Iron Sulfide scale is a significant problem in oil and gas industry where the iron sulfide depositions have adverse impact to the production operations. Typically, iron sulfide scale formed as result of the reaction between the hydrogen sulfide and iron. Iron sulfide scale has several forms; pyrrhotite (Fe7S8), troilite (FeS), pyrite (FeS2), greigite (Fe2S4), and mackninawite (Fe9S8). Pyrite, which has good thermal stability, tends to deposit at shallower places downhole and is inert to acid. Other iron sulfides react with acids. Thermal stable species are much harder to be removed by acids. Thus the iron sulfide downhole are mainly pyrrhotite, pyrite, marcasite and mackinawite, in which pyrite cannot be removed by acid treatment. Both pyrite and marcasite have very low solubility in HCl, and the only available method of removal in the oil industry is to mill these types of scale.
In this paper we introduced new formulations that can be effectively used to remove the pyrite and marcasite scales with removal efficiency reaches 85%. The new formulation will not release H2S during the removal of pyrite scale or any other iron sulfide scale as HCl, therefore, no need for H2S scavenger and also no HSE consideration are required during the removal because the new formulation can be considered as environmentally friendly. No need for additives such as corrosion inhibitors because these fluids pH is above 11 and they are not corrosive. The new formulation consists of high pH chelating agents such as DTPA (di ethylene tri amine penta acetic acid) at pH range from 11 to 14 and catalytic or converting agent such as potassium carbonate (K2CO3), (Cesium Carbonate) Cs2CO3, or (Cesium formate) CsCOOH. The optimum concentration of the chelating agents is 20 wt % DTPA and 7 wt% for the catalyst/converter. In the oilfield treatment water wetting surfactant or solvent should be injected first to remove the organic scale that covers the iron sulfide scale and then DTPA/Converting agents can be injected. No gases will be released during the scale removal process and this will not increase the pressure of the well during the treatment process.
Currently the removal efficiency of the pyrite scale type is maximum 20 % by HCl. Using the new formulations the solubility of actual field samples of pyrite reached 85%. The new formulation can be used effectively to remove all types of iron sulfide scales especially those cannot be removed by HCl. One more advantage of the new formulation is that the H2S will not evolve during the treatment and that will cut the cost of safety considerations bedside the cost of H2S scavengers.