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SPE 98236

Sustaining ConductivityJ.D. Weaver, D.W. van Batenburg, M.A. Parker, and P.D. Nguyen, Halliburton Energy Services Group

Copyright 2006, Society of Petroleum Engineers

This paper was prepared for presentation at the 2006 SPE International Symposium andExhibition on Formation Damage Control held in Lafayette, LA, 15–17 February 2006.

This paper was selected for presentation by an SPE Program Committee following review ofinformation contained in a proposal submitted by the author(s). Contents of the paper, aspresented, have not been reviewed by the Society of Petroleum Engineers and are subject tocorrection by the author(s). The material, as presented, does not necessarily reflect anyposition of the Society of Petroleum Engineers, its officers, or members. Papers presented atSPE meetings are subject to publication review by Editorial Committees of the Society ofPetroleum Engineers. Electronic reproduction, distribution, or storage of any part of this paperfor commercial purposes without the written consent of the Society of Petroleum Engineers isprohibited. Permission to reproduce in print is restricted to a proposal of not more than 300words; illustrations may not be copied. The proposal must contain conspicuous

acknowledgment of where and by whom the paper was presented. Write Librarian, SPE, P.O.Box 833836, Richardson, TX 75083-3836, U.S.A., fax 01-972-952-9435.

AbstractRapid loss of fracture conductivity after hydraulic fracture

stimulation has often been attributed to the migration of

formation fines into proppant pack or the generation of finesderived from proppant crushing. Findings presented in this

paper suggest that diagenesis-type reactions that can occur

between proppant and freshly fractured rock surfaces can lead

to rapid loss of proppant-pack porosity and loss of

conductivity. Generation of crystalline and amorphous porosity filling minerals can occur within the proppant pack

because of chemical compositional differences between the

proppant and the formation, and the compaction of the proppant bed due to proppant pressure solution reactions.

This damage mechanism is applicable to all propped,

fracture-stimulated wells; however, it is more significant inhigh temperature and high stress wells. It provides a possible

explanation for the difference often observed between

reservoir simulation of production after fracturing and actual

production.

Studies indicate as little as 25% of the initial proppant- pack porosity may remain after only 40 days at 300°F and

6,000-psi closure stress. The rate of porosity loss can be

influenced by the surface treatment of the proppant, which

indicates that some control of this process may be

accomplished.Significance of this discovery has great impact on the

economic life of a fracture-stimulation treatment. It affects thechoice of proppant composition and post-fracture cleanup

procedures, and adds an additional dimension to the

appropriate laboratory determination of fracture conductivity

that might be expected with the use of a particular proppant.

IntroductionLehman et al .1 reported that the use of surface-modification

agents (SMA) to coat proppants used in propping hydraulicfractures resulted in sustained and more uniform production

from wells. Fig. 1 taken from that publication shows the

production decline curves from some of their data, and it does

appear to show a significant change in decline rate compared

to the use of untreated proppant.

Initial use of this type of SMA treatment was promoted asa method to increase the conductivity of proppant owing to its

ability to prevent close packing of the proppant, which can

result in increased porosity and permeability of the pack by

rendering the proppant surface tacky. Subsequent studiesindicated that its use provided proppant-pack protection from

fines infiltration and migration. This mechanism has beenemployed to explain the observations that sustained

production results from the use of SMA on proppants. This is

further substantiated by long-term results obtained in a single

field study known for fines production problems. That both

mechanisms are active has been well established through

laboratory studies, but they alone do not completely explainthe reduction in production decline rate as reported.

A field study of SMA-treated proppant was reported to the

Arkansas Oil and Gas Commission 2004 CBM Workshop thadisclosed long-term results on gas production. These were

CBM wells in the San Juan Basin that typically required

refracturing each year to produce at an economical rate. With

the SMA-treated proppant, no refracs have been required, andas shown in Fig. 2, production has remained essentially

constant for 5 to 6 years. This longevity was initially attributed

to prevention of fines invasion into the proppant pack

however, it is possible that there are additional mechanisms

operational.

