Abstract
Geosteering horizontal wells in the flank of a flatly dipping structure is the focus of a PDO waterflood development project since 2006. The reservoir is a carbonate Shuaiba formation in central north Oman. As the crestal areas have been fully developed, the future development focus is in the flank, which is located within the transitional oil column. This column has a low oil saturation that can vary from 35% to 50% in the first top 2m of the reservoir. Consequently, to avoid leaving attic volumes, wells need to be placed as close as possible to top of reservoir while avoiding penetration of the highly unstable overlying shale.
This field has unique geosteering challenges. The reservoir-caprock interface is an erosional surface which is complicated further by resistive and cemented layers, faults and fractures. These features can deflect the drilling bit for a few degrees in inclination, which seriously affects the geosteering capability. Regarding log analysis implications, the ultra-low resistivity (~0.5 ohmm) in this reservoir provides little resistivity contrast with the shale roof making it very difficult to interpret the upper bed boundary. The ability to interpret the bed boundary is only compensated by the shale anisotropic effect on the propagation resistivity, which is limited to some 1m maximum depth of detection. In addition to the above, insufficient hole cleaning and tortuous trajectory will contribute to high drilling torque that poses a challenge to reach the desired well length. With these multiple challenges, experienced geosteering personnel in this field are an important factor in successful well placement.
The standard practice for geosteering in this field is to use the real time resistivity response and compare it with forward resistivity models to interpret the well position. This had been successfully done by three service companies with experienced geosteering personnel in the ongoing drilling campaign. The standard LWD suite is density, neutron, gamma ray and propagation resistivity. From the propagation resistivity tool, various compensated resistivities are being transmitted in real time and used for the well placement. Gamma ray provides a secondary indicator to indicate if the well is near to the shale caprock.
In conclusion, we are now routinely successful in geosteering horizontal wells in ultra-low resistivity and flank of a flatly dipping structure. This has proved the economic viability of transitional oil column in the flank and thus adding reserves to the field.
