Well-Stimulation Well-Stimulation Well-Stimulation

Well-Stimulation

1. :: WELL STIMULATION::

2. WELL STIMULATION
Wellstimulationistheprocessby
whichtheproductivityofawellis
increased.Productivityismeasured
intermsofproductivityindexorJ
O
J
O
= q / P
Productivity index (PI) is defined a
sth ration of flow of oil per unit draw
down (P = P
R
-P
Wf
)

3. WELL STIMULATION:
OBJECTIVES
Stimulation is a catcall term for a variety of
techniques designed to enhance well
performance in new wells, or restore
production in older wells. Stimulation of oil
and gas reservoirs, with a view to increasing
well productivity
Initially applied in carbonate reservoirs, the
technique was extended to more complex
mineralogy

4. WELL STIMULATION: METHODS
i. Hydraulicfracturing
ii.Acidizing
iii.Notroshooting
iv.De-paraffination

5. WELL STIMULATION
HYDRAULIC FRACTURING

6. HYDRAULIC FRACURING:
Objectives
The objectives of hydraulic fracturing for well
stimulation is to increase productivity of
producing zone by creating the highly
conductive path (compared to the reservoir
permeability) some distance away from the
well bore in to the formation

7. HYDRAULIC
FRACURING:Objectives
A fracturing treatment consists in breaking
down a producing section hydraulically with a
sand carrying fluid, the sand being used to
prop the resulting fracture. Usually
conductivity is maintained by propping with
sand to hold the fracture face apart

8. HYDRAULIC
FRACURING:OBJECTIVES
i.Toincreaseproductivitybypassingdamage
zone.

ii.To increase productivity in low permeability
formation by creating deep penetration fracture
(i,e, massive fracturing)

9. HYDRAULIC
FRACURING:MECHANISM
i.Pressure parting in water injection
wells.
ii.Lost circulation during drilling
operations.
iii.The break down of formation during
squeeze cementing operations.

10. HYDRAULIC
FRACURING:APPLICATIONS
i.Inalowpermeability,homogenousrock,
fracturingissimilarineffecttoincreasingthesize
ofthehole,
ii.Fracturingwilleliminateformationdamagedueto
invasionofdrillingmud,depositionofmineral
matter,orswellingofclays.
iii.Fracturesradiatingfromthewellboreactas
gatheringlinesconnectingpermeableand
poroussystemsthatareotherwiseisolatedform
thewellbyimpermeablebarriers.

11. INITIATION OF FRACTURES
A hydraulic fracturing treatment is accomplished
by pumping a suitable fluid in to the formation at
a rate faster than the fluid can leak off in to the
rock. Fluid pressure or stress is built up sufficient
to overcome the earth compressive stress
holding the rock material together. The rock then
parts or fractures along the plane perpendicular
to the minimum compressive stress in the
formation stress in the formation matrix.

12. PARAMETERS FOR HYDRAULIC
FRACTURING
For water pressure gradient is 0.426 psi/ft.
For brine pressure gradient is 0.433 psi/ft.
For over burden 1 psi/ft. 125 –200 lbs/ft
3
average 144 lbs / ft
3
.
i.The fracture will be created in a plane, which is
perpendicular to minimum principal stress.
ii.In deeper formation, tear is much easier than
lifting.
iii.In shallower formation lifting is much easier
than tear apart.

13. INITIATION OF FRACTURES
i.Pressure required to create a Horizontal
fracture
P
f
(h) = 
Z
=1.0 x D
Where D is the depth in feet
ii.Pressure required to create a vertical fracture
P
f
(v)= [ 2 / (1 -) ] 
Z
+ S
t
Where is Poisson’s ratio = lateral strain / axial
strain varies between 0.18 to 0.27 for the
average type of rocks.
S
t
= tensile stress.

14. OBSERVATION OF WELL
STIMULATION
i.Success of the well stimulation
job can be found by evaluating
productivity index (PI) before and
after the job that is
PI (after job) / PI (before job)

15. HYDRAULIC FRACTURING
The well is
obstructed by silt,
sediments,
deposits.oil can’t flow
to the borehole

16. HYDRAULIC FRACTURING
Water is
injected at a
very high
pressure

17. HYDRAULIC FRACTURING
The obstructions
are forced out of
the oil producing
formation. Oil can
now flow freely!

18. USE OF PROPPANTS
i.Fracture created in sandstone formation is
required to be propped by suitable propping
agents (or proppants).
ii. Proppants are specially selected sands of
20 –40 mesh size.
iii. The proppants should be of uniform size
and spherical shape as possible.

