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3-Dimensional Ultradeep Azimuthal Resistivity, a Tool for Id

Some mature chalk fields of the Norwegian Continental Shelf have a long history of enhanced oil recovery programs utilizing water injection. Small-scale depositional heterogeneity, diagenetic changes, and subseismic faults created barriers or conduits for fluids resulting in variable fluid distribution across the field with complex sweep patterns. Imaging and mapping the fluid distributions play key roles in optimizing well placement and maximizing production. In this partially swept setting, evaluation of the formation and fluid resistivity around development wells plays a major role in the identification of bypassed reserves and the planning of additional target wells to intercept these zones.
This paper illustrates the application of three-dimensional (3D) UDAR inversion in one of the major chalk reservoirs to map the variability of fluids, enabling bypassed pay zones to be targeted, thereby improving economic production. The one-dimensional (1D) UDAR inversion was deployed in combination with conventional LWD logs in real time to optimize well placement. The target well encountered a significant section of the waterflooded reservoir. The 1D UDAR inversion only shows the vertical distribution of the fluids as mapped from their resistivity values; however, UDAR images indicated laterally displaced hydrocarbons to the north-west of the wellbore. A 3D UDAR inversion was performed post-well to map the lateral distributions of the fluids.
During real-time operations, 307 ft of oil-bearing formation was identified in the formation evaluation (FE) logs. The 3D UDAR post-well inversion identified an additional 407 ft of hydrocarbon mapped to the north-west of the wellbore, extending up to 120 ft laterally from the wellbore while drilling in a relatively low-resistivity environment (1 O·m). Three major structural trends correlating with seismic features were identified as the potential cause for the fluid confinement. To increase hydrocarbon recovery in the area, a sidetrack well was planned to intersect the hydrocarbon volume identified from the 3D reprocessing. Anticipating the challenging topology of the new target zone, the 3D UDAR inversion was utilized during the real-time operation. The planned sidetrack successfully intersected the hydrocarbon body, where it was originally detected from the previous well. Additionally, 442 ft of bypassed hydrocarbon was identified from the sidetrack well logs, exhibiting a preferential distribution of the oil-bearing formation to the north-west of the wellbore. In this case, the additional pay was not evident from the seismic analysis, demonstrating the importance of data provided by the 3D UDAR inversion to deliver more detailed subsurface imaging for well planning and real-time operations.
Understanding the lateral extension, fine-scale reservoir architecture, and confinement of the reservoir in three dimensions has a significant impact on mature field development. The 3D UDAR analysis allows the identification of bypassed pay, even at great distances from the wellbore (within the range of 100 ft under the conditions of this case study). This is crucial information for field development planning, supporting justification for additional sidetrack or infill well opportunities that have not been identified in 3D or four-dimensional (4D) seismic, increasing the economic ultimate recovery.
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