Composition analysis of hydrocarbon gas and liquid streams is required by the Production Department for a variety of reasons including:
- Facility design (Engineering)
- Economic forecasting (Exploitation)
- Field development and reservoir engineering (Exploitation)
- Metering (Production Operations)
- Product allocation (Production Operations)
- Product pricing (Production Operations)
In each of these areas, composition information often has a significant impact on the commitment or allocation of large amounts of money.
Composition is usually determined by laboratory analysis of a fluid sample. The usefulness of the laboratory result is critically dependent on the sample being representative of the stream from which it was obtained. Great care must therefore be taken in obtaining the sample to ensure that it is representative.
A variety of sampling methods, each with its own advantages and disadvantages, may be used. The following section provides general guidelines for routine sampling and well test sampling.
RECOMMENDED GUIDELINES FOR SAMPLING
Testing New Wells
Obtaining representative gas and liquid samples is an extremely important part of any well test. If the laboratory finds the sample quality poor, there will generally be no opportunity to re-sample until permanent gathering facilities are installed.
Individual well samples can be taken as well as test separator samples while an individual well is on test. The following procedures are recommended for sampling during the testing of new gas wells.
- Have an experienced laboratory representative or an experienced employee obtain all samples. Contact the laboratory in advance to inform them of operating pressures and temperatures, gas and liquid rates, sour or sweet, and whether the liquid is formation fluid or load fluid.
- Obtain samples early in the test, but after the well flow has stabilized, so that if problems are encountered, it may still be possible to re-sample before the end of the test.
- Have the laboratory representative obtain duplicate high pressure gas and high pressure liquid samples if applicable.
- Confirm with the laboratory that the samples are good before ending the well test.
- If the gas is sour, the H2S content must be determined in the field. If the H2S concentration is very small (less than 0.5%), it can be determined using a H2S detector.
- If free water is produced, a water sample should be obtained in a plastic bottle from the separator.
Routine Sampling in Existing Operations
Company personnel, provided they are properly trained, may obtain routine samples. A standard accepted method is to be used by company personnel for both gas and liquid sampling that is simple, fast, and generally reliable.
If problems with bad or questionable samples are experienced in specific cases of routine sampling, other sampling methods should be utilized. This should be done initially by an experienced laboratory representative, who can then train company personnel if an alternate method is to be used on a routine basis.
Record and provide the laboratory with all pertinent information regarding the sample: location, date, sample point, pressure, temperature, production rates, name of sampler, and sampling method. Also inform the laboratory of the purpose of the analysis. The laboratory can then choose the best methods of analysis for that purpose, and can often provide valuable assistance with interpreting the results of the analysis when required.
Review Questions :
- Write down the simple steps of sampling new wells in your own way?
- What are the information, you have to add regarding the sample?
Do not locate sample points on dead legs or cavities. For gas, a valve with sample line / point on top of a horizontal line or vessel is best to minimize interference due to any liquid carry-over.
For liquid, the valve and sample line / point should be on the on bottom of a vessel or horizontal pipe so that no gas is present. The sample line should be in the turbulent flow area of the pipe. If it is in a laminar flow area the sample may not be truly representative of the overall flow.
Designing the sample point is important to:
- Obtain a representative sample from the middle of the process line
- Provide a knockout pot for removal of condensate if gas samples are taken
The sample line should avoid the following:
- Long runs with large diameter tubing
- Potential condensation problems by using insulation or heat tracing
There are many crude oil sample points in a field and production plant. Under normal operating conditions the outside operator collects samples of crude oil from sample points at:
- Individual Well
- Production Header
- Outlet of Separators
- Outlet of a Desalter
The supervising operator may tell the outside operator to collect samples from other sample points in the production plant for example, under upset conditions.
The outside operator can collects crude oil samples from different types of sample points for example:
- Sample Pot
- Sample pipe with a connection to the sewer
- Sample pipe without a connection to the sewer
- Gauge glass drain on a separator
Open the sample point valve to remove any debris before connecting a sample line.
Sample Lines and Valves
Keep sample lines short, 0.6m (2 ft) or less, to avoid condensation in gas or flashing in liquid due to temperature change through the line. Use a close nipple connection if possible.
With gas sampling, use a side vent purge valve and connect the cylinder as close to the sample line as possible to minimize dead space at the cylinder valve, as shown in Figure 2.
Purge the sample line thoroughly before admitting sample to the cylinder. If the sample line is not completely purged, the sample will be contaminated with air.
When sampling sweet gas, the sample line may be eliminated if there is sufficient space to connect the sample cylinder directly to the sample valve using a close nipple. The connection is purged by bleeding gas through the sample valve as the cylinder is being screwed on. This arrangement is particularly recommended (where possible) when gas is being sampled at a high pressure, to help avoid condensation between the sample point and the sample cylinder.
The sample point should be equipped with a sample probe so that the sample is drawn from a point away from the wall of the vessel or pipe.
Gas Sampling Apparatus
With gas sampling, it is usual for a sample conditioning system to be used to keep the sample representative of the process, free of solids and/or liquids. Some form of coalescer is used. Some problems can be commonly encountered when sampling. These can be summarized as follows:
Improper drawing of gas samples from process lines
- Avoid sources of possible contamination
- Avoid areas of turbulence
- Avoid areas where “pooling” may occur
Improper sample conditions
- Keep the sample representative of the process
- Requirements for filtration – conductive liquids, pipe scale, salts, oil
Efficient filtration system
- Compatible with the process
- Efficient removal of solids and liquids
Begin with an evacuated container for all sampling methods. This ensures the container is free of all foreign substances.
Where H2S is a known contaminate, the H2S content should be determined in the field since H2S may be absorbed into the sample cylinder wall or react with water vapor, reducing the concentration by the time the sample reaches the laboratory. Duplicate samples should always be obtained.