GAS CUSTODY MEASUREMENT ACCURACY IMPROVED IN SAUDI ARABIA

Iftekhar Ali
Saudi Arabian Oil Co.
Dhahran, Saudi Arabia

To comply with new and more accurate gas measurement standards Saudi Arabian Oil Co. (Saudi Aramco) modified software for existing flow computers and installed an online gas chromatograph for measuring natural gas and ethane-rich gas sales. The new standards are covered in AGA (American Gas Association) Report 8 and the latest revision of AGA Report 3.

For gases of varying composition (e.g., ethane-rich gas), a knowledge of the pressure, volume, and temperature (PVT) relationship is required for determining supercompressibility factors. The BWR-Starling equation of state was determined to best represent ethane-rich gas properties and is programmed in the new flow computers.

GAS SALES

Saudi Aramco sells natural gas and ethane-rich gas locally to the chemical industry, power plants, and saline-water conversion plants. Natural gas is used as fuel and the ethane-rich gas is used as feedstock and fuel.

The natural gas contains 75-80% methane, and the ethane-rich gas contains 9095% ethane. Gas supplied exceeds 800 bcf/year. Gas associated with crude oil as well as nonassociated gas is collected from gas/oil separation plants (GOSPs) near oil fields and then is sent to several gas plants for treatment and fractionation (Fig. la).

Main trunk lines transport this sweet gas and ethane-rich gas to the industrial area.

Custody transfer metering stations are located along the main trunk line. Each metering station has a skid-mounted orifice flow meter. On-line flow computers receive field data and calculate gas volume at standard conditions and total BTU content.

PREVIOUS INSTALLATIONS

Custody transfer metering station design and installation are based on AGA Report 3 issued in 1978.1 Fig. 1b shows typical piping and instruments consisting of a double chamber orifice (senior) fitting, straightening vanes, and a pressure-reducing station. Data from the dual range (1-20 ft and 0-100 in.) differential pressure, static pressure, and temperature (RTD) transmitters are sent to local flow computers housed in air-conditioned buildings at each location.

At the central locations for natural gas, an on-line calorimeter determines heating value (BTU/scf) and an analyzer measures specific gravity.

For the ethane-rich gas, samples are collected from automatic samplers and sent to a laboratory that determines composition, heating value, and specific gravity.

Data from the local flow computers are also transmitted to a central computer located in the main office by remote terminal units (RTU).

PREVIOUS CALCULATIONS

As per AGA Report 3 (1978), the standard gas volume is calculated by Equation I in the equation box.

The orifice flow coefficient (C') is made up of many factors (Equation 2) that are functions of flowing conditions, gas properties, and physical makeup of the orifice meter.

Methods to calculate these factors are in AGA Report 3. Most of the calculations are straightforward and do not change for varying gas composition.

However, the supercompressibility factor (Fpv) depends on the gas composition, flowing temperature, and pressure. The Fpv factor is defined by Equation 3.

For natural gas, supercompressibility factors are calculated from AGA Report NX-19 (Equation 4).2

For ethane-rich gas, NX-19 is not applicable.

A correlation developed by Hall and Yarborough (H-Y correlation) from Z-factor charts published by Standing-Katz,3 was used (Equations 5, 6, and 7).

The parameters A, B, C, and D are functions of composition, reduced temperature, and pressure. The equation is solved by using the Newton-Raphson iteration technique.

REVISED CALCULATIONS

The two major revisions made to the AGA flow calculations method are the calculation of the supercompressibility factor and the orifice constant.

AGA in 1985 published Report 8 for calculating the supercompressibility factors of natural gas.8 The report was based on studies at the University of Oklahoma by Starling and his associates. Several methods in the report can calculate the supercompressibility factor. However, the most accurate method requires gas composition and flowing temperature.

The equation is shown as Equation 8. The differences between the supercompressibility factors calculated by the two methods (NX-19 and AGA Report 8) are shown in Fig. 2 for varying pressures at constant methane and varying methane at constant pressure.

ETHANE-RICH GAS

AGA Report 8 equations do not apply to ethane-rich gas.

AGA recommends using the appropriate equation of state for the calculation of supercompressibility factors for gases other than natural gas, i.e., for ethane 20%.

Saudi Aramco conducted studies, and a correlation proposed by Starling 5 was considered.

This correlation (Equation 9) is a modified BWR equation of state.

The 11 parameters, (Bo, Ao, Co, etc.) are calculated by the mixing rule as shown below Equation 9.

Equation 9 is a function of pressure and is solved by Newton-Raphson or other iteration techniques assuming an initial density value.

The differences of a tertiary mixture (C1, C2, and C3) at several pressures between Equation 9 and the H-Y correlation are shown in Fig. 2 for varying pressure at constant ethane and varying ethane at constant pressure.

ORIFICE-FLOW CONSTANT

An API revised flow equation (Equation 10) was published in 1991.6

The compressibility factor (Z) is calculated from AGA Report 8 for natural gas and BWR-Starting equation for ethane-rich gas as discussed previously.

FIELD MODIFICATIONS

To implement these changes, all field flow computers were modified. Because existing flow computers were obsolete and spare part availability was a problem, the computers were replaced with new ones programmed with the new equations.

An on-line gas chromatograph was also installed at a central station to replace the existing calorimeter that required constant maintenance.

The on-line gas chromatograph provides the new flow computers with gas composition, heating value, and specific gravity data.

The new configuration is shown in Fig. 1c.

ACKNOWLEDGMENT

The author thanks the Ministry of Petroleum & Mineral Resources and Saudi Arabian Oil Co. (Saudi Aramco) for approving and allowing this article to be published.

REFERENCES

AGA Report 3, Orifice Metering of Natural Gas and Other related Hydrocarbons, 1969.
AGA Par Research Project, NX-19, Compressibility of Natural Gas.
Standing, M.B., and Katz, D. L., AIME Petroleum Development Technology, Vol. 146, 1942, P. 140.
AGA Report 8, Compressibility and Supercompressibility for Natural Gas and Other Hydrocarbon Cases, December 1985.
Starling, K.E., Hydrocarbon Processing, Vol. 50, No. 3, 1971, p. 101.
API Manual of Petroleum Measurement Standard, Natural Gas Applications, Chapter 14.3 Part 3.