![]() Short-turn horizontal holes generally are different from other horizontal classifications. Multiple, small diameter horizontal holes are drilled extending radially from the same wellbore with some systems, but usually not with more than two holes. It is common to drill the pattern in cased holes with smaller diameters. Horizontal section lengths range from about 100 to a maximum of about 800 ft in a few cases. Short turn: Short-turn horizontal holes have a turn radius of a few feet to 60 ft and angular build rates of 95°/100 ft to greater than 1000°/100 ft MD. It occurs in horizontal drilling, but it is not a severe problem. Hole closure by caving formations was not a major problem in early slant-hole drilling and later ERD. There were early concerns regarding horizontal holes remaining open. These problems cause high risks in horizontal drilling operations and emphasize the importance of planning and prudent operations. Good hole cleaning often is difficult to attain, but a clean hole solves many problems. ![]() In horizontal drilling, high stresses in equipment and tubulars are common. A few of these are summarized for emphasis and special applications to horizontal drilling. Problems increase with increasing depth, higher angles, and longer horizontal sections. The major problems encountered in directional drilling as described in Chapter 4 also occur in high angle and horizontal drilling, often more frequently and with a higher degree of severity. Most drilling problems found in other forms of drilling occur in horizontal drilling operations. Mud logging equipment is run on most wells to aid in drilling, support hole guidance, and help in formation evaluation. But the procedure should be used sparingly because it increases the difficulty of drilling a pattern that often is already complex. 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. 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). 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. Horizontal drilling is a special form of directional drilling.
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