高含硫气田套管腐蚀情况实验研究及建议

An experimental study on casing corrosion in a high-H_2S gas field

在我国近年对高温高压酸性气田的开发过程中,天然气中的H2S和CO2对套管造成了严重腐蚀,套管在服役过程中腐蚀穿孔和刺漏现象时有发生,对油气井安全生产威胁极大。为此,在实验室条件下采用自行设计的高温高压循环流动釜,模拟研究了四川盆地某气井中的套管在超高温高压高含H2S和CO2的条件下的腐蚀情况——井深4 944～5 738m井段流速太低、套管壁会有水膜、元素硫附着、积水及腐蚀严重。实验以该气田在役使用的3种套管材料为研究对象,分别研究其在不同采气期(携水采气期和积水采气期)的腐蚀情况。根据实验获得的数据并结合API579适用性评价方法,计算出被腐蚀套管的抗挤强度和抗内压强度,进而预测套管的安全服役寿命。结果表明:①该井的套管在携水采气期的腐蚀程度较积水采气期的腐蚀程度轻微;②随着开采年限的增加,套管的抗内压强度和抗外挤强度均呈直线下降;③在实验环境下根据剩余强度曲线图预测3种套管的安全服役寿命至少还有10年。建议在气田的开采过程中应尽量增加携水采气期的开采时间,从而延长套管的安全服役寿命。

During the development of HPHT sour gas fields in China in recent years,serious corrosion occurs so easily on casings by H2S and CO2 in the natural gas that corrosion perforation and corrosion piercing are rather common,which has posed a great threat to the safety of an oil/gas well.In view of this,a self-designed ultra-HPHT circulation flow kettle is thus developed for an experimental study to simulate the corrosion on casings in a gas well of a gas field in the Sichuan Basin under such conditions as high pressures,high temperatures,and high H2S and CO2 contents.In such a case,at the well depth of 4944-5738 m,the fluid flow is rather slow and water filming,the settlement of element sulfur,water accumulation,and corrosion are all found on the casing-pipe wall.In this experiment,three kinds of casing materials used in the said field are selected as the research object to study the corrosion on casings at different gas production periods of water-carrying gas production and water-accumulation gas production.From the experimental data and the Applicability Evaluation Method in API 579,the collapse strength and the burst strength of the corroded casings are calculated so that the safe service life of the casings can be predicted.The results show that(1)the corrosion on casings is not so serious during the water-carrying gas production period as that during the water-accumulation gas production period,(2)the collapse strength and the burst strength of the casings drop sharply with the increase of casing service years,and(3)the residual strength curves obtained from the experiments indicate the residual service lives of such 3 kinds of casings are predicted to be at least 10 years.In the end,it is suggested that the water-carrying production period should be extended as much as possible to prolong the safe service life of casings.