PETROLEUM SOClElY OF CIM PAPER NO. 98-01

APPLICA nON OF HEAT TREATMENTTO ENHANCE PERMEABILITYIN TIGHT GAS RESERVOIRS

A.K.M. Jamaluddin, D.B. Bermion, F .B. Thomas, T. Y . Ma

Hycal Energy Research Laboratories Ltd.

results showed promise. The primary mechanisms ofdie FHT process are to vaporize blocked water.dehydrate clay-bound water. destroy clay lattices andpossibly create miaofractures due to dtennal inducedstresses.

ABSfRACT

During drilling and completion phases, die primarymeclJaniSlnS of near-wellbore formation damage can beattn"buted to die following factors: I) pore throatconstriction caused by clay swelling, defloccu1ation dueto incompatible fluids or clay migration; 2) waterblocking resulting in a reduction in relative permeabilityto hydrocarbons; 3) plugging with drill solids and mudproducts; and 4) loading of die ~oir with drilling orcompletion fluids. In tight resavoirs, phase trappingand water-blocking are believed to be the primarycauses of near-wellbore formation damage, resulting invery low productivity. Clay swelling and phase b"appingin tight gas reservoirs during drilling and completionhave long been identified as major problems.Preventive measures have been discussed in literature;however, prevention of clay damage and phase trappingis not always possible or effective and curative measuresmay ilien become necessary. Several curative methodshave been attempted and presented in literature wiilimixed success.

The objective of this laboratory study was toevaluate the feasibility of applying the fonnation heattreatment process on cores taken from a tight gasresavoir. The results indicate dJat the FHT stimulationat 650°C resulted in a 2100/0 improvement inpermeability from d1e baseline undamaged value and675% improvement from the damaged (water-trapped)value. The post FHT waterflooding of the core stillshowed 500/0 improvement in permeability from thebaseline value and 275% more than the water-trappedvalue. Laboratory results along with the field logisticswill be presented in this paper.

INTRODUCTION

Fonnation damage can occur at any time during dIehistory of a well - from the initial drilling andcompletion of the wellbore through to the depletion ofthe reservoir during production. Operations such as

A fonnation heat treatment (FHT) process has beendevelooed in the last four years and initial field test

drilling, completion, workovers and stimulation, whichexpose the formation to a foreign fluid, may causefonnation damage because of adverse wellbore-fluid tofonnation interactions. Such damage is usually severein horizontal wells, because of the longer exposure ofthe wellbore to the offending fluids.' During the drillingand completion phases, the primary mechanisms ofnear-wellbore fonnation damage can be explained by thefollowing factors:

Thennal Curative Processes

One of the earliest reports of in-situ thennaltreatment was that of Albaugh,16 on a field test that wascarried out in an oil well in California. During the fieldtest, an electrical heater was lowered into a 6.5 indiameter well and positioned close to the fonnation face.Natural gas was injected to push the oil back into diereservoir and subsequently, die well was heated to375 °C (733 oF) for 6 days. After this time, the heatingceased and when the temperature had decreased to175°C (373°F), the well was put on production. Thepre-b'eatment rate was 21 bbl/day, while die post-treatment oil rate was increased to 37 bbl/day. Anincremental production of 16 bbl/day was acllieved andmaintained for several mondis.

Pore throat constriction, caused either by clayswelling due to incompatible fluids or by clay

migration.Water blocking due to reduction in relativepenneability to hydrocarbon.Plugging with drill solids and mud products.Loading of dte reservoir with drilling or completionfluids. Since dten, many other curative dterma1 processes

have been desaibed for a variety of purposes includingdie removal ofwax'7 or asphaltene" buildups, dtermalfracturing of dte formation. 19 and dte consolidation of

unconsolidated formations.20 More specifically relatedto phase trapping and water blocking damage aremethods aimed at removing water by evaporation at high

temperatures.

