Logged C/O and Sigma in First Horizontal Well, Revealed Precise Up-Dip Oil Saturation
Study quantitatively the oil saturation, monitor the injection effect, and precisely estimate up-dip oil saturation across a mixed salinity formation. The horizontal well suffered from high water cut with 160 STB/D oil. Some perforations were perforated at high up-dip structure with the inclination build up to 91°, creating a water sump zone and increased water cut.
In consultation with the customer, Weatherford experts decided to record carbon/oxygen (C/O) and sigma data with a wireline tractor. Sigma has a deeper depth of investigation while a shallow investigation log was needed, such as C/O, which is not affected by the mixed salinity due to the injection from the surface in nearby wells.
The tools were deployed via a Raptor Cased-Hole Evaluation System, a pulsed neutron wireline-logging device that provides advanced analysis of oil, water, and gas saturation behind casing.
The sigma log was selected to estimate the formation mixed salinity using a salinity inversion model, while an advanced C/O processing approach using the C/O data from three detectors was used to quantify the oil saturation across all the zones of interest including the high up-dip structures.
The C/O advanced interpretation showed oil at the top of the main sand formation (Upper Burgan 3, UB3) and the rest of zones were watered out as clearly seen from C/O.
Across the same section, sigma was affected by the higher depth of investigation and the effect of the high dip-structures. As a result, it was reading high capture units, showing water mainly, while C/O showed good oil saturation of 17.5 % around the well drainage area of the UB3 formation.
The results of the sigma salinity inversion model were very helpful to accurately estimate the mixed formation salinity profile across the zones of interest with a salinity ranging from 50 to 200 kppm.
The data revealed that sigma was not good in highly deviated/horizontal wells where oil-water contact is nearby, as the deeper investigation is highly affecting and manipulates the capture cross section. A better method is to log C/O with sigma as C/O has shallower depth of investigation and provides more accurate saturations.
The oil gain was 490 STB/D from a total of 2,800 STB/D from intervention of the last workover in October 2022. Previously, the oil gain was only 160 STB/D of oil from a total of 4,259,490 STB/D.
Value to Customer
The advanced C/O processing approach precisely estimated up-dip oil saturation because of using an advanced characterization technique and the C/O data from three different detectors rather than the two detectors traditionally used by most PNC tools for C/O processing.
The first detector from the bottom was closer to the source which helped quantify the oil saturation accurately in the well drainage area with an oil gain of 490 STB/D.