Iron Control CONTENTS 1.
Introduction.
1
2.
Oxidation State Of Dissolved Iron.
3
3.
Methods Of Iron Control.
4
3.1.
Chelating Agents.
6
3.1.1. Citric Acid. 3.1.2. EDTA Acid. (Ethylene-Di-Amine-Tetra-Acetic Acid) 3.1.3. Tetra Sodium EDTA. 3.1.4. Di-Sodium EDTA .. 3.1.5. Tri-Sodium NTA .. 3.1.6. Nitrilo -Acetic Acid.
6 6 7 7 7 7
3.2. Reducing Agents. 3.2.1. Sodium Erythorbate . 3.2.2. Erythorbic Acid. 3.2.3. Ferrotrol 260L. 3.2.4. Ferrotrol 270 And Ferrotrol 271. 3.3. Ph Control Agents (Buffers). 3.3.1. Acetic Acid And Acetic Anhydride.. 3.3.2. Sodium Acid Pyrophosphate (SAPP). 3.4. Sulphide Scavengers. 3.4.1. Ferrotrol HS-A And Ferrotrol HS-B. 4. Acid Treatment Designs.
4.1. Pipe Pickling Treatments. 4.2. Sequestering Acid Systems (SA-Systems). 4.3. Hydrogen Sulphide Scavenger Acid (HSSA). 4.3.1. HSSA Corrosion Inhibition. 5. Non-Acid Treatments Using Chelating Agents.
Page i
8 9 9 9 10 10 11 11 11 11 12
12 12 16 18 19
Iron Control
TABLES
Table 1:
Sources of Iron.
2
Table 2:
Ferrotrol Products (Iron Control Chemicals).
5
Table 3:
Conversion Factors, Ferrotrol Chelating Agent Equivalents.
7
Table 4:
Ferrotrol 270 and Ferrotrol 271 Guidelines up to 20% HCl.
9
Table 5: Pounds of Iron Sequestered per 1000 gallons of SA-Acid System .
13
Table 6: Effectiveness of Various Iron Sequestering Agents in Spent Acid.
15
Table 7:
Limitations of Ferrotrol Agents in Acid.
16
Table 8:
HSSA System Options Based on Well Conditions.
17
Table 9:
HSSA Treatment Fluids.
17
FIGURES
Figure 1: Effect of Increasing pH on Various Sequestrants.
Page ii
13
Iron Control 1. Introduction. { XE "Introduction" } { XE "Iron Scale:Introduction} { XE "Iron Carbonate:Introduction }{ XE "Iron Sulphide:Introduction" } { XE "Water Insoluble Scales:Introduction" } { XE "Complexes:Introduction" } { XE "Acidizing:Introduction" } { XE "Iron Chloride:Introduction" } { XE "Ferric Iron:Introduction" }{ XE "pH:Spent Acid" } { XE "pH:Introduction" }{ XE "Ferric Iron:Introduction" } { XE "Ferric Iron:pH" } { XE "Ferric Iron:Hydrolysis" }{ XE "Hydrolysis:Ferric Iron" } { XE "Ferric Iron:Spent Acid" } { XE "Spent Acid:Hydrolysis" }{ XE "Spent Acid:Ferric Iron" } { XE "Hydrolysis:Ferric Iron" }{ XE "Precipitates:Introduction" } { XE "Precipitates:Iron Hydroxide" } { XE "Iron Hydroxide:Introduction" } { XE "Iron Hydroxide:Precipitation" } { XE "Iron Hydroxide:pH" } The presence of undesirable metal ions in production or injection wells, whether in the formation fluids, or in treating solutions, is an expensive problem. One of the most successful answers is that offered by chelation chemistry. Specific materials are added to treatment fluids which tie-up metal ions in a complex molecule so that their presence is no longer harmful. Those specific materials are called "chelating" and sequestering agents.
Iron scale deposits on tubing and casing in producing and injection wells have been a continuous and costly problem to the petroleum industry since the earliest days. These water-insoluble scales; iron oxides and salts such as iron carbonate and iron sulphide, not only restrict production or injectivity directly, but can also produce undesirable effects during acid stimulation. When an acid treating solution is injected through tubing or casing it will dissolve part of any existing iron scales present (rust { XE "rust" }, iron sulphide, iron carbonate { XE "iron carbonate:introduction" } etc.). The acid will then carry these iron compounds, that are put into solution as iron chlorides, deep into the formation. Additionally, iron compounds or minerals may be present in the formation itself and will also be dissolved by the acid (Table 1). Various studies have shown that hydrochloric acid entering the formation during a treatment, may contain concentrations of iron in solution as high as 10,000 parts per million. The presence of this iron can create three types of problems. First, interaction of the live acid containing ferric ions with reservoir hydrocarbons, specifically asphaltenes { XE "Asphaltenes" }{ XE "Organic Deposits" }, can cause the deposition of organic materials (sludge { XE "Sludge" }s) that are almost impossible to remove Second, as the acid becomes spent by reacting with the formation, the pH (acidity alkalinity value), of the solution increases. As the pH rises above 2.0, the ferric iron in solution will undergo hydrolysis and begin to re-precipitate as iron hydroxide. Formation of insoluble iron hydroxide increases as the pH of the treating solution continues to rise towards a neutral pH of 7.0.
Page 1
Iron Control
The insoluble iron hydroxide thus formed, is a gelatinous type precipitate that adheres strongly to rock surfaces. Such reprecipitation can reduce formation permeability and plug flow channels, reducing oil or gas production in producing wells or increase the injection pressure and reduce the injection volume in water-flood wells. The third problem is one of reprecipitation in a hydrogen sulphide (sour gas) environment. This involves iron sulphide scale { XE "Iron sulphide scale" } removed from the tubulars by the acid being re-deposited in the formation due to precipitation { XE "Precipitates:Iron Sulphide" }{ XE "Sour Gas:Hydrogen Sulphide" }{ XE "pH:Iron Sulphide Scales" }{ XE "Hydrogen Sulphide:Iron Sulphide Scales" } as the pH of the spent acid rises above 1.9.
Page 2
Iron Control Table 1: Sources of Iron. Scale Deposit
Chemical Formula
Oxidation State
Casing, tubing and equipment{ XE "Casing, tubing and equipment:Sources of Iron" }.
Iron oxide{ XE "Iron oxide:Sources of Iron" } Rust{ XE "Rust:Sources of Iron" } Ferric oxide { XE "Ferric oxide:Sources of Iron" } Iron carbonate { XE "Iron carbonate:Sources of Iron" } Iron sulphide{ XE "Iron sulfide:Sources of Iron" }
FeO
+3
Fe2O3 Fe2O4
+3 +2
FeCO3
+2
FeS
+2
Formation{ XE "Formations:Sources of Iron" }.
