WorldOil

Acquiring low-contamination samples in high-pressure differential reservoirs

Eni Petroleum saved over half of the rig cost by reducing the time to collect a clean sample from 5 hr to less than 2 ½ hr. This was achieved by minimizing invasion, plugging the near-wellbore pore space with a special mud, and optically monitoring the sample cleanup in real time. Filtrate contamination from Synthetic-Based Mud (SBM) systems tends to obscure and influence several properties of the reservoir fluid characterization analysis. Contaminated PVT samples require correction for reliable test results. The flow assurance samples had to be as clean as possible to obtain reliable analysis, typically less than 10% contamination.1 Downhole optical sensors have been used to distinguish mud filtrate from formation oil and to estimate the contamination percentage while pumping.2, 3 Sample quality is a function of several factors including mud cake quality, time after drilling and overbalance.4 High overbalance induces more filtrate invasion and makes quality samples difficult to obtain.