19. PROPPING THE FRACTURS
The objectives of the propping is to maintain
desired fracture conductivity economically.
Horizontal fracture sand used as a propping
agent tends to be crushed or embedded in
the fracture faces and recommended the use
of rounded crushed wall nut shells

20. PROPPING THE FRACTURS
In present design sand concentration ranges
fro, 85 to 137.5 lb / ft
2
of fracture are using
either 20 –40 or 10 –20 mesh sand.
However greater fracture capacity is required
the design are for 70 % 10 –20 mesh sand
followed by 8 –12 mesh angular sand are
recommended as propping agents,

21. WELL STIMULATION
ACIDIZING

22. WELL STIMULATION:
ACIDIZING
Acidisusedtoremovedamage
nearthewellboreinalltypeofthe
wells.Incarbonateformationacid
maybeusedtocreatelinearflow
systemsbyacidfracturing.Acid
fracturingisnotapplicableinSand
stonewells.

23. WELL STIMULATION:
ACIDIZING
(1) Matrix acidizingis performed below
fracturing rate and pressure. Acid flow is
through the matrix with reactions being in
existing pores and natural fractures.
(2) Fracture acidizingis performed above
fracturing rates and pressures. Etching of the
created fractures provides well stimulation,
not just damage removal

24. WELL STIMULATION:
MATRIX ACIDIZING
Itisprimarilyappliedtoremoveskindamage
causedbydrilling,completion,workoverorwell
killingfluidsandbyprecipitationofdepositsfrom
producedwater.
Duetoextremelargersurfaceareacontactedby
theacidinamatrixtreatment,spendingtimeisvery
short.Thereforeitisdifficulttoaffecttheformation
morethanafewfeetfromthewellbore..

25. WELL STIMULATION:
MATRIX ACIDIZING
Removalofsevereplugginginsandstone,
limestoneordolomitecanresultinaverylarge
increaseinwellproductivity.
Ifthereisnoskindamage,amatrixtreatmentin
limestoneordolomitecouldstimulatenatural
productionnomorethanoneandone-half
times.
Matrixtreatmentstendtoleavezonebarriers
intactofpressurearemaintainedbelowfracture
pressure.

26. MATRIX ACIDIZING: USE OF ACID
HCl(10–15%)isusedforacidwashingjobsin
whichbottomholeiscleanedbyacidreaction
withscale(CaCO
3
)etc.
Thedepositionontheinsidewalloftubingand
perforationpluggingmaybecleanedbyacid
washing.Asmallamountofacidisprovidedby
to&frocirculation.

27. ACIDS USED IN MATRIX
ACIDIZING
Theacidispumpedatapressurelowerthan
fracturedpressureoftheformation.
Theacidisintendedtoenterintothematricesof
therock.
Forcarbonate(Limestone&Dolomite)formation
10–15%HClisused.
Forsandstoneformation,acombinationof12%
HCl+3%HFcalledmudacidisused.

28. ACIDS USED IN MATRIX
ACIDIZING
HClprovidesreactionwithcarbonatepresent
inthematricesincludingwithsilica+clays.
Matrixacidizingisaimedatremovalof
formationdamage, therebyrestore
productivity.

29. FRACTURE ACIDIZING
Fracture acidizing or Acid Fracturing is an
alternative to hydraulic fracturing and propping
in carbonate reservoirs.
In fracture acidizing, the reservoir is
hydraulically fractured and then the fracture
faces are etched with acid to provide linear
flow channels to the well bore.

30. FRACTURE ACIDIZING
Fracture acidizing has no
application is sand stone wells.
Break down of a sand stone well
with acid at fracture pressure tends
to break down natural vertical
permeability barriers to adjacent
unwanted zones.

31. FRACURE ACIDIZING

32. SAND STONE ACIDIZING
Theprimaryreactiontoacidizingsandstone
wellsistoincreasewellpermeabilityby
dissolvingclaysnearthewellbore.
Theseclaysmaynaturallyoccurringformation
claysormayhaveintroducedfromdrilling/
completionorworkoveroperations.
HFcandissolvecalciumcarbonate,sand,clays,
shaleandfeldspars.However,theHFisusedto
removeclaydepositsformtherockmatrix.