Terminology

Conductivity

Hydraulic conductivity is simply the ability of a conduit to

transmit a fluid. It is a function of the fluid properties and theconduit geometry. It is determined by measuring the pressure

drop and fluid rate for a specific fluid through a conduit ofixed length with respect to the cross-sectional flow area. I

the conduit is a pipe with fixed length, conductivity is usually

presented by friction-drop-per-length tables for a specific fluidand is calculated using the Darcy-Weisbach equation. The key

parameters in determining any hydraulic conductivity are

conduit geometry, fluid rate, pressure drop, and fluidviscosity.

Fracture Conductivity

A fracture generated during a hydraulic-fracturing treatment is

a fluid conduit and has conductivity. This conductivity is

responsible for the difference in the pre- and post-fracturing



2 SPE 98236

well productivity. In practice, the exact geometry of a

hydraulically generated fracture is not known exactly;

therefore, the actual fracture conductivity cannot be calculateddirectly. Sophisticated well test methods have been developed

that utilize transitory pressure measurements to provide

estimates of fracture dimensions and conductivity. Proppant . Most wells stimulated by hydraulic-fracturing

methods use granular agents to sustain fracture geometry afterrelieving the hydraulic pressure applied during the fracturing

operation. These materials range in composition from simplequartz to high-strength ceramics and are carefully classified by

size and shape to be as monosperse as feasible to maximize

permeability and strength. Proppant partial monolayer . On some occasions, proppant

is used below a concentration required to achieve a completemonolayer of proppant in the fracture. This is referred to as a

partial proppant monolayer. Partial monolayer propped

fractures provide very high fracture conductivities because of

their high porosity. They are limited however by the diameterof the proppant grain and generally cannot stand much closure

stress, thus mostly limiting their application to low stress

wells. Proppant pack . The hydraulic conductivity of a proppant

pack more than a monolayer thick is limited by the porosity of

the pack. This is typically 38–42% for a well-classified

proppant. Small changes in pack porosity result in significant

changes in pack permeability and fracture conductivity.Fracture conductivity is designed by controlling concentration

of proppant used to hold the fracture width open and is limited

by the porosity of the pack. Certain tackifying agents can beapplied during frac treatments that enhance proppant pack

porosity by causing the proppant to resist forming tight packs

resulting in higher than expected porosity.

Proppant Bed ConductivityThis is a principal parameter used in numerical fracturing

simulators to optimize fracture conductivity. The American

Petroleum Institute implemented a standard proppantconductivity measurement method for comparing proppants.

The proppant conductivity values derived by this method are

used in most modern fracture design simulators. This term issometimes also called fracture conductivity and can lead to

confusion as it is only one factor of several that has direct

impact on fracture conductivity.Proppant bed conductivity is determined by measuring the

pressure drop of a fluid through a uniformly distributed

proppant bed in a cell with fixed length and height. The width

varies with proppant concentration and closure stress. Theflow capacity of this proppant bed is typically measured with

respect to closure stress for a particular fluid and temperature.To ensure commonality between testing labs, API RP-61 has

been adopted as the standard method.2 The principle use for

proppant bed conductivity values is for the economicoptimization of fracture treatment designs in numerical

simulators developed to permit the simulation of fracture

geometry. The fracture geometry predicted by these simulators

is used to optimize fracture conductivity based on treatmentdesign criteria.

Proppant embedment . The interface area (where proppant

pack contacts the formation face) carries the overburden load

and stresses may not be well distributed in this area. It is

believed that most damage to conductivity occurs in this

region. Examination of the formation core faces afterconductivity measurements reveal insight into the embedmen

of proppant into the core material. Very soft formation

material may be imbedded one or two proppant grains deep

while on hard rock, only minor embedment is observed. The

size and distribution of embedment footprints provide somequantification of this effect.

