SURFACE SAMPLING 1. General

Surface samples are taken after the well has been conditioned for later recombination in the laboratory. Gas and oil samples should be taken simultaneously forming paired or

‘companion’ samples. It is important that accurate gas and oil production rates are known at the time of taking the samples. Refer to API RP44 for further details.

Before any separator sampling begins, the following procedures should be carried out:

1) Sample bottles should be made ready by having the gas bottles checked to ensure that they have an absolute vacuum and plugs available for each port.

2) Oil sample bottles need to be checked to ensure they are evacuated above the piston, and that the piston is at the top of the bottle. The fluid below the piston should be checked to make sure that there is no air present, as this can give extraneous readings when measuring the fluid flow whilst sampling is in progress. This will cause problems later when an attempt is made to determine the pressure (P_b) in the PVT laboratory.

3) The sampling manifolds should be prepared with gauges to suit the expected sampling pressure already fitted. Liners should be cleansed and made ready. An oil sample bottle stand should be readily available, together with a 600cc measuring cylinder. Sampling manifolds should be kept as simple as practically possible with as small an internal volume as is reasonably possible but with liners that are long enough to avoid any possibility of straining the connections to the sampling point and to the sampling manifold.

4) A bucket of clean water and a supply of rags should also be readily available for leak testing full sample bottles and for wiping clean the bottles before shipping to the PVT laboratory.

5) For gas, sampling should be conducted using evacuated sample bottles. These are clean and easy to use as no flushing is required, hence contamination is unlikely. A vacuum pump is required and care should be taken that no valves become plugged with hydrates.

6) Oil should be sampled using piston bottles. These are clean, easy to use, have a known volume and are mercury-free. They are also relatively easy to use in forming the gas cap for safety during transportation.

7) All samples must be labelled immediately after being taken using Agip sample labels, if available. The following information must be recorded:

• Well number.

• DST number.

• Choke size.

• Perforation interval.

• Time of sampling and duration.

• Oil/condensate and gas rate at time of sampling.

• Stock tank oil/condensate, temperature, gravity and shrinkage, pressure.

• Gas temp, gravity, static and differential pressures, orifice size and meter run size.

• BS&W.

8) All samples should be loaded into an empty container and shipped to base as soon after the test as possible. Record on the morning report, the container in which the samples are being shipped to shore. Do not ship all samples in one container, split samples into two representative batches and ship in separate containers.

9) It is vital when taking samples that any problems are recorded, highlighted and fully documented.

Note: More specific sampling requirements may be detailed on individual well testing programmes.

14.3.2. Sample Quantities

Separator samples should always be taken simultaneously as matched sets of oil and gas samples, thus being sampled under identical conditions. At least two sets of separator samples (2 x oil and 2 x gas) should be taken, so that there is comparability between sets of samples. The ratio of gas samples to oil samples is dependent upon the GOR - hence being one of the reasons stable separator conditions is required.

GOR equal or less than 1,500scf/stb = 1:1

GOR greater than 1,500scf/stb, but less than 3,000scf/stb = 3:2

GOR greater than 3,000scf/stb = 2:1

14.3.3. Sampling Points

The sampling points on a separator should be very carefully chosen as samples taken from the wrong point on a separator will not be truly representative of the produced fluids.

The gas sample point should be:

• Upstream of the Daniels box in the gas line.

• As close to the separator vessel, as possible.

• Not immediately downstream of thermal wells or ports in the flowline.

• Not immediately after a bend in the flowline.

• Ideally the sampling point should protrude into the centre of the gas flowline and face upstream. However, a pipe into the stream is acceptable.

Note: The sampling point should not be on the lower half of the flowline cross

section, due to any possibility of free liquid/liquid carryover being present. If the sampling point has to be fitted flush to the inside surface of the flowline then it is preferable that it is on the top of the line and not on the side.

The oil sampling point should be:

• As close as possible to the exit of the oil flowline from the main vessel and upstream of meters.

• Not immediately downstream of thermal well or bends in the flowline.

• Ideally the sampling point should protrude into the centre of the flowline with the mouth facing upstream. However a pipe into the centre of the flowline is acceptable.

• It should be upstream of any increase in flowline diameter.

• It is preferable that samples are not taken from the bottom of the oil sight glass, as the level in the sight glass does sometimes falls, especially if there is much rig movement which can allow free gas to enter the sampling line.

Note: The sampling point should not be on the upper half of the flowline cross section, due to any possibility of there being free gas. If the sampling point is on the wall of the flowline then it is preferable that it is on the side, rather than on the top or the bottom, due to possibility of free gas or water being in the flowline.

14.3.4. Surface Gas Sampling

The following is the procedure for taking a gas sample:

1) Any flushing should be done through a hose directly downwind, or to sea level, to prevent any risk of poisoning due to gasses such as H2S.

2) Record the bottle number.

3) It is preferable, for the sake of safety, to take gas samples with the bottles lying horizontally unless it can be securely fastened upright or held in a stand.

4) The manifold should be flushed before use, then attached either to the top valve (V1), or to one of the end valves (V1, V2) if the bottle is lying on its side (Refer to figure 14.a).

The manifold valve (V3) should then be opened slowly to test for any leaks. If there is a leak, then close the manifold valve, and remake the connections to the bottle.

Note: No manifold or gauge should be attached to the second valve (V2) under any circumstances. This is to prevent the loss of any of the heavier components of the gas which might have condensed in the bottle when exposed to a vacuum.

5) The bottle valve (V1) may now be slowly cracked open. Even with the noise around a separator, it is still quite easy to hear the gas ‘hissing’ into the bottle and this can also be heard even when wearing a BA set. Sometimes the gauge needle can be seen to slightly dip on the initial opening.

If there is just one gas bottle being filled to one oil bottle, then the sampling time should be about 30 minutes. This length of time means there is less chance of an invalid sample being taken.

If the ratio of gas samples to oil samples is greater that 1:1, then the fill time should be worked out to still allow the oil samples to take about 30 minutes.

6) When the sample bottles are full and the sampling time has elapsed, shut the bottle valve (V1) and the valve on the separator sampling point (V3).

7) Record the pressure on the gauge, and bleed off about 30psi (using V4) then open the bottle valve (V1). The gauge should now read the original sampling pressure. If it doesn’t then check the manifold and the bottle valve for blockages or icing-up. If possible clear the obstruction, take up a fresh bottle, and re-sample both the oil and gas samples. If the pressure returns to near the original, then the sample is good and the separator sampling point valve (V3) may be reopened for a few moments to allow the pressure in the bottle to return to the sampling pressure.

8) Record the final sampling pressure and temperature, as they will be needed for the sampling sheets. The bottle and manifold valves (V1, V3) may now be closed, and the connecting line broken.

9) Plug the valves, and both valves checked in a bucket of water for any leaks. Now place the bottle safely aside.

10) Prepare for the next bottle for sampling.