The successful penetration of oil and gas formations by a horizontal well depends on the accuracy of the forecast of the depth and angle of the layers’ dip at the entry point. Methods and mathematical algorithms for predicting the geometric behavior of formations during drilling of a horizontal well at the stage of its approach to the entry point into target productive horizons are developed.

The relationship between the formation dip, their stratigraphic thickness, and apparent vertical thickness in vertical and sub-horizontal wells is considered. It is shown that even small angles of inclination can lead to a significant influence on the prediction of the point of formation opening by a horizontal well. A detailed correlation of the offset well section with a horizontal well one while drilling was used for the analysis. A method for predicting the depth of disclose of the target formation by a horizontal well based on the change in the apparent vertical thickness is shown. A mathematical algorithm for calculating the apparent bedding angle on the basis of initial and while drilling data has been obtained. The calculated bedding angle allows predicting the depth of the target formation penetration with a horizontal well.

The proposed method for predicting the horizontal well landing point depth allows avoiding errors associated with non-horizontal layering. The use of the proposed technique when drilling a number of horizontal wells in the oil fields of the Dnieper-Donets Basin (DDB) and the Pre-Carpathian Foredeep made it possible to determine with high accuracy the apparent bedding angle, even at their small values. The calculations made it possible to predict the depth of entry into the target formation during drilling with high accuracy. This is especially important in the context of small oil deposits, where it is impossible to make significant adjustments to the lateral position of the horizontal part of the wellbore. The predicted depths of the entry points into the formations were confirmed by the drilling results.

The use of the proposed method makes it possible to perform high-quality geosteering while drilling horizontal wells at the stage of approaching the target formation entry point using the minimum data set.

The simplicity of the method allows you to quickly analyze the geological section penetrated by a horizontal well and determine its geometric behavior. This approach makes it possible to successfully open pay formations with horizontal wells even without using a pilot well.

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