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Obtaining representative fluid samples is a prerequisite for being able to satisfactorily appraise a hydrocarbon accumulation. The expense, knowledge requirements and limitations of sampling methods needs to be carefully considered when designing sampling strategies

There are four main sampling methods. Open hole sampling by RFT or MDT allows relatively small samples to be obtained. The advances in MDT design now mean that larger fluid volumes can be withdrawn before the sample is taken and allows more control of sample rates. The disadvantages are small sample size, filtrate contamination compromising the use of the sample in PVT experiments, and localised nature of the sample.

DSTs have considerable cost implications but in addition to the reservoir information obtained, allow samples to be taken downhole, at surface both isokinetically and from the separator. The DST samples fluid from a chosen interval and invariably involves a drawdown pressure which can compromise the fluids taken from systems at saturation pressure. There is also often the possibility of unwanted fluid comingling from other intervals. The produced fluids are generally not contaminated by external sources and the large fluid volumes obtained allow detailed PVT and flow assurance studies to be performed. A careful analysis of PVT results can also allow the nature of any downhole fluid commingling to be understood.

A significant problem when analyzing DSTs is to determine how representative recorded GORs are, often due to measurement error from liquid carryover, errors in separator conditions or fluids metering. An EOS analysis of separator products can often identify the causes of such errors and lead to a correction or a quantification of errors in these results.

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