RIASSUNTO
Abstract
Efficient operations are critical to the successful development of North Slope, Alaska, prospects where an abbreviated drilling season and rig-time costs constrain project planning. Logging While Drilling (LWD) provides the real-time data used to make essential well construction decisions. In addition, data are acquired in troublesome formations with complex well profiles where acquisition of wireline data would be overly risky or sometimes impossible. This paper highlights a particular case in which one operator used LWD technology and strategic evaluation to manage risk and significantly reduce operating costs while drilling a well in the Sag River formation.
The Sag River formation is fine-grained sandstone with variable permeability controlled by porosity and lithology. The permeability profile causes a significant transition zone at the oil/water contact. Reservoir engineering studies require an accurate understanding of the formation permeability to determine the optimal well placement strategy, and horizontal wellbores are necessary to maximize exposure in the most productive zones. Considering these factors, it is critical to understand permeability, pore size distribution, traditional porosity, and saturation properties for successful development.
Because of considerable wellbore stability risk in the Kingak shale, LWD services were used to acquire reservoir data. The LWD services used to evaluate this reservoir measured bulk density, neutron porosity, gamma ray, resistivity, spectroscopy, capture sigma, magnetic resonance porosity and T2 spectrum, permeability, pore size distribution, annular pressure, formation pressure, and a computed mobility. Acquiring the necessary formation data with LWD tools saved 3 days of rig time and eliminated stuck tool and fishing risks associated with wireline evaluation. Furthermore, a casing string that would have been used to facilitate prolonged reservoir evaluation operations after drilling was eliminated.
Data interpretation showed that the Sag River formation at this location was oil bearing and that the higher-than-expected water saturation was actually bound water. The spectroscopy results helped refine the clay volume as well as the volume of non-quartz minerals. The results of this evaluation will be used in subsequent reservoir modeling to identify the best approach to exploiting this reservoir. A comparison of the core evaluation with the petrophysical interpretation is also presented.