SPE 75158

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SPE 75158 AN ALTERNATIVE MODEL FOR FILTER CAKE
FORMATION IN FRACTURES Randy Seright New
Mexico Petroleum Recovery Research Center
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• TOPICS COVERED IN SPE 75158
• New model for filter cake formation in fractures.
• Effects of temperature and gel composition on
  extrusion for Cr(III)-acetate-HPAM gels.
• Extrusion behavior for a guar-borate gel.
• Effects of gel composition, fracture width, and
  addition of particulates on washout of gels from
  fractures (during brine injection).

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• WHAT ARE GEL PROPERTIES DURING EXTRUSION THROUGH
  FRACTURES?
• GEL USED Cr(III)-acetate-HPAM
• 0.5 Ciba Alcoflood 935 HPAM,
• 0.0417 Cr(III)-acetate,
• 1 NaCl, 0.1 CaCl2, pH6.
• Gelation time 5 hrs at 41C.
• Aged 24 hours at 41C before use.
• Injected into brine-saturated fractured
• cores, 41C.

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• PROPERTIES OF Cr(III)-ACETATE-HPAM
• GEL DURING EXTRUSION THROUGH FRACTURES
• A minimum pressure gradient must be met before a
  formed gel will extrude through a fracture.
• Once the minimum pressure gradient is met, the
  pressure gradient during gel extrusion is not
  sensitive to injection rate.
• Once the fracture is filled with gel, pressure
  gradients are stable (no progressive plugging).
• The pressure gradient for gel extrusion varies
  inversely with the square of fracture width.

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• PROPERTIES OF Cr(III)-ACETATE-HPAM
• GEL DURING EXTRUSION THROUGH FRACTURES
• Gels dehydrate, thus retarding the rate of
  movement of the gel front.
• Although water leaks off through the fracture
  faces, crosslinked polymer cannot.
• Dehydrated (concentrated) gel is immobile.
• Mobile gel is the same as the injected gel.
• Mobile gel wormholes through immobile gel.

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Leakoff rate total matrix production
rate total fracture area
wf 0.04 in.
Average Injection Flux
ul 0.05 t -0.5
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WHY DOES ul 0.05 t -0.5 ? Analogy with leakoff
from hydraulic fracturing ul C1 t -0.5 from
SPE Monograph Volume 2 (1970), p. 33. Basis
Assume formation of a filter cake of uniform
thickness, L, and permeability, k, with pressure
drop, ?p. ul ?p k / (? L) and L C2 ? ul
dt Solution ul C1 t -0.5 C1
?pk/(?C2)0.5
fluid flow in fracture
filter cake, k
L
?p
water leakoff
fracture face
porous rock
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• WHY DOES
• ul 0.05 t -0.5 ?
• Our problem is slightly different
• The filter cake is not areally uniform.
• Fresh gel wormholes through concentrated gel.

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BASIS OF NEW MODEL (Model details are in SPE
75158) On first contact, fracture area is
covered with mobile gel. With time, more and more
concentrated gel accumulates. Area contacted by
mobile gel decreases with time. Mobile gel forms
wormholes through immobile gel. Dominant leakoff
is from mobile (wormhole) gel.
Fraction of fracture area covered by immobile
concentrated gel
30 minutes
Fraction of fracture area covered by mobile
wormhole gel
1 minute
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• CONVENTIONAL FILTRATION MODEL
• VERSUS THE NEW MODEL (SPE 75158)
• CONVENTIONAL MODEL ASSUMES
• Areally uniform coverage with filter cake.
• Leakoff rate decreases with time because the
  filter cake becomes thicker with time.
• NEW MODEL ASSUMES
• Two types of areal coverage
• Mobile gelthe dominant water source.
• Immobile gelminor water source.
• Leakoff rate decreases with time because the area
  covered by mobile gel decreases with time.

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Summary of all tests to date at 41C.
Gel 0.5 HPAM, 0.0417 Cr(III)-acetate, 41C.
Flux 129 to 66,200 ft/d, Lf 0.5 to 32 ft, hf
1.5 to 12 inches, wf 0.02 to 0.16 inches.
wf, inches
New model ul um / 1 ? ul dt /wf
ul 0.05 t -0.5
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Conventional filter cake model
Vertical views intersecting the fractures
New model
Active flow path
Filter cake or concentrated gel
Fracture height
Porous rock
Fracture width


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Pressure gradients required to extrude a
Cr(III)-acetate-HPAM gel through fractures
dp/dl0.02/wf2
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Conventional filter cake model
Vertical views intersecting the fractures
New model
Active flow path
Filter cake or concentrated gel
Fracture height
Porous rock
Fracture width


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Gel injection rate 200 cm3/hr or 413 ft/d, Lf
x hf x wf 48x1.5x0.04-in.
Pressure Behavior in the Fracture During Gel
Injection
After 1st gel contact, the conventional model
predicts that flow channel width should decrease
and pressure gradient should increase with gel
throughput. The new model predicts that they
should remain about the same.
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• CONCLUSIONS
• For extrusion of Cr(III)-acetate-HPAM gels
  through fractures in water shutoff applications,
  a new filter cake model was developed that
• is based on fracture area in contact with
  immobile concentrated gel increasing with time
  rather than filter cake thickness increasing with
  time.
• predicts leakoff rates similar to those for the
  conventional filter cake model.
• correctly predicts that pressure gradients during
  gel extrusion do not increase with throughput.
• may be of interest in other areas (i.e.,
  hydraulic fracturing, produced water
  re-injection).

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LEAKOFF RATES Guar-borate vs.
Cr(III)-acetate-HPAM
Average dp/dl 21.7 psi/ft, Breakthrough at 9.2
fracture volumes
New model for Cr(III)-acetate-HPAM
0.36 guar, 0.018 NaBO2, 0.24 tallow soap, 0.1
surfactant, aged 1 day at 40C, Lf x hf x wf
6x1.5x0.04 in., Injection flux 4,130 ft/d.
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0.36 guar, 0.018 NaBO2, 0.24 tallow soap, 0.1
surfactant, aged 1 day at 40C, Lf x hf x wf
12x12x0.04 in.
Flow direction
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• Compared to the conventional filter cake model,
  if dp/dl and the total filter cake volume are
  fixed, the new leakoff model predicts
• similar leakoff rate behavior,
• higher average fluid velocities,
• higher shear rates at the fracture wall,
• higher shear stresses at the fracture wall.

 
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PROPERTY COMPARISONS FOR A POWER-LAW FLUID New
Model Prediction Divided by Conventional Model
Prediction
??w uave ?w dp/dl
Constant pressure gradient (ww /wc)1/(n1) (ww /wc)(n2)/(n1) ww /wc 1
Constant injection rate wc /ww 1 (wc /ww)(n1) (wc /ww)(n2)
wc flow opening for conventional filter cake
model ww wormhole width ww/ wc gt 1 -1 ? n ? 0