Description and Benefits of Drilling Horizontal Exploration and Delineation Wells, Supported by Deployment of New Sensors and Digital Technologies

The necessity for drilling multiple vertical or sub-vertical delineation wells to appraise the extent of a discovery has been overcome in this project by the innovative combination of the latest logging-while-drilling (LWD) formation properties and reservoir mapping tools along horizontal trajectories. By having horizontal wells, operators can gain extended benefits in delineating the hydrocarbon prospects and reservoir structures. Another advantage is the ability to reach and detect lateral boundaries/extension of the reservoir which would be a critical input for hydrocarbon volume calculation in place. Mapping intra-reservoir heterogeneities, current oil-water contact, and/or gas-oil contact would also be possible by drilling a horizontal appraisal well. Lastly, minimizing the number of overburden section drilling could result in significant savings and reduce the environmental impact of the campaign. Øst Frigg Field was produced, from both the Alfa and Beta structures, for gas until 1997, when it was abandoned. A relatively thin oil rim of 7 and 10 m on Alfa and Beta, respectively, was detected by the initial exploration wells drilled on the Øst Frigg Alfa and Beta structures. However, oil was never produced through former development wells. Three horizontal wells were planned to appraise the Beta, Alfa, and Epsilon structures within the Yggdrasil area. The main objective of the project was to improve understanding of the hydrocarbon resource potential, particularly the presence of oil in the Frigg sand reservoir for the three structures. Well A was the first well drilled along the Beta structure, which propagates towards the west side of the Øst Frigg Beta structure with 3,010 m MD of reservoir section drilled to TD point. The ultradeep azimuthal resistivity tool (UDAR) provided delineation of the conductive roof and the conductive base of the hydrocarbon-filled Frigg sand reservoir. The well revealed a continuous oil column up to 25 m thick and the presence of an oil-water contact. Well C, the second well drilled, appraised the northern part of the Beta and explored the Alfa structure, which propagates towards the north side of the field. The well proved the northern extent of the oil accumulation and mapped the oil-water contact. Advanced logging-while-drilling (LWD) NMR and spectroscopy measurements fed continuous petrophysical interpretations, identifying the lithology and special minerals like siderite using a variable autoencoder (VAE) in a neural network (NN). This was confirmed through X-ray diffraction (XRD) analysis done on cuttings, validating the spectroscopy results. Continuous LWD spectroscopy and NMR data (real time and post-TD) drove the quantification of residual gas vs. oil in the produced zones. Both were used as input to multimineralogical models (QElan). The well was called early TD after 1,955 m MD length to avoid the risk of drilling extensively in the shaly section. Well B was the third well drilled, targeting the Epsilon structure towards the east side of the field with 6,012 m MD of reservoir section drilled to TD point. It revealed an oil column up to 15 m in thickness. Up to 30 m of an overlying gas cap in the Epsilon structure was identified by density-neutron and delineated with UDAR interpretation. Evidence of oil even until 6,000+ m MD lateral length at TD point was delivered, leading to estimate the volumetric oil in place. This case study demonstrates the capacity to reduce the number of exploration and delineation wells as the latest technological development in LWD has overcome past limitations for their deployment in horizontal wells drilled for exploration and delineation. By the time the third horizontal well reached TD, the team had set a record for the longest exploration well ever drilled on the Norwegian Continental Shelf (6,012 m MD length), and in total, approximately 11,000 m of horizontal reservoir data was acquired. Preliminary calculations show 53 to 90 million recoverable oil equivalents*.