Vertical survey uncertainty presented a challenge for the development of a thin oil column by horizontal ERD wells in the Chayvo field off the northeast coast of Sakhalin Island, Russia. A novel technique was developed to reduce uncertainty by using a well-established pressure vs depth gradient from a vertical offset well as an independent, constant vertical depth reference. Pressures were acquired in aquifers above the target reservoir and the well landing depths were adjusted to match the measured pressure gradient with the reference pressure gradient. A near-horizontal pilot hole was drilled on the first development well to establish the oil-water and gas-oil contacts and confirm the target landing depth for the subsequent production wells. This technique reduced the vertical uncertainty between wells and improved the ability to land wells as close as possible to the desire depth within the oil column.


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