Well stimulation

1. WELL STIMULATION
 Well stimulation is the process by
which the productivity of a well is
increased. Productivity is measured
in terms of productivity index or JO
JO = q / P
 Productivity index (PI) is defined a
ratio of flow of oil per unit draw down
(P = PR - PWf)

2. WELL STIMULATION:
OBJECTIVES
 Stimulation is a 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

3. WELL STIMULATION: METHODS
 i. Hydraulic fracturing
 ii. Acidizing
 iii. Notroshooting
 iv. De-paraffination

4. 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/ft3
average 144 lbs / ft3.
 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.

5. INITIATION OF FRACTURES
 i. Pressure required to create a Horizontal
fracture
 Pf (h) = Z = 1.0 x D
 Where D is the depth in feet
 ii. Pressure required to create a vertical fracture
 Pf (v) = [ 2  / (1 -  ) ] Z + St
 Where  is Poisson’s ratio = lateral strain / axial
strain varies between 0.18 to 0.27 for the
average type of rocks.
 St = tensile stress.

6. 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)

7. WELL STIMULATION
ACIDIZING

8. WELL STIMULATION:
ACIDIZING
 Acid is used to remove damage
near the well bore in all type of the
wells. In carbonate formation acid
may be used to create linear flow
systems by acid fracturing. Acid
fracturing is not applicable in Sand
stone wells.

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

10. WELL STIMULATION:
MATRIX ACIDIZING
 It is primarily applied to remove skin damage
caused by drilling , completion, work over or well
killing fluids and by precipitation of deposits from
produced water .
 Due to extreme larger surface area contacted by
the acid in a matrix treatment, spending time is very
short. Therefore it is difficult to affect the formation
more than a few feet from the well bore. .

11. WELL STIMULATION:
MATRIX ACIDIZING
 Removal of severe plugging in sandstone,
limestone or dolomite can result in a very large
increase in well productivity.
 If there is no skin damage, a matrix treatment in
limestone or dolomite could stimulate natural
production no more than one and one - half
times.
 Matrix treatments tend to leave zone barriers
intact of pressure are maintained below fracture
pressure.

12. MATRIX ACIDIZING: USE OF ACID
 HCl (10 – 15%) is used for acid washing jobs in
which bottom hole is cleaned by acid reaction
with scale (CaCO3) etc.
 The deposition on the inside wall of tubing and
perforation plugging may be cleaned by acid
washing. A small amount of acid is provided by
to & fro circulation.

13. ACIDS USED IN MATRIX
ACIDIZING
 The acid is pumped at a pressure lower than
fractured pressure of the formation.
 The acid is intended to enter in to the matrices of
the rock.
 For carbonate (Limestone & Dolomite) formation
10 – 15% HCl is used.
 For sand stone formation, a combination of 12%
HCl + 3% HF called mud acid is used.

14. ACIDS USED IN MATRIX
ACIDIZING
 HCl provides reaction with carbonate present
in the matrices including with silica + clays.
Matrix acidizing is aimed at removal of
formation damage, thereby restore
productivity.

15. 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.

16. 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.

17. FRACURE ACIDIZING

18. SAND STONE ACIDIZING
 The primary reaction to acidizing sandstone
wells is to increase well permeability by
dissolving clays near the well bore.
 These clays may naturally occurring formation
clays or may have introduced from drilling/
completion or work over operations.
 HF can dissolve calcium carbonate, sand, clays,
shale and feldspars. However, the HF is used to
remove clay deposits form the rock matrix.

19. 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-
 First a pre-flush of about 10% HCl is pumped followed by
mud acid (12% HCl + 3% HF) volume that is called Pad
Volume. The pad volume is followed by a spocer flush or
after flush of 5% HCl.

20. HYDROCHLORIC ACID (HCl)
CaCO3 + 2HCl --------- CaCl2 + H2O + CO2
Lime stone+ AcidSoluble salt +Water +(gas evolved)
CaMg(CO3)2 + 4HCl -- CaCl2 +MgCl2+ 2H2O + 2CO2
Dolomite+Acid -- Soluble salts+ Water + (gas evolved)

21. HYDROCHLORIC ACID (HCl)
 i. Normally 15% HCl by weight is used in the field but the
concentration varies between 5% to 35%. The freezing
point of the 15% acid is –27deg F, less than –70degF for
20 29% and – 36 deg F for 35% acid.. HCl will dissolve
limestone, dolomite and other carbonates.
 ii. A thousand gallons of 15% HCl will dissolve 1,840 lb or
01.5 cu.ft of zero porosity lime stone (CaCO3). This
reaction will produce 2,050 lbs of Calcium chloride
(CaCL2), 812 lb or 6,600 cu.ft. of Carbon dioxide (CO2)
gas at standard conditions of temperature and pressure.
In addition to the 7,600 lb of water injected as a carrier
fluid for acid, 333 lbs of water is also produced as the
part of the reaction.

22. HYDROCHLORIC ACID (HCl)
 III. In practice, after spending in lime stone, 1000
gal of 15% HCl becomes 1020 gal of 20%
solution of Calcium Chloride weighting 9.79 ppg.
 IV. A 1000 gal of 15% HCl will dissolve 1,7010
lb 0r 9.6 cu.ft. of Dolomite CaMg(CO3)2. The
spending of 1000 gal of 15% HCl in dolomite will
produce 1020 gal of mixture of Calcium Chloride
and 9% Magnesium Chloride weighing 9.7ppg.

