Factors Governing Oil Well Corrosion

Most crude oils are non-corrosive and as long as weU bore and surface equipment are in an oU-wet condition the production system is protected. This condition will persist as long as oU remains the external phase of the produced liquids. The phase relationship between the oil and water will generally invert between a cut of 25-35% so that water becomes the continuous phase. 

With the inversion the well bore equipment wiU change to a water-wet condition. The time required for equipment to become water-wet is a function of the tenacity and thickness of the oil film. However, once the phase inversion has occurred, eventually the system will become water-wet. 

It is suggested when the cut approaches 25% analyses be reviewed or tests conducted to evaluate the potential corrosivity of the wells. In most production areas waters from the same formation will be roughly comparable as to corrosivity. Also where the produced gas contains either hydrogen sulfide and/or carbon dioxide, it should be anticipated that the produced water will be corrosive. 

The installation of corrosion coupons (corrcoupons) at this time is highly desirable. If significant corrosion occurring, the coupon will give an indication of severity. After a corrosion program has been started a comparison of before and after results are a measure of the treatment effectiveness. 

A step-wise procedure can be followed in evaluating the corrosive possibilities in a well. With produced waters having a pH of 6 or lower, serious corrosion is inevitable once the system becomes water-wet. If the pH ranges between 6.0 and 7.0 corrosion will also occur once the water becomes the external phase and inhibition would be desirable when the attack is of the pitting type or over 0.127 mm/yr (50 mpy). 

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Table 6.1 Rules of thumb" to use in interpreting factors governing oil well corrosion...

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