In tight gas reservoirs, fonnation damage, due tophase uapping and water blocking, has long beenidentified as a major problem. Preventive measuresagainst this type of damage is not always possible oreffective, and CW'8tive measures may d1en becomenecessary. Several CW'8tive methods have beenattempted and presented in the literature.I-IS

It is a well known facrs that the lattice structure ofalmost all clay minerals responds to thermal shock andthat die degree of change in the lattice structure ofvarious minerals depends on dietem perature level. It isbelieved that these mechanisms of the FHT processwould be beneficial to gas production improvement anddlus a new matrix stimulation concept was designed andtested in the laboratory. The process involves theapplication of heat for the treatment of near wellboredamage. Bench scale heating tests were calTied out oncores taken from a tight gas-bearing formation todetermine the effect of heat on perDleability, fluidsatumion, and mineralogy (i.e. degradation of in-situminerals). The experimental stimulation results arepresatted in this paper.

Non-thermal Curative Processes

One approach is to bypass die near weliboredamage using hydraulic fracturing. This technique isvery effective in sandstone formations and in verticalwells. However, there arc situations where hydraulicfracturing is not desirable (eg. in water or gasfloodingsituations, mnes containing active bottom water or gascaps) or not economical (eg. in some horizontal wells).

Another approach is to snmulate die near wellboreregion using acids, which dissolve die surroundingformation rodc (HCI and HF acids). Matrix snmu1ationtechniques using acids have been applied to carbonatereservoirs for productivity improvement. In some cases,however, die reaction of HF' (or HBF .)10-11 acid withfeldspars and other minel'als can result in variousinsoluble precipitates. In addition, many acids have alimited effective penetration due to rapid spending whenin contact with the formation.9 In both hydraulicfracturing and acid stimulation techniques, die properdesign of the fluid system is very important. When thefluid systems contain fresh water, the chances of furtherphase trapping and water blocking exist.

(FHT)FORMATIONCONCEPT

HEAT TREATMENT

The concept of applying intense heat for thetreatment of near wellbore fonnation damage wasevaluated. The process26 consists of exposing thefonnarion to an elevated temperature to cause:

Vaporization of blocked waterDehydration of die clay structure

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RESERVOIR DESCRIPTION AND FORMATIONEVALUATION

Partial destruction of the clay mineralsPossible microfracturing of the fonnation in thenear wellbore area due to thermal induced stresses

Two core samples were selected to conduct FHTstimulation based on their initial penneability andporosity (Table I). Petrographic studies were carriedout to characterize the core samples. X-raydiffractometry (XRD), and scanning electronmicroscopy (SEM) ~ used in d1ese characteriDtionstudies. Pre- and post- FHT XRD and SEM analyseswere conducted on Sample A. Pre- and post-FHT XRDanalyses were conducted on Core Sample B. A briefdescription of the results is presented below.

The dehydration and vaporization of bound andblocked water occur at temperatures higher d1an thesaturation temperature corresponding to the reservoirpressure. The extent of clay destruction also depends onthe heating temperature.

We based the selection of our experimentaltemperature conditions on X-ray diffraction (XRD)results from isolated kaolinite and smectite samples inthe laboratory published earlier'. The spectra from akaolinite sample treated at five different temperatureswere presented2'. One can see that heating to 107°Cand 300°C produces no change in the XRD spectrum,OOIDpareci to that of the air dried sample. This indicatesthat heat treatment up to dtese temperatures does notaffect dIe kaolinite sbUcture. However, at 550°C, it isobserved that all the kaolinite peaks disappear,indicating complete destruction of the kaolinite stn1ctw'eat this temperature. The AI-hydroxyl bonds becomedehydrated. and the well ordered crystalline formdegenerates into an amorphous arrangement. The onlyremaining peak is at 3.52 A. which COne5ponds toanatase, a TiO2 phase that was present in the original

sample.

XRD results indicate that bodt Samples A and Bcontained 6.3% and 5.8% total clays, respectively.These clays are mostly comprised of kaolinite and illite:

Sample A: illite 64.2%, kaolinite 35.8%Sample B: illite 68.4% and kaolinite 31.60/.

