Oil Samples
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
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
3656æ MD
11-5-82
13:15 hrs
220 units
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.
For shipping instructions, contact Mark McCaffrey
tel: (214) 584-9169, e-mail: mccaffrey@OilTracers.com