A comparison of wireline and drillstem test fluid samples from a deepwater gas-condensate exploration well

摘要

Successful sampling of gas-condensates is not straightforward with wireline fluid sampling tools, and ambiguous results are often obtained. As a result well testing has often been regarded as the only method of ensuring representative samples. However, testing is expensive and environmentally unfriendly so to be able to rely on wireline fluid samples would be of great value in exploration and appraisal. Few examples exist where both test and wireline samples have been obtained and compared. In a recent deepwater exploration well offshore Mid-Norway (6305/7-1 Barden) which confirmed the presence of hydrocarbons in the southern extension of the Ormen Lange Dome structure, BP took both wireline and DST samples. This study compares the two sample types and discusses the sampling operations. The drillstem test was justified by the requirement for high quality PVT samples which were needed to give an accurate assessment of the liquid content of the gas. This is critical to determining the processing requirements in a future development of the discovery. The well was drilled in 860m of water and the cold wellbore conditions were considered a key risk to wireline fluid sample quality. The well was drilled with water-based mud and a relatively dry gas (5-30 bbls/mmscf) was expected; the dew point was unknown. The MDT tool was used to take wireline samples using a'low shock' sampling technique (controlled drawdown). The test samples were taken isokinetically from the separator and just upstream of the choke. The two sets of samples show a remarkable similarity in fluid properties, encouraging us to believe we may be able to rely on wireline samples in future wells. The reasons for the success are discussed. This experience demonstrates that, where water based-mud is used, representative fluid samples can be obtained from undersaturated gas-condensate wells using the MDT wireline fluid sampling tool, even in a cold deepwater environment. This experience could enable operators in future to minimise or avoid the need for well testing.