Trace amounts of chlorite materials were also seenin both samples. The illite component loses itshygroscopic water after one hour of heating attemperatures of 125 - 250°C. The kaolinite group isreplaced by amorphous meta-kaolin after one hour ofheating at temperatures of 575 - 625°C.

The post-FHT XRD analyses on Sample A indicatedJat die total clay content decreases from 6.2% to 1.5%.The total clay content decreases from 5.8% to 2.70/0 inSample B after FHT stimulation at 649°C. It appearsdlat in Sample A. kaolinite clays are eidier completelydestroyed or converted to meta-kaolin after FHTstimulation at 649°C. On die odier hand, illite materialsappear to be reduced from 68.4% to 37.2% after FHTstimulation at 64~C. These results are presented inTable 2.

Similarly. six XRD spectra from a sm~ samplewere also presente(J27. The top specbum was dtat of thewet sample. The peak at 19.09 A corresponded to theswelled state of smectite, in which each Ca + ion on the

clay surface was surrounded by four layers of water. Asthe sample was heated up to 300°C. this peak shiftsprogressively to the right, indicating progressivevaporization of the water. No major clwlge in the XRDSpecbum was observed between the 300°C and SSO°Cb'eatDlents, revealing that the intel"iayeI" dewateringprocess is complete after heating to 300°C. At 800°C.the peak intensities decreased significantly. As withkaolinite, the crystallinity of smectite was destroyed,resulting in an amorphous structure dtat does notgenerate any strong XRD peak.

EXPERIMENTALPROCEDURE

EQUIPMENT AND

A conventional appanttus was used to measure dieeffective penneability of die core samples. The sampleswere dien mounted in a tri-axial core holder andconrmed at a nominal overi>urden pressure. Theportions of die core holder directly adjacent to dieinjection and production ends of die core were equippedwidi radial distribution plates to ensure evenlydistributed nitrogen flow into and out of die corespecimen. Pressure, temperature, and velocity of dienitrogen flow dlrough the core were measured using a

Based on die above results and die literature data ,various temperatures (i.e. 120, 31S, 480 and 6S0°C)were selected to evaluate die effect of intense heating ondie penneability of die two test cores discussed next.

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Klinkenberg corrections were made by plotting themeasured penneability values versus the inverse ofmean pressures in atmospheric units. Subsequently, theKlinkenberg corrected penneability values are read asthe penneability at zero inverse of mean pressure.

nitrogen perrneameter. Rotameter-style flow meterswere used to facilitate the flow measurements. Allpenneability measurentents were carried out using thisprocedure. Perrneabilities were measured at three ratesfor Klinkenberg corrections.

Heat cycling was conducted by placing therespective core samples into an Inconel reactor andheating the reactor in a high temperature oven. Aschematic diagram of the apparatus used during the heatcycling is presented in Figure t. A constant pressure of51 MPa was maintained inside the reactor using aregulated nitrogen source and a backpressure regulator.Temperature was monitored with a thermocouple thatdisplayed the internal reactor temperature. The testingprocedure was as follows:

Results of the FHT Stimulation on Sample A

Sequential penneability changes in Core Sample Aduring ilie laboratory procedure are presented in Table3 and the values are graphically presented in Figures 2and 3. As seen in Table 3, the initial water saturationwas detennined to be 21.3%. The con"espondingeffective nib'ogen penneability was 0.02 InD.Subsequently, we flooded dIe core widt fresh water anddesaturated the core using nitrogen flow to a saturationof28.2%. The effective nib"ogen penneability at 28.2%water saturation was 0.008 mO. This corresponds to apenneabi1ity reduction of 600/0. This penneabilityreduction is the result of added water trapping in dIecore.

