Design Considerations for High-Pressure Gas Production Facilities
Authors: Wild P.C. 
Published: December 2007

This paper presents design considerations for high pressure gas production facilities. Topics covered include methods for production facilities. Topics covered include methods for hydrate prevention, equipment design and operation, and an investigation into the determination of the critical erosional velocity. Critical erosional velocity field data is compared to published values.

Introduction

Operational considerations associated with gas well production at high flowing tubing pressures (FTP), e.g., pressures greater than 5,000 psig, include:Hydrate preventionEquipment design and operationDetermination of the critical erosional velocity

Appropriate design of the surface facilities can minimize or eliminate operational problems associated with hydrates, high pressures, erosion, etc. pressures, erosion, etc. Controlling the flowrate and surface pressures at the wellhead choke can produce hydrates at the wellhead and in the flowlines. Methanol injection may solve the hydrate formation problem while creating injection pump problems and increasing operating costs. By controlling the flowrate and pressure at the indirect heater choke, the hydrate formation problem and the need for methanol injection are eliminated.

For wells where the flowrate and pressure are controlled at the indirect heater choke, downhole tubulars, wellhead and surface tubulars upstream of the indirect heater choke should be rated for the same pressure. A guide to equipment design and operation to best meet these requirements is presented.

Several sources suggest erosional velocity limitations ranging from 22-80 feet per second. This paper presents field data and compares the data to published recommended critical velocities. Additionally, several equations of state are evaluated for determining dense phase compressibility and density.

The information and recommendations included in this paper pertain to gas production in the Delaware Basin of West Texas. Particularly, this paper deals with wells completed in Particularly, this paper deals with wells completed in the Pennsylvanian-age Morrow formation, approximately 17,000 feet deep.

HYDRATE PREVENTION

An initial design question centers around hydrate formation. If the gas composition is dominated by light hydrocarbons (methane and ethane) and free water is expected, then hydrate prevention is a primary objective in the design of the production facilities. Table 1 shows a typical West Texas Morrow gas composition (on a dry basis at standard conditions, where hydrates are anticipated.

A hydrate formation curve and gas expansion curves are helpful in the initial facility design. Additionally, these curves can be used to set operating conditions. These curves can be found in Section 15 of the Gas Processors Suppliers Association (GPSA) Engineering Data Book.

Hydrate expectancy curves can also be used in conjunction with thermometers to predict surface flowing temperature. In remote locations where power is unavailable for thermocouples and the flowing pressure exceeds the 6000 psig limits on the NPT threads of a thermowell, the following estimating procedure appears fairly accurate.