Proppant stress cycling . Conductivity studies are often performed by cycling the closure stress and flow rates to

simulate flowing wells at different drawdown pressures

Generally, there is a significant loss of conductivity each time

stress is increased until the pack is well stabilized. This type of

testing is often used with soft formations to induce fines

movement from the formation into the pack. Proppant crushing . Proppant crushing can occur at many

sites during fracturing operations. While proppant is wel

classified at the manufacturing site, it is transferred at leastthree times before going downhole. Cracking and chipping can

occur during each of these transfers and efforts should be

taken to minimize this exposure. However, the major sourcefor crushing is formation closure, particularly where the

proppant is not well distributed. Examination of proppan

packs after conductivity studies indicates that crushing is mos

prevalent at the interface and less significant toward the cente

of the pack.Fines infiltration. Rublized formation material and sof

formation material can be produced back after a fracture is

packed with proppant. If these fines are too large, or too highin concentration, they filter out on the proppant pack and

create a pack that is significantly lower in permeability

Infiltration of fines into a pack in effect reduces the

conductive width of the fracture and provides a source of fines

that may migrate upon stress cycling.Fines invasion. Proppant size used in soft formations is

often dictated by the size required to mechanically prevent

formation fines from invading the proppant pack. Theappropriate use of proppant surface modification agents allows

significantly larger proppant to be used in these applications.Fines migration. The free movement of fines through a

proppant pack generally does not impact conductivity much

but may result in significant equipment problems during

production. However, pack plugging can occur even when

fines are small enough to flow freely through a pack byfloculating to form larger particles, which can dramatically

reduce permeability of the pack. Floculation can be induced

by slight changes in surface chemistry and ionic compositionof the producing fluids.

Reactive surface. The hydraulic-fracturing process

actually creates new highly reactive surfaces by mechanically

breaking the chemical bonds of the formation and exposing

fresh surfaces. Upon closure, some of the proppant grains placed in the fracture to support the closing fracture walls wil

actually break, again creating highly reactive surfaces. It is

these newly exposed surfaces that are available to react with

fluids and minerals. Diagenesis. Diagenesis is the alteration of sediments into

rock at temperatures and pressures that can result in significan

changes to the original mineralogy and texture. It is generally



SPE 98236 3

accepted that sediments become compacted as they experience

higher overburden loads caused by successive sedimentation.

The porosity of the sediment is gradually infilled partially withmineral deposits that cement the particles to form the rock.

This process is generally thought to proceed slowly, requiring

centuries to manifest the change. Proppant-pack diagenesis. Clean proppant packs placed

into hydraulically created fractures in formations of hightemperature and stress undergo rapid diagenic-type reactions

that result in dramatic reduction in pack porosity. It has beendiscovered3 that dissolution-mediated compaction reactions

are accelerated from a few centuries (normally expected with

diagenesis) to a fraction of a year as the temperature is

increased, resulting in decreased porosity (15–25% of the

starting porosity).

Proppant StabilityProppant stability under realistic downhole conditions is an

area generally ignored. During the early development ofanalytical methods,4-6 to qualify proppant, long-term testing at

high stress and temperature indicated that conductivitycontinues to decrease with time and exposure. Fig. 3, taken

from McDaniel4 shows clearly that conductivity declines with

time. In the API method2 adopted for classifying proppants,

only short-term conductivity is used. It has been generally

believed that most damage to conductivity that occurs during

testing is due to failure of the proppant caused by crushing,which results in reduced fracture width and proppant bed

permeability. This mechanism probably is predominating

during the early time at a stress; however, this does notexplain the gradual decline in conductivity with the longer

time exposure to high-stress conditions.

Cobb et al. (Fig. 4) reported data7 collected with aspecially designed system aimed at eliminating all other

permeability-damaging mechanisms other than proppantcrushing, especially potential system corrosion. These tests

were performed during a much longer timeframe (70–80 days

under realistic conditions) and demonstrated that conductivity

continued to decline at a fairly steady rate throughout theentire testing program at 7,000 psi closure stress and 212°F.

