Getting the Best Out of Fluid Samples
Authors: Williams John M. 
Published: December 2007

E&P Exchange

PVT measurements provide vital data for many reservoir engineering and production applications, but the fundamental importance of having valid samples may be overlooked during preparation of the laboratory study and totally ignored when results are interpreted. While fluid-sampling procedures and PVT experimental techniques are well-documented, no industry guidelines are in widespread use for quality control of laboratory samples, for selection from duplicate samples, or for verification and averaging of sampling data to be used for a study.

This article proposes verifications that should be used and evaluates the implications for the laboratory and the client (the client could be the operating company for the in-house PVT laboratory). The primary objectives must be selection of a fluid that is most representative of the reservoir fluid and identification of any serious quality problems related to the samples or the sampling data; these problems must be communicated to the client before proceeding with the fluid study.

Poor sample quality can arise from such sources as sampling nonrepresentative fluid, human error during sampling or field transfers, contaminated sample containers, and leaks during shipment. Sampling essentially involves collecting surface samples of the oil and gas from the production test separator, bottomhole samples from the wellbore, or both types of samples.

For separator gas samples, the quality checks that should be made when the sample bottles have been heated to, or slightly above, separator temperature are (1) determination of opening pressure; (2) compositional analysis, including air content; and (3) determination of residual liquids, possibly from carry-over. Separator liquids transported with a gas cap must be homogenized by pressurization and agitation. In this instance, the controls that must be performed are (1) determination of initial opening pressure; (2) determination of bubblepoint pressure at separator temperature; (3) flash separation to give GOR, shrinkage, gas gravity, or composition; and (4) a check for presence of sediments or aqueous phase.

Bottomhole samples should be checked in the same way except that bubblepoint pressure can be measured at either ambient or reservoir temperature. Measurement at reservoir temperature takes longer but is preferable for comparisons with downhole static or flowing pressures. Ambient bubblepoint pressure estimates are usually available from field transfers, but should be used only as a guide because thorough mixing of the sample may not be achieved during recompression and temperatures may be unstable. In bottomhole samples of a highly volatile oil or a gas condensate, no "break point" will be seen on the recompression curve, and a saturation pressure must be determined in a windowed PVT cell.

The following are parameters that should be used, in order of preference, when a sample is selected on the basis of sample quality alone.An adequate sample volume or pressure.A bottomhole sample bubblepoint pressure lower than downhole pressure during sampling.Contamination levels lower than, or similar to, duplicate samples.Bottle opening pressures that agree with sampling data (leaks unlikely).Surface sample bubblepoint pressures that agree with separator data.A close correlation between laboratory measurements on duplicate samples.One sample that represents "average" properties of duplicates.

Production test data are most critical for surface samples because portions of oil and gas samples are recombined in the laboratory on the basis of the separator GOR measured in the field. For proper control of the data, the PVT laboratory must have access to the production test report or other documentation in addition to the sampling sheets that accompany the sample bottles. Because a high proportion of sampling sheets contain inconsistencies, the report should be studied with the following objectives.To identify what well conditioning has been performed.To look for the stability of gas and liquid rates when the surface samples were taken and, possibly, to calculate averages at the time the samples were taken.To ensure that the GOR is based on oil flow rate at separator conditions.To determine which gas gravity and nonideality factors were used and the reference pressure and temperature.To verify reservoir temperature and static pressure.

If all samples meet the quality criteria, the choice can be made on the basis of field data alone, although the selection is an optimization process in some cases. The client may have additional data on the reservoir and should be consulted if any doubt exists concerning sample selection or data (e.g., the reservoir temperature and pressure that should be assigned for the PVT study).Primary emphasis should be given to (1) samples collected after proper well conditioning, (2) surface oil and gas samples taken simultaneously or close together, (3) a bottomhole sample that was collected above its bubblepoint that compares well with the bubblepoint pressure for duplicate samples, (4) a good bottomhole sample in preference to a recombined surface sample, and (5) are combined surface sample if doubt exists about the bottomhole sample.

In cases where bottomhole samples have been backed up by surface samples (an excellent practice), creation of a recombined surface sample from the best surface samples might be worthwhile, especially if there are only one or two. This allows comparison with the bottomhole samples. In fact, in important wells, complete analyses on the two types of reservoir fluid samples might be useful. Such an approach would give a high level of confidence in the data and could provide a crosscheck of separator GOR's.

Inadequate checking of sample quality or simple errors in recorded data can lead to PVT studies that are useless or even misleading, and this could have important financial penalties. The validation process described will enhance the quality and confidence in PVT study data greatly; but, of course, it takes effort. It is reasonable that the additional work and reporting that lead to higher-quality fluid studies be compensated by an appropriate increase in cost. All validation data do not have to be included in the PVT report; however, a minimum of one page should be assigned to explain sample selection and to detail any quality or field data problems.

A conflict of interest may exist when the sampling company and the laboratory are within the same organization because of the reluctance to report sample quality problems. In contrast, however, there is the advantage of ready access to necessary data. In such circumstances, a solution might be to have the quality-control data interpreted and the relevant field data validated by a client employee or an independent consultant. Sample and sampling data quality control are effectively part of PVT quality assurance. They complement normal laboratory checking procedures used during a study and in the final report, which still can occasionally cast doubt on apparently valid samples (e.g., saturation pressure above reservoir pressure or extremely stable oil/water emulsions).

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