Quality control in reservoir Sampling

modified EAGE template

CONTENTSSELECTING SAMPLES FOR PVT STUDIESXXXXXX

SAMPLINGINTRODUCTION

Nr. L. GanzerINTRODUCTIONThis chapter highlights the principal controls that should be performed and gives guidelines for selecting which samples are most likely to be representative.

Poor sample quality can arise from such sources

as sampling a non-representative fluid, human error during sampling or field transfers, contaminated sample containers, and leaks during shipment.SAMPLINGINTRODUCTION

Nr. L. GanzerBOTTOM HOLE SAMPLE OR SURFACE SAMPLEFor undersaturated reservoirs the recommended sampling procedure is bottom hole sampling. Undersaturated reservoirs have initial (or current) reservoir pressure above the bubble point pressure. Surface samples are taken as backup.

For saturated reservoirs bottom hole and surface sampling should be utilised. Saturated reservoirs have initial (or current) reservoir pressure below the bubble point pressure there is either the presence of an original gascap or the presence of a secondary (induced) gascap due to production.SAMPLINGINTRODUCTION

Nr. L. GanzerSAMPLE VOLUMESEstimated minimum Volumes needed for analysisPreliminary AnalysisTwo samples sets are required, each set to consist of:1 litre stabilised crude or condensate10 litre gas, 1 litre of waterIdentification AnalysisTwo samples sets are required, each set to consist of:2 litre stabilised crude (for oil reservoirs) or 10 litre condensate5 litre gas, 5 litre of waterEvaluation AnalysisTwo samples sets are required, each set to consist of:20 litre stabilised crude (for oil reservoirs) or 20 litre condensatePVT Analysis (bottom hole samples)Three samples sets are required, each set to consist of:0.6 litre reservoir fluid or one sample chamber if volume differsPVT Analysis (surface samples)Two samples sets are required, each set to consist of:0.6 litre crude (for oil reservoirs) or 0.6 litre condensate (for gas reservoirs) or one sample chamber if volume differs20 litre gasSAMPLINGINTRODUCTION

Nr. L. GanzerINTRODUCTIONSeperator SamplesThe following QC checks are recommendedDetermination of opening pressureCompositional analysis with focus on air contentDetermination of residual liquids (possibly from carry-over)

Seperator liquids (usually with gas cap) Must be first homogeneized by pressurization, thenDetermination of opening pressureDetermination of bubble-point pressure at seperator temperatureCheck for water phase presence (or sediments)Flash separation to provide GOR, gas gravity, shrinkage


Nr. L. GanzerINTRODUCTIONDownhole SamplesMust be checked as seperator liquid samples, except that bubble-point determination can be made at reservoir temperature

Note that in samples of highly volatile oil or gas condensate fluids no real break point will be seen on the compressibility (between single phase and 2-phase state) and hence the Psat must be determined using a visual window cell.


Nr. L. GanzerINTRODUCTIONSelecting the best Sample from DuplicatesThe following are parameters that should be used, in order of preference, when a sample is selected on the basis of sample quality alone (i.e., when samples are essentially duplicates collected at the same time and under the same conditions):An adequate sample volume or pressureA downhole sample bubblepoint pressure lower than downhole pressure during samplingContamination levels lower than, or similar to, duplicate samplesbottle opening pressures that agree with sampling data (i.e., leaks are unlikely)Surface sample bubblepoint pressures that agree with separator dataA close correlation between laboratory measurements on duplicate samples


Nr. L. GanzerOil-Based Mud Contamination Effect on GC


Nr. L. GanzerSAMPLE VALIDATION CARRIED OUT BY PVT LABGas sample: The sample cylinder is brought to a temperature as close as possible to the on site sampling temperature and the opening pressure of the cylinder is measured. This pressure must be reported in the PVT (analysis) report. Any significant deviation from the original sampling pressure indicates leakage during transit and the sample should be rejected.During analysis, the concentration of air present in the sample is determined. It is not uncommon for small amounts of air to be present due to failure to thoroughly purge the sample bottle prior to sampling and ingress during sample transfer procedures. This may not be a problem since it is common practice to adjust the measured nitrogen concentration in the sample for the nitrogen present due to air contamination (air being approximately 78% nitrogen by volume) and report the composition on an air free basis.The presence of air is indicative of poor sampling techniques.SAMPLINGINTRODUCTION

