Gas Sampling Fundamentals

The gas sampling fundamentals of natural gas and NGL has become more important in the last few years because of the increase in price and scarcity of

1. Sampling

The sampling of natural gas and NGLs has become more important in the last few years because of the increase in price and scarcity of gas products. The difference in composition (methane, ethane, propane, butane, gasoline, etc.) and heating value of different streams of natural gas make proper sampling increasingly important. As defined by the Gas Processors’ Association (GPA), sampling is “obtaining a representative sample of a hydrocarbon stream under investigation.” The key word in this definition is representative. A representative sample is a small duplicate of the product as it flows through the pipeline.

2. Importance of Sampling

In today’s market natural gas contracts often contain confusing terms about composition and heating value of the gas. Differences between the seller’s and buyer’s analyses of gas components can result in the buyer refusing to purchase the seller’s gas. An inaccurate representative  can cost a Company millions of dollars in lost revenue, contribute to the improper design of plants, or cause incorrect plant or pipe line balances. Therefore, accurate and representative samples of the gas and liquids flowing through the pipeline are important. It is no longer sufficient to just “grab a sample.”

3. Sample Analysis

From the field, gas and NGL samples are delivered to a laboratory for testing. Here lab technicians draw portions of the sample to measure the percentages of each component in the gas stream. Gas is also evaluated for heating value, measured in British Thermal Units (BTUs). These tests reveal the percentages of each hydrocarbon and, when requested, other components such as H2S, C02, water, etc. All of this information is processed and decisions regarding sales contracts, reservoir studies, property evaluations, enhanced recovery programs, and engineering for new facilities are made.

4. Sample Cylinders

Samples are collected in a device called a sample cylinder. Sample cylinders are often ignored when taking samples, but a cylinder should be of proper working pressure and made of a material that will not react with the sample, usually stainless steel. The cylinder should be cleaned properly with a solvent, such as acetone or trichloroethane, then steam cleaned and air dried. If a sample cylinder has not been cleaned properly, oil, grease, or other contaminates in the cylinder may absorb some of the heavy hydrocarbons in the sample resulting in an inaccurate sample.

Hand valves and relief valves (rupture disks) installed on sample cylinders should be checked for the proper working pressure and be non-reactive with the products being sampled. Proper maintenance of these valves can not be over stressed because of the problems encountered when a leak occurs. A leak of any type creates a sample which is not representative of the flow.

When transported, all sample cylinder should be in a container that will protect the valves and cylinder from damage or loss of the sample. This is a requirement of the Department of Transportation (DOT) because improper transportation of a sample cylinder is a safety hazard. In addition, the sample should be accompanied by properly completed DOT shipping forms.

Expansion of a sample occurs as the ambient temperature increases. Problems due to expansion usually occur when dealing with NGL samples. This can be a safety hazard if the sample has no room to expand within the sample cylinder. The result could be a ruptured cylinder. If a relief valve or pressure safety element is used, then expansion within a cylinder can result in the loss of the sample through the relief valve.

To allow for expansion, outage is usually necessary for NGL samples. Outage simply refers to creating a space within the sample cylinder by bleeding displacement fluid.

Mixtures of hydrocarbons stratify or layer if left long enough in the sample cylinder. A test sample drawn from a stratified cylinder is a non-representative sample; there-fore, the sample in the cylinder must be mixed before a test sample is drawn. In addition, the sample cylinder should be analyzed as quickly as possible after it is taken from the pipeline.

8. Types of Samples

There are three types of samples: spot, incremental, and continuous. In spot sampling, a sample is collected at a particular location for one specific time. Spot sampling is the most used. Incremental sampling involves taking a sample in predetermined quantities over a predetermined length of time, usually 2, 4, 6 or 8 hours. This method is more common in test situations.

Continuous sampling uses a specifically constructed sampler which collects a set quantity of sample in proportion to the flow volume,

e.g. 0.5 cc/40 bbls. past the sample point. The sample which is delivered to the laboratory is a portion of the composite sample.

Although continuous sampling is not as common as spot sampling, it is the most accurate sampling method. Because of continuous sampler cost, this method typically is used for high volume or ethane-rich flows.

• [accordion]
• 1. Gas Sampling Procedure
• 
• 2. ISO 19230-2020 Gas analysis Sampling guidelines
• 3. ASTM D4057 - 22 Standard Practice Manual Sampling of Petroleum