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![Chemistry of Hydrofluoric Acid Systems
• HF is the only common, inexpensive mineral acid able to dissolve
siliceous minerals.
• For any acid system to be capable of damage removal, it should
contain HF in some form.
• The most common formulation :
1- Ammonium bifluoride NH4HF2
It attacks silica component:
2- By diluting concentrated HCl-HF formulations.
Treatingfluids
10
SiO2 + 4 [NH4][HF2] → SiF4 + 4 NH4F + 2 H2O](https://image.slidesharecdn.com/sandstoneacidizingsadeqrajabi-190317190834/85/Well-Stimulation-2019-10-320.jpg)
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Well Stimulation 2019
- 1.
- 2. Review • Well stimulationis a technique used to improve the flow of oil or gas from the reservoir by dissolving the rock or creating new channels around the wellbore. below fracture pressure. above fracture pressure. Review 2 Matrix Acidizing Fracturing Acidizing
- 3. • Formation Damage: Anarea of decreased permeability around the wellbore, It is often called skin damage. • Some source of formation damage *A Successful Stimulation treatment Review 3 Drilling Fluid •Weighting Agents (Barite) •Fluid Loss Additives (CaCo3) Formation Clay •Swelling (Semitic) •Disintegration (Illite) •Mobile (Kaolinite) Fluid Incompatibilities •Precipitates •Change the formation wettability •Emulsion •Clay sewlling Skin Factor Reduced Effective Wellbore Radius Increased Drawdown
- 4. • The mostcommon Acids which used in acidizing operation included : Review 4 MineralAcids Hydrochloric Acid HCL Hydrofluoric Acid HF Phosphoric Acid H3PO4 Nitric Acid NHO3 OrganicAcids Acetic Acid CH3COOH Formic Acid HCOOH
- 5. Sandstone Constituents Remaining PoreSpace SandstoneConstituents 5 *Quartz *Feldspars *Mica *Chert *Secondary Cement (Carbonates) *Clays Pore Filling and Lining (Kaolinite, Illite)
- 6. • Al-Harthy (2008/2009) (A) Grainsof quartz cemented by overgrowth of carbonates (B) Quartz (C) Feldspar (D) Porosity reduction occurs from pore-filling clays such as kaolinite (E) Pore-lining clays such as illite SandstoneConstituents 6
- 7. • Sandstone Formation •Matrix acidizing mostly used in sandstone formations. • Are mostly formed by quartz , Also have some carbonate particle • Usually use HF as a quartz solvent Highly Toxic, Highly Corrosive, Toxic and Corrosive vapors * SiO2(Silica, Quartz) will not react wit Hydrochloric HCL acid • It is difficult to affect formation more than a few feet from the wellbore. SandstoneFormation 7 6HF + SiO2 H2SiF6 + 2H2O Mud Acid = HF + HCL
- 8. Sandstone Acidizing Mechanism HF+ Mineral + HCL → ALFX + H2SiF6 H2SiF6 + Mineral + HCL → Silica Gel + ALFX ALFX + Minerals → AlFY + Silica Gel ; X>Y • When sandstone formation is treated with mud acid: (Mahmoud et al. 2011) • Formation of potassium and sodium silicate precipitates • Formation of calcium fluoride precipitates • Formation of hydrated silica precipitates AcidizingMechanism 8
- 9. Hydrochloric Acid ChemistrySystem • The use of HCl alone causes significant damage. • Weak organic acid reduces the damage. • Zeolite minerals are sensitive to HCl and strong mineral acids. • All fluids that are injected should have an organic Acid included to maintain a low-pH environment. * An HCl preflush is always injected in sandstones prior to the HF Treatingfluids 9 The Zeolite Family Mineral Description Heulandite Dissolve in contact with HCL Occur in shallow environment No gelatin formed Chabazite Dissolve in contact with HCL Occur in medium-depth environment No gelatin formed Natrolite Dissolve in contact with HCL Occur in deeper environment gelatin formed Analcime Dissolve in contact with HCL Occur in deeper environment gelatin formed
- 10. Chemistry of HydrofluoricAcid Systems • HF is the only common, inexpensive mineral acid able to dissolve siliceous minerals. • For any acid system to be capable of damage removal, it should contain HF in some form. • The most common formulation : 1- Ammonium bifluoride NH4HF2 It attacks silica component: 2- By diluting concentrated HCl-HF formulations. Treatingfluids 10 SiO2 + 4 [NH4][HF2] → SiF4 + 4 NH4F + 2 H2O
