Chemical Injection Prescription for Success

the methanol injection process involves the injection of methanol into a reservoir through small-diameter parallel tubing (or a tubing/casing annulus) with the aid of an injection pump. Methanol acts as a freezing inhibitor to minimize interfacial tension between oil and water. The rate and amount of methanol injected into the reservoir is strongly dependent on characteristics of the reservoir and project economics.

A methanol injection’s surface system includes a methanol injection pump, a recycle valve and a flash tank (Figure 1). The pump supplies the methanol to the respective wells via block valves. Due to the output pressure and constant flow rate of the pump and the “inhibitor” requirements of the individual wells fluctuating, the recycle valve plays a critical role. It recycles excess methanol back into the flash tank. Any problems associated with the recycle valve not only increase the work load for the methanol injection pump, but also damages the pump if the valve can’t relieve excess pressure in the injection line in time.

A problem commonly associated with the methanol injection system is damage to the valve’s trim due to cavitation. When the pressure of the liquid flowing across the valve’s trim is reduced below its vapor pressure, bubble formation occurs. There’s a reduction in potential energy of the system, and an increase in the system’s kinetic energy. The increase in kinetic energy directly translates into an increase in fluid velocity. However, in the absence of an anti-cavitating trim, when the fluid exits the trim (and the outlet pressure is above the vapor pressure), the bubbles implode and result in cavitation. Energy released by this implosion of bubbles causes significant damage to the valve’s trim, causing it to vibrate and rendering it incapable of controlling the fluid flow adequately.

The methanol injection system on Norsk Hydro’s Troll C platform faced a similar situation when problems associated with the methanol recycle valve decreased the efficiency of the methanol injection process. A detailed analysis of the valve revealed its trim was unable to eliminate cavitation or reduce the fluid velocity exiting the trim, causing severe erosion.

In order to provide a valve that would be equipped with the proper ammunition to combat cavitation and reduce the trim velocity, a disk stack comprising of individual disks equipped with tortuous, expanding passageways was designed.

The fluid flows through these expanding passage-ways, losing energy each time it encounters a right-angled turn, and finally exits the trim at a velocity approximately 80% lower than the original valve’s trim exit velocity. This velocity of liquid exiting the valve’s trim is represented by the equation above.

The 2-in. x 2-in. (6-cm x 6-cm), 1500 ANSI valve provided to Norsk Hydro consisted of a 1.5-in (4-cm) trim equipped with 20 pressure reducing stages utilizing DRAG® technology. The result was an 80% decrease in trim exit velocity; from a calculated trim exit velocity of almost 90 m/s to a trim exit velocity of almost 18 m/s, a level lower than ISA’s recommended trim exit velocity of 23 m/s.

In order to provide added protection to the trim materials, the plug assembly and the disk stack were constructed out of Inconel 718 and stellited 316 stainless steel. The objective in using Inconel 718 was to provide added protection to the valve by coupling the principle of velocity control with a hard trim material. Minimization of trim exit velocities was the sole selection criterion for the methanol pump recycle valve. As indicated earlier, maximum reliability of the pump recycle valve is critical to the efficient operation of the methanol injection system. By deploying a valve equipped with an anti-cavitating, velocity-reducing trim, Norsk Hydro anticipates the final result of eradicating cavitation, and ensuring the smooth and safe operation of the methanol injection system.

Published in Solutions Winter 2002