Study Of Corrosion To Drilling Rigs By Solid Free Composive Salt Drilling And Completion Fluids And Its Prevention Technology

Solid free composive salt drilling and completion fluids are used increasingly for their good oil layer preservation ability, however, they can cause corrosion of the drilling tool due to large amount of monovalent and bivalent organic or inorganic cations and anions adding to the system for maintaining its density.By using electrochemistry and weight loss method to test the corrosion rate of steel G105 in different salt water:in the single salt water, the corrosion rate sequence is found as : ZnCl₂〉ZnBr₂〉NaCl >NaBr〉CaBr₂( larger than NaCl at high temperature)〉MgCl₂〉KCl〉CaCl₂〉sodium formate, potassium formate. Temperature and dissolved oxygen are the main factors affecting the corrosion rate; In the the composite salt water ,concentration of Cl-1 has great effect on the corrosion rate while concentration of Br-1has slightly less effect on the corrosion rate. The concentration change of Ca²⁺has slightly more influence on the corrosion rate that that of Mg²⁺. Ions interaction analysis suggests that when ratio of the Mg²⁺ and Ca²⁺ is 15, the solution has the least corrosion rate. Corrosion rate is affected mainly by the dissolved oxygen at ambient temperature. The dissolved oxygen is less when the density of the composite salt water is higher, thus a slightly less corrosion rate is observed. When other experiment condition is the same, the corrosion rate increases with temperature for low density composite water (density less than 1.3 g/cm³). Corrosion rate will increase for high density composite salt water as the density is increasing because the pH decreases fast and the salt corrosion mechanism changes to acid corrosion mechanism.The corrosion pattern of salt water for drilling tools at different concentration and different salt species is investigated by SEM and X-ray diffraction. It shows that the corrosion product mainly isβ-FeOOH. After the corrosion product membrane is removed, no spot corrosion is found. The corrosion is therefore uniform corrosion and G105 has rather strong resistance of spot corrosion.Using electrokinetic potential scanning polarization curve, alternating current resistance and dynamic corrosion experiments, the corrosion inhibitors are screened and formulated. The corrosion inhibitors that are compatible with solid free composite salt drilling fluid and completion fluid are selected.The HTHP dynamic evaluation test and in-field test shows the corrosion inhibitors are well compatible with drilling fluid additives and have little influence on the drilling and completion fluid performance. Corrosion inhibition rate reaches over 80%. All the corrosion rates of the drilling fluids are less than 0.05mm/a.