- 11. • Numerous mineralspecies react with HF, and they generate aluminum silica fluoride complexes. (Smith et al., 1965). . Treatingfluids 11 Chemical composition of typical sandstone minerals Classification Mineral Chemical Composition Quartz SiO2 Feldspar Microcline Orthoclase Albite Plagioclase KAlSi3O8 KAlSi3O8 NaAlSi3O8 (Na,Ca)Al(Si,Al)Si2O8 Mica Biotite Muscovite Chlorite K(Mg,Fe2+)3(Al,Fe3+)Si3O10(OH)2 KAl2(AlSi3)O10(OH)2 (Mg,Fe2+,Fe3+)AlSi3O10(OH)8 Clay Kaolinite Illite Smectite Chlorite Al2Si2O5(OH)4 (H3,O,K)y (Al4 ⋅ Fe4 ⋅ Mg4 ⋅ Mg6)(Si8 – y ⋅ Aly)O20(OH)4 (Ca0.5Na)0.7(Al,Mg,Fe)4(Si,Al)8O20(OH)4 ⋅ nH2O (Mg,Fe2+,Fe3+)AlSi3O10(OH)8 Carbonate Calcite Dolomite Ankerite Siderite CaCO3 CaMg(CO3)2 Ca(Fe,Mg,Mn)(CO3)2 FeCO3 Sulfate Gypsum Anhydrite CaSO4 ⋅ 2H2O CaSO4 Chloride Halite NaCl Metallic oxide Iron oxides FeO, Fe2O3, Fe3O4 1000 gal of 2% HF 350 lbm of clay
- 12. Solubility of by-products •When minerals are dissolved by HF, numerous by-products can form. • Calcium fluoride Some carbonates may remain after preflushing Amount of carbonate cementing material in the sandstone The carbonates’ initial protective siliceous coating * CaF2 readily forms when calcite contacts HF. This can lead to substantial damage: Solubilityofby-products 12 Solubility in water at room temperature of HF reaction by-products Secondary Product Solubility (g/100 cm3) Calcium fluoride (CaF2) 0.0016 Sodium fluoaluminate (Na3AlF6) Slightly soluble Sodium fluosilicate (Na2SiF6) 0.65 Aluminum fluoride (AlF3) 0.559 Aluminum hydroxide (Al(OH)3) Insoluble Ferrous sulfide (FeS) 0.00062 CaCO3 + 2HF ↔ CaF2 + H2O + CO2
- 13. Kinetics: Factors AffectingReaction Rates • Weak cementation → reduced-strength mud acid (1.5% HF) → avoid crumbling (Fogler et al., 1976) • Fluoboric H3OBF4 → low concentration of HF • Regular mud acid (12% HCl – 3% HF) is the normal concentration to use to remove damage in clean quartzose sands. • Field experience has shown that weaker concentrations (0.5% to 1.5% HF) can be effective for other sands. • If the combined percentage of clay and feldspar is more than 30%, 1.5% HF or less should be used. Kinetics 13 Hydrofluoric Acid Concentration Hydrochloric Acid Concentration
- 14. Mud Acid Volumeand Concentration • Gidley (1985) • Less may be used where only shallow damage exists around new perforations(75 gal/ft) The HCl:HF ratio and concentration are selected to prevent or reduce the formation of damaging precipitates. MethodsofControlling Precipitates 14 More than 125 gal/ft of mud acid Acid use guidelines for sandstone acidizing (McLeod, 1984) Condition or Mineralogy Acid Strength (blend) HCl solubility > 20% HCl only High permeability (>50 md) High quartz (>80%), low clay (<5%) 12% HCl–3% HF High feldspar (>20%) 13.5% HCl–1.5% HF High clay (>10%) 10% HCl–1% HF High iron/chlorite clay (>15%) 10% acetic acid–1% HF Low permeability (≤10 md) Clay (<10%) 6% HCl–1% HF Clay (>10%) 6% HCl–0.5% HF
- 15. The dissolution ofminerals is a thermally activated phenomenon; thus, the rates increase greatly as a function of temperature and the penetration depths of live acid diminish. Figure : The variation of the reaction rate of mud acid with silica (more reactive than quartz) as a function of both HF concentration and temperature Kinetics 15 Temperature
- 16. The relatively hightotal specific surface area of sandstone rocks is the primary parameter determining mud acid spending. Clays react much faster than feldspars, which react much faster than quartz • An increase in pressure speeds up the overall dissolution reaction slightly. Kinetics 16 Mineralogical Composition and Accessible Surface Area Pressure Relative surface areas of sandstone minerals Mineral Surface Area Quartz <0.1 cm2/g Feldspar Few m2/g Kaolinite 15–30 m2/g Illite 113 m2/g Smectite 82 m2/g
