of otherparticulates, including kaolinite clay, can be susceptible todetlocculation induced damage and this can have significantresults with respect to impaired productivity.

Wax and Asohaltene Problems

Many heavy crude oils contain a high concentration ofaspbaltene. This may not necessarily be a significant problem ifthe asphaltenes are peptized (suspended or solubilized in the



Bacteria thrive best at a temperature range betweenapproximately 40°C to approximately 70-800C but can activelypropagate at tempertures as low as 200C and at temperatures ofup to 13SoC for very hardy strains. There are three majorproblems associated with the introduction and propagation ofbacteria in porous media. These being:

oil reservoirs by hot water, steam or in-situ combustion enhancedoil recovery mechanisms. The nature of mechanisms ofthermally induced formation damage would fall into thefollowing classifications:

Mineral Transformations

Plugging - bacteria produce extremely high molecular weightpolyacharride polymer and fonn a biofilm upon the surfaceof the fonnation to protect themselves from fluid shear. Thephysical adsorption of this biofilm can cause a significantreduction in injectivity or productivity of a given well overan extended period of time. Oxidants such as bleach orperoxide are commonly used to both reduce and desorb thepolymer and kill colonies of growing bacteria.

At temperatures in excess of approximately 200°C, the potentialfor mineral transformations is present. This occurs when arelatively inert clay such as kaolinite is transfonned into a freshwater sensitive clay such as a smectite. This reaction has beendocumented by various authors(19.Z0). Subsequent contact of freshsteam condensate with the newly transfonned fresh water reactiveclay can result in significant swelling and expansion and largereductions in apparent penneability of the pore system. Thisphenomena has been well documented in many thennaloperations throughout the world.2 Co"osion problems - bacteria, when colonized on metal

surfiM:es, form small electrochemical cells which result in ahydrogen reduction reaction which causes the corrosion andpitting problems on surfaces such as downhole tubing, pumpsand in surf~ facilities.

SolubiliDtion and Precioitation

Another potential mechanism ofthennally induced damage is thatthe solubility of both carbonates and silica increases in aqueoussolution as temperature is elevated. This can result in thedissolution of portions of the forDlation. This can have a twofoldeffect. Firstly, the dissolution may dissolve partially solubleclasts of carbonaceous or silicate material, thereby releasingpreviously immobilized fmes which subsequently migrate to porethroats and cause reductions in perDleability and productivity.Secondly, the mineral saturated brine, as it moves further into theforDlation, encounters colder forDlation material and subsequentlycools and loses the ability to maintain the materials dissolved insolution. This results in re-precipitation of calcium, magnesiumor silicate based solids and the magnitude and location of this re-precipitation can also result in potential reductions inperDleability and productivity. A schematic illustration of boththese phenomena appears as Figure 9.

3, Toxicity concerns - sulphate reducing bacteria. a particularlytroublesome family of anaerobic bacteria. metabolizeelemental sulphate which may be present in naturallyoccurring formation water or injection waters and createtoxic hydrogen sulphide gas as a by-product. This HzS gasis highly soluble in oil or water and can be potentially toxicor lethal to human beings in concentrations of greater thanapproximately 1000 ppm. Documented cases of sulphatereducing bacteria producing in-situ HzS concentrations inexcess of 20,000 ppm have been illustrated in areas such asthe East Wilmington field in California and the Kuparik fieldin the North Slope in Alaska.

Biological damage problems are extremely difficult to remediate,particularly with sour gas once the gas has propagated aconsiderable distance into the reservoir. Therefore, the besttechnique associated with biologically induced damage is toensure continuous and diligent monitoring of surface anddownhole bacterial levels using rapid detection field kits and anaggressive biocide and treating program for not only continuouslyinjected fluids such as injection water, but also any fluids usedfor drilling, completion, workover or stimulation operations is

rigorously implemented.

Wettabilitv Alterations

Wettability of porous media is strongly controlled by the physicaladsorption of heavy polar constituents on the surface of the rock.This adsorption is governed by temperature considerations. Astemperature becomes higher, the amount of physical adsorptiondecreases and many of the heavy polar constituents tend to bephysically desorbed from the surface of the rock. This generallyresults as temperature increases in formations tending to becomemore and more water-wet. This is schematically illustrated inFigure 10. This generally has favorable connotations in that therelative permeability to oil is generally increased while therelative permeability to water is reduced. This is particularlybeneficial in cyclic stearnflood projects where the mobility ratio