33. ACID USED IN FRACTURE
ACIDIZING
In case of carbonate rocks 12-15% HCl is used for acid
fracturing job that is done at pressure more than fracture
pressure of the reservoir rock. For matrix acidizing, the
following procedure is required-
Firstapre-flushofabout10%HClispumpedfollowedby
mudacid(12%HCl+3%HF)volumethatiscalledPad
Volume.Thepadvolumeisfollowedbyaspocerflushor
afterflushof5%HCl.

34. HYDROCHLORIC ACID (HCl)
CaCO
3
+2HCl---------CaCl
2
+H
2
O+ CO
2
Limestone+AcidSolublesalt+Water+(gasevolved)
CaMg(CO
3
)
2
+4HCl--CaCl
2
+MgCl
2
+2H
2
O+2CO
2
Dolomite+Acid--Solublesalts+Water+(gasevolved)

35. HYDROCHLORIC ACID (HCl)
i.Normally15%HClbyweightisusedinthefieldbutthe
concentrationvariesbetween5%to35%.Thefreezing
pointofthe15%acidis–27degF,lessthan–70degFfor
20-29%and–36degFfor35%acid.HClwilldissolve
limestone,dolomiteandothercarbonates.
ii.Athousandgallonsof15%HClwilldissolve1,840lbor
01.5cu.ftofzeroporositylimestone(CaCO
3
).This
reactionwillproduce2,050lbsofCalciumchloride
(CaCl
2
),812lbor6,600cu.ft.ofCarbondioxide(CO
2
)
gasatstandardconditionsoftemperatureandpressure.
Inadditiontothe7,600lbofwaterinjectedasacarrier
fluidforacid,333lbsofwaterisalsoproducedasthe
partofthereaction.

36. HYDROCHLORIC ACID (HCl)
III.Inpractice,afterspendinginlimestone,1000
galof15%HClbecomes1020galof20%
solutionofCalciumChlorideweighting9.79ppg.
IV.A1000galof15%HClwilldissolve1,7010
lb0r9.6cu.ft.ofDolomiteCaMg(CO
3
)
2
.The
spendingof1000galof15%HClindolomitewill
produce1020galofmixtureofCalciumChloride
and9%MagnesiumChlorideweighing9.7ppg.

37. HYDROFLUORIC ACID (HF):
i.HFisusedinoil,gasorservicewellsinnormally3%
HFcombinedwith12%HClalsocalledasMudAcid.Itis
employedexclusivelyusedinSandstonerockmatrix
treatmentstodissolveeithernaturalclaysorclayswhich
havemigratedintotheformation.
ii.1000galof4.2%HFacidwilldissolve700lbofclay.
iii.FastreactiontimeandprecipitationmakeHFacid
undesirableincarbonatecontainingsandshavingmore
than20%solubilityinHCl.HFacidshouldneverbeused
incarbonateformation.

38. HYDROFLUORIC ACID (HF):
SiO
2
+6HF---H
2
SiF
6
+2H
2
O
Sand Fluo-silicicAcidsolution
Al
2
Si
4
O
10
(OH)
2
+36HF---4H
2
SiF
6
+12H
2
O+2H
3
AlF
6
Clay Fluo-Aluminicacidsolution

39. HYDROFLUORIC ACID (HF):
i.Reactionrateonsandandclayare
dependentontheratioofthesurfaceareaof
therocktovolumeofacidinthesandstone
matrix.
The acid produced by the reaction of HF acid
on sand stone will react with NaCl or KCl in
the sand around the well bore to produce
insoluble precipitates.

40. HYDROFLUORIC ACID (HF):
Fluo-silicicAcid
H
2
SiF
6
+2Na
+
Na
2
SiF
6
+ 2H
+
H
2
SiF
6
+2K
+
K
2
SiF
6
+ 2H
+
Precipitate

41. HYDROFLUORIC ACID (HF):
Fluo-Aluminicacid
H
2
SiF
6
+ 3Na
+
--Na
3
AlF
6
+ 3H
+
H
2
SiF
6
+ 3K
+
--K
3
AlF
6
+ 3H
+
i.TheinsolubleprecipitateformedareNa
2
SiF
6
,
K
2
SiF
6
,Na
3
AlF
6
,
K
3
AlF
6
.Thesefluoridesaregelatinoustype
materialsandoccupyalargevolumeofpore
space.Theyalsoadherestronglytorock
surfaces.Theresultsareverydamagingtowell
productivity.