Nr. L. GanzerSAMPLE VALIDATION CARRIED OUT BY PVT LABHydrocarbon liquid samplesUpon receipt in the laboratory the cylinder opening pressure must be checked to asses any loss of gas due to valve or seal failure during transit.Restoration to reservoir conditions, rocking frequently or continuously, for at least 3 days (5 days, if the sample may be destined for Flow Assurance analyses) sample saturation pressure (i.e., bubble point pressure) must be determined at sampling temperature.The measured saturation pressure of a downhole sample should be at or above reservoir pressure.The measured saturation pressure of a separator sample should correspond with the sampling pressure when collecting the sample from the liquid outlet of a gas liquid separator. In each case, a significantly lower saturation pressure would indicate sample leakage with loss of light ends and the sample should be rejected.All sample containers should be checked for contaminants (e.g., water or mud).SAMPLINGINTRODUCTION

Nr. L. GanzerCHECKS FOR PVT LAB RESULT QUALITYBefore any experimental PVT data are used for design or study purposes, it is necessary to ensure that there are no errors or major inconsistencies that would render any subsequent work useless. Two such means of data validation are the Campbell Diagram (Buckley plot) and the Mass Balance Diagram. These techniques are described below:


Nr. L. GanzerCHECKS FOR PVT LAB RESULT QUALITYBuckley Plot (or Campbell Diagram)

For hydrocarbon gas and liquid phase in equilibrium, there is a linear relationship between the log of the individual component K-values (mole fraction of a component in the gas phase divided by the mole fraction of the same component in the liquid phase) and their respective critical temperatures squared (see Diagram). This technique is described by Campbell. Note that as components become less paraffinic in nature, the greater will be the deviation from linearity. Thus, it is common for the components heavier than C5 to show such deviations.

For the light components, any significant deviations from the linear relationship indicate possible non equilibrium separation, suspect analyses or numerical errors in the data reporting. SAMPLINGINTRODUCTION

Nr. L. GanzerCAMPBELL DIAGRAM


Nr. L. GanzerChapter 5b QC for Fluid SamplingLeonhard Ganzer

PVT and Phase Behavior of Reservoir FluidsSAMPLINGINTRODUCTION

Nr. L. GanzerMASS BALANCE DIAGRAM


Nr. L. GanzerSAMPLE TYPES


Nr. L. GanzerSAMPLING GUIDELINESContamination: where OBM (oil based mud) is used during the drilling of reservoir sections, OBM samples should be collected at the wellsite and have their composition determined. This will allow for removing contamination of reservoir fluid samples at future dates. Additionally, if oil is used during completion of the well prior to sampling, completion fluid should also be collected and analysed to allow for future contamination deconvolution.

PVT samples should be taken as soon as practical after clean up and preferably without closing in the well. The well must be flowing at the lowest practical stable rate and a minimum of 2 tubing volumes to have been produced at this rate.

Single phase sampling is preferred if the well fluid is known to be monophasic at either the perforations (bottom hole sampling) or the well head (surface sampling).

Surface recombination sampling is preferred for all multiphase oil and gas tests.SAMPLINGINTRODUCTION

Nr. L. GanzerSAMPLING GUIDELINESIf wax or asphaltenes are anticipated in an oil or gas-condensate well, or scale formation suspected on an aquifer test, then down hole constant pressure sampling is essential.

Down hole sampling is desirable when the down hole flow is monophasic, but not at surface. In low permeability reservoir and or when the reservoir is close to the saturation pressure, MDT sampling may provide more representative PVT samples than down hole sampling during a production test (because the drawdown may drop below saturation pressure).

Down hole sampling is not recommended where free water production is occurring.SAMPLINGINTRODUCTION

Nr. L. GanzerSAMPLIG GUIDELINESAll down hole sampling should be performed under flowing conditions. However, if this is not possible and samples are taken from a static well, the samples should be taken from the mid point of the oil column (a static gradient survey may be required to establish fluid gradients).

All parameters related to separator and separation efficiency while surface sampling the well shoul be measured, recorded and reported. This should be clearly communicated to and performed by the Operations team, as the sampling contractor will be busy with the sampling operation itself.

Note: Any chemical injection upstream of the sampling points should be stopped prior to sampling.The production test separator and well head conditions must be stable prior to sampling.Once sampling has commenced there must be no changes in conditions at which the test separator is operating.SAMPLINGINTRODUCTION

Nr. L. Ganzer

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Quality control in reservoir Sampling