Nanopartículas

Hartman et al. (2003), applied 10% acetic acid which proved to be useful in sandstone acidizing

at a higher temperature and showed better results compared to 10% HCl. This acid is only effective at low temperature. Martin (2004), insisted on using the non HF-based system because of the damaging nature of this acid and complex reaction mechanism. Fluosilicic acid (H2SiF6)

plays an important role in removing the formation damage. It was applied successfully to stimulate sandstone in two injector wells in off-shore Brazil in 1999 by(Da Motta and C.M.

Dos Santos, 1999). Kalfayan and Metcalf (2000), achieved200% increment in permeability by

applying the same acid. During sodium fluoride manufacturing, H2SiF6 formed as a byproduct,

which is also considered as a viable option for sandstone acidizing operations because of its low cost.This acid can cause precipitation reactions at high temperatures. Table 2.7 shows the summary of acids development with time.

In order to get better results from the acidizing job and to extend the life of the acidizing treatment, one of the alternative technique used as a remedy is the acid blends instead of conventional HCl-HF acid systems. Phosphonic acid complex replaced HCl acid in one of the improved chemistry systems. A number of available hydrogen ions in phosphonic acid are five, which can dissociate at different stoichiometric conditions. Therefore, this Phosphonic acid complex can also be named as five-hydrogen (HV) complex (Nwoke et al. 2004; Rae and Lullo,

2007 and Nasr-El-Din, 2016). The HF acid can be obtained by reacting HV acid with

ammonium bifluoride (NH4HF2). 1000 gal of water mixed with 20 gals of HV when reacted

with almost 123 lbs of NH4HF2 can produce 1% HF acid solution (Uchendu et al. 2006). HBF4

is the retarded acid where this self-generating reaction of HF acid lessen the rate acid of reaction with the rock and therefore, allows live HF acid to penetrate deep into the formation. Due to

61 this slow reaction, the risk of formation deconsolidation decreases, unlike conventional acids which are the reason for the deconsolidation of rock near the wellbore.

In past 30 years, several research publications have appeared which are related to the development of an expert system for designing sandstone acidizing (Abel et al. 1990; Van

Domelen et al. 1992; Chiu et al. 1993, and Nitters et al. 2000). Robust rule-based systems have

been introduced by (Van Domelen et al., 1992 and Xiong and Holditch, 1995) specifically for damage type formations acid fluid selection but depend predominantly on the fluid injected and the reservoir rock mineralogy. A matrix stimulation model has been developed by (Bartko

et al. 1996) that helps in diagnosing the type of formation damage, optimization of acidizing

treatment and pressure differential due to skin acid treatment. All these guidelines and systems developed, however, do not include or describe the recent technology development in acidizing like a phosphonic acid blend or chelating agents developed. Instead, mostly the selection of acid through guidelines based on mud acid and organic mud acid. Also, in the discussed rule- based systems, the presence of clay and the interaction of oil and acid in the rock has been totally ignored.

(Nasr-El-Din et al. 2002; Nasr-El-Din et al. 2007; Rae and Lullo, 2007 and Urraca, 2009), developed another acidic chelate-based blend to mitigate secondary and tertiary reactions and satisfy long-lasting effects of simulation in sensitive sandstone formations. But the application of these chelates is restricted to high-temperature formations with a high content of dolomite and low clay. Table 2.7 represents the summary of different acids developed during the last few decades.

62 Table 2.7. Summary of acids developed and under active research

Name, year, author Experiments performed Advantages Disadvantages / Gaps

Mud acid (HF – HCl)

1965 - Smith and Hendrickson

Fo NMR, ICP, CT scan, solubility, SEM, XRD, EDS, core

flooding on Berea sandstone

Dissolves quartz, Removes damage, HF acid dissolves silicates while HCl helps in controlling precipitation reaction

Corrosive, precipitation reactions, fast reaction, inefficient due to early consumption especially at temperatures higher than 200oF.

Retarded mud acids

1996 - Al-Dahlan 2012 – Al-harbi

Fo NMR, ICP, solubility, SEM, XRD, EDS, Core flooding on

Berea sandstone

Reduces the reaction rate for penetration

Same problems as mud acid at high temperatures and formation

of precipitates (KBF4)

HCOOH-HF acid (Organic mud acid)

1996 - Shuchart

Fo NMR, ICP, CT scan, solubility, SEM, XRD, EDS, core

flooding on Berea sandstone

Less corrosion rate, useful in HCl sensitive clay at temperatures

more than 350oF

Expensive, some precipitates formed at high temperature

CH3COOH – HF acid

2003 - Hartman Core flooding, SEM, ICP analysis

Good results at a higher temperature – more than 100o_F

Only applicable where dolomite percentage is high, mostly applied in carbonate acidizing

Na3HEDTA and HEDTA

2002 – Ali and Frenier

Fo NMR, ICP, CT scan, solubility, SEM, XRD, EDS, core

flooding on Berea sandstone

Better acidizing results at high temperature (400o_F)

No fluoride ion, no dissolution of quartz, only applicable where

63

Fluosilicic acid

1999 - Da Motta

Permeability, porosity, corrosion tests, core flooding

Decent permeability increment, Byproduct of HF acid and silica

reaction

Not used at high temperature, corrosive, can form a precipitate

at high temperatures

Single stage acid

2013 – Goma and Cutler

Core flooding, corrosion, productivity

Eliminates the use of preflush and after flush stages

Expensive, reaction mechanism is not clear

Phosphoric-HF & Fluoboric-formic

2013 - Shafiq

Porosity, permeability, strength,

SEM, EDS, saturation, solubility More permeability increment and less corrosion

Not used at high temperature, reaction mechanism unknown

Emulsified Acids

1933 – De Groote

Core flooding, permeability, CT scan

Slow reaction rate, stable at high temperature

Not applied to a sandstone formation

GLDA

2008 – Heus

Fo _{NMR, ICP, CT scan,}

solubility, SEM, XRD, EDS, core flooding on Berea sandstone

Environment-friendly, iron controlling agent, applied at high

temperatures successfully

No fluoride ion, no dissolution of quartz, only applicable where

quartz percentage is less

EDTA and HAc

1997 – Fredd and Fogler

Fo NMR, ICP, CT scan, solubility, SEM, XRD, EDS, core

flooding on Berea sandstone

Effective in creating wormholes, less corrosive, control iron precipitates and remove scales

Not soluble in the less acidic environment, not readily

biodegradable

MSA

2017 - Kankaria

Core flooding, corrosion, CT scan, ICP

Applied at high temperatures, suitable in the form of a blend

Expensive, not effective when quartz percentage is high

In-Situ foam

1964 - Bernard and Holm

Core flooding, CT scan

Create uniform wormholes in carbonates, effective in dolomite

No fluoride ion, no dissolution of quartz, only applicable where

64 acidizing, effective in low

permeability formation also

HV complex

Nwoke, Uchendu et al. 2004

Core flooding, strength, complexation of ions

Slow reaction rate, more penetration of acid into the

reservoir, less formation deconsolidation

No fluoride ion, no dissolution of quartz, only applicable where

65