Oxygen, carbon dioxide and hydrogen sulfide gases are problematic due to their reactions with steels. Oxygen has the lowest solubility. In contrast, the solubility of carbon dioxide and hydrogen sulfide is hundred times higher than oxygen.
Oxygen can hardly be measured with an oxygen meter. However, hydrogen sulfide gas and carbon dioxide can be quickly detected by the destructive surface they cause in the alkaline state of the drilling fluids. Sometimes they also cause the drilling fluid to become acidic. The presence of oxygen in the drilling fluid causes the formation of iron oxide on the rings. But carbon dioxide causes formation of iron carbonate and hydrogen sulfide causes formation of iron sulfate.
Factors affecting corrosion in the drilling industry
The most important factors affecting corrosion in the drilling industry are as follow:
PH: In general the corrosion rate increases with decreasing pH.
Temperature: Most corrosion rates increases with temperature rise.
Velocity: Increasing the velocity of fluids may increase the rate of corrosion generally.
Heterogeneity: Differences in the concentration of elements in the microscopic structure of pipes may increase the corrosion rate.
High Stress: Areas affected by more stress are corrode faster. As a result, drill pipes located just above the Drill Collars will corrode faster.
Many of the mud used today is more corrosive than ever before. Oxygen absorbers should be used to control the amount of dissolved oxygen in the mud. In practice, if the amount of dissolved oxygen in the mud is more than one PPM, corrosion will be observed. Decreasing the pH of the mud and increasing its electrical conductivity causes corrosion rate to increases.
Oxygen enters the mud system in different ways. As a result most of the mud is saturated with oxygen when it enters the pump. Therefore oxygen corrosion is obtained in drilling pipes and mud pumps. This type of corrosion occurs in areas where the metal surface is covered with mud or protective rubber. If the drill pipes are not well cleaned of mud, a pile will form in the area where the mud sticks to the surface of the pipe, which will cause corrosion flow.
Factors affecting oxygen corrosion
Factors affecting oxygen corrosion are oxygen solubility and ambient pH. In general, the colder the water, the higher the oxygen solubility. Oxygen solubility in water at zero degrees Celsius is equal to 14.6 ppm. But the oxygen solubility at 25C is 7.8ppm. According to a dynamic simulation of the drilling system in the laboratory, it is found that with increasing temperature in the depths of the well, the concentration of dissolved oxygen in the fluid does not decrease. Because oxygen cannot leave the drilling fluid to enter the atmosphere. Therefore, in winter, when the air is cold and the solubility of oxygen increases, and the corrosion rate naturally increases.
Oxygen corrosion occurs at any alkaline pH. In general the corrosion rate decreases at high pH. Experiments have shown that the solubility of oxygen in salt-saturated solutions naturally decreases. Therefore, less oxygen absorbers and inhibitors are needed in these muds.
Recognizing and monitoring oxygen corrosion
Recognizing and monitoring oxygen corrosion. Accuracy is the key to detecting oxygen corrosion. Are environmental conditions suitable for oxygen corrosion? Demonstration of oxygen corrosion is possible by measuring the corrosion rate and oxygen sensitive devices.
Oxygen corrosion treatment
Oxygen absorbers are used to reduce the corrosion of steels in contact with the drilling fluid. These materials are added to the drilling fluid through the suction area of the mud pump.
When the above materials are used to absorb oxygen, the amount of sulfide (SO3) in the drilling fluid measured from the Flow Line should be kept in the range of 150-50mg / l. Method of oxygen uptake by sulfide:
½ O2 + SO3 -> SO4
Special kits are used to determine the amount of excess sulfide in drilling mud.
Steps to remove oxygen corrosion
- The injection of oxygen absorber should be done directly through the mud pump suction. Injection can also be done by special equipment for injecting chemicals continuously during drilling.
- The amount of oxygen absorber for injection should be determined according to the amount of oxygen present in the mud and its circulation speed.
Determining the exact amount of adsorbent required is possible by measuring the amount of sulfide in the flow line. The amount of sulfide in the drilling fluid (flow line) should not exceed 50-150ppm.
When oxygen absorbing materials cannot be used in the drilling fluid, inhibitory materials can be used. Liquid chromite inhibitors are available in 55 gallon barrels. Injecting inhibitors into the mud circulation system is similar to injecting oxygen absorber. When using chromatic inhibitors (CrO4) it is recommended to keep at least 500 ppm chromite in the fluid to prevent corrosion. To determine the amount of chromite in the drilling fluid, special kits shall be used. Chromite inhibitors should not be used with sulfide oxygen absorbers.