42. HYDROFLUORIC ACID (HF):
i.HClcannotdissolveclayorsandbutcan
dissolvecarbonatespresentinsandstone
formation.HFacidreactswithlimestoneand
precipitatescalciumfluorideaninsoluble
finewhitepowder.
CaCO
3
+2HF CaF
2
+H
2
O+CO
2
Precipitate

43. HYDROFLUORIC ACID (HF):
Toavoidprecipitationinsandstoneapre-flush
ofHClisusedtodissolvethelimestoneand
preventcalciumionsfromcontactingHFacid.
Sandstoneformationshave20%moresolubility
inHClshouldnormallybetreatedwithHClonly.

44. ACETIC ACID (CH
3
COOH) :
i.Aceticacidisaweakionised&slowreacting
acid.A1000galof10%aceticacidwilldissolve
about1,110lboflimestone.Thecostof
dissolvingagivenweightoflimestoneisgreater
thanHCl.
Acetic acid is very easy to inhibit against
corrosion and can usually be left in contact with
tubing or casing for days without danger of
serious corrosion. Therefore it generally used as
perforating fluid in lime stone wells

45. ACETIC ACID (CH
3
COOH) :
i.Aceticacidisnaturalsequesteringagent
againstironprecipitation.Itdoesnotcause
embrittlementorstresscrackingofhighstrength
steel.
ii.Itwillnotcorrodealuminium.
iii.Itwillnotattackchormeplatingupto200
deg.F.Thereforeitmaybeconsideredwhen
acidizingisdonewithanalloypumpinthehole.

46. FORMIC ACID (HCOOH):
i.Formicacidisaweakionisedslow
reactingorganicacid.Ithassimilar
propertiestoaceticacid.However
formicacidismoredifficulttoinhibit
againstcorrosionathigher
temperaturesanddoesnothave
widespreadacceptance.

47. ACID ADDITIVES
i.Surfactants: To reduce surface / interfacial tension
to prevent emulsions to water wet formations.
Surface tension of 15% HCl is 72 Dynes/cm that can
be reduced up to 30 Dynes/cm by addition of the
additives.
Suspending Agents: Most of the carbnate formations
contain fines which nay cause blockage in formation
pores or fractures if fines released by the acids are
allowed to settle and form bridges. Halliburton’s HC-
2 in concentration of about 5 Gal / 1000 gal of acid
mat be used to suspend fines for more than 24 hours
& possibly up to 7 days.

48. ACID ADDITIVES
i.Sequestering Agents: to inhibit precipitation of iron
as hydrochloric acid spends. Acetic acid, Citric acid/
oxalic acid and lactic acid are common sequestering
agents.
ii.Anti Sludge Agents: To prevent sludge formation
by keeping colloidal materials dispersed.
iii.Corrosion Inhibitors: To slow down the rate of
corrosion. arsenic acid is more effective inhibitor
than organic inhibitor.

49. ACID ADDITIVES
i.Alcohols: Methyl alcohol or IPA at the
concentration of 5% -2% are used with acid to lower
the surface tension. The use of alcohol accelerates
the well clean up and improves clean up particularly
in dry wells.
ii.Fluid Loss Controller: It may be required to
reduce acid leakage in fractures etc.
iii.Diverting or Bridging Agents: These are used to
aloe relatively uniform acidizing of various porous
zones open to well bore.

50. OTHER IMPORTANT ADDITIVES:
i.Corrosioninhibitorsinsmallconcentrationofabout
0.5%ofacidvolumeisaddedinallthreestagesof
treatmenttopreventcorrosionChromates,Di-chromates
ofPotassiumororganicinhibitorsareusedforthe
purpose.
ii.Surfactants1%byvolumeofacidisusedtoreduce
interfacialtensionbetweenoil,waterandacidsothatany
formationofacidsludgeisprevented.
iii.Stabilizingagent3%byvolumeofacidsuchasacetic
acidisusedtostabilizeformationclaysandotherfines.

