Geochemical Sampling Procedures
Hydrocarbon Liquid, Produced Gas, Mud Gas, and Side Wall Core Sampling Procedures
Production samples should be collected at the well head. Well head samples are preferable to header samples, because even header samples may be contaminated by leaking valves, corrosion inhibitor, etc. Flush the sampling point, since taps and needle valves are dead spots in the flow stream and may contain non-representative fluids. Samples should be collected in glass bottles or metal cans, avoiding contact with any plastic and avoiding contact with any oil-contaminated sampling equipment. Plastic bottles contain plasticizers that contaminate the sample. It is NOT necessary to try to separate oil from water, or to try to break emulsions. Minimum sample sizes for archival purposes are 2 drams (~ 4 ml), but larger containers up to quart size are preferred.
After sample collection, the containers should be tightly sealed, and should be labeled with:
- The sampling date and time (ESSENTIAL)
- The well name (ESSENTIAL)
- The field name (ESSENTIAL)
- The sampling point (well head, or header, or separator, etc.) (ESSENTIAL)
- Other location information (OPTIONAL. Possible information might include: country, state, county, basin, latitude/longitude, and/or section/township/range, etc.)
- Operator (OPTIONAL)
- Depth(s) (OPTIONAL)
- Test number (OPTIONAL)
- Reservoir (name and/or lithology and/or age) (OPTIONAL)
- GOR. (OPTIONAL)
- Condensate Samples
Extra care to avoid contamination should be taken when sampling condensates because they typically have low biomarker concentrations and can be easily contaminated by fluids left in the separator from a previous test. To minimize contamination, start with clean equipment and sample near the end of the test. If a condensate is waxy, it should be sampled near the well head upstream of any suspected wax precipitation in the flow lines.
RFT samples are often vented at the surface, thereby losing the gas fraction of the sample. If a DST or PVT sample will not be taken over the interval, any RFT chamber that is believed to contain hydrocarbons should be vented into an evacuated cylinder as described below in order to preserve the gas for possible future analysis.
Produced Gas Samples
Produced gas samples can be taken at the well head, from the production stream, or from a test separator. If associated gas is sampled from an oil well, the sample should be taken upstream of the stock tank (e.g., the test separator) rather than from the top of the stock tank. Samples should be taken in 300 ml metal cylinders (supplier information can be obtained from Weatherford Laboratories, 214-584-9169). Cylinders should have been sent evacuated! If not, evacuate them, and sample as follows:
Locate appropriate sampling port for taking gas sample. A location on or as near as possible to the primary separator is preferred. Avoid low spots in the offgas line for taking gas samples to keep from collecting condensed liquids. Locate appropriate fittings required to rig a sample line from port to cylinder. Check sampling port gas pressure to ensure it is less than 1800 psi. In the unlikely event that sampling port gas pressure exceeds 1800 psi, the pressure needs to be reduced with an additional regulator.
Remove plug from sample cylinder valve. Purge sample line with gas to blow any liquids out of the sampling line by cracking valve on sampling port. Loosely hook up fitting at end of sampling line to sample cylinder and purge again; tighten fitting while purging. Open valve on sampling port all the way.
Open valve to sample cylinder. Confirm that cylinder was originally evacuated by listening for hissing sound as cylinder fills. Wait a minute or so to allow pressures to equilibrate, then close valve on sample cylinder. Close valve on sampling port; disconnect sampling line from cylinder.
Plug cylinder valve and label cylinder with date, time, well name, test number, separator temperature and pressure, perforation depths, and GOR. Record cylinder number in sampling notes (number is stamped on cylinder).
Mud Gas Samples
Mud gas is sampled from the mudlogging gas line during drilling. Samples are collected using IsoTubes, a product of Isotech Laboratories. Contact Weatherford Laboratories or Isotech Laboratories for sampling procedures.
Typically, samples are collected at regular sampling intervals. The interval is well specific, but the example below is typical:
- 1st Depth Range: casing point to top of target -30 m intervals
- 2nd Depth Range: target interval - 5 m intervals
- 3rd Depth Range: 30 m intervals
- Shows: should be sampled at 3-5 m intervals for the duration of the show (e.g., 20 m show = 4 bags).
Each IsoTube should be labeled with well name, depth of sample, date, time, and total gas units recorded. For example,
Prudhoe Bay State -1
Any important notes should also be labeled on the samples, e.g. first sample below casing, mud weight increased, mud additive added etc.
Side Wall Cores
Immediately following collection, side wall cores (SWC) should be handled as follows. All of the elements in this protocol have two goals:
- To minimize evaporation of light ends from the samples, and
- To avoid contamination of the samples with plasticizers or other organic substances.
After being removed from the bullet, the SWC should immediately be wrapped tightly in tin foil, and placed back in the SWC jar. Additional, crumpled foil can be added to the jar to prevent the SWC from excessive movement during transit. The exterior of the jar lid should be wrapped tightly with Teflon tape. The jar should then immediately be put in the freezer. When it is time to ship the samples, they should be shipped frozen, by putting them in a cooler along with frozen gel-packs or frozen plastic bottles of water.