Accurate measurement volumes are heavily dependent on representative gas analysis data. To obtain representative data, your measurement personnel need to be armed with standard operating procedures (SOPs) and equipped with measurement training to effectively use equipment and execute measurement tasks.

When measurement operations are optimized, then you can improve accuracy and decrease measurement uncertainty. However, an absence of training and natural gas sampling procedures can quickly lead to errors and discrepancies that expose your company to increased audit risk and financial losses.

Challenges Addressed With Sampling Procedures and Training

A gas chromatograph can only report on the sample provided. If that sample collected by your measurement technicians is not representative of the gas flowing, then all volumes calculated using that analysis will be reported incorrectly.

Providing a representative sample to the lab is dependent on the sampling equipment used, the procedures used, and the quality of training. When performed correctly and consistently, natural gas sampling helps reduce uncertainty, builds confidence in the balances, and protects your company from disputes.

Incorrect sampling equipment, improperly maintained equipment, or failure to properly execute sampling procedures can introduce bias into the volume calculations, create disputes with the other parties, and expose your company to financial losses.

That’s why it’s imperative for personnel to be trained on how to follow natural gas sampling procedures. This way, field technicians will have the guidance they need to consistently perform proper sampling and constantly deliver representative samples to your lab.

What is a Representative Sample in Natural Gas Sampling?

When performing sampling, field personnel need to understand how to obtain a “representative” sample of natural gas flowing in a system. This starts with technicians developing a working knowledge of the accepted sampling equipment and procedures outlined in both API 14.1 and GPA 2166.

– API 14.1 (Collecting and Handling of Natural Gas Samples for Custody Transfer)

provides a comprehensive guideline for how to properly collect, condition, and handle representative samples of natural gas that are at or above their hydrocarbon dew point.

For reference, the hydrocarbon dew point is the temperature at which the hydrocarbon components of a hydrocarbon-rich gas mixture, such as natural gas, will start to condense out of its gaseous phase. Understanding how to calculate the dew point is critical for obtaining a representative sample of product flowing in a system.

API 14.1 also includes recommendations for how to identify special areas of concern or importance for each sampling method. It is intended for custody transfer measurement systems and may be applicable to allocation measurement systems.

– GPA 2166 (Obtaining Natural Gas Samples for Analysis by Gas Chromatography) includes the recommended procedures for obtaining samples flowing from natural gas streams that represent the composition of the vapor phase portion of the system being analyzed.

This standard includes detailed sampling procedures using equipment such as a gas chromatograph (GC). GPA 2166 helps measurement personnel understand how to obtain representative samples that can be transported to a laboratory and analyzed for compositions and/or trace contaminants or analyzed on-site by a GC.

Gaining a deeper understanding of API 14.1 and GPA 2166 will help measurement personnel gain confidence in the industry best practices. Then, they can select the proper procedure to satisfy gas quality, pressure, and contractual requirements. This allows the technician to select the proper sampling procedure for each sample location and sample type.

Sampling Techniques to Capture a Representative Sample

Once the building blocks are in place, then measurement personnel need to understand the common sample types used in the field. The sample type selection requires consideration of flow patterns, flow rates, and the required set forth in contracts or tariffs. Consider the three types of samples:

• Gas Spot Sampling. Tip: use for low-volume measurement points where consistent flow is expected.
• Composite Sampling. Tip: use at higher-volume custody transfer points.
• On-Line Gas Chromatograph (GC) Sampling. Tip: use for continuous analysis of gas at high-volume custody transfer points.

Gas Spot Sampling

Spot Sampling provides a representative sample for a specific point in time, so it may not provide a representative sample if flow rates vary significantly or flow is composed of gas from multiple sources. This technique is commonly used where consistent flow rates can be expected. This sample type is typically reserved for low-volume measurement points.

Composite Sampling

Composite sampling is commonly used for higher volume wells, pipeline interconnects, and other intermediate volume custody transfer points. The sample is collected over time and stored in a local cylinder at the sample site. At the designated date, the sample cylinder is removed and set to the lab, and an empty cylinder is put in its place.

When coupled with a sample pump controlled by a flow meter, it provides for a representative sample for the period (typically a month), even if the makeup of the gas stream varied during the period.

On-Line Gas Chromatograph (GC) Sampling

High volume custody transfer meters and process control points may require the installation of dedicated equipment to continually analyze the gas quality.

An on-line gas chromatograph installation includes the piping and equipment to withdraw samples directly from the pipeline and route them to the chromatograph. When properly installed and maintained, the on-line chromatograph provides a representative, near real-time gas analysis for use in volume calculations.

While on-line gas chromatographs are the preferred sampling method for high-value custody transfer points, the reality is that gas chromatographs require expertise to operate and maintain. Training for technicians supporting on-line or portable gas chromatograph begins with the standards defined by GPA 2145, GPA 2166, and GPA 2172, but must also include vendor training for the specific devices.

– GPA 2261 (Analysis for Natural Gas and Similar Gaseous Mixtures by Gas Chromatography) defines the analysis process to derive the mole percentages of the individual components of the gas stream.

– GPA 2145 (Table of Physical Properties for Hydrocarbons and Other Compounds of Interest to the Natural Gas and Natural Gas Liquids Industries) provides the physical constants used in the GPA 2172 calculations.

 – GPA 2172 (Calculation of Gross Heating Value, Relative Density, Compressibility and Theoretical Hydrocarbon Liquid Content for Natural Gas Mixtures for Custody Transfer) provides the calculation methods used to convert the mole analysis data into the derived values

Standards.

– Tip: Ensure that you have up-to-date and relevant measurement SOPs for best sampling practices. We can help through our dedicated program, combined with training and software tools to support your measurement activity.

Increase Measurement Certainty Working with GCI

At Gas Certification Institute (GCI), we provide a combination of measurement SOPs, training, and software tools that will help your personnel perform their duties in the field in an efficient manner.

• Measurement SOPs define the requirements for how measurement will be performed in the field, including the sample types and the procedures used to collect samples.
• Measurement training provides the fundamental knowledge needed to properly and constantly implement the SOP requirements.
• Field operations software (via Muddy Boots) enables personnel to get the job done efficiently, with powerful tools that provide sample scheduling, validation of sample data, and reporting of overdue samples.

GCI helps your personnel confidently apply their learning in the field to decrease measurement uncertainty, reduce losses, and support your company’s measurement objectives.

Schedule a consultation with GCI to discuss this opportunity to grow your measurement capabilities through the implementation of natural gas sampling procedures, training, and software tools. We would appreciate the opportunity to support your measurement program.