Haemetite { XE "Haemetite:Sources of Iron" } Magnetite Pyrite { XE "Pyrite:Sources of Iron" } Siderite Chlorite { XE "Chlorite:Sources of Iron" } Mixed Clays { XE "Mixed Clays:Sources of Iron" }
Page 3
Fe2O3 FeO3, Fe2O3 FeS
+3 +2 and +3 +2
FeCO3 -
+2 +2
-
+2
Iron Control 2. Oxidation State of Dissolved Iron. { XE "Oxidation State of Dissolved Iron" }{ XE "Oxidation State" }{ XE "Oxidation State:Ferric Iron" }{ XE "Oxidation State:Ferrous Iron" } { XE "Ferric Iron:Oxidation State" } { XE "Ferrous Iron:Oxidation State" }{ XE "pH:Ferric Iron" } { XE "pH:Ferrous Iron" }{ XE "Ferric Iron:Precipitation" }{ XE "Ferrous Iron:Precipitation" } { XE "Precipitates:Ferric Iron" } { XE "Precipitates:Ferrous Iron" }{ XE "Oxidation State:Precipitation" }{ XE "Precipitates:Oxidation State" }{ XE "Solubility:Iron Compounds" }{ XE "Solubility:Oxidation State" }{ XE "Acidizing:Oxidation State" } { XE "Oxidation State:Acidizing" } +3 +2 Iron in solution is found in two oxidation states, ferric (Fe ) and ferrous (Fe ), which depends upon the source of the iron compound which is dissolved by the acid (see Table 1). The oxidation state is important to iron control because, the two states form compounds of different solubilities, with the same anions. The oxidation state also determines the pH at which precipitation of the compound will occur. For example; ferric ion present in solution will precipitate as ferric hydroxide at a pH of about 2.5. However, ferrous hydroxide from ferrous iron remains in solution up to a pH of about 7.5.
The chemistry of dissolved iron is further complicated by a changes to the oxidation state of the iron anion that take place down hole. This change occurs by the oxidation-reduction balance of the solution. For example; dissolved ferrous iron is readily oxidized to ferric iron in oxygen-containing waters. Conversely, ferric iron is reduced to ferrous iron in the presence of a reducing agent, such as hydrogen sulphide (H2S) and can even produce elemental sulphur. During Acidizing treatments dissolved iron will be present in both oxidation states (ferrous and ferric). Analysis conducted on return fluids after acid treatments has shown 2+ that the ferrous state (Fe ) is the dominant form. The ratio of ferrous to ferric state iron present can vary greatly depending on well conditions, but commonly fall between the range of 5:1 and 10:1 ferrous to ferric iron. •
2+
Ferrous iron (Fe ).
Precipitates occurs at pH of 7.0 or greater. •
3+
Ferric iron (Fe ).
Precipitates occurs at pH of 2.0 to 3.0. Understanding of the environment in which the acid is to be pumped is essential to determining the appropriate materials and quantities of iron control (Ferrotrol) agents to be used.
Page 4
Iron Control 3. Methods of Iron Control. { XE "Methods of Iron Control" } Several chemical additives are used to inhibit the precipitation of iron from spent acid. These comprise four main methods and are used as follows
Chelation and Sequestration. { XE "Chelation:Methods of Iron Control" } { XE "Methods of Iron Control:Chelation" }{ XE "Sequestration:Methods of Iron Control" }{ XE "Methods of Iron Control:Sequestration" }
•
Complexion of metal ions by organic or inorganic molecules. These additives form stable, water soluble, metal-complexes. By complexing with iron, its activities are reduced and it is prevented from reacting as normally expected. Most often these additives are organic compounds. Reducing Agents. { XE "Methods of Iron Control:Reducing Agents" }{ XE "Reducing Agents:Methods of Iron Control" }
•
These additives function primarily by converting ferric iron present in solution to the more soluble ferrous state. pH Control Agents (Buffers). { XE "Methods of Iron Control:Buffering (pH Control)" }{ XE "Buffers:Methods of Iron Control" } { XE "pH:Methods of Iron Control" }
•
Organic acids such as acetic acid are used as buffering agents to control and maintain a low pH within the acid system. They retard the precipitation of insoluble iron particularly after the treatment has spent. Sulphide Scavengers. { XE "Methods of Iron Control:Sulphide Scavengers)" }{ XE "Sulphide Scavengers:Methods of Iron Control" }
•
Chemicals that form stable complexes with sulphide ions. Used in conjunction with ferrous iron complexing agents, prevent the formation of iron sulphide. The choice of iron control agent is usually determined by well conditions. The addition of a chemical agent in acidizing should not be done without design, because the equality of effective iron control is not the same for all chemicals. Their effectiveness is influenced by: • • •
• •
Page 5
The pH{ XE "pH:fluid systems" } of the system in which they are utilized. Temperature{ XE "Temperature" }. Concentration{ XE "Additive Concentrations" } at which the additive is used. Other additives present in the system. Presence of oxygen{ XE "Oxygen:Reservoir fluids" } in the reservoir fluids.
Iron Control •
Presence of hydrogen sulphide { XE "Hydrogen sulphide:reservoir fluids" }.
The data shown in Table 2 for the Ferrotrol agents may be used as a guide for selecting an iron control agent. Table 3 shows conversion factors for the different Ferrotrol chelating agents equivalents. Often these materials are used in conjunction with each other to optimize their benefits.
Table 2: Ferrotrol Products (Iron Control Chemicals).
