Thermodynamic+Inhibition

=﻿ Thermodynamic Inhibition =

Thermodynamic inhibitors are able to prevent the formation of barite scaling by decreasing the solution supersaturation. This is done by decreasing the ionic activity product by either lowering the solution pH or through a method called **chelating metal ions.**

The major problem for offshore oil recovery companies is the formation of Barium Sulphate (barite) in the tubes. This occurs when the SO4²- rich sea water comes in contact with the Ba2+ contained in the brine to form the insoluble Barium Sulphate. This section will discuss the reaction mechanism behind the formation of Barium Sulphate and the different types of chemical reagents needed to remove Barium Sulphate.

Formation:
The formation of Barium Sulphate is a simple exothermic addition reaction that’s given by the following equation. Note: Other precipitates such as Calcium Carbonate and Calcium Sulphate are also formed in the offshore oil recovery process; however, the removal techniques for these scales is a simple acid dissolution. So, we'll focus our attention here on the mechanisms of how to remove Barium Sulpahte scale using the chelating technique.



What type of reaction conditions would increase the rate of preciptation of Barium Sulphate? =Applying Le Chatelier's Principles=
 * 1) Since the overall enthalpy of the reaction is negative we have an exothermic reaction; as a result, lower temperatures would increase the rate of barite precipitiation. This becomes a major problem for offshore drilling because the deeper you drill the colder the surrounding temperature gets.
 * 2) Also, since they are 2 mols of reactants to 1 mol of products, increasing the pressure would also increase the rate of precipitation of barite. Again, when drilling at large depths you are dealing with extremely high pressures.
 * 3) Moreover, the addition of either the Sulphate or Barium ions can drive this reaction to completion faster. Since the oil companies use Sulphate rich sea water, to maintain the pressure in the well, they are actually increasing the rate at which barite forms on the inner surface of the pipes.



Characteristics of Barium Sulphate:

 * 1) The Barium Sulphate is a rhomboildal shape structure that’s held together very closely and strongly
 * 2) The pH varies from 6.5-8.0 (close to neutral)
 * 3) The solubility in H2O(l) ≤ 0.2% ,and 0.6% in acetic acid
 * 4) It has an extremely high melting point of 1580 ⁰C
 * 5) The porosity of the barite is very low, which is shown in the above space filling diagram

Due to these characteristics of Barium Sulphate, an acid dissolution is a very ineffective way to remove this type of scale. A more effective approach is to use a chelating agent when you have a very insoluble compound such as barite. Researchers have found that the best chelating agents for removing barite scale are dethylene triamine pentaacetic acid (DTPA) and ethylene diamine tetraacetic acid (ETDA).

 <span style="color: black; font-family: 'Times New Roman','serif'; font-size: 10.5pt; line-height: normal; margin: 6pt 0in 6pt 0.5in; text-indent: -0.25in;">﻿  <span style="color: black; font-family: 'Times New Roman','serif'; font-size: 10.5pt; line-height: normal; margin: 6pt 0in 6pt 0.5in; text-indent: -0.25in;">

Introduction to EDTA and DPTA:
The purpose of using chelating agents such as **EDTA** and **DTPA** is to turn the insoluble Barium Sulphate into either Barium Carbonate or other compounds that are easier to dissolve using an acid. You can control the stability of the metal complex by controlling the hydrogen ion concentration of the enviroment (i.e. pH). This is most commonly done by dissovling **sodium hydroxide** in solution to increase the pH of the enviroment. An increase in pH will promote the deprotonation of the **EDTA** and **DTPA** molecules (i.e the removing the hydrogen atoms) to form **(EDTA)4-** or **(DTPA)6-** ions. These ion complexes can now adhere to the Barium Sulphate and remove the Ba2+ ions; as a result, leaving the SO42- ions in solution. Another unique characteristic about **EDTA** and **DTPA** is that each nitrogen atom can donate its lone pairs to the Ba atom to form an additional single bond. Since **DTPA** has 5 carboxylic acidfunctional groups and 3 amine functional groups it can form 8 bonds to the Ba, whereas **EDTA** can only form 6. The full octet (i.e. 8 bonds) stabilizes the the Ba better then the 6 bonds, making **DTPA** a better chelating agent then **EDTA** for Barium Sulphate. An example of the **(EDTA)4-** ion complex stabilizing a metal ion is shown in (Figure 4). Also, the stereochemistry of the **Ba(EDTA)** and **Ba(DTPA)** molecules make it very difficult to remove the Ba atom, because of the large oxygen atoms staggered around the Ba. This prevents any sort of nucleophilic substitution or nucleophilic elimination reaction to occur.



**The following video illustrates how an EDTA complex brings Barium Sulphate to dissolution:**

media type="file" key="EDTA complex.swf" width="460" height="423" align="center"

<span style="font-family: Arial,Helvetica,sans-serif;">When using chelating agents such as EDTA and DTPA along with sodium hydroxide in solution to promote the deprotonation of the EDTA or DTPA you end up with the following chemical reaction. Where the Na+(aq) are merely [|spectator ions] for the following reaction.

<span style="font-family: Arial,Helvetica,sans-serif;"><span style="font-family: Arial,Helvetica,sans-serif;">﻿ From the above schematic it’s clear that the chelating technique is a multiple stage industrial process.

**The Process Steps**
<span style="font-family: Arial,Helvetica,sans-serif;">**Step 1:** The first step of the process is to prepare the EDTA solution by adding Sodium Hydroxide, distilled water, Carbonate ions, and the ETDA powder together. Try to maintain a pH of around 8 when preparing this solution, so that you can dissolve the ETDA powder, because EDTA is only soluble at a pH of around 8.

<span style="font-family: Arial,Helvetica,sans-serif;">**Step 2:** Add the EDTA solution into the pipe. At this stage the Barium Sulphate will start to convert into Barium Carbonate, and the Sulphate ions and Sodium ions can be removed in solution.

<span style="font-family: Arial,Helvetica,sans-serif;">**Step 3:** You're left completely with a Barium Carbonate precipitate in the pipe that’s easier to remove than the Barium Sulphate. At this stage you could be left with Calcium Carbonate or any other precipitate depending on how you prepared your EDTA solution.

<span style="font-family: Arial,Helvetica,sans-serif;">**Step 4:** Addition of an acid will bring the Barium Carbonate to dissolution, which then can be removed by flushing it out of the pipe in solution. You’ve now got a clean pipe! <span style="font-family: Arial,Helvetica,sans-serif; margin: 0in 0in 10pt;">This process may need to be repeated several times to get the majority of the barite removed; however, this is really the only technique to remove insoluble scales such as barite through a chemical approach. Each time this process is done you’ll notice that the efficiency of the oil recovery operation will increase. <span style="font-family: Arial,Helvetica,sans-serif;">Back to Top of Page