Thermally Induced Formation Damage

Thermally induced formation damage is a family of potentialdamage mechanisms which are unique to the production of heavy


is dramatically increased resulting in higher oil production on theproduction cycles. If the relative permeability to water isseverely depressed by temperature induced wettability alterations,it may result in reductions or restrictions in injectivity of hotwater or steam into the formation. Generally, this is not aconcern as at high temperature the viscosity reduction associatedwith the increased temperature with the injection water or steamis sufficient to overcome the corresponding reduction usuallynoted in the relative permeability.

induced damage in unconsolidated sandstone fonnations. Ingeneral, however, the use of hydrochloric acid or hydrofluoricacid in predominately quartose heavy oil bearing reservoirs is notadvised as a primary completion treatment due to the potentialproblems which can often be associated by adverse interaction ofacid both with the fonnation and with the in-situ crude in theporous media. These problems would tend to include:

A propensity for most acids, particularly when spent and inthe presence of high concentrations of iron, to spontaneouslyemulsify with many heavy crude oils creating extremely highviscosity tight emulsions and asphaltic based sludges whichtend to have a plugging effect in the porous media.

There have also been some isolated incidences of significant

long-term wettability alterations caused by steamfloodingoperations. In this situation it is postulated that, when steamtemperature is achieved and the connate water layer present inthe porous media is vaporized and removed, a portion of theresidual oil saturation is allowed to directly contact the rock andestablishes an oil-wet film on the surface of the formation. Thiscauses a rapid transition from a strongly water-wet to a stronglyoil-wet scenario and a large increase in the water phase relativepermeability and subsequently degrades the performance of anytype of a cyclic steam operation resulting in high produced watercuts and poor recovery to oil. This phenomena has beendocumented in the laboratory on both a permanent and atransitory basis and is discussed in Bennion(19).

2. Potential precipitation problems associated with the use ofhydrofluoric acid and reaction with carbonate or otherpotential materials which are present in the porous media. Ifhydrofluoric acid is to be considered, a detailed compatibilityprogram and an extensive design program and staged acidjob, complete with pre-flush treatment of carbonaceousmaterial, should likely be conducted in order to minimizepotential concerns associated with fluoride based precipitates.

HeaVY Oil Production and Horiztonal Wells

Absolute PerDleabilitv Chane:es Heavy oil production in many areas of the world has increasedsubstantially in recent years with the advance of horizontaldrilling. Horizontal wells are particularly susceptible to a varietyof fonnation damage induced mechanisms due to their geometryand completion characteristics. Horizontal wells are moresusceptible to fonnation damage for the following reasons:

Conflicting evidence exists in the literature on the effect oftemperature on absolute permeability. In general, increases intemperature result in an increase in compression on the grain tograin contacts caused by thermal expansion effects associatedwith the porous media. This generally has a moderate to slightreducing effect on the effective absolute permeability of the rock.Previously mentioned problems such as dissolution and re-precipitation can also result in motion of in-situ particulates andchanges in pore geometry which may have increasing or reducingeffects on the apparent absolute permeability of the porousmedia. For this reason, elevated temperature absolutepermeability measurements are difficult to conduct in areproducible fashion due to the fact that a considerable amountof permanent physical alteration in the rock character occursfrom the start to the conclusion of the test sequence, eliminatingthe possibility for reproducible repeat control permeabilitymeasurements.

Increased propensity for invasion of both liquids and solidsduring overbalanced drilling operations due to extendedexposure times.

2. Even shallow invasive damage can often be significant inhorizontal wells due to the fact that the majority ofhorizontal wells, particularly in heavy oil applications, areeither openhole or slotted liner type completions andproduction must occur through the damaged zone.

3. If damage does occur, obtaining penetrating stimulationtreatments is difficult and expensive due to the size of theexposed area and the limited ability to obtain zonal isolationin many situtations where slotted liners are present.Acidiziol! in HeaVY Oil Reservoirs

Well designed acid treabnents can be of benefit in somesitutations when heavy oil reservoirs are being produced fromcarbonate or dolomite fonnations or from sandstone formationscontaining a high fraction of carbonaceous material. In somesituations, well designed hydrofluoric acid treatments have alsobeen effective in decomposing and removing clay and drilling

4. Fonnation damage effects are accentuated by penneabilityanisotropy effects in lower K to ~ penneability ratiofonnations. This is often the case in many heavy oilsituations where, due to depositional environment, laminatedvertical permeability barriers exist. Additional detaileddiscussion on potential severity of permeability impairment



with respect to anisotropic permeability impainnent iscontained in the referencesfll-24).

