A case history is presented for a horizontal injector drilled at the base of a moveable oil column on top of a tar mat in a carbonate oil reservoir in the Middle East. The well was placed utilizing real-time Logging-While-Drilling (LWD) Nuclear Magnetic Resonance (NMR) oil viscosity correlations and formation tester mobility data.
As a pilot water injector, placed at an oil/tar interface with restricted historic oil viscosity versus depth data, obtaining calibration oil samples was considered critical. Both LWD and pipe-conveyed (TLC) formation tester data sets were acquired, which made direct comparisons of LWD and TLC acquired formation pressures and formation mobilities possible. The comparison proves the reliability of the LWD formation mobilities. The LWD measured formation pressures were supercharged while the TLC formation tester formation pressures were largely in line with expected formation pressures.
The oil viscosity results from the TLC formation tester in-situ viscosity fluid analyzer and the NMR viscosity correlation compared favorably with the laboratory results from the acquired TLC formation tester fluid samples. This indicates that accurate real-time in-situ fluid property determination is possible with modern formation tester and NMR tools.
In this reservoir, at an early phase of acquiring oil/tar transition zone oil viscosity versus depth data, the main lesson learned was that the deeper section of the case study well contained higher asphaltene content than expected, which caused wellbore plugging that prevented reservoir access after suspending the well for tie-in. A clean-out operation was unsuccessful as plugging reoccurred. The well is now planned to be re-sidetracked in 3-20 centiPoise (cP) oil at the top portion of the oil/tar transition zone.