Ferrotrol Product Code
Type Function
Chelation Ratio*
Amount to Retain 1000 ppm Ferric Iron in Solution
Product Solubility in 15% HCl (lbs/1000 gal)
200
Reducing Agent
1.0 : 2.5
9.45 ppt
Very Soluble
210
Reducing Agent
1.0 : 1.7
7.7 ppt
Very Soluble
260L
Reducing Agent
3.0 : 3.1
1.3 gpt
Very Soluble
270L
Reducing Agent
2.02 : 1.0
1.8 gpt
Very Soluble
271L 300
Catalyst for Ferrotrol 270L Chelation
3.53 : 1.0
29.45 ppt
> 300
300L
Chelation
3.53 : 1.0
5.89 gpt
> 300 (60 gal)
700
Chelation
5.30 : 1.0
44.23 ppt
68
800
Chelation
5.19 : 1.0
43.44 ppt
> 400
800L
Chelation
5.19 : 1.0
8.64 gpt
> 400 (85 gal)
810
Chelation
3.61 : 1.0
30.10 ppt
Very Soluble
900
Chelation
8.33 : 1.0
69.95 ppt
100
900L
Chelation
7.45 : 1.0
14.13 gpt
100 (22.7 gal)
1000
Chelation
6.70 : 1.0
55.91 ppt
89
HS-A
Sulphide
-
See HSSA
Very Soluble
HS-B
Scavenger System pH Control
-
Acetic Acid
Page 6
Very Soluble
-
Very Soluble -
-
Iron Control { XE "Ferrotrol Products:Iron Control Chemicals" }{ XE "Ferrotrol Products:Function" }{ XE "Iron Control Chemicals:Usage" }{ XE "Ferrotrol Products:Usage" } { XE "Usage:Ferrotrol Products" } { XE "Solubility in Acid:Iron Control Chemicals" }{ XE "Solubility in Acid:Ferrotrol Products" }{ XE "Solubility:Iron Control Chemicals" } Notes:
* Chelation Ratio 1. 2. 3. 4.
=
lbs of Ferrotrol product per 1.0 lb of Iron
Ratios are based on spent acid with the exception of Ferrotrol 260L, Ferrotrol 270 and Ferrotrol 271. Most Ferrotrol chelation agents are less effective in live acid. 3+ 2+ Control Ratio for Reducing agents is to reduce Fe to Fe . Ferrotrol 270L and Ferrotrol 271L must always be used together.
3.1. Chelating Agents.{ XE "Chelating Agents.:Methods of Iron Control" }{ XE "Methods of Iron Control:Chelating Agents" }
Chelating agents { XE "Chelating agents" }{ XE "Chelating agents:Citric Acid" }{ XE "Chelating agents:NTA" }{ XE "Chelating agents:EDTA" } are chemicals that for stable, water soluble complexes with ferric and ferrous iron ions. By complexing the iron, its reactivity is reduced and its normal insoluble products are inhibited from forming. Organic acids and their salts such as Citric Acid { XE "Citric Acid:Chelating agents" }, NTA and EDTA { XE "EDTA:Chelating agents" }, are the materials currently used in this capacity. The iron complexes { XE "iron complexes:Chelating agents" }{ XE "iron complexes:stability" } formed will remain in solution in the spent acid, preventing the formation of ferric hydroxide even at pH values approaching 7.0. However, these complexes will break down with time (see Table 6), and therefore, the chelating agent should be selected to provide sufficient protection time to allow treatment of the well and recovery of the fluid. Under normal circumstances fluid recovery { XE "fluid recovery time" } should begin within one to four hours following a treatment. If Hydrogen sulphi de { XE "Hydrogen sulphide:Chelating agents" } (H2S) is present then the use of hydrogen sulphide scavengers { XE "hydrogen sulphide scavengers:Chelating agents" } should be used in conjunction with the chelating agents.
3.1.1. Citric Acid{ XE "Citric Acid:Chelating Agents" }{ Agents:Citric Acid" }.
XE
"Chelating
The most commonly used chelating agent is citric acid. This is Ferrotrol 300 { XE "Ferrotrol 300:Citric Acid" }{ XE "Citric Acid:Ferrotrol 300" }{ XE "Citric Acid:Chelating Agents" }{ XE "Ferrotrol 300" } (powder) or Ferrotrol 300 L{ XE "Ferrotrol 300 L" } (50% aqueous solution containing 5 pounds of citric acid per gallon). It can be used in all acid solutions including 28% HCl and HCl:HF systems. Often combinations of citric acid with other Ferrotrol agents (chelating and reducing types) and or buffers are used. Page 7
Iron Control
At concentrations greater that 50 pounds per 1000 gallons the possibility exists for the formation of calcium citrate { XE "calcium citrate" }. This may occur when the iron concentration is insufficient to consume all of the citric acid and sufficient calcium has dissolved in the acid. At a pH value above 4.5 the solubility of calcium citrate is very low in spent acid or water. Ordinarily the precipitation of calcium citrate will not occur in the formation due to the pH effect of dissolved carbon dioxide in the treating fluid, formed by the reaction of acid with carbonates. The calcium citrate will most likely form in areas where the greatest pressure drop occurs, such as the perforation tunnels, the down hole pump or at the surface. Recommended concentrations: Ferrotrol 300 Ferrotrol 300L
25 to 250 pounds per 1000 gallons. 5 to 50 gallons per 1000 gallons.
3.1.2. EDTA Acid{ XE "Chelating Agents:EDTA Acid" }{ XE "EDTA Acid:Chelating Agents" } (Ethylene-Di-Amine-Tetra-Acetic Acid{ XE "Ethylene-Di-Amine-Tetra Acetic Acid" }).
This product (Ferrotrol{ XE "Chelating Agents:Ferrotrol 700" } 700{ XE "Ferrotrol 700:Chelating Agents" }{ XE "Ferrotrol 700:EDTA" }{ XE "EDTA:Ferrotrol 700" }) is seldom recommended for use as an iron control agent due to its extremely low solubility in water and very slow rate of dissolution in acid at surface conditions. Due to this various salts of EDTA are commonly used instead and are mentioned below.
3.1.3. Tetra Sodium EDTA.{ XE "Tetra Sodium EDTA:Chelating Agents" }{ XE "Chelating Agents:Tetra Sodium EDTA" }{ XE "Chelating Agents:Ferrotrol 900" }{ XE "Ferrotrol 900:Tetra Sodium EDTA" }{ XE "Tetra Sodium EDTA:Ferrotrol 900" }
Ferrotrol 900 is Tertra-Sodium-Ethylene-Di-Amine-Tetra-Acetate, Tetra-Hydrate (EDTA Na 4 .4H2O) and Ferrotrol 900L { XE "Ferrotrol 900L" } is the 42% by weight solution (approximately 4.5 pounds of Ferrotrol 900 per gallon. These materials are not recommended for use in acids containing hydrofluoric acid or in hydrochloric acid that is stronger than 15% HCl. The solubility in 15% HCl is limited to approximately 100 pounds per thousand gallons or 22.7 gallons per thousand gallons respectively. These are the least efficient chelating agents available for controlling iron compounds in spent acid.