REFERENCES

Sydansk, R.D.: "Stabilizing Clays With PotassiumHydroxide", JPT (Aug. 1984) 1366.Laboratory Tests to Evaluate Fonnation Dama2e Mechanisms in

Heavy Oil Reservoirs2. Sloat, B.F.: "Field Test Results With Alkaline

Potassium Solutions to Stabilize Clays Pennanently",SPERE (May 1990) 143.

A wide suite of avajlable laboratory technology exists to evaluatemany of the potential fonnation damage mechanisms which havebeen discussed in this paper. A proper understanding of thepetrology and geology of the reservoir coupled with the flowcharacteristics of the rock, wettability, emulsification potential ofthe crude oil and specific tests with relationship to pore sizedistribution and solid size distribution of the fluid systems andcomposition of the fluid systems proposed for drilling.completion or stimulation can be evaluated in controlledlaboratory situations to obtain a significant degree of confidencein the proposed viability of specific drilling, completion,production or operation scenarios. Additional information onspecific laboratory procedures utilized for both conventional andheavy oil formation damage operations is contained in theliterature(2I-24).

), Saucier, R.J.: "Considerations in Gravel Pack Design",JPT (Aug. 1974), 205.

4. Coberly, C.J. and Wagner, F.M.: "Some Considerationsin the Selection and Installation of Gravel Packs For OilWells", JPT (Aug. 1938).

s. Hill. K.E.: "Factors Affecting The Use of Gravel in OilWells", API Drilling and Production Practice (1941)134.

6. Stein, N.: "Sand Control Methods Using a ParticulatePack With External and Internal Particle SizeDistribution Relationships", U.S. Patent 3, 434, 540(March 25, 1969).CONCLUSIONS

A wide range of different potential types of fonnation damagemechanisms have been discussed for heavy oil reservoirs. Thesehave been broadly classified into the subclassifications ofmechanical, chemical, biological and them1al1y induced formationdamage. In the majority of operations in heavy oil, much of theformation damage is typified to be of the mechanical nature withphysical invasion of solids and the formation of emulsions beingsome of the more problematic offenders in many operationalsituations. A proper understanding of the rock character involvedin a particular heavy oil reservoir coupled with a proposed fluidprogram and the interaction of the fluids and contained solidswithin the porous media is essential in understanding andminimizing potential problems associated with formation damagewhich will result in apparent reduction in the productivity orinjectivity .

7. Bennion, D.B., Sanders, R. and Thomson, W.S.:"Minimizing Formation Damage to Gravel Packs:Laboratory and Field Case Studies in the BattrumField", paper SPE 27355 presented at the SPESymposium on Formation Damage Control, Lafayette,Louisiana, February 7-10, 1994.

8. Bennion, O.B., Bennion, O. W., Thomas, F .B. and Bietz,R.F.: "Injection Water Quality, A Key Factor toSuccessful Waterflooding", paper CIM/AOSTRA 94-60presented at the 46th ATM held June 1994, Calgary,Alberta.

9. Bennion, D.B., Bietz, R.F., Thomas, F .B. and Cimolai,M.: "Reductions in the Productivity of Oil and LowPermeability Gas Reservoirs Due to Aqueous PhaseTrapping", JCPT (Nov. 1994) 44.ACKNOWLEDGEMENTS

The authors wish to express appreciation to the management ofHycal Energy Research Laboratories Ltd. for pennission topublish this paper.

10. Bennion, D,B., Thomas, F,B., Bietz, R.F, and Bennion,D. W ,: "Water and Hydrocarbon Phase Trapping inPorous Media - Diagnosis, Prevention and Treatment",paper CIM 95-69 to be presented at the 46th AnnualTechnical Meeting of the Petroleum Society of CIM.May 14-17, 1995, Banff, Alberta.


Maini, B.B., Sanna, HoK. and George, A.Eo:"Significance of Foam-Oil Behavior in PrimaryProduction of Heavy Oils", paper CIM 92-77 presentedat the CIM ATM, June 7-10, 1992, Calgary, Alberta.

20. Hutcheon, I. et at: "A Comparison of FolmationReactivity in Quartz Rich and Quartz Poor ReservoirsDuring Stearn-Assisted Thennal Recovery", 4thUNITAR/UNDP Heavy Crude and Tar SandsConference, Vol. 2, paper 235,747.

12. Kraus, W.P., McCaffery, W.J. and Boyd, G.W.:"Psuedo-Bubble Point Model for Foamy Oils", paperCIM 93-45 presented at the May 9-12, 1993 ATM,Calgary, Alberta.