Visual inspection of proppant packs after exposure tosimulated closure stress and after API conductivity testing

reveals that the layer of proppant adjacent to the simulated

formation (usually Ohio sandstone) sustains some damage in

that some grains are cracked with a few shattered (Fig. 5).This is observed perhaps two or three proppant grains deep,

provided the closure stress conditions were below the crush

strength of the proppant being tested. Generally, very littledamage is observed for any proppant in the interior of the

proppant pack. However, it has been observed that significantquantities of fines seem to be generated in some of these tests,

and this was most often attributed to proppant crushing, but

little laboratory data was gathered to support this conclusion.Fig. 6 is an example of some of this porosity filling material

that forms during the testing at stress. This material does not

seem to form at low closure stresses.Fig. 7 is a collection of micrographs from a test in which

efforts were made to identify this material formed during the

testing.3 Zooming in by electron dispersive X-ray (EDX) on

various areas of interest in the sample provided considerable

insight. The silica-to-aluminum ratio observed for the

proppant was 0.9, as is typical for the ceramic proppant, while

that for the Ohio sandstone was 8.4. The porosity filling precipitate was found to be 4.9, or an intermediate

concentration of these metals. Visual inspection of the

proppant packs after exposure reveals numerous areas o

crystalline growth, as shown in the last two micrographs. The

2.8 ratio of Si/Al is characteristic of some clay minerals.It is apparent from these observations that some sort of

geochemical reactions are taking place when high mechanicalstress is applied to the proppant by Ohio sandstone in aqueous

media, and that these reactions seem to be attenuated by

temperature.

Proppant DiagenesisClassical diagenesis occurs when permeable sandbeds are

buried by subsequent deposits, resulting in exposure to high

closure stress at high temperature for centuries. The sandbeds

through geochemical reactions, are converted to low-porositylow-permeability rock.

Most hydrocarbon-bearing formations that requirehydraulic fracturing to produce economically are mature, have

already undergone diagenesis, and typically have high closure

stress and temperature conditions. When the rock is cracked

and packed with virgin proppant, conditions are right to

promote geochemical reactions that cause diagenic reaction

to begin filling the porosity of the proppant pack. Thesereactions are surprisingly faster than normally expected.

Yasuhara et al . reported8 that “at effective stresses of 5,000

psi, with temperatures in the range 170–570°F, the rates o porosity reduction and ultimate magnitudes of porosity

reduction increase with increased temperature. Ultimate

porosities asymptote to the order of 15% (570°F) to 25%(170°F) (of the original porosity) at the completion of

dissolution-mediated compaction and durations are acceleratedfrom a few centuries to a fraction of a year as the temperature

is increased.”Figs. 8–10 show the significance of proppant size

reservoir temperature, and closure stress on the rate at which

compaction and porosity loss can occur. The starting porosity

of each of these curves was 37%, and the plots show the percentage of retained porosity based on a geochemica

compaction model. According to this model, in low-

temperature shallow wells, compaction and porosity loss may

not be a significant issue, but as temperature and stress

increase, the possibility for this mechanism to contribute tofracture conductivity loss is significant. For reservoirs near

390°F and 7,000 psi, only 17% of the initial pack porositywould be expected after just 10 days of post-fracturing.

The model just described assumes that the materials in

contact are silica-based and are the same; therefore, there is no

great potential chemical difference between the formation and

the proppant.Engineering properties of proppant strength, embedment

fines plugging, and the like have been well studied and are

mostly understood. However, the chemistry associated with ahydraulically generated fracture packed with proppant having

a very different mineralogy from that of the reservoir has not

been well studied. Additional knowledge and understanding o



4 SPE 98236

this chemistry could give rise to the development of many new

porosity-filling minerals.

Pressure Solution and Compaction Mechanism

The solubility of quartz is about 50 ppm at room temperature

and increases proportionally with temperature. However,

when two quartz grains are brought into contact, and a high

mechanical stress is applied, the solubility at the contact pointsis greatly increased because of the strain placed on the

molecular bonds. As the soluble silica diffuses through thewater film to the pore space, the solution in the pore space

becomes supersaturated because it is no longer under high

mechanical stress and subsequently precipitates, thereby

reducing the pore volume (Fig. 11).This results in two effects: (1) removal of material from

between the grains flattens the surface between them and leads

to compaction, which causes a loss of fracture width if the

proppant is supporting a packed fracture (Fig. 12) and (2) a

reduction of porosity resulting in reduced permeability andfracture conductivity. Both of these mechanisms depend on

the presence of a wetting water film for the reactions to occur.This model is quite simple, invoking only the use of silica

materials. When alumina, zirconium, titanium, calcium, iron,

and other ions are present in the proppant, the formation of

clay-like minerals is very likely.9 Recognizing that there is a

considerable chemical potential energy difference between the proppant and the formation mineral leads one to a similar

pressure solution and compaction mechanism. As an analogy,

consider the system similar to the galvanic corrosion that

occurs when two dissimilar metals are brought into contact.However, this reaction only occurs when a conductive water

film is present. To prevent these reactions, either a dielectric

or hydrophobic film is placed between the surfaces.

Reduction of Proppant Diagenesis ReactionIt has been found that coating proppant with a dielectric

material such as SMA can significantly inhibit the

geochemical reactions that lead to diagenesis and porosityloss. While no analytically accurate method has thus far been

developed to satisfactorily quantify the change in diagenesis

rate within a reasonable timeframe, it is apparent from visualinspection that a significant effect can be provided by the

application of certain hydrophobic SMA films. The best

results seem to be produced when both the proppant and the

formation face are coated with the SMA material.Early applications of SMA in hydraulic fracturing (as

shown in Fig. 2) involved adding the material directly to the

fracturing fluid-proppant blend. Only about 70% proppant-coating efficiency was achieved with this method; the

remainder was dispersed in the aqueous frac fluid. It is possible that a significant portion of this dispersed material

coated the formation face and may have contributed

significantly to reduction of subsequent geochemicalreactions. Later applications of SMA have been improved so

that the proppant-coating efficiency is greater than 90%. A

neural net study and review of field results is planned to

determine if this has had an effect on production rate decline

curves as might be expected from diagenesis-type reactions.Insertion of the compaction and pressure solution model

into fracture-stimulation production models shows the

dramatic effect expected to occur to the production decline

profile (as shown in Fig. 13). This profile indicates that at low

stress, diagenic reactions have a minimal effect, but astemperature and stress increase, they become a predominan

factor.

These geochemical reactions and their impact on fracture

width and porosity may provide an explanation as to why

some formations require frequent refracs to achieve suitable production rates. At the highest level, one might conside

these reactions nature’s way of “healing” the hydraulicallygenerated fracture. The use of dissimilar minerals for

proppants may exacerbate the rate of porosity loss.

Surface Modification of Proppant

To combat proppant diagenesis, a new SMA material has been

developed specifically for use with aqueous fracturing

operations. This material helps ensure treatment of the

formation face as well as the proppant to create a hydrophobic

film to minimize geochemical reactions that require a waterfilm to proceed. In addition to providing a dielectric film to

protect the proppant surface from attack by aqueous reactions

this material also provides a tacky surface for excellent controof fines by preventing invasion from the formation and

migration through the pack. The conductivity enhancemen

derived from use of this material is similar to that previously

described for nonhardening surface modification agents.10-15

Laboratory StudiesMost tests were performed using 3-in. diameter radia

conductivity cells fitted with Ohio sandstone core wafers onthe top and bottom of the proppant bed. A proppant loading of

2 lb/ft2 was used with 2% KCl as the fluid medium. It is

important to recognize that for these tests, no flow wasallowed, only static conditions for the test time. For most

studies, a time at conditions of 140 hr was used. Following thetime at conditions, the cells were carefully disassembled, and

the Ohio sandstone wafers were examined to determine

proppant embedment by optical microscopy. The proppan

pack was examined by ESEM with particular attention tosurvey the proppant layer next to the Ohio sandstone wafer in

comparison to the center of the proppant pack (see Figs. 5–7)High-quality quartz sand and commercially available ceramic

and bauxite proppants (20/40 mesh) were used. During the

ESEM examination, areas of geochemical change were

identified, and high magnification micrographs with EDX

scans were obtained. Figs. 14–18 show some results with and

without SMA treatment after about 140-hr exposures at 250°F

and 10,000-psi closure stress.

API Conductivity Cell Testing

The objective of this testing was to compare the performance

of conventional and new types of SMA material. Comparison

were used to determine whether in addition to reducingdiagenesis, the new SMA can effectively control or mitigate

the invasion of formation fines into the proppant and maintain

permeability and conductivity of the proppant pack.A 5 lb/ft2 proppant pack of 20/40-mesh ceramic proppan

was sandwiched between the frozen, unconsolidated silica

wafers, which in turn were installed inside the Ohio sandstone

core wafers in linear conductivity cells (as shown in Fig. 19)



SPE 98236 5

The frozen, unconsolidated silica wafers were prepared using

wet silica flour with particle sizes of 325-mesh or smaller to

simulate unconsolidated formation faces of a soft formationand were molded into the proper shape and frozen to permit

cell assembly. The cells were then brought to an initial stress

of 2,000 psi and 180°F. An initial conductivity was obtained

by flowing through the proppant pack in the conventional

linear direction. Comparisons of conventional SMA with newSMA materials were performed by injecting the proppant pack

in the reverse direction with 3 pore volumes of treating fluid.

The initial conductivity was determined at 2,000-psi closurestress. After stable flow was achieved, flow from the core

wafers was introduced and the effluent fluid was captured to

examine for fines production. The conventional SMA and the

new SMA performed similarly, showing significantly reduced

fines after 48 hr compared to the untreated test (Fig. 20). The pack was then cycled from 2,000 to 4,000 psi several times

with a doubling in inflow rate with each cycle to try to

destabilize the pack. Fig. 21 shows the untreated proppantloses all conductivity very early in the test. However, the

SMA-treated proppants both show stable conductivity with

stress cycling.

ConclusionsGeochemical reactions can lead to rapid, dramatic loss of

porosity of proppant packs exposed to high temperature andstress conditions, leading to significant loss of fracture

conductivity. This mechanism is functional at lower

temperatures and closure stresses, but may be sufficiently slow

to not be a significant factor in production economics.The use of high-strength proppants may actually

exacerbate porosity-filling reactions by forming clay-like

minerals. This may partially mitigate the advantage of usingstronger proppants. Additional studies are needed to

understand the significance of this damage mechanism.Coating proppant with a hydrophobic film reduces the

action of water on the proppant and reduces the diagenetic,

geochemical reactions that lead to compaction.Coating both the proppant and the formation face with a

hydrophobic film provided by a new SMA appears to provide

the best protection against geochemical reactions that lead toloss of fracture conductivity due to porosity filling and

compaction mechanisms.

AcknowledgementsThe authors wish to thank the management of Halliburton for

their permission to publish this paper. Special thanks are

expressed to Gerard Glasbergen for conducting the production predictions and to Dr. Ray Loghry for his ESEM evaluations

and Mr. Bobby Bowles and Mr. Mike Gideon for theirdevelopment of testing protocols and management of long-

term conductivity testing.

References1. Lehman, L.V., Shelley, B., Crumrine, T., Gusdorf, M. and

Tiffin, J.: “Conductivity Maintenance: Long-term Results fromthe Use of Conductivity Enhancement Material,” paper SPE82241, 2003 European Formation Damage Conference, The

Netherlands, May 13-14.

2. API RP-61, Recommended Practices for Evaluating ProppanConductivity.

3. Weaver, J.D., Nguyen, P.D, Parker, M.A. and van BatenburgD.: “ Sustaining Fracture Conductivity,” paper SPE 94666, 6thSPE European Formation Damage Conference, ScheveningenThe Netherlands, 25-27 May 2005.

4. McDaniel, B.W.: “Conductivity Testing of Proppants at HighTemperature and Stress,” SPE 15067, 56th California RegionaMeeting, April 2-4.

5. McDaniel, B.W.: “Realistic Fracture Conductivities o

Proppants as a Function of Reservoir Temperature,” paper SPE16453, 1987 Low Permeability Reservoirs Symposium, DenverCO, May 18-19.

6. Parker M.A., and McDaniel, B.W.: “Fracturing Treatmen

Design Improved by Conductivity Measurements under In-SituConditions,” paper SPE 16901, 1987 Technical Conference andExhibition, Dallas, TX, September 27-30.

7. Cobb, S.L. and Farrell, J.J.: Evaluation of Long-term ProppanStability,” paper SPE 14133, 1986 International Meeting onPetroleum Engineering, Beijing, China, March 17-20.

8. Yasuhara, H., Elsworth, D., and Polak, A.: “A MechanisticModel for Compaction of Granular Aggregates Moderated by

Pressure Solution, Journal of Geophysical Research, Vol. 108 No. B11, 2530, November 18, 2003.

9. Schott, J., and Oelker, E.H.: “Dissolution and Crystallization

Rates of Silicate Minerals as a Function of Chemical AffinityPure & Applied Chem., Vol 67, No. 6 pp. 903-910, 1995.

10. Nguyen, P.D., Dewprashad, B.T., and Weaver, J.D.: “A NewApproach for Enhancing Fracture Conductivity,” paper SPE

50002, 1998 Asia Pacific Oil and Gas Conference andExhibition, Perth, Australia, October 12-14.

11. Dewprashad, B., Weaver, J.D., Nguyen, P.D., Blauch, M., andParker, M.: “Modifying the Proppant Surface to Enhance

Fracture Conductivity,” SPE 50733, 1999 InternationaSymposium on Oilfield Chemistry, Houston, TX, February 1619.

12. Weaver, J., Blauch, M., Parker, M., and Todd, B.: “Investigation

of Proppant-Pack Formation Interface and Relationship toParticulate Invasion,” paper SPE 54771, 1999 EuropeanFormation Damage Conference, The Hague, The NetherlandsMay 31-June 1.

13. Blauch, M., Weaver, J., Parker, M., Todd, B., and Glover, M.“New Insights into Proppant-Pack Damage Due to Infiltration oFormation Fines,” paper SPE 56833, Annual TechnicaConference and Exhibition, Houston, TX, October 3-6.

14. Nguyen, P.D., Weaver, J.D., Dewprashad, B.T., Parker M.A.

and Terracina J.M.: “Enhancing Fracture Conductivity throughSurface Modification of Proppant,” paper SPE 39428, 1998

Formation Damage Control Conference, Lafayette, LAFebruary 18-19.

15. SPE 48897, Surface-Modification System for Fracture-

Conductivity Enhancement, P.D. Nguyen, J.D. Weaver and B.TDewprashad. International Conference and Exhibition, BeijingChina, 2-6 November 1998.



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Fig. 1—Test production data published3 by Lehman et al . for two adjacent wells stimulated with the same size

and type of fracturing treatment using 20/40 U.S. mesh ceramic proppant.

0 500 1000 1500 2000 2500

Well 1

Well 2

Well 3

Well 4

Well 5

Well 6

Well 7

Well 8

Well 9

Well 10

Well 11

Well 12

Well 13

Gas Production Rate, MCFD

Initial Post-frac Production(Frac Treatments, Mar 1997-Mar 1999) Production, May 2004

Fig. 2—Survey of wells fractured or refractured using SMA-coated proppant showing stability of production withtime. Uncoated proppant-fractured wells generally had to be refraced each year to sustain production.



SPE 98236 7

Fig. 3—Comparison conductivity provided by ceramic and quartz-based proppantswith respect to time at 6,000-psi closure stress and 275°F.

Fig. 4—Long-term conductivity measurements made using specially designed flowsystem to eliminate corrosion as a source of conductivity loss.



8 SPE 98236

Fig. 5—Alumina-based proppant (20/40 mesh, 2 lb/ft2) before and after exposure to 10,000-psi closure stress.

Micrograph on the right is of the proppant pack face that was forced against an Ohio sandstone core material.

Fig. 6—Ceramic proppant (20/40 mesh, 2 lb/ft2) after exposure to

10,000-psi closure stress at 250°F for 140 hr in 2% KCl solution understatic flow condition. Note the formation of porosity-filling debris thatdoes not appear to be derived from the proppant. This materialappears throughout the proppant pack.



SPE 98236 9

Fig. 7—Series of micrographs showing the apparent embedment of a 20/40-mesh ceramic proppant into Ohiosandstone that occurred during conductivity testing at 6,000-psi closure stress and 225°F. Top left–Ceramicproppant grain. Debris surrounding the grain was found not to be Ohio sandstone or ceramic, but rather a new,high-in alumina material. Top right–Closeup showing how some of the new material is actually bonded to theproppant grain. Lower left–Area where a crystalline overgrowth has started growing. Lower right–Closeup of thecrystalline overgrowth.

Fig. 8—Plot showing the impact of closure stress on compaction derived bypressure solution and precipitation reactions.



10 SPE 98236

Fig. 9—The impact of reservoir temperature on compaction derived bypressure solution and precipitation reactions.

Fig. 10—The impact of proppant size on compaction derived by pressuresolution and precipitation reactions.

Fig. 11—A compaction process by pressure solution mechanism.6 At the grain-to-grain contacts, the mineral

dissolves into the water film owing to the high localized stress, causing an increase in the solubility product ofthe mineral. The solute diffuses through the water film into the pore space where it becomes supersaturated andthen precipitates, resulting in reduced porosity.



SPE 98236 11

Fig. 12—Schematic drawing showing how a packed fracture with uniform-sized proppant might undergodiageneic compaction resulting in loss of fracture width, and pack porosity and permeability.

Fig. 13—Fracture production simulation performed for a 1 mD, 300°F reservoir with a starting reservoir pressureof 3,000 psi using 10-mesh proppant with porosity filling data

8 from Yasuhara, et al .

Fig. 14—Authigenic crystal growth apparent near the edge of a craterformed by untreated ceramic proppant embedded into Ohio sandstone.



12 SPE 98236

Fig. 15—High magnification of bottom of SMA-treated proppantembedment crater in Ohio sandstone showing no apparentcrystal growth.

Fig. 16—Crystal growth apparent in the crater formed in Ohiosandstone under an embedded, untreated quartz proppant grain.

Fig. 17—Apparent crystal growth protruding from ceramic proppant formed after 140 hr at 250°F and 10,000-psiclosure stess in 2% KCl against Ohio sandstone.



SPE 98236 13

Fig. 18—Bottom of a untreated ceramic proppant embedment crater showing considerable diagentic activity afterexposure for 140 hr at 10,000-psi closure stress and 250°F in 2% KCl.

Fig. 19—Schematic of modified API linear conductivity apparatus for determining the effect of fines invasion fromthe formation into proppant packs.



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Untreated Proppant New SMA Conventional SMA

F i n e s P r o d u c e d i n E f f l u e n t , m g / L

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Fig. 20—Silica flour fines produced during conductivity study performed to compare untreated totreated proppants.

Fig. 21—Conductivity comparison of 20/40-mesh ceramic proppant with conventional SMA treatment and

treatment with a new SMA material with cyclic stress and flow conditions.

sustaining conductivity.pdf

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