Facility Chokes on Sand – CCI Comes to the Rescue

Bean chokes have traditionally been used in the “oil-patch” to control hydrocarbon flow rate from hydrocarbon reservoirs. Depending on the required flow rates or reservoir conditions, the “bean” sizes are adjusted manually by field operators. While bean chokes seem a viable solution for a long duration of steady flow rates, they tend not to be the optimum solution in cases where the flow rates vary due to the fluctuations in demand. In addition to lack of controllability and rangeability, these single-stage, labor-intensive bean orifices, such as those shown here, become a high-cost maintenance item due to rapid erosion as a result of high, pressure differentials (severe service conditions) across the choke.

The Katy Gas Storage facility in Katy , Texas is Aquila Energy’s (a fully owned subsidiary of Utilicorp) gas storage facility that supplies gas to neighboring power plants. For some time now, the production/injection chokes at this facility were conventional bean chokes, single-stage fixed orifices to control flow. Recent advances in gas trading and deregulation in the utilities’ markets have given rise to reservoir management optimization at gas storage facilities. Hence, in order to maximize profitability at Katy Gas Storage, the engineers at Aquila Energy investigated several means of optimizing this facility’s operations.

In collaboration with the application engineers at CCI, the engineers at Aquila Energy completed a feasibility study to optimize gas production using advanced production technologies. The solution was to replace the bean chokes with a multistage control valve. The benefits expected to be attained from the modification were as follows:

• Increase in profits by eliminating production downtime resulting from constantly changing the bean size.
• Increase in profits by eliminating production downtime due to constant replacement of the bean trim as a result of erosion from sand particles traveling at high velocities across the valve.
• Profit optimization by tightly controlling flow rates across the control valve.
  In order to calculate the increase in profits due to elimination of production time, the feasibility study involved the following steps:
• Determining flow rates by conducting a detailed analysis of the Inflow Performance (IPR) Curves of individual wells.
• Estimating the production downtime associated with changing the beans.
  
• Translating the information from the above-mentioned two points to lost revenue using the equation:

Lr = QTGp (1)
Where: Lr = Lost revenue ($)
Q = Gas flow rate (Mscf/day)
T = Production downtime (days), and
Gp = Gas price ($)

Finally, in order to tightly control flow through the wide range of flow rates, modulating chokes containing multistage trims were provided. In order to design chokes that would control flow through the wide range of flow rates, a two-flow regime had to be considered – production and injection gas. Hence, the chokes were designed to handle bidirectional flow associated with highly variable system control pressures and pressure differentials in the presence of sand particles.

Solution Description
The eleven chokes provided to Aquila Energy encountered flow rates ranging between 240 and 2400 MMSCFH (0.7 to 70 MMSCMH) at inlet pressures that varied between 475 to 2650 psig (33 to 183 BarG), and differential pressures across these chokes varied between 10 psiG to as high as 1300 psiG (0.7 BarG - 90 BarG). These chokes operated at temperatures around 130 F (38 C). The chokes were custom-designed to meet some 30 specific operating conditions within these flow, pressure, and pressure-differential ranges.

These 11, 6-in. x 6-in. (150-mm x 150-mm), 1500 ANSI angle valves meet these severe and wide-ranging operating and reliability requirements through both special designs and materials.

Design Concept and Details

Using the concept of reduction of trim outlet energy to minimum levels in order to minimize trim damage, each disk within the disk stack consisted of a maximum of 4 pressure reducing stages. The disks were bonded at high temperature to prevent inter-disk erosion. The passage dimension of each disk was designed to be 0.25-in. (6-mm). The purpose of a comparatively large passage dimension was to ensure that sand from the reservoir did not plug up the passages, while meeting the necessary noise requirements.
Rugged guiding and trim-dampening devices were supplied to avoid high frequency resonant vibration of the moving parts. Considering the fact that sand was present in the fluid stream, the disk stacks, plug assemblies and seat rings were manufactured from solid Tungsten Carbide. In addition, dynamic guiding surfaces and dynamic contact surfaces for balance seals were also constructed out of solid Tungsten Carbide.

In the event any maintenance is required, the chokes are characterized with top entry bonnets. The purpose of the top entry bonnets is to ensure that any maintenance work is performed with the utmost ease. The trims for the chokes are of quick change design for ease of maintenance. The bonnets for the valves were designed to be a bolted type rather than the screwed type. The bolted design not only ensures ease of maintenance but ensures against any body replacement charges resulting from damage to the screw threads due to corrosion.

Conclusion

It should be noted that the fundamentals underlying the success of the project was a two-pronged approach, comprising of a strong knowledge of gas storage mechanics and economics combined with a solution custom- designed to achieve the desired objectives.

While an in-depth study of gas storage reservoir mechanics and field economics at Katy Storage triggered the need for advanced, custom-designed products, it was the ability of the custom-designed products to live up to expectations that helped bring the concept to fruition.

Published in Solutions Winter 2002