The Challenges of Evaluating Hydrocarbon-Water Contacts with Residual Hydrocarbon Effects on Resistivity Measurements
Many of the fields in the Highlands of Papua New Guinea (PNG) have residual hydrocarbon intervals below the live hydrocarbon column. This is likely caused by modification of the trap spill point due to continued evolution of the structure post-hydrocarbon charge. The residual hydrocarbon left behind after the water contact has moved to higher structural levels results in a higher resistivity response compared to the resistivity response below the paleo contact (in the interval where no hydrocarbon has been trapped). In some wells the magnitude of the difference in the resistivity responses between the live hydrocarbon interval versus the residual hydrocarbon interval is small. It can be difficult to determine the movable fluid type without the integration of valid formation pressure measurements. A perfect example of this effect was seen on the resistivity responses from the Hides-PWD1-BP1 well. The resistivity-derived water saturation reaches as low as 40% (i.e. as high as 60% hydrocarbon saturation), although a set of valid formation pressure measurements clearly establishes this interval is in the residual hydrocarbon leg with water as the continuous / movable phase. The residual hydrocarbon (residual gas in this well) becomes mobile as the reservoir pressure declines, as demonstrated by the formation gas captured in the downhole samples. Another example of this effect was seen on the resistivity responses from the Muruk-1ST1 well, below 550m TVDSS. This interval also has anomalously high resistivity responses, similar to the responses seen in the Hides-PWD1-BP1 well. No valid formation pressure measurements were acquired in the Toro A repeat interval, as the wireline tool became differentially stuck in another interval. It was difficult to be certain of the movable fluid type in this interval until valid formation pressure measurements were acquired in the Muruk-1ST2 well over the same interval. These valid formation pressure measurements (below 550 mTVDSS) clearly establish this interval is in the residual hydrocarbon leg with water as the continuous / movable phase. Integration of valid formation pressure measurements is required to determine the movable fluid type for any given interval in the Highlands of PNG. The movable fluid type cannot be determined from porosity and resistivity responses alone, due to the residual hydrocarbon effect on the resistivity measurements.
AAPG Datapages/Search and Discovery Article #90371 © 2020 AAPG Asia Pacific Region, The 1st AAPG/EAGE PNG Geosciences Conference, PNG’s Oil and Gas Industry: Maturing Through Exploration and Production, Port Moresby, Papua New Guinea, February 25-27, 2020