7/28/2019 Getting the Best Out of Fluid Samples

http://slidepdf.com/reader/full/getting-the-best-out-of-fluid-samples 1/1

E&P Exchange

Getting the Best Out of Fluid SamplesJohn M. Williams, SPE, Science Support Services

752 September 1994 • JPT 

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 resultsare interpreted. While fluid-sampling procedures and PVT

experimental techniques are well-documented, no industry guidelinesare 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 clientcould be the operating company for the in-house PVT laboratory).The primary objectives must be selection of a fluid that is mostrepresentative 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 withthe fluid study.

Poor sample quality can arise from such sources as sampling non-representative fluid, human error during sampling or field transfers,

contaminated sample containers, and leaks during shipment.Sampling essentially involves collecting surface samples of the oiland gas from the production test separator, bottomhole samples fromthe wellbore, or both types of samples.

For separator gas samples, the quality checks that should be madewhen 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 liquidstransported with a gas cap must be homogenized by pressurizationand agitation. In this instance, the controls that must be performed are(1) determination of initial opening pressure; (2) determination of 

 bub-blepoint 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 bubble point pressure can be measured at either ambient or reservoir temperature. Measurement at reservoir temperature takeslonger 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 aguide because thorough mixing of the sample may not be achievedduring 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 asaturation 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.1. An adequate sample volume or pressure.2. A bottomhole sample bubblepoint pressure lower than down-

hole pressure during sampling.3. Contamination levels lower than, or similar to, duplicate

samples.4. Bottle opening pressures that agree with sampling data (leaks

unlikely).5. Surface sample bubblepoint pressures that agree with separator 

data.6. A close correlation between laboratory measurements on

duplicate samples.

7. 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 thesampling sheets that accompany the sample bottles. Because a high

 proportion of sampling sheets contain inconsistencies, the reportshould be studied with the following objectives.

1. To identify what well conditioning has been performed.2. To look for the stability of gas and liquid rates when the surface

samples were taken and, possibly, to calculate averages at the timethe samples were taken.

3. To ensure that the GOR is based on oil flow rate at separator conditions.

4. To determine which gas gravity and nonideality factors wereused and the reference pressure and temperature. 5. To verifyreservoir temperature and static pressure. If all samples meet the

quality criteria, the choice can be made on the basis of field dataalone, although the selection is an optimization process in some

cases. The client may have additional data on the reservoir andshould be consulted if any doubt exists concerning sample selectionor data (e.g., the reservoir temperature and pressure that should beassigned for the PVT study). Primary emphasis should be given to

(1) samples collected after proper well conditioning, (2) surface oiland gas samples taken simultaneously or close together, (3) a

 bottomhole sample that was collected above its bubble-point thatcompares well with the bubblepoint pressure for duplicate samples,

(4) a good bottomhole sample in preference to a recombined surfacesample, and (5) a recombined surface sample if doubt exists aboutthe 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 withthe 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 dataand 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, andthis could have important financial penalties. The validation processdescribed will enhance the quality and confidence in PVT study datagreatly; 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 validationdata do not have to be included in the PVT report; however, aminimum 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 thereluctance 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 datainterpreted and the relevant field data validated by a client employee

or an independent consultant. Sample and sampling data qualitycontrol are effectively part of PVT quality assurance. Theycomplement normal laboratory checking procedures used during a

study and in the final report, which still can occasionally cast doubton apparently valid samples (e.g., saturation pressure above reservoir 

 pressure or extremely stable oil/water emulsions).