- 17. Other Acidizing Formulations •Problems related to the use of mud acid to remove damage in sandstone formations Rapid spending provides only a short penetration. Destabilization of Fines The high dissolving power of mud acid • New sandstone acidizing systems are designed to alleviate these shortcomings. Fluoboric acid Alcoholic mud acid Mud acid plus aluminum chloride for retardation Organic mud acid Self-generating mud acid Buffer-regulated hydrofluoric acid OtherAcidizingFormulations 17
- 18. Summary of acidsdeveloped and under active research Name, year, inventor Advantages Disadvantages Year of research Mud acid, 1965 Smith and Hendrickson Dissolves quartz, remove damage Corrosive, precipitation reactions, fast reaction 1965– present Retarded mud acids, 1996, Al- Dahlan Reduces the reaction rate for penetration (200 F) Same problems at high temperature and formation of precipitates KBF4 1996– present Organic-HF acids, 1996, Shuchart Less corrosion rate, useful in HCl sensitive clay (350 F) Expensive, some precipitates formed at high temperature 1996– present 10% acetic acid, 2003, Hartman Good results at higher temperature (100 F) Only applicable where carbonate percentage is high 2003 Na3HEDTA and HEDTA (chelating agents), 2002 Ali, Frenier Better results at high temperature (300 F) No fluoride ion 2002– present Single-stage acid. (Goma, Cutler 2013) Eliminates the use of pre-flush and after flush Expensive, reaction mechanisms not clear 2013– present Phosphoric–HF and Fluoboric– formic, 2013 Shafiq More permeability increase and less corrosion Not used at high temperature, reaction mechanism unknown 2013– present OtherAcidizingFormulations 18
- 19. Methods of ControllingPrecipitates • The methods used to control the precipitates caused by acidizing: MethodsofControlling Precipitates 19 Proper Acid Staging Lower Acid Concentration Correct Usage Preflush and Sufficient Overflush
- 20. Preflush Stage The preflushdisplaces formation brine away from the wellbore to prevent it from mixing with reacted mud acid and causing a damaging precipitate. To dissolve any Na, K and Ca ions that may produce insoluble silicates when reacted with the silica. Preventing the live HF acid to enter into a high pH region. Pre-flush also provides a low pH region reducing the risk of precipitate formation. • Preflush with MethodsofControlling Precipitates 20 1. 5% to 15% HCl Acetic Acid
- 21. MethodsofControlling Precipitates 21 Fluid selection guidelinesfor preflush fluids Mineralogy Permeability >100 md Permeability 20 to 100 md Permeability <20 md <10% silt and <10% clay 15% HCl 10% HCl 7.5% HCl >10% silt and >10% clay 10% HCl 7.5% HCl 5% HCl >10% silt and <10% clay 10% HCl 7.5% HCl 5% HCl <10% silt and >10% clay 10% HCl 7.5% HCl 5% HCl If the formation contains more than 1% to 2% carbonate Solution: HCl preflush is necessary to dissolve the carbonate, prevent the waste of mud acid. If completion brines such as seawater, potassium chloride (KCl), calcium chloride (CaCl2) have been used in the well prior to acidizing. Solution : Preflushing the mud acid with HCl or brine containing ammonium chloride (NH4Cl) to dilute the brines Preflushes can also be used to displace and isolate incompatible formation fluids (either brine or crude oil)
- 22. To dissolvethe quartz, clay, feldspar and silicates. This acid may also dissolve the remains of carbonates present after the pre- flush stage MethodsofControlling Precipitates 22 Fluid selection guidelines for mud acid fluids Mineralogy Permeability >100 md Permeability 20 to 100 md Permeability <20 md <10% silt and <10% clay 12% HCl–3% HF 8% HCl–2% HF 6% HCl–1.5% HF >10% silt and >10% clay 13.5% HCl–1.5% HF 9% HCl–1% HF 4.5% HCl–0.5% HF >10% silt and <10% clay 12% HCl–2% HF 9% HCl–1.5% HF 6% HCl–1% HF <10% silt and >10% clay 12% HCl–2% HF 9% HCl–1.5% HF 6% HCl–1% HF Main acid stage