3.1.4. Di-Sodium EDTA.{ XE "Chelating Agents:Di-Sodium EDTA" }{ XE "Chelating Agents:Ferrotrol 1000" }{ XE "Di-Sodium EDTA:Ferrotrol 1000" }{ XE "Ferrotrol 1000:Di-Sodium EDTA" }{ XE "Ferrotrol 1000:Chelating Agents" }{ XE "Di-Sodium EDTA:Chelating Agents" }
Page 8
Iron Control
Ferrotrol 1000 is Di-Sodium-Ethylene-Di-Amine-Tetra Acetate, Di-Hydrate (EDTA Na2 .2H2O). This material is not recommended for use in acids containing hydrofluoric acid or for use in hydrochloric acid stronger than 15% HCl. Solubility in 15% HCl is limited to approximately 89 pounds per thousand gallons. Ferrotrol 1000 is a slightly more efficient chelating agent than Ferrotrol 900 or 900L.
3.1.5. Tri-Sodium NTA. { XE "Chelating Agents:Tri-Sodium NTA" }{ XE "Chelating Agents:Ferrotrol 800" }{ XE "Tri-Sodium NTA:Ferrotrol 800" }{ XE "Ferrotrol 800:TriSodium NTA" }{ XE "Ferrotrol 800:Chelating Agents" }{ XE "Tri-Sodium NTA:Chelating Agents" }
Ferrotrol 800 is Tri-Sodium-Nitrilo-Acetate, Mono-Hydrate (NTA Na 3.H2O) and Ferrotrol 800L{ XE "Ferrotrol 800L" } is a 43% by weight solution (approximately 4.7 pounds per gallon). This product can be used in a broad range of acid strengths from 3.0% to 28% HCl and can be placed in either the mix water or the acid. However, its solubility at surface conditions is limited to 50 pounds per thousand gallons in 28% HCl. This product is not recommended for use in Hydrofluoric acid systems due to the sodium content. Tri-sodium NTA will react with calcium to form a salt that is soluble in water and spent acid. Recommended concentrations: Ferrotrol 800 Ferrotrol 800L
25 to 350 pounds per 1000 gallons (3.0% to 20% HCl). 5 to 75 gallons per 1000 gallons.
3.1.6. Nitrilo-Acetic Acid{ XE "Nitrilo-Acetic Acid" \t " See NTA Acid" }.{ XE "Chelating Agents:NTA Acid" }{ XE "Chelating Agents:Ferrotrol 810" }{ XE "NTA Acid:Ferrotrol 810" }{ XE "Ferrotrol 810:NTA Acid" }{ XE "Ferrotrol 810:Chelating Agents" }{ XE "NTA Acid:Chelating Agents" }
Ferrotrol 810 is NTA Acid and is the second most effective chelating agent available. This product can be used in a broad range of acid strengths from 3.0% to 28% HCl and most commonly used strengths of HCl:HF acid mixtures. Ferrotrol 810 is only slightly soluble in water and should always be added to acid and not the mix water. NTA reacts with calcium ion to form a salt that is soluble in spent acid and water. Recommended concentrations: Ferrotrol 810
25 to 350 pounds per 1000 gallons.
Table 3: Conversion Factors, Ferrotrol Chelating Agent Equivalents. { XE "Ferrotrol Products:Conversion Factors" }{ XE "Conversion Factors:Ferrotrol Products" }
Page 9
Iron Control To Replace One Pound of
Use Weight in Pounds of Ferrotrol 300
700
800
800L
810
900
900L
1000
Ferrotrol 300
---
1.51
1.47
2.26
1.02
2.36
5.23
1.90
Ferrotrol 700
0.66
---
0.93
2.15
0.65
1.58
3.40
1.26
Ferrotrol 800
0.68
1.07
---
2.34
0.70
1.70
3.65
1.36
Ferrotrol 800L
0.44
0.46
0.43
---
0.30
0.74
1.58
0.59
Ferrotrol 810
0.98
1.54
1.44
1.44
---
2.44
5.24
1.96
Ferrotrol 900
0.42
0.63
0.59
0.59
0.41
---
2.15
0.80
Ferrotrol 900L
0.19
0.29
0.27
0.27
0.19
0.47
---
0.37
Ferrotrol 1000
0.53
0.79
0.73
0.73
0.51
1.25
2.68
---
Note:
Density (ppg) : : :
Ferrotrol 300L Ferrotrol 800L Ferrotrol 900L
= = =
10.0 ppg (50% Active) 11.0 ppg 10.9 ppg
3.2. Reducing Agents{ XE "Reducing Agents:Methods of Iron Control" }{ XE "Reducing Agents" }{ XE "Methods of Iron Control:Reducing Agents" }.
Reducing Agents function to convert ferric iron in solution to ferrous iron and to maintain this oxidation state. The elimination of ferric ions prevents the precipitation of ferric hydroxide{ XE "ferric hydroxide:Reducing Agents" } as the acid spends and the pH rises above 2.5. In addition, removal of the ferric ions reduces the risk of asphaltene flocculation and precipitation. Anytime asphaltenes { XE "asphaltenes:Reducing Agents" } are present in the crude oil, a reducing agent should be used. The presence of dissolved oxygen{ XE "dissolved oxygen:Reducing Agents" }{ XE "Oxygen:Reducing Agents" } ( maximum 8.0 parts per million) will react with these reducing agents and therefore, the amount of reducing agent used in the acid should compensate for this. Erythorbic acid { XE "Erythorbic acid:Reducing Agents" } and its salts are commonly used for this application.
3.2.1. Sodium Erythorbate.{ XE "Reducing Agents:Sodium Erythorbate" }{ XE "Ferrotrol 200:Sodium Erythorbate" }.{ XE "Sodium Erythorbate.:Reducing Agents" }{ XE "Sodium Erythorbate.:Ferrotrol 200" }
Page 10
Iron Control
Ferrotrol 200{ XE "Reducing Agents:Ferrotrol 200" }{ XE "Ferrotrol 200:Reducing Agents" } is Sodium Erythorbate (NaC 6H7O6.H2O). The products of this reducing agent are believed to be complexes with the ferrous ions. Ferrotrol 200 is very soluble in water and 15% HCl. It is less stable as the acid strength is increased and if added to 28% HCl, degradation will occur, producing a fine carbon like product. Stability is also affected by temperature. Storage time in acid should be minimized to ensure the chemical stability. Usage of this material in HCl:HF acid systems should be minimized. This product is more effective in spent acid.
Recommended concentrations: Ferrotrol 200
10 to 40 pounds per 1000 gallons.
