The horizontal section of the first development well was placed an average of 1.6 m from the top of the reservoir by geosteering it with the PeriScope bed boundary mapper.
“AWE was impressed with the performance of the PeriScope tool and the capacity of the inversion software to accurately image the top of the reservoir. We believe that we could not have achieved the same result with any other well placement method.”
Eric Matthews
New Zealand
Exploration Manager
AWE
AWE accesses attic oil using PeriScope mapper to place horizontal sections close to reservoir roof in the Tui project
Challenge
Place wellbore close to top of formation to access attic oil without exiting reservoir, while maximizing horizontal length and minimizing dogleg severity.
Solution
Use PeriScope bed boundary mapper to guide geosteering decisions.
Results
Drilled 1,850-m horizontal section in 1 run, placing 95% of it within reservoir. Positioned wellbore an average of 1.6 m from reservoir roof, with maximum dogleg severity of 3.2°/30 m.
Maximizing attic oil recovery a challenge
The success of AWE’s development plan for the Tui project in the Taranaki basin offshore New Zealand hinged on the ability to place four horizontal wells as close as possible to the top of the reservoir. That placement—necessary to increase horizontal section length and drain the maximum amount of attic oil—presented a challenge.
Density imaging could not provide the control required because the reservoir sequence was massively bedded, and the synthetic-base mud used to drill the long offset horizontal wells further limited the choice of geosteering tools. Because LWD induction tool polarization horns only give detectable responses approximately 1 to 1.5 m from a boundary, the use of such tools for geosteering would have risked frequent collisions with the reservoir roof and deep excursions into the reservoir. That would have resulted in suboptimal recovery and might have made it impossible to “thread” liners to TD.
Well placed close to reservoir roof
AWE achieved its placement objectives using a PeriScope bed boundary mapper. The relatively high resistivity contrast between the oil-bearing reservoir and the roof made the mapper, with its directional, deep imaging capabilities, ideal for this application. Detailed prejob modeling of more than 20 different geological scenarios indicated that PeriScope mapping could keep the wellbore 0.5 to 3 m below the top of the reservoir.
AWE and Schlumberger, working in close association, created a decision tree to use throughout the drilling. Revisions to the decision tree and prejob modeling were made right up to the point when drilling the pilot hole for the first well began.
The PeriScope mapper was run in the pilot hole to calibrate its response to the relevant geological boundaries. This calibration included imaging the roof from both above and within the reservoir, and imaging the oil/water contact from above. During drilling of the well’s long horizontal section, real-time PeriScope mapping enabled the team of well placement engineers and directional drillers to make the geosteering decisions needed to accurately place the well close to the reservoir roof.
Results exceeded expectations
Use of the PeriScope mapper to guide geosteering decisions enabled the team to place the horizontal section of the first development well an average of 1.6 m from the top of the reservoir, with a maximum dogleg severity of only 3.5°/30 m. The 1,850-m horizontal section was drilled in 1 run, and 95% of it was placed within the reservoir. Wellbore placement in the other three wells was similarly successful, and results exceeded expectations in all four wells.
Directional, deep imaging with the PeriScope mapper accurately determined the depth of the reservoir roof along the full distance of the wellbores. These profiles were used to remap the geological structure, which reduced volumetric uncertainty in postdrill assessment of reserves.
