Research On The Corrosion Mechanism And Protection Technology For The Casing In Changqing Oil Field

Casing damage is a ubiquitous trouble in most oil fields in the world wide, which severely hinders the oil extraction. In Changqing oil field, it was found the accumulative total of casing damage exceeded one thousand at the end of 2006, which is about ten percent of the total amount of oil wells. There are many causes of casing damage, but corrosion results in the case in Changqing oil field because of the famous corrosive environment.In this thesis, the corrosion behaviors of casing in changing oil field were investigated by damaged casings, hanging samples in oil wells and laboratory research. On the basis of corrosion characters, special geographical entironment, inherent characteristic of "low pore, low permeability, and low salinity", two types of anti-corrosion technologies for outer corrosion and other two for inner corrosion of casing were designed and put in practice. They integrate the anti-corrosion network of the whole oil field.Investigations discovered the inner corrosion caused by the flowing liquid in Jurassic reservoir and severe outer corrosion resulted from corrosive groundwater in Luohe. It discovered that effect of oxygen on the corrosion rate was less than that of sulfide hydrogen and carbon dioxide. Evident pits were observed. The corrosion rate was affected HCO₃^-, mineralization and temperature. Four equations, De Waard-Milliams, OLI, modified Shell and NORSOK, were adopted to predict the corrosion rate of casing in Changqing oil field. The results show that the OLI software introduced by Changqing Company needed be modified for accurately calculate casing corrosion rate.As a patent technology, an anti-corrosion measure combing cold-tangled epoxy belt and zinc anode was proposed and applied in Changqing oil field. Based on the comparison of electrochemical performance and cost, high purity zinc and epoxy were chose as sacrificial anode and cold-tangled belt, respectively. The actual generate electricity of high purity zinc in simulant solution reach to 98.9%. Cold-tangled epoxy technology was confirmed as following after optimization process: 8% to 10% of dosage of curing agent, five minutes of churn time and fifteen minutes static reaction time. Locale pretreating equipment and cure-repairing technique were developed to support the anti-corrosion method. The tests show that all the performances of anti-corrosion coating satisfied the industry technical criterion. The vertical potential distribution indicated the stable protection of casing was come into being in a short time.An impressed current cathodic protection technology applied to protect multiple drilling wells casing corrosion has been proposed and put in practice because of great amount of wells and absent groundwater. It brings several neighboring wells into one protection system, which means the anode well is public. In order to balance the potential in every protected well, protection potential is controlled by strong current electron element, and the special circuit of current distribution. The high silicon cast iron and MMO is used as complex assistant anode. The protection potential at the well port and supplied current for every well are set as -1.5V to -1.0V and 4A to 6A, respectively. Excellent protection for casing corrosion was explained by the detected graph by the self-design vertical potential distribution instrument. This protection technology was applied in more than two thousand oil wells. No casing damage was found. For the sake of satisfy the atrocious environment in northern of Shanxi province, the protection mode of circuit, transformation mode of transformer and configuration of equipment outdoors were progressed. The remote-supervision system of cathodic protection was come true.DPC is an epoxy-bakelite coating sintered at high temperature. It has many advantages such as high temperature endurance, corrosion endurance and strong binding, so it is a candidate for inner anti-corrosion coating on casing. Laboratory data discovered that DPC coating assuredly had excellent mechanical performance, acid and alkali durability, anti-corrosion of carbon dioxide and hydrogen sulfide under high temperature and high pressure. The corresponding technologies such as coating the screw thread after unloading hoop, installing uprighter to prevent wear and adopting hydraulic packer settled the leak of solution and coating damage well. Application in new wells in severe inner corrosion area and old oil field proved DPC coating was a good method to control the inner corrosion of casing in Changqing oil field.Z-3 inhibitor was talent showing itself from six inhibitors by stimulant corrosion experiments in solution and oil-solution. It was suggested to prevent the inner corrosion of casing, but the pumping trouble must be well settled. An inhibitor dropper in well was developed. It hides the shortcoming of traditional continuous or intermittent pumping mode. The Z-3 inhibitor was found steady-going work in long time by analyzing the concentrations of iron ion and inhibitor in water produced. Z-3 inhibitor injected by the dropper could decrease drastically corrosion rate of casing and restrain the pit corrosion. These results were observed from hanged sample inner casing in wells. Field application discovered the inhibition efficiency was eighty-four percent at least.The four anti-corrosion technologies mentioned above present great efficiency after applicating in the oilfield. The corrosion inducing break of casing decreased drastically. They can satisfy different oil wells in the oil field, so application is strongly accelerated. Thus far, prominent economic benefit and strong technical support is achieved.