Page 250 - Formation Damage during Improved Oil Recovery Fundamentals and Applications
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Formation Damage by Inorganic Deposition                     221


              5.2.1 Carbonate scales
                                                           22
              Carbonate scales include precipitates containing CO , which are mainly
                                                           3
              barium carbonate ðBaCO 3 Þ, strontium carbonate ðSrCO 3 Þ, calcium
              carbonate ðCaCO 3 Þ, and iron carbonate (FeCO 3 Þ. Of the carbonate
              scales, calcium carbonate scale is probably the most commonly encoun-
              tered scale in oil and gas production. Many of them can precipitate in
              different mineralogical forms. For example, calcium carbonate (CaCO 3 )
              can exist as calcite, aragonite, vaterite, or witherite. The equilibrium of
              carbonate scales can be summarized by considering the two stoichiometry
              equilibria, with calcium used as an example:
                                C 11  1 2 HCO 3 Þ2C a HCO 3 Þ
                                                    ð
                                       ð
                                 a                         2
                           C a HCO 3 Þ -C a CO 3 Þk 1 CO 2 m 1 H 2 O
                                          ð
                             ð
                                     2
                 From the above equilibria, we can see that the presence of CO 2 or
              increasing CO 2 partial pressure leads to the increase of CaCO 3 solubility
              in water. When CO 2 comes out of solution as the pressure decreases,
              CaCO 3 solubility in water decreases. As CaCO 3 solubility in water
              decreases with the increase of temperature, changing temperature can also
              move the equilibrium to the right in the equation and increase the risk of
              carbonate scale forming. The solubility of CaCO 3 also increases as the
              level of dissolved salts increases (Plummer and Busenberg, 1982).
                 Many carbonate scales are acid soluble, which generally makes it easier
              to remediate than sulfate scales. However, damage in downhole frac-
              packs is very difficult to remove without very good treatment diversion
              or the use of coiled tubing. Therefore, a proactive approach to downhole
              carbonate scale prevention is essential.



              5.2.2 Sulfate scales
              The concentration of sulfate in formation water is usually low, but it is
              high in sea water as shown in Table 5.1. When sulfate-rich water is
              injected to flood the barium/strontium-rich formation water, the
              tendency of forming sulfate scales increases. The sulfate scales include
              barium sulfate ðBaSO 4 Þ, strontium sulfate SrSO 4 Þ like celestite, and
                                                      ð
              calcium sulfate CaSO 4 Þ. calcium sulfate has three different crystalized
                            ð
                                                                        1
              forms known as gypsum (CaSO 4 U2H 2 O), hemihydrate (CaSO 4 U H 2 O)
                                                                        2
              and anhydrate (CaSO 4 ). Gypsum is the most common calcium sulfate
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