ABSTRACT

In subsea oil and gas production, both corrosion inhibitors and hydrate inhibitors are used for chemical treatment due to the severe conditions encountered. Performance is traditionally measured separately due to the low CO2 corrosion rates at temperatures where hydrates form, resulting in plugged flowlines. However, hydrates under certain conditions can alter the rate of metal loss due to changes in fluid chemistry and potentially due erosion and erosion corrosion. The effects corrosion inhibitors, methanol, and low dosage hydrate inhibitors (LDHI's) on the measured corrosion rate under low shear and high shear conditions in the presence of hydrates are concurrently measured. Under the test conditions, significant quantities of hydrates are formed without an inhibitor present. The difference in measured corrosion rates and the qualitative trends between methanol, hydrate inhibitors, a corrosion inhibitor, and their combinations are illustrated. The role of both temperature and shear stress in hydrate and corrosion inhibition is experimentally measured in the laboratory. The role of erosion due to hydrate formation is also explored.

INTRODUCTION

With increasingly severe producing conditions encountered as deeper and deeper waters are explored, control and maintenance of oil and gas production are critical issues. Integrity aspects including corrosion, and flow assurance issues such as hydrate plugging, must be addressed. Specialty chemicals such as inhibitors and dispersants may be used to remedy production problems. Of particular interest is the interplay among production issues and the chemicals that are utilized to solve the problem. One such example is the formation of natural gas hydrates in a subsea system where CO2 corrosion is also occurring.

Natural gas hydrates 1 can pose a threat to any production system that encounters low temperatures and high pressures. Unlike waxes and asphaltenes, hydrates can plug off a flowline during normal production operations, with little or no warning. More vulnerable are the transient operations, such as shut-in and start-up, where temperatures tend to be lower, pressures can be higher, and water has time to accumulate in low spots. Hydrate particles, which are solids, have been speculated contribute to erosion and erosion corrosion.

The traditional method to inhibit hydrate plugging is to prevent hydrate formation altogether. Techniques include insulating the flowline to keep the produced fluids warm, injecting thermodynamic inhibitors such as methanol to lower the hydrate stability temperature, and/or blowing the system down to low pressures during shut-ins.

As an alternative to thermodynamically preventing hydrate formation, low dosage hydrate inhibitors (LDHI' s) can be used to control the nucleation and crystallization process, thereby preventing hydrate plugging. Some LDHI's allow hydrates to form, but control the nucleation and crystallization process, thereby inhibiting hydrate plugging. These products have now been proven in the field to mitigate hydrate plugging 2'3. In this work LDHI refers only to those hydrate inhibitors that alter the particle size and dispersion characteristics and does not include inhibitors that reduce the rate at which hydrates form (kinetic inhibitors).

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