After the FHT stimulation at 121°C, thepenneability did not change significantly (0.01 mD).This was due to the inability to vaporize the blockedwater at the test temperature. Penneability did increasesignificantly after heating to temperatures of 315 and482°C. The FHT stimulation at 482°C increased thecore penneability to 0.035 mD and this is a permeabilityimprovement of 75% from the base line value of 0.02mO. As seen in Table 3, FHT stimulation at 482°C alsovaporized all the water from the core. Subsequentsaturation and de-satw"ation with water resulted in awater saturation of 15.3% and the correspondingnitrogen perDleability was 0.02 InD. This correspondsto the baseline permeability at an initial water saturationof 21.3%. However, this permeability is a 1500/0improvement over the water-trapped permeability of0.008 mD at a water saturation of 28.2%. SubsequentFHT stimulation to 64goC resulted in a significantimprovement in permeability (0.062 mD). Thisimprovement COrTesponds to 2100/0 from die baselinepenneability (0.02 mD) and 675% from the water-trapped permeability (0.008 mD). After water flush, theregain saturation was 5.2% and die correspondingpermeability (0.03 mD) is 500/0 more than the baselinepermeability (0.02 mD) and 275% more than the water-

trapped permeability (0.008 mD).

Subject pre-FHT samples to XRD and SEM

analyses.Fix Swi with 2% KCI fluid.Measure gas penneability to humidified nitrogen atroom temperature and full reservoir net overburdenpressure at three rates.Flush core with 2 pore volumes of fresh water (tosimulate drilling and completion induced damage)Desaturate sample by nitrogen flooding toirreducible fluid saturation; measure gaspenneability .Heat core sample to specified temperature atconstant pressure (S I MPa) inside lnconel reactor,kept at temperature for four hours, depending ondesiredtem perature level, while circulating nitrogenslowly through reactor.Cool reactor to room temperature prior to removing

sample.Measure humidified nitrogen permeability.Flush sample again with fresh water.Desaturate sample to irreducible fluid saturationprior to measuring gas permeability.Subject samples to post-FHT X-ray diffraction(XRD) and scanning electron microscopy (SEM)

analyses.

A multiple temperature heating cycle was carriedout wid1 both core samples.

EXPERIMENTAL RESULTS

The following is a summary of results from theFHT tests on Samples A and B. Penneability valueswere measured dtree times at each step. The measuredvalues were con"ected for Klinkenberg effects. The

Results oftbe FHT Stimulation on Sample B

Sequential penneability changes in Core Sample B

.{

during the laboratory procedure are presented in Table4 and the values are graphically presented in Figures 4and 5. As seen in Table 4, similar results were alsoobtained in Core Sample B. A reduction of 56% innitrogen permeability was obtained in this core sampledue to water trapping. However, in this sample theinitial water saturation was 19.2%, giving an effectivenitrogen permeability value of 0.016 mO. After waterflooding and desaturation, the water saturation was24.5%. At this water saturation. the correspondingeffective nitrogen penneability was 0.07 mO. The FlITstimulation at 315°C increases the core penneability to0.025 mO. This is a permeability improvement of 56%from the baseline value of 0.016 mO. Subsequentstimulation at 482°C resulted in an improvement inpenneability of 878/0 from the baseline permeability of0.016 mO. As seen in the table, FlIT stimulation at 315and 482°C also vaporized all water from dte core.Subsequent saturation and desaturation with waterresulted in a water saturation of 15.5% and thecorresponding permeability was 0.02 mO. This is a25% improvement over the baseline permeability of0.016 mO at a water saturation of 19.2%. However, thisis still a 185% improvement over the water-trappedpermeability of 0.007 roD.

the application of heat may also have generatedsubstantial thennal stresses in the rock. Such hightemperatures may have been sufficient to exceed d1eyield strength of the constitutive grains or cementingmaterial. which in turn would have introducedmicrofractures into the system. These microfractureswould allow increased fluid flow and thus serve as asecondary mechanism of permeability enhancement.However, this microfracture mechanism was notapparent from the petrographic studies on the heattreated samples.

If the in-situ results are comparable to thoseobtained to date in the laboratory, the FHT processwould be most suitable in situations where conventionaltreatment methods are not effective (i.e. horizontalwells) or not desirable (i.e., hydraulic fractures invertical wells with active bottom water). The processwould be applicable in sandstone formations withmoderate permeability, to ensure proper nitrogeninjectivity, with high reservoir potential, and containingswellable clays and shales. Reservoirs where fluidblocking is a cornmon phenomenon would also besuitable candidates for the FHT process.