3.2.2. Erythorbic Acid. { XE "Reducing Agents:Erythorbic Acid" }{ XE "Ferrotrol 210:Erythorbic Acid" }.{ XE "Erythorbic Acid.:Reducing Agents" }{ XE "Erythorbic Acid.:Ferrotrol 210" }{ XE "Ferrotrol 210:Reducing Agents" }{ XE "Reducing Agents:Ferrotrol 210" }
Ferrotrol 210 is Erythorbic Acid (C 6H8O6.). The products of this reducing agent are believed to be complexes with the ferrous ions. Ferrotrol 210 is very soluble in water and 15% HCl. It is less stable as the acid strength is increased and if added to 28% HCl, degradation will occur, producing a fine carbon like product. Stability is also affected by temperature. Storage time in acid should be minimized to ensure the chemical stability. This product is more effective in spent acid.
Recommended concentrations: Ferrotrol 210
10 to 40 pounds per 1000 gallons.
3.2.3. Ferrotrol 260L.{ XE "Ferrotrol 260L:Reducing Agents" }{ XE "Reducing Agents:Ferrotrol 260L" }
This is a proprietary blend of chemicals in liquid form which can be readily mixed with hydrochloric acid at concentrations ranging from 3.0% to 28% HCl and most common HCl:HF acid mixtures. This system is quite expensive however, 1.3 gallons per thousand gallons will reduce 1000 parts per million of ferric iron. This product works equally well in live acid or spent acid. Recommended Concentrations: Ferrotrol 260L
Page 11
1.5 to 15 gallons per thousand gallons.
Iron Control
3.2.4. Ferrotrol 270 and Ferrotrol 271. { XE "Ferrotrol 270L:Reducing Agents" }{ XE "Reducing Agents:Ferrotrol 270L" }{ XE "Ferrotrol 271L:Reducing Agent Catalyst" }{ XE "Reducing Agents:Ferrotrol 271L" }{ XE "Catalyst:Reducing Agents" }
These are proprietary blends of chemicals in liquid form which can be readily mixed with hydrochloric acid at concentrations ranging from 3.0% to 20% HCl and most common HCl:HF acid mixtures up to 12%:3.0% HCl:HF. The system has two components with Ferrotrol 270 being added to the acid at the time of loading, and Ferrotrol 271, the catalyst, being added just prior to pumping. Failure to pump the fluid immediately after adding the catalyst may result in more material being required to provide adequate reducing capabilities. 1.8 gallons per 1000 gallons of Ferrotrol 270 and 1.0 gallons per 1000 gallons Ferrotrol 271, together will reduce 1000 parts per million of ferric iron. These products work equally well in spent or live acid. Table 4 lists guideline concentrations for these products for use in hydrochloric acid up to 20% concentration. Recommended Concentrations: Ferrotrol 270L Ferrotrol 271L
2.0 to 16 gallons per thousand gallons. 1.0 to 2.0 gallons per thousand gallons.
Table 4: Ferrotrol 270 and Ferrotrol 271 Guidelines{ XE "Ferrotrol 270 and Ferrotrol 271 Guidelines" } up to 20% HCl. Ferric Iron (ppm)
Ferrotrol 270 (gpt)
Ferrotrol 271 (gpt)
1000
2
1
2500
5
1
5000
8
2
10000
16
2
3.3. pH Control Agents (Buffers).{ XE "pH Control Agents (Buffers).:Methods of Iron Control " }{ XE "Methods of Iron Control:pH Control Agents" }{ XE "Methods of Iron Control:Buffers" }{ XE "Buffers:Methods of Iron Control" }
These materials act as buffering agents to maintain a low pH and retard the precipitation of insoluble iron compounds. Proper buffering agents help control the pH of an acid treating fluid once it begins to react on the formation or other acid soluble materials. The use of buffers without chelating agents and or reducing agents is generally not recommended. Buffers should also not be used in well with a bottom hole temperature greater than 160 °F (71 °C). Acetic Acid, Acetic Anhydride or Sodium Acid Page 12
Iron Control
Pyrophosphate (SAPP) are the primary materials used as buffers in acid treating solutions. The effect of pH on the precipitation of iron compounds with various pH control agents is shown in Figure 1 on page 12
3.3.1. Acetic Acid{ XE "Acetic Acid:pH Control Agents" }{ XE "Acetic Acid:Buffers" }{ XE "Buffers:Acetic Acid" }{ XE "pH Control Agents:Acetic Acid" } and Acetic Anhydride.{ XE "Acetic Anhydride:pH Control Agents" }{ XE "Acetic Anhydride:Buffers" }{ XE "pH Control Agents:Acetic Anhydride" }{ XE "Buffers:Acetic Anhydride" }.
These are both liquids that mix rapidly in all normal acid stimulation fluids and should be readily available in most areas. Recommended Concentrations: Acetic Acid
10 to 20 gallons per thousand gallons.
3.3.2. Sodium Acid Pyrophosphate{ XE "Sodium Acid Pyrophosphate" \t " See SAPP" } (SAPP{ XE "SAPP:Sodium Acid Pyrophosphate" }{ XE "SAPP:Buffers" }{ XE "SAPP:pH Control Agents" }{ XE "pH Control Agents:SAPP" }{ XE "Buffers:SAPP" }).
SAPP is a white solid material which dissolves slowly in acid treating solutions, but is not corrosive or difficult to handle. Recommended Concentrations: SAPP
1.0 to 10 pounds per thousand gallons.
3.4. Sulphide Scavengers{ XE "Sulphide Scavengers:Methods of Iron Control" }{ XE "Methods of Iron Control:Sulphide Scavengers" }.
Sulphide scavengers are chemicals which form stable complexes with sulphide ions. This complexation in conjunction with the complexation of the ferrous ions { XE "Ferrous ions:Sulphide scavengers" } by one of the chelating agents, prevents the precipitation of iron sulphide{ XE "Iron sulphide:Sulphide scavengers" } in a formation containing hydrogen sulphide { XE "hydrogen sulphide:Sulphide scavengers" }, when the acid spends. Iron sulphide scale dissolved by hydrochloric acid will precipitate out of spent acid as the pH of the fluid rises above 1.9. BJ Services’ sulphide system consists of Ferrotrol HS-A { XE "Ferrotrol HS-A:Sulphide scavengers" } and Ferrotrol HS-B { XE "Ferrotrol
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Iron Control
HS-B:Sulphide scavengers" } to provide control of the sulphides in solution, whilst Ferrotrol 300 { XE "Ferrotrol 300:Sulphide scavengers" } (300L), Ferrotrol 800 { XE "Ferrotrol 800:Sulphide scavengers" } (800L) or Ferrotrol 810 { XE "Ferrotrol 810:Sulphide scavengers" } can be used with these additives to control the ferrous ions. 3.4.1. Ferrotrol HS-A and Ferrotrol HS-B.
