The First Horizontal Carbon Oxygen Logging with an Advanced Processing Approach to Precisely Estimate Up-Dip Oil Saturation Across a Mixed Water Salinity Reservoir, Case Study, Kuwait
Authors: Elias M. , Al-Quoud K. , Sergeev E. , Raffi M. , Ikande P. , Tagarieva L. 
Published: April 2024

A horizontal well suffered from high water cut (WC). Some perforations were located across high up-dip structures with inclination buildup to 91 degrees and going back to 89 degrees which created water-sump zones near to the wellbore, leading to an increase in the WC. The objective was to quantify the oil saturation, monitor the surface water injection, and precisely estimate up-dip oil saturation across a mixed-salinity reservoir using advanced carbon/oxygen (C/O) processing approach in a horizontal section for the first time in Kuwait.

It was decided to record Pulsed-neutron capture in C/O (for low salinity water pulse neutron saturation measurements) and sigma (for high saline water content) modes. The main challenge was to properly estimate the water saturation across the up-dip structures water-sump zones. Sigma mode has a deeper depth of investigation; however, a shallow investigation log was needed (C/O) to precisely estimate the oil saturation of the up-dip structures. In addition, C/O is not affected by the mixed salinity due to the water injection in nearby wells. A sigma log was used to estimate the mixed formation water 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.

The C/O advanced interpretation with three detectors showed oil at the top of the main sand formation (UB3) while the other zones were saturated with water. Across the same section, due to the nature of the sigma measurements and the high dip-structures, the sigma readings were affected; 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 with a salinity range of 50-200 kppm. For oil-saturation estimation, sigma was not good in this highly horizontal well due to its nature of measurements and the high dip-structure presence across the horizontal section. The additional acquisition of C/O helped to give more accurate saturations. As a result of this detailed characterization, a workover was done in October 2022 to perforate in UB3 formation, the oil gain was 490 STB/D from a flow rate of 2800 STB/D after the workover intervention as compared to the previous oil production of 160 STB/D of oil from a total of 4259 490 STB/D.

The advanced C/O processing approach precisely estimated up-dip oil saturation by using an advanced characterization technique and the C/O data from three different detectors rather than the two in most of the PNC tools in the industry for C/O processing. The first detector from the bottom is closer to the source which helped to quantify the oil saturation accurately in the well drainage area with an oil gain of 490 STB/D.