51. OTHER IMPORTANT ADDITIVES:
i.Sequesteringagents(3%byvolume)suchAsorganic
acids(aceticacid/oxalicacid/citricacid)helptoprevent
re-precipitationofsaltsofaluminium%iron.
CaCO
3
+2HClCaCl
2
+ H
2
O+ CO
2
Limestone+AcidSolublesalt+Water+(gasevolved)


52. OTHER IMPORTANT ADDITIVES:
CaMg(CO
3
)
2
+4HClCaCl
2
+MgCl
2
+2H
2
O
+2CO
2
Dolomite+AcidSolublesalts+Water+(gas
evolved
CaCO
3
+2HFCaF
2
+ H
2
O+ CO
2
Limestone+Acid Precipitate+Water+(gas
evolved)

53. OTHER IMPORTANT ADDITIVES:
i.AModeraterateofpre-flushistoprevent
reactionofcarbonatewithHF.
ii.HFreactswithsilica&claysanddissolved
themintotheHFsolution.
iii.OverflushpushesHCl+HFdeeperintothe
formationtofacilitatetreatmentinlargerrock
volume.
iv.Adequatemudacidisrequiredforeffective
treatment.

54. WELL STIMULATION
NITROSHOOTING

55. WELL STIMULATION:
NITRO SHOOTING
Liquidexplosivecallednitro-glycerinewas
pumpeddownhole&detonated.Thisresults
intohighpressuregaswhichshattersrock
aroundbottomholeresultinginincreased
permeability.Theonlyproblemisthatthe
wholeprocessisuncontrolled.Thismethod
hasbeendiscontinued.

56. DEPARAFFINATION
Removal of wax or paraffin deposits from the
bore hole or the down hole equipment is
known as De-paraffination.
Primary cause is cooling of oil due to
expansion of gas or oil, loss of heat etc.
Paraffins can precipitate from crude on
changing the equilibrium conditions

57. WELL STIMULATION
DE-PARAFFINATION

58. DEPARAFFINATION
Asphaltenes, fines and corrosion products
present in the system act as nucleating
materials and increase the binding force of
solid to get deposited.
Deposition of asphaltenes on the formation
sand grains near the well bore that is oil wet
zone causes the plugging the permeable
zone and reduces the production

59. DEPARAFFINATION
The most common methods of removing
paraffin deposits form wells are
i. Mechanical removal: use of scrappers and
cutters. Soluble plugs of microcrystalline wax
or insoluble plugs of hard rubber or sharp
edged plastic spheres have been found
useful in removing deposits.

60. DEPARAFFINATION
ii. Use of solvents: Chlorinated Hydrocarbon
such as Carbon Tetra Chloride, Carbon di-
sulphide are the various solvents to remove
the deposits.
iii. Use of dispersants: Water soluble
dispersants in the concentration of 2 –10%
have been found effective

61. WELL STIMULATION
WITH SURFACTANTS

62. Well Stimulation using surfactants
Surfactants are chemicals that can be used to
improve the flow around the well bore. They
can
i.Raise or lower surface and interfacial tension.
ii.Make, break, weaken or strengthen an
emulsion.
iii.Change the wettability of reservoir rock, casing,
tubing and flow-lines.
iv.Disperse or floculate clays and other fines.

63. Well Stimulation using surfactants
The real problem in emulsion removal from
sandstone formation with surfactant is the
near impossibility of getting the surfactant in
infinite contact with the emulsion droplets.
Surfactant stimulation treatment form finger
or channel through a viscous emulsion.
The majority of the untreated emulsion can
re-block the channels if not treated properly.

64. Well Stimulation using surfactants
If the damage is oil wetting this can be
treated by injecting a strong water wetting
surfactant in to the formation.
Cationics are very difficult to remove so the
use of cationics in sandstone should be
avoided.

65. Well Stimulation using surfactants
Stimulation is usually carried out with a dilute
solution of surfactant 2 % -3% in filtered oil or
filtrate salt water 2% KCl.
Treatment size should be equal or greater the
size of fluid that has damaged the formation.
An average treatment is 100 gal per foot (1.5 kL
per meter) of interval treated for the radius fo 3-5
ft. from the well bore with min.24 hours retention
time.

66. THANK YOU