Ferrotrol HS-A and Ferrotrol HS-B should be added to the acid prior to pumping. The Ferrotrol HS-A may be added as much as twenty four hours prior to pumping, however, Ferrotrol HS-B must not be added until just prior to pumping. These two components mixed in acid release the active ingredient which controls the sulphide. This ingredient is very volatile and will eventually evaporate from the acid solution. Ferrotrol HS-A and Ferrotrol HS-B concentrates should never be mixed directly together. Ferrotrol HS-A is corrosive and Ferrotrol HS-B is flammable. Recommended Concentrations:
4.
Ferrotrol HS-A 5.0 to 15 gallons per thousand gallons. Ferrotrol HS-B 3.0 to 10 gallons per thousand gallons. Acid Treatment Designs.
4.1.
Pipe Pickling Treatments.
The most effective means of controlling tubular iron as a source of problems is to perform a “pipe pickling { XE "pipe pickling treatments" }” treatment. This involves pumping a volume of inhibited acid { XE "inhibited acid" }, down the tubing and then reversing it out of the well, allowing the acid to remove the majority of iron deposits. This prevents the main acid treatment from dissolving as much iron and carrying it into the formation. It is generally recommended that a volume of 100 gallons per 1000 ft of tubing be used to perform this “pickle”. The fluid should consist of at least 10% hydrochloric acid and preferably 15%. It is further recommended that a solvent phase (xylene { XE "xylene:pipe pickling" }) be used either ahead of the acid or dispersed within the acid (One Shot Acid Plus { XE "One Shot Acid Plus:Pipe Pickling" }), to facilitate removal of organic materials and pipe dope { XE "pipe dope" } that may be in the tubing. If casing is to be “pickled”, the volume to use may need to be adjusted depending on the extent of the iron scale present.
4.2. Sequestering Acid Systems (SA-Systems). { XE "Sequestering Acid Systems" }{ XE "Precipitates:Sequestering Acid Systems" }{ XE "Acetic Acid:Sequestering Acid Systems" }{ XE "Citric Acid:Sequestering Acid Systems" }{ XE "Lactic Acid:Sequestering Acid Systems" }{ XE "Organic Acids:Sequestering Acid Systems" }{ XE "pH:Precipitation" }{ XE "pH:Sequestering Acid Systems" }{ XE "pH:Effect on Sequestrants" }{ XE "SA-10:Sequestering Acid
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Iron Control
Systems" }{ XE "Effect of pH on Various Sequestrants" }{ XE "Iron Hydroxide:Sequestering Acid Systems" }{ XE "Precipitates:Iron Hydroxide" }{ XE "Iron Hydroxide:Precipitation" } { XE "Hydrochloric Acid:Sequestering Acid Systems" }{ XE "pH:Hydrochloric Acid" }{ XE "Hydrochloric Acid:pH" }{ XE "Spent Acid:pH" }{ XE "pH:Spent Acid" } Chemicals called sequestering agents are used in acidizing to control the secondary precipitation of iron deposits from spent acid solutions in non hydrogen sulphide environments. Added to hydrochloric acid or HCl:HF systems, these sequestrants prevent the customary, iron-spent acid reaction for extended periods of time. Most sequestering agents are organic acids such as acetic, citric or lactic acid, or a mixture of these acids. However, not every organic acid is equally effective for reducing or controlling the activities of iron. The effectiveness of each acid is influenced by many factors, such as the pH of the system in which it is used, its concentration and the external temperature. For these reasons sequestrants should be laboratory tested at different concentrations under simulated pH and temperature conditions of actual use. Figure 1 shows the pH range at which iron re-precipitates in neutralized SA-10 compared to three other sequestrants in spent acid. As can be seen the alternative sequestering systems, will retard the reprecipitation of iron hydroxide up to a pH of about 5.0, whereas the SA-10 will prevent this up to a pH of nearly 8.0. BJ Services Sequestering Acid systems (SA-2, SA-4, SA-6, SA-8 and SA-10), will effectively prevent water insoluble iron hydroxides, resulting from the chemical reaction of hydrochloric acid and iron compounds, from forming and precipitating out into the formation or well-bore by sequestering (chemically "tying up") these hydroxides.
Figure 1: Effect of Increasing pH on Various Sequestrants{ XE "Effect of Increasing pH on Various Sequestrants" }.
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Iron Control
SA-10 ACETIC AND CITRIC ACID * ACETIC ACID (GLACIAL)
IRON REPRECIPITATES
LACTIC ACID (44%) pH OF NEUTRALISED ACID 0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
pH SCALE * 10 gallons Acetic Acid & 50 lbs Citric Acid per 1000 gallons.