21. Beatty, T. et al: "Minimizing Formation Damage inHorizontal Wells: Laboratory and Field Case Studies",paper CIM 93-16, presented at the 1993 AnnualTechnical Meeting, May 9-12, 1993, Calgary, Alberta.

Smith, G.E.: "Fluid Flow and Sand Production inHeavy Oil Reservoirs Under Solution Gas Drive", SPE15094 (1986).

22. Bennion, D.B. and Jones, W.: "Procedures forMinimizing Drilling and Completion Damage inHorizontal Wells - Laboratory and Field Case Studies inthe Virginia Hills Belloy Sands", presented at theSPFlCIM 4th Annual One Day Conference onHorizontal Wells, Nov. 22, 1994, Calgary, Alberta.

14. Loughead, D.l. and Saltuklaroglu, M.: "LloydrninsterHeavy Oil Production - Why So Unusual", 9th OilSands Technology Symposium (March 11, 1992),Calgary, Alberta.

23. Bennion. D.B. et al: "Effective Laboratory CorefloodTests to Evaluate and Minimize Fonnation Damage inHorizontal Wells", presented at the 3rd InternationalConference on Horizontal Well Technology, Nov. 12-14, 1991, Houston, Texas.

1S. Bennion, D.B., Chan, M., Sarioglu, G., Courtnage, D.,Wansleeben, J. and Hirata, T.: "The In-Situ Fonnationof Bitumen-Water Stable Emulsions in Porous MediaDuring Thermal Stimulation", paper ClM 93-46,presented at the 1993 A TM (May 9-] 2, 1993), Calgary,Alberta. 24. Bennion, D.B. et al: "Underbalanced Drilling of

Horizontal Wells: Does It Really Eliminate FormationDamage?", SPE 27352, presented at the SPEInternatjonal Symposium on Formatjon Damage Control,Feb. 7-10, 1994, Lafayette, Loujsjana.

16. Bennion, D.B., Scott, JA. and Bennion, D. W.:"Detailed Laboratory Studies of Chemically andBiologically Induced Formation Damage in the EastWilmington Field", paper CIM 92-44 presented at theATM of the Petroleum Society of CIM (May 7-10,1992),

7. Thomas, F.B., Bennion, D.W., Bennion, D.B., Hunter,B.E.: "Experimental and Theoretical Studies in SolidsPrecipitation From Porous Media", JCPT (1992).

18. Thomas, F .B., Zhou, X., Moore, R.G.: "Modelling ofSolids Precipitation From Reservoir Fluids", paper CIM95-46, to be presented at the 46th Annual TechnicalMeeting of the Petroleum Society of CIM, May 14-17,1995, Banff, Alberta.

19. Bennion, D.B., Thomas, F .B. and Sheppard, D.A.:"Fonnation Damage Due to Mineral Alteration andWettability Changes During Hot Water Injection andSteam Injection in Clay-Bearing Sandstone Reservoirs",paper SPE 23783 presented at the 1992 Symposium onFormation Damage Control, February 26-27, 1992,Lafayette, Louisiana.

FIGURE 1IllUSTRATION OF EFFECT OF WETTABIUTY

ON FINES MOBILIZATIONFIGURE 2

SOLIDS INVASION INTO AHOMOGENEOUS PORE SYSTEM

\"""- 10__-14_--- ..I

""~

c..1.~...~ to t-a_-

~-~~ 10__-

~ 10__&-

r-----AIr_-&- -

\It-/

FIGURE 3MECHANISM OF SUSPENDED SOLIDS

ENTRAINMENT

FIGURE 4MECHANISM OF

AQUEOUS PHASE TRAPPING

FIGURE 5SEVERITY OF AQUEOUS PHASE TRAPPING

~:a~m~Q.

~

100 20 30 40I~ Water Saturation

50 70S)

FIGURE 7EXPANSION OF SWELLING CLAYS

Stabilized - Expended -Dehydrated ~ted

".,

Water&ms~

/fI - 0

H.H

---

~

--+ +-""

--t,.-k_-tQnC! t.,-k_-tQnC!

.~(N*:~ It8WkIg8 -- die kIjed8cI oils ~ almpatible n n-.abIe ~ die

~ ~ - . - 8dcIIkJfW dam8ge mey be ~)

FIGURE 8MECHANISM OF CLAY DEFLOCCULATION

-High8aIk*Y

-~-~

- Low I8InIy

-~-DeIDcxxMI8d

FIGURE 9EXAMPLE OF FORMATION DAMAGE

MINERAL DISSOLUTION

FIGURE 10INFLUENCE OF TEMPERATURE ON

RELATIVE PERMEABILITY/WETTABILITY(Illustrative Example)