Gravity pulls the mass below the stabilizer to the low side of the hole, which decreases the hole angle (Fig. When a stabilizer is run at or above the point of tangency, the stabilizer produces a plumb bob or pendulum effect. The reverse of the fulcrum effect caused by the drillstring is the pendulum principle. Small drill collars and high bit weights are often used to increase drift tendencies. Sizes and utilizing properly located stabilizers, moderate bit weights, and higher rotary speeds. 10-14 Pendulum principle (Courtesy Wilson Directional Drilling) 10-13 Fulcrum principle (Courtesy Wilson Directional Drilling) Fig. ![]() This tendency is reduced by increasing drill collar Fig. The result is that the change in deflection is additive, which tends to increase the hole deflection. The point of contact usually is on the low side of the hole in high-angle holes. The angle formed by this point of contact and the clearance between the drill collars and wellbore approximate the change in deflection that would accompany the next increment of drilled hole (Fig. When a stabilizer is run below the point of tangency, it has a fulcrum effect, which causes the hole to pick up angle. This point of tangency distance from the bit depends on the collar OD and stiffness, the diameter of the hole, and the amount of bit weight. At a certain distance above the bit, the collars touch and rub the sidewall. The bend will normally seek the low side of the hole. To the same bending as a column under static load. 10-12 Downhoie motor (Courtesy Norton Christensen) 10-10 Whipstocks used to kick-off a well (Courtesy Wilson Directional Drilling) Fig. With the fulcrum principle, a string of drill collars under torsional strain and compression is not generally subject to helical buckling, although it is subject Fig. Experience and knowledge arc necessary tools in BHA selection. Likewise, proper stabilizer positioning can increase the hole angle via the fulcrum principle, or it can decrease via the pendulum principle. A packed-hole assembly uses a sufficient number of stabilizers so that hole angle changes are difficult to achieve, which is desirable after the final hole angle has been obtained. The drillstring and bottom-hole assembly play an important role in directional drilling. Motors and turbines are provided by service companies in a wide range of sizes, operating speeds, and torque characteristics for various types of bits and drilling conditions.ĭrillstring Considerations. 10-12).ĭownhole mo tors/turbines are receiving a significant amount of use because they can be used in connection with bent subs and steering tools. The conventional rotary system has the bit connected directly to the drillstring, and the entire combination is rotated from the surface, Downhole motors, used with a stationary drillstring, turn the bit via rotation induced by mud flow within the motor (Fig. In directional drilling, as with all types of rotary drilling, the bit is turned with the rotary system or a downhole motor device. A steering tool is commonly used with the bent sub. ![]() The sub has Vf-Vh" of bend in it that will deflect the motor in the desired direction (Fig. After drilling 12-15 ft below the whipstock, the entire assembly is removed and a regular drillstring assembly is run into the well.īent subs are used with downhole motors. The whipstock is oriented and drilling is initiated. it is a wedge-shaped tool that forces the bit in the desired direction (Fig. The whipstock is a very simple device used to kick off the wcli. A survey was taken at this point and jetting was restarted until the desired well path was achieved. After washing 6-8 ft of hole, the rotary was used to drill the rest of the joint. The jetting action of the mud from the large jet would wash a small cavern into the formation that the drillstring would tend to follow. The drillstring was held stationary while the mud was circulated. The bit was oriented so the large jet faced in the desired direction of the wellbore. It involved the use of a large bit jet and two smaller jets. Jetting was a widely used technique several years ago. Geology affects the decision as well as the desire to use a steering tool. Hole angles are usually kicked off by jetting, whip-stocks, or some type of bent sub, downhole motor tool. Related subjects such as mud and bit selection will not be considered at this point, although they are important to the success of the effort. ![]() The topic of directional drilling techniques can be separated into the following categories: Service companies experienced in directional drilling are usually required to prepare adequately for the upcoming well. In addition to a variety of available equipment, other variables such as bits, mud, and geology must be considered. Directional drilling equipment and techniques have become an involved discipline.
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