Sequestering acid systems prevent the severe plugging and permeability damage that might otherwise result where iron scales or minerals are present. The specific SA-Acid system selected depends upon the quantity of iron solids present that must be tied up in solution. SA-2 can sequester 2000 ppm of ferric iron whilst SA-10 will complex about 10,000 ppm iron. Typical quantities of common iron salts that are sequestered by the different SA-Acid systems is shown in Table 5. { XE "Sequestering Acid Systems:Iron Hydroxide" }{ XE "Sequestering Acid Systems:Hydrochloric Acid" }{ XE "Sequestering Acid Systems:pH" }{ XE "pH:Hydrochloric Acid" }{ XE "Hydrochloric Acid:pH" }{ XE "Spent Acid:pH" }{ XE "pH:Spent Acid" }{ XE "Iron Scales:Sequestering Acid Systems" }{ XE "Iron Salts" } Table 5: Pounds of Iron Sequestered per 1000 gallons of SA-Acid System{
XE Pounds of Iron Sequestered per 1000 gallons of SA-Acid System } { XE "Ferrous Iron " " Sulphide:SA Acid Systems" }{ XE "Iron Sulphide:SA Acid Systems" }{ XE "Ferrous Iron Carbonate:SA Acid Systems" }{ XE "Iron Carbonate:SA Acid Systems" }{ XE "Ferric Iron Oxide:SA Acid Systems" }{ XE "Ferric Oxide:SA Acid Systems" }{ XE "Iron Oxide:SA Acid Systems" }. SA-ACID SYSTEM SA-2
FERROUS IRON SULPHIDE FeS 26.2
FERROUS IRON CARBONATE FeCO3 34.6
FERRIC IRON OXIDE Fe2O3 23.8
SA-4
52.5
69.2
47.6
SA-6
78.8
103.8
71.5
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Iron Control SA-8
105.0
138.1
95.4
SA-10
131.3
172.9
119.1
Sequestering agents, such as Ferrotrol 300, control metal ions by blocking the reactive sites of the metal ion and preventing them from entering into their normal (and in many cases undesirable) reactions. Those sequestering agents forming the most stable bonds with metal ions will be the most effective in preventing metal ion activity. { XE "Ferrotrol 300:SA Acid Systems" }{ XE "Ferrotrol 300:Sequestrant" }{ XE "Ferrotrol 210:Reducing Agent" } { XE "Reducing Agents:Ferrotrol 210" }{ XE "Sequestering Agents:Ferrotrol 300" } 3+ 2+ Ferrotrol 210 is a reducing agent. This product works by reducing Fe to Fe , based on a chemical reaction where an electron is donated to the ferric iron thus reducing its oxidation state. When mixed with hydrochloric acid, Ferrotrol 210 also exhibits weak chelating properties that combine with its reducing properties to provide enhanced activity. In general, a molar ratio of 1.0 to 1.0 sequestrant is sufficient to maintain solubility of the iron. Using this ratio, one pound of iron requires: { XE "Molar Ratio of Sequestrant to Iron Solubility" }{ XE "Citric Acid:Weight Ratio to Iron" }{ XE "EDTA:Weight Ratio to Iron" }{ XE "NTA:Weight Ratio to Iron" } { XE "Sequestrants:Molar Ratio" } { XE "Sequestrants:Performance" }{ XE "Iron Control Chemicals:Performance" } 3.5 lbs Citric Acid 5.2 lbsEDTA (H4 EDTA) 7.5 lbs EDTA Tetra-Sodium Salt (Na4 EDTA) 17.9 lbs EDTA Na 4 solution 3.5 lbs NTA Na 3 (Tri-Sodium Salt) • • • • •
The performance of sequestering agents is not always equal. Their effectiveness at forming stable metal chelates can be influenced by
• •
• •
The pH of the system. Temperature. Concentration at which they are used. Metal ion or ions that are present in the system.
The best approach in selecting a sequestering agent is to compare their performance under simulated actual use conditions. Table 6 shows the effectiveness of various sequestering in spent hydrochloric acid at different temperatures. Studies to compare the performance of iron control agents under similar well conditions have shown the relative order of stability of the soluble complexes to be; Citrate > EDTA > NTA > Acetic.
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Iron Control Table 6: Effectiveness of Various Iron Sequestering Agent s in Spent Acid. { XE "Effectiveness of Various Iron Sequestering Agents in Spent Acid" }{ XE "Citric Acid:Effectiveness" "Acetic Acid:Effectiveness" "Lactic }{ XE }{ XE Acid:Effectiveness" }{ XE "EDTA:Effectiveness" }{ XE "NTA:Effectiveness" }{ XE "Gluconic Acid:Effectiveness" }{ XE "Ferric Iron:Amount Stabilized by Various Sequestrants" } { XE "Stability of sequestrants" } { XE "Complexes:Stability" } Sequestering Agent
Additive Concentration (Lbs/1000 Gals)
Temperature (°F)
Ferric Iron Stabilized (ppm)
Time Before Reprecipitation
Citric Acid (Ferrotrol 300) Acetic Acid
35
200 75 150 200
1,000 10,000 5,000 10,000 5,000
Over 48 Hours 24 Hours 2 Hours 10 Minutes 20 Minutes
75
10,000 5,000 10,000 5,000 10,000 5,000
48 Hours 7 Days 24 Hours 7 Days 15 Minutes 30 Minutes
Mixture Of Citric And Acetic Acid
174
50 87
150 200
Tetra-Sodium Salt of EDTA (Ferrotrol 900) Tri-Sodium Salt of NTA (Ferrotrol 800) Lactic Acid
225
All Ranges
4,300
Over 48 Hours
50
Up to 200
1,000
Over 48 Hours
65
75 150 200
1,700 1,700 1,700
24 Hours 2 Hours 10 Minutes
Gluconic Acid
103
150
1,500
20 Hours
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Table 7: Limitations of Ferrotrol Agents in Acid{ XE "Limitations of Ferrotrol Agents in Acid " }. Ferrotrol Agent
Common Name
Ferrotrol 200{ XE Sodium Erythorbate { XE "Ferrotrol "Sodium 200:Limtations" } Erythorbate:Limtations" } Ferrotrol 210{ XE Erythorbic Acid { XE "Ferrotrol "Erythorbic 210:Limtations" } Acid:Limtations" } Ferrotrol 260L { Proprietary Blend XE "Ferrotrol 260L:Limtations" } Ferrotrol 270L { Proprietary Blend XE "Ferrotrol 270L:Limtations" } Ferrotrol 271L { Proprietary Blend XE "Ferrotrol 271L:Limtations" } Ferrotrol 300{ XE Citric Acid { XE "Citric "Ferrotrol Acid:Limtations" } 300:Limtations" } Ferrotrol 300L { Citric Acid Solution XE "Ferrotrol 300L:Limtations" } Ferrotrol 700{ XE EDTA Acid { XE "EDTA "Ferrotrol Acid:Limtations" } 700:Limtations" } Ferrotrol 800{ XE Tri-Sodium Salt of NTA { "Ferrotrol XE "Tri-Sodium 800:Limtations" } NTA:Limtations" } Ferrotrol 800L Tri-Sodium Salt of NTA Solution Ferrotrol 810{ XE NTA Acid{ XE "NTA "Ferrotrol Acid:Limtations" } 810:Limtations" } Ferrotrol 900{ XE Tetra-Sodium Salt of EDTA { XE "Tetra"Ferrotrol 900:Limtations" } Sodium Page 19
Limitations
Not recommended for acid stronger than 20% HCl or with HF. Not recommended for acid stronger than 20% HCl. Expensive.
Must be used together and not in acid
stronger than 20% HCl.
Needs sufficient iron to consume.
Needs sufficient iron to consume.
Not recommended due to low solubility.
Not recommended for acid stronger than 20% HCl or with HF. Not recommended for acid stronger than 20% HCl or with HF. None
Solubility
Iron Control
Ferrotrol 900L { XE "Ferrotrol 900L:Limtations" } Ferrotrol 1000 { XE "Ferrotrol 1000:Limtations" } Ferrotrol HS-A { XE "Ferrotrol HS A:Limtations" } Ferrotrol HS-B { XE "Ferrotrol HSB:Limtations" } SAPP{ XE "SAPP:Limtation s" } Acetic Acid{ XE "Acetic Acid:Limtations" } or Acetic Anhydride
EDTA:Limtations" } Tetra-Sodium Salt of EDTA Solution
Solubility
Di-Sodium Salt of EDTA { XE "Di-Sodium EDTA:Limtations" }
Solubility
Proprietary Blend
Must be used together and with one of
Proprietary Blend
the Chelating agents above.