- 23. Postflush or Overflush It cleans the formation rapidly by removing the spent acid. To displace mud acid reaction products away from the wellbore Is used to keep the wettability of the formation to the original state To displace non-reacted mud acid into the formation MethodsofControlling Precipitates 23 Typical overflushes for mud acid treatments are • Water containing 3% to 8% ammonium chloride • Weak acid (3% to 10% HCl) • Diesel oil (oil wells only and only following a water or weak acid overflush) • Nitrogen (gas wells only and only following a water or weak acid overflush)
- 24. Matrix Acidizing DesignGuidelines • McLeod etal. (1983), who recommended the following steps for treatment design: 1) Estimate safe injection pressures. 2) Estimate safe injection rate into the damaged formation. 3) Select stages required for fluid compatibility. 4) Calculate volume of each stage required. 5) Select acid concentrations according to formation mineralogy. MatrixAcidizingDesignGuidelines 24 Formation brine displacement HCL stage or acetic acid stageCrude oil displacement Mud acid stage Overflush stage Allowable safe injection pressure at both the wellbore and at the surface Present bottomhole fracturing pressurePresent fracturing gradient
- 25. Calculation • Fracturing pressure •Matrix treatments are defined as fluid injection occurring below fracturing pressure. If the fluid is injected above fracturing pressure, the acid may bypass the damage. • Injection rates: • qi,max is the injection rate in bbl/min • k is the effective permeability of the undamaged formation in md • h is the net thickness in ft • gf is the fracture gradient in psi/ft • H is the depth in ft • Δpsafe is the safety margin for the pressure in psi (usually 200 to 500 psi) • p is the reservoir pressure in psi • μ is the viscosity of the injected fluid in cp • re is the drainage radius in ft • rw is the wellbore radius in ft • s is the skin effect factor • B is the formation volume factor and has a value of 1 for noncompressible fluids MatrixAcidizingDesignGuidelines 25
- 26. • Friction pressureestimation Accurate fluid friction pressure is a difficult paramete to obtain. Because the tubular arrangement can be different in each case • ppipe friction is the friction pressure in psi/1000 ft • ρ is the density of the fluid (specific gravity) in g/cm3 • q is the pump rate in bbl/min • D is the diameter of the pipe in in • Fluid volumes • Vp is the pore volume for the distance s in gal/ft • φ is the fractional porosity • rs is the distance it is necessary to penetrate the damaged or displaced section in ft • Volume HCL • VHCl is the volume of HCl required in gal/ft • XHCl is the fraction of the bulk rock dissolved by HCl • β is the dissolving coefficient expressed as the amount of rock dissolved per gallon of acid and is related to the acid strength MatrixAcidizingDesignGuidelines 26
- 27. Acid Treatment DesignConsiderations Selection of Fluid Sequence Stages • The preflushes, main stage and overflush should be matched to the type of damage. Cleaning (Tubing Pickle) • remove rust, scale and organic deposits *Scale Occur in wellbore, Usually results in a sharp drop of production Such as Iron Sulfide, Iron carbonate *Organic deposits, Commonly located in tubing Such as Wax, Paraffins or Asphaltenes • dissolve oily films and pipe dope that could plug the downhole equipment and perforations • limit the amount of iron that gets into the formation and contacts the crude oil. AcidTreatmentDesign Considerations 27
- 28. Acid treatment sequenceand fluid options Stage Fluid System 1. Preflush Brine Hydrocarbons HCl 2. Main fluid HCl-HF formulation 3. Overflush HCl or NH4Cl 4. Diverter Foam or slug OSR 5. Repeat stages 1–4 as necessary with 1–3 as the last fluid sequence 6. Fluoboric acid With diverter solvent for OSR or foam- weakening agent (mutual solvent) 7. Fluoboric acid diverter Fluoboric acid–based fluid system, either foamed or slug OSR 8. Fluoboric acid Fluid left at the perforations AcidTreatmentDesign Considerations 28 Diversion techniques • This is an excellent way to ensure that the main fluid stages are properly isolated by the preflush and overflush fluids. • Diversion should be matched to formation characteristics and the type of treating fluid. Typical Sandstone Acid Job Stages