23. HYDROFLUORIC ACID (HF):
 i. HF is used in oil, gas or service wells in normally 3%
HF combined with 12% HCl also called as Mud Acid. It is
employed exclusively used in Sandstone rock matrix
treatments to dissolve either natural clays or clays which
have migrated into the formation.
 ii.1000 gal of 4.2% HF acid will dissolve 700 lb of clay.
 iii.Fast reaction time and precipitation make HF acid
undesirable in carbonate containing sands having more
than 20% solubility in HCl. HF acid should never be used
in carbonate formation.

24. HYDROFLUORIC ACID (HF):
 SiO2 + 6HF --- H2 Si F6 + 2H2O
Sand Fluo-silicic Acid solution
 Al2 Si4 O10 (OH)2 + 36 HF--- 4 H2 Si F6
+ 12 H2O + 2H3Al F6
 Clay Fluo-Aluminic acid solution

25. HYDROFLUORIC ACID (HF):
 i.Reaction rate on sand and clay are
dependent on the ratio of the surface area of
the rock to volume of acid in the sandstone
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.

26. HYDROFLUORIC ACID (HF):
 Fluo-silicic Acid
 H2 Si F6 + 2Na +  Na2Si F6 + 2 H+
 H2 Si F6 + 2K +  K2Si F6 + 2 H+
Precipitate

27. HYDROFLUORIC ACID (HF):
Fluo-Aluminic acid
 H2 Si F6 + 3Na + -- Na3 Al F6 + 3 H+
 H2 Si F6 + 3K + -- K3 Al F6 + 3 H+
i. The insoluble precipitate formed are Na2Si F6 ,
K2Si F6 , Na3 Al F6 ,
K3 Al F6. These fluorides are gelatinous type
materials and occupy a large volume of pore
space. They also adhere strongly to rock
surfaces. The results are very damaging to well
productivity.

28. HYDROFLUORIC ACID (HF):
 i. HCl can not dissolve clay or sand but can
dissolve carbonates present in sand stone
formation. HF acid reacts with lime stone and
precipitates calcium fluoride an insoluble
fine white powder.
 CaCO3 + 2HF  CaF2 + H2O +CO2
Precipitate

29. HYDROFLUORIC ACID (HF):
 To avoid precipitation in sand stone a pre-flush
of HCl is used to dissolve the lime stone and
prevent calcium ions from contacting HF acid.
 Sand stone formations have 20% more solubility
in HCl should normally be treated with HCl only.

30. ACETIC ACID (CH3COOH) :
 i. Acetic acid is a weak ionised & slow reacting
acid. A 1000 gal of 10% acetic acid will dissolve
about 1,110 lb of lime stone. The cost of
dissolving a given weight of lime stone is greater
than HCl.
 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

31. ACETIC ACID (CH3COOH) :
 i. Acetic acid is natural sequestering agent
against iron precipitation. It does not cause
embrittlement or stress cracking of high strength
steel.
 ii. It will not corrode aluminium.
 iii. It will not attack chorme plating up to 200
deg.F. Therefore it may be considered when
acidizing is done with an alloy pump in the hole.

32. FORMIC ACID (HCOOH):
 i. Formic acid is a weak ionised slow
reacting organic acid. It has similar
properties to acetic acid. However
formic acid is more difficult to inhibit
against corrosion at higher
temperatures and does not have
widespread acceptance.

33. ACID ADDITIVES
 i. Surfactans: To reduce surface / interfacial tension
to prebvent 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.

34. 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.

35. 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.

36. OTHER IMPORTANT ADDITIVES:
 i. Corrosion inhibitors in small concentration of about
0.5% of acid volume is added in all three stages of
treatment to prevent corrosion. Chromates, Di-chromates
of Potassium or organic inhibitors are used for the
purpose.
 ii. Surfactants 1% by volume of acid is used to reduce
interfacial tension between oil, water and acid so that any
formation of acid sludge is prevented.
 iii. Stabilizing agent 3% by volume of acid such as acetic
acid is used to stabilize formation clays and other fines.

37. OTHER IMPORTANT ADDITIVES:
 i.Sequestering agents (3% by volume) such As organic
acids (acetic acid/ oxalic acid/ citric acid) help to prevent
re-precipitation of salts of aluminium % iron.
 CaCO3 + 2HCl  CaCl2 + H2O + CO2
 Lime stone+Acid  Soluble salt +Water+(gas evolved)


38. OTHER IMPORTANT ADDITIVES:
Ca Mg (CO3)2 + 4HCl  CaCl2 + MgCl2 + 2H2O
+ 2CO2
Dolomite +Acid  Soluble salts+ Water+ (gas
evolved
CaCO3 + 2HF  CaF2 + H2O + CO2
Limestone+Acid Precipitate +Water+(gas
evolved)

39. OTHER IMPORTANT ADDITIVES:
 i. A Moderate rate of pre-flush is to prevent
reaction of carbonate with HF.
 ii. HF reacts with silica & clays and dissolved
them in to the HF solution.
 iii. Over flush pushes HCl + HF deeper in to the
formation to facilitate treatment in larger rock
volume.
 iv. Adequate mud acid is required for effective
treatment.

40. WELL STIMULATION
NITROSHOOTING

41. WELL STIMULATION:
NITRO SHOOTING
 Liquid explosive called nitro-glycerine was
pumped down hole & detonated. This results
in to high pressure gas which shatters rock
around bottom hole resulting in increased
permeability. The only problem is that the
whole process is uncontrolled. This method
has been discontinued.

42. 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

43. WELL STIMULATION
DE-PARAFFINATION

44. 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

45. 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.

46. 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

47. WELL STIMULATION
WITH SURFACTANTS

48. 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.

49. 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.

50. 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.

51. 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.