It is possible to plug back and sidetrack medium- and long-turn holes in either the curved or horizontal sections. There are indications that drilling straight sections with drift angles of 700–900 are similar. Horizontal and extended-reach drilling (ERD) described here includes angles greater than about 600, more commonly about 700–900. The discussion referring to horizontal drilling generally applies to high-angle extended-reach patterns unless otherwise noted. Horizontal and high-angle drilling operations generally are similar to directional drilling but more complex because of higher build rates and drift angles, and tangent and horizontal sections. Other major applications of horizontal drilling are related to naturally fractured tight reservoirs, thin dry reservoirs, and less wells needed to develop a field. Horizontal wells not only increase the exposure of pay zone but also help avoiding the gas and water coning problem by providing the less drawdown pressure.
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These classifications are summarized in Table 3.1. Hence, horizontal wells have been classified based on the radius of built as short radius, medium radius, long radius, and ultralong radius. The BUR of horizontal well can vary depending on the objectives of the well. Sometimes, the horizontal well can have a straight section between two build sections ( Fig. 3.9).
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Horizontal well trajectory begins with a VS, then a build section that starts from an angle of 0° from the vertical and continues to build gradually until 90°, and then a horizontal or lateral section that penetrates the formation of interest ( Fig. 3.8). It is usually applied to thin formation with good vertical permeability or formations with high ratio of vertical to horizontal permeability. The long laterals of the well increase reservoir–well contact, thereby improving reservoir productivity. It involves drilling a well through a formation at well inclination of 90° from vertical.
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Horizontal drilling is a special form of directional drilling.