Sodium Acid Pyrophosphate
Not recommended as a Buffer at temperatures greater than 160 °F
Acetic Acid
Not recommended as a Buffer at temperatures greater than 160 °F
4.3. Hydrogen Sulphide Scavenger Acid{ XE "HSSA:Hydrogen Sulphide Scavenger Acid" }{ XE "Hydrogen Sulphide Scavenger Acid" \t " See HSSA" } (HSSA{ XE "HSSA" }).
The HSSA{ XE "HSSA:Sour Wells" }{ XE "Sour wells:HSSA" } system is a special acid system for use in sour wells to help control the re-precipitation of iron sulphide { XE "Iron sulphide:HSSA" }, iron hydroxide and the formation of elemental sulphur { XE "Elemental sulphur:HSSA" } during acid stimulation. The system incorporates Ferrotrol HS-A { XE "Ferrotrol HS-A:HSSA" }, Ferrotrol HS-B { XE "Ferrotrol HS-B:HSSA" }, and Ferrotrol 300{ XE "Ferrotrol 300:HSSA" }, Ferrotrol 800 { XE "Ferrotrol 800:HSSA" } or Ferrotrol 810 { XE "Ferrotrol 810:HSSA" }, which provide a synergistic effect to control these very damaging acid reaction precipitates. The system may be prepared using 3.0% to 28% hydrochloric acid. The following tables show the conditions for usage. Table 8: HSSA System Options Based on Well Conditions.{ XE "HSSA System Options Based on Well Conditions." } Severe Conditions
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System Options
Iron Control
AIron Sulphide{ XE "Iron A- Higher Concentrations of additives in the spearhead fluid with reduced Sulphide:HSSA" XE "Pipe }{ quantity in body of treating fluid Pickling:HSSA" "Hydrogen }{ XE Sulphide:HSSA" } Scale present. B- Unable to pickle tubulars. or C- Moderate to high H 2S (Greater than 5.0%).
B- Treat entire fluid with same high concentrations.
Moderate Conditions
System Options
A- Moderate H2S (2.0% to 3.0%) B- Able to pickle tubulars
A- Treat entire fluid with same concentration.
C- Iron Silphide Scale may be present. Normal Conditions
A- Low to moderate H 2S (0.5% to 2.0%). B- Able to pickle tubulars.
System Options
A- Treat entire fluid with same concentration.
C- Iron Sulphide Scale may be present
Table 9: HSSA Treatment Fluids.{ XE "HSSA Treatment Fluids." }{ XE "HSSA:Additive quantities" } Conditions
Quantities of Additives
Severe
Spearhead :
Acid + 15 gpt Ferrotrol HS-A + 10 gpt Ferrotrol HS-B + Ferrotrol 800 or 200 ppt Ferrotrol 300 ( 40 gpt Ferrotrol 300L) or Body of Treatment:
Acid + 8.0 gpt Ferrotrol HS-A + 6.0 gpt Ferrotrol HS-B + Ferrotrol 800 or 150 ppt Ferrotrol 300 ( 30 gpt Ferrotrol 300L) Moderate
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Iron Control
Acid + 7.5 gpt Ferrotrol HS-A + 5.0 gpt Ferrotrol HS-B + 100 ppt Ferrotrol 800 or 100 ppt Ferrotrol 300 ( 20 gpt Ferrotrol 300L) Normal
Acid + 5.0 gpt Ferrotrol HS-A + 2.5 gpt Ferrotrol HS-B + 50 to 75 ppt Ferrotrol 800 or Ferrotrol 300 ( 10 to 15 gpt Ferrotrol 300L)
4.3.1. HSSA Corrosion Inhibition{ XE "Corrosion Inhibition:HSSA" }{ XE "HSSA:Corrosion Inhibition" }.
Use recommened inhibitor loadings based on acid strength at temperatures up to 180 °F (82.2 °C). Above this temperature 25% more corrosion inhibitor should be utilized to provide adequate protevtion of tubulars.
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Iron Control 5. Non-Acid Treatments Using Chelating Agents. { XE "Non-Acid Treatments Using Chelating Agents" }{ XE "Calcium Sulphate:Removal" } { XE "Gypsum:Removal" } { XE "Mud Damage:Removal" } { XE "Water Based Mud Damage:Removal" }{ XE "Gypsol:Non Acid Treatments" }{ XE "Scale:Gypsum" }{ XE "Versol:Non Acid Treatments" }{ XE "Acid Sensitive Formations" } Chelating agents are also used in non-acid treating solutions for the removal of damage from water based mud and for the removal of calcium sulphate scales (gypsum).
Gypsol systems are water based systems containing chelating agents and surfactants that chemically disperse the scale into more simple acid soluble constituents by a dissociation process, or dissolve the scale by complexing the calcium ion and retaining it in solution in its ionic state. Both these systems are applied by using soaking of up to 24 hours and then swabbing the treatment back. These systems are useful where it is undesirable to perform an acid job. Gypsol 1 for example, can be used when there is the risk that an acid treatment might stimulate water production. Gypsol 1 dissolves the calcium sulphate scale by complexing the calcium ion and retaining it in its ionic state. This removal of the calcium provides the mechanism by which the scale material dissolves, usually requiring the solution to be left in contact for between 16 and 24 hours. Gypsol 2 is a "converter type" solvent, that converts calcium sulphate from an acid insoluble scale to one that can be easily dissolved with acid. Normally this solution is spotted across the scale deposit and left to soak for up to 24 hours, and is then swabbed or pumped out of the well. This treatment is then followed with a conventional hydrochloric acid soak to remove the converted scale. Versol 1 and Versol 2 are non acid reactive solvents which consist of a mixture of chelating agents and surfactants. They are used in the removal of formation damage caused by drilling muds, to break emulsions and water blocks, and to lower the viscosity of drilling fluids. Versol solvents are particularly useful in place of acid when treating, acid sensitive formations or wells with deep high temperature formations where excessive corrosion of tubular goods may be a problem. Versol treatments range from 100 to 200 gallons per foot of formation, with a recommended radial penetration of 4.0 to 6.0 feet from the well-bore. For best results these solvents should be allowed to soak for between four and six hours.
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