- 29. Typical stage sequencefor a sandstone acidizing treatment Stage Number Stage Reason for Stage Information Source Stage Composition Stage Volume 1 Crude oil displacement To prevent oil sludge formation by the acid Acid–crude oil sludge test Aromatic solvent To achieve 3-ft radial displacement 2 Formation water displacement To prevent scale deposition HCO3 and SO4 contents from formation water analysis Ammonium chloride (NH4Cl) at 3%–8% depending on the salinity of the formation water To achieve 3-ft radial displacement 3 Acetic acid Iron compounds in formation (pyrite, siderite, hematite), chlorite, clay, zeolites X-ray-diffraction (XRD) analysis 3%–10% acetic acid CaCO3 (%) Volume (gal/ft) 0–5 5–10 10–15 15–20 25 50 75 100 4 Hydrochloric acid CaCO3 or other HCl- soluble minerals HCl solubility test and/or XRD analysis According to core mineralogy: 3%–15% HCl Calculated on the basis of HCl solubility and porosity or this schedule: HCl Solubility Stage Volume of HF (%) (gal/ft) <5 5–10 10–20 50 100 200 5 Hydrofluoric acid (not used for carbonates and sandstones where HCl solubility > 20%) To remove clay, other formation fines and mud damage XRD analysis, SEM analysis HCl:HF solubilities According to formation mineralogy: 3%–13.5% HCl with 0.5%–3% HF 75–100 gal/ft 6 Overflush To spend acid and flush spent acid away from the near-wellbore area Always used 3%–8% NH4Cl or 3%–5% HCl in all wells followed by nitrogen (gas wells), kerosene (oil wells) or 5% HCl (water injection wells) One to two volumes of the HCl:HF volume or to achieve 5-ft radial displacement 7 Diversion To improve injection throughout the interval Used as required for heterogeneous formation permeability OSR for oil or low gas/ oil ratio wells, foam for either oil or gas wells and water-soluble resins for water injector wells 29 AcidTreatmentDesign Considerations
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- 31. WhyAcidizingisPreferredOverHydraulicFracturinginSandstoneFormation? • Sandstone Acidizingcan be used instead of hydraulic fracturing in many cases like High permeability formation and naturally fractured reservoirs. Sandstone acidizing can be utilized in depleted sandstone reservoirs for CCS. • carbon capture storage is the process of capturing waste carbon dioxide (CO2) from large point source, such as fossil fuel power plant, transporting it to a storage site, and depositing it where it will not enter the atmosphere, normally an underground geological formation Acidizing can be used at different temperatures and pressures. (Abdelmoneim and Nasr-El-Din 2015). 79% of the stimulation jobs were acid treatments, they are a low cost than hydraulic fracturing treatment, they only consumed 20% of the money spent for well stimulation. (Collier 2013) Whyacidizingispreferredover hydraulicfracturing? 31
- 32. 32 Damage Identification Knowledge of Chemical Reaction Select of Appropriate Types and Volumes of Preflushes and Overflushes Use Numerical Simulator Evaluating The Executed Acid Treatment Conclusion
- 33. References Reservoir Stimulation, ThirdEdition By Michael J. Economides, Kenneth G. Nolte Production Enhancement with Acid Stimulation, Second Edition By Leonard Kalfayan Oilfield Review, Sandstone Acidizing Harry O. McLeod, Conoco, USA William David Norman, Schlumberger Dowell Oilfield Review, Schlumberger, wniter 2008/2009 High-Temperature Well Stimulation, Salah Al-Harthy Houston, Texas, USA Sandstone matrix acidizing knowledge and future development, 2017 Mian Umer Shafiq Hisham Ben Mahmud References 33
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