TENTATIVE FIELD SCHEMESimilar to Core Sample A, FHT stimulation at

649°C resulted in a significant improvement inpermeability of 0.047 OlD. This improvmlmtcorresponds to 194% from die baseline penneability(0.016 OlD) and 571% from die water-trappedpenneability(O.OO7mD). Following die water flush. dieregain saturation was 10.5% and die correspondingpelmeability (0.022 OlD) is 378/0 more d1an die baselineperDleability(O.O 16 OlD) and214% more than die water-

trapped penneability (0.007 OlD).

The field implementation of the FHT process wouldinvolve the placement of a tubing- or wireline-conveyedheating device aaoss d1e perforations or d1e producingsand face, and the injection of an inert gas (eg. nitrogen)into the weUbore, through or around the heating device.The heating device can be made of an electricalresistance heating element or any other device that cangenerate heat downhole. This downhole heater raisesthe temperature of the injection gas, which in turn heatsup the formation.

DiscussionThis heating process could be designed for both

cased and open holes, vertical and horizontal wells, aslong as the formation and die casing are able to sustaindIe thennal stresses generated by dIe heat source. In dIecase of tubing-conveyed heaters, reduction of wellboreheat losses can be achieved by injecting cool gasthrough dIe annular space.

At this point, it is appropriate to summarize dtevarious mechanisms which have affected the effectivepenneability of these cores upon intense heating. Theforemost effect of heat was to vaporize the b'8ppedwater. Secondly, the hygroscopic water of the claymaterials were also vaporized. Finally, it was evidentfrom the petrographic studies that various clay mineralswere degraded at very high temperatures, and that thiseffect contributed to the drastic increase in effectivepenneability observed after the high temperaturetreabnents.

CONCLUSIONS

Experimental results obtained to date in thelaboratory indicate d1at the application of intense heatvaporizes bound and blocked water. destroys claylattices. and ultimately increases the penneability ofBesides water vaporization and clay degradation,

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9clay-rich formations. A significant increase inpermeability can be achieved in cores exposed to649°C. This intense heating process can be used toincrease the penneability of cores taken from light gasreservolfS.

Lund, L., Fogler, H.S. and McCune, C.C.:"Predicting the Flow and Reaction ofHCl/HF AcidMixtures in Porous Sandstone Cores", SPE J..Trans. A/ME, Vol. 261, pp. 248-260,1976.

10. Wamser, C.A.: "Hydrolysis of Fluoboric Acid inAqueous Solution",J. Am. Chem. Soc., Vol. 70,pp1209-1213,1984.

ACKNOWLEDGEMENT

The authors wish to dIank Chevron USAProduction Company for the permission to publish thiswork and also Kerby Aslesen of Chevron USA forassisting in the design of d1ese experiments.

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14. Schaible, D.F.: "Identification, Evaluation, andTreatment of Fonnation Damage, OffshoreLouisiana", SPEpaper 14820, 1986.Borchardt, J.K., Roll, D.L. and Rayne, L.M.: "Use

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3,

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No. 1,1998.

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TABLEtFHT FEASffiILITY STUDY

PERMEABILITY AND POROSITY DATA8

Penneability_(mD)

Porosity(OJ.)

Sample ID

AB

0.280.31

8.708.30

TABLE 2FlIT FEASIBILITY STUDY

XRD RESUL 1'8

TABLE 3FHT FEASIBILITY STUDY

SUMMARY OF PERMEABILITY VALUES ON CORES - SAMPLE A

TABLE 4FHT FEASIBILITY STUDY

SUMMARY OF PERMEABILITY V ALVES ON CORES - SAMPLE B

RCKME1FHT FEASlBIUlY STUDY

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FHT FEASlBlUTY STUDY - 8AM'lE A~ :-.. ~7 . 0-02 IICJ)

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