Economic Impact of Different Gas Flow Measurement Standards
Authors: Baldwin Stephen D. , Starling Kenneth E. , Luongo Juan F. , Goforth Myron E. 
Published: December 2007

The volume of gas passing through an orifice meter is commonly calculated following the standards dictated in AGA Report #3 published in 1985 or AGA Report #3 published in 1992. whether the 1985 standards or the 1992 standards are used depends on the contractual agreement between the parties involved. The purpose of this study is to show the volumetric differences that result from comparing the 1985 standards with the 1992 standards and to relate those differences to a dollar value. Natural gases with different compositions are compared and an effort has been made to develop rules of thumb to estimate the direction in which the volumetric flow rate difference will move as a function of composition. Two options to calculate volumetric flowrates using orifice meters are compared:

- AGA Report #3, 1985 together with NX19, 1962 (Gravity, CO2, N2),

- AGA Report #3, 1992 with AGA Report #8, 1992 (Gravity, CO2, N2).

Several different gas streams were considered. The differences in volumetric flow rate ranged from -.08% to .81%. In general, the earlier standards, when compared to the more accurate 1992 standards, calculate lower volume flowrates for rich gases. For some lean and/or high inert and acid gases, the earlier standards calculate higher flowrates than the 1992 standards.

The dollar figure associated with the differences in the calculated volumes is directly related to the price of gas and the flowrates of the gas streams, therefore real life examples where taken to demonstrate the economic impact that the differences have. The results demonstrate that close attention to the choice of standard is well worthwhile.

Introduction

Over the years standards for measurement of natural gas flow have been implemented and revised in an effort to represent the true value as closely as possible. Any time a revision is made there are differences with the previously used standard. The measurement of the gas is done according to the contractual specifications of the parties involved. Some contracts might directly specify what standard the parties have agreed to use to measure the gas and some others might simply state that the measurement shall be performed according to the latest revision of the standard. The purpose of this paper is to show the economic impact of choosing one standard versus another. In order to show these differences we will compare two commonly used standards: AGA Report #3, 1985 together with NX19, 1962 (Gravity, CO2, N2) which we will refer to as Method 1 and AGA Report #3, 1992 with AGA Report #8, 1992 (Gravity, CO2, N2) which we will refer to as Method 2.

Because many companies still using Method are in the process of updating their contracts to reflect the newer standards we felt it would be of use to develop a rule of thumb to estimate the impact of such changes based on gas composition.

Generating Representative Data

Samples from four different fields located in different areas of the United States were chosen as the basis of the study. The fields are labeled 1,2,3, and 4 in Table 1. Then the Specific Gravity, Mole percent of Carbon Dioxide and Mole percent of Nitrogen were allowed to vary around the sampled values as shown in parenthesis in Table 1. The total number of generated points was 5324, corresponding to 11 variations in every variable for every field.

The data was generated in Data - Base format, which allowed easy manipulation and sorting.