sustaining conductivity.pdf
TRANSCRIPT
7/21/2019 sustaining conductivity.pdf
http://slidepdf.com/reader/full/sustaining-conductivitypdf 1/14
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
7/21/2019 sustaining conductivity.pdf
http://slidepdf.com/reader/full/sustaining-conductivitypdf 2/14
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
7/21/2019 sustaining conductivity.pdf
http://slidepdf.com/reader/full/sustaining-conductivitypdf 3/14
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
7/21/2019 sustaining conductivity.pdf
http://slidepdf.com/reader/full/sustaining-conductivitypdf 4/14
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)
7/21/2019 sustaining conductivity.pdf
http://slidepdf.com/reader/full/sustaining-conductivitypdf 5/14
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.
7/21/2019 sustaining conductivity.pdf
http://slidepdf.com/reader/full/sustaining-conductivitypdf 6/14
6 SPE 98236
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.
7/21/2019 sustaining conductivity.pdf
http://slidepdf.com/reader/full/sustaining-conductivitypdf 7/14
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.
7/21/2019 sustaining conductivity.pdf
http://slidepdf.com/reader/full/sustaining-conductivitypdf 8/14
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.
7/21/2019 sustaining conductivity.pdf
http://slidepdf.com/reader/full/sustaining-conductivitypdf 9/14
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.
7/21/2019 sustaining conductivity.pdf
http://slidepdf.com/reader/full/sustaining-conductivitypdf 10/14
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.
7/21/2019 sustaining conductivity.pdf
http://slidepdf.com/reader/full/sustaining-conductivitypdf 11/14
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.
7/21/2019 sustaining conductivity.pdf
http://slidepdf.com/reader/full/sustaining-conductivitypdf 12/14
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.
7/21/2019 sustaining conductivity.pdf
http://slidepdf.com/reader/full/sustaining-conductivitypdf 13/14
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.
7/21/2019 sustaining conductivity.pdf
http://slidepdf.com/reader/full/sustaining-conductivitypdf 14/14
14 SPE 98236
0
10
20
30
40
50
60
70
80
90